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Sample records for ferromagnetic insulating state

  1. Interface driven states in ferromagnetic topological insulator heterostructures

    NASA Astrophysics Data System (ADS)

    Lauter, Valeria; Katmis, Ferhat; Moodera, Jagadeesh

    The broken time reversal symmetry (TRS) states can be introduced into a topological insulator (TI) material by ferromagnetic ordering at the interface. Recently we demonstrated a fundamental step towards realization of high temperature magnetization in Bi2Se3-EuS TI-FMI heterostructures through observation of magnetic proximity-induced symmetry breaking on the Bi2Se3 surface via the exchange interaction by depositing EuS film on the top of the Bi2Se3 surface.Here we show that we can independently break the TRS on both surfaces of a TI, which brings the long-range induced magnetism on either or both surfaces of a TI in a controlled way. We provide a depth-sensitive data on details of magnetic proximity effect in hidden interfaces by Polarized Neutron Reflectometry. The proximity coupling strength and penetration depth of magnetism into TI are extracted as functions of temperature, magnetic field and magnetic history. The large neutron absorption of Eu atoms serves as the element sensitivity and enables us to identify such magnetism in TI as proximity magnetism. This provides a next step to realization of complex heterostructures of TI and FMI leading to wide applications in TI-based next generation spintronic devices. Supported by U.S. DOE, Office of Science, BES, MIT MRSEC award DMR-0819762, NSF Grant DMR-1207469, ONR Grant N00014-13-1-0301, NSF Grant DMR-1231319.

  2. Surface-State-Dominated Spin-Charge Current Conversion in Topological-Insulator-Ferromagnetic-Insulator Heterostructures

    NASA Astrophysics Data System (ADS)

    Wang, Hailong; Kally, James; Lee, Joon Sue; Liu, Tao; Chang, Houchen; Hickey, Danielle Reifsnyder; Mkhoyan, K. Andre; Wu, Mingzhong; Richardella, Anthony; Samarth, Nitin

    2016-08-01

    We report the observation of ferromagnetic resonance-driven spin pumping signals at room temperature in three-dimensional topological insulator thin films—Bi2Se3 and (Bi,Sb ) 2Te3 —deposited by molecular beam epitaxy on Y3 Fe5 O12 thin films. By systematically varying the Bi2 Se3 film thickness, we show that the spin-charge conversion efficiency, characterized by the inverse Rashba-Edelstein effect length (λIREE ), increases dramatically as the film thickness is increased from two quintuple layers, saturating above six quintuple layers. This suggests a dominant role of surface states in spin and charge interconversion in topological-insulator-ferromagnet heterostructures. Our conclusion is further corroborated by studying a series of Y3 Fe5 O12 /(Bi,Sb ) 2Te3 heterostructures. Finally, we use the ferromagnetic resonance linewidth broadening and the inverse Rashba-Edelstein signals to determine the effective interfacial spin mixing conductance and λIREE.

  3. Surface-State-Dominated Spin-Charge Current Conversion in Topological-Insulator-Ferromagnetic-Insulator Heterostructures.

    PubMed

    Wang, Hailong; Kally, James; Lee, Joon Sue; Liu, Tao; Chang, Houchen; Hickey, Danielle Reifsnyder; Mkhoyan, K Andre; Wu, Mingzhong; Richardella, Anthony; Samarth, Nitin

    2016-08-12

    We report the observation of ferromagnetic resonance-driven spin pumping signals at room temperature in three-dimensional topological insulator thin films-Bi_{2}Se_{3} and (Bi,Sb)_{2}Te_{3}-deposited by molecular beam epitaxy on Y_{3}Fe_{5}O_{12} thin films. By systematically varying the Bi_{2}Se_{3} film thickness, we show that the spin-charge conversion efficiency, characterized by the inverse Rashba-Edelstein effect length (λ_{IREE}), increases dramatically as the film thickness is increased from two quintuple layers, saturating above six quintuple layers. This suggests a dominant role of surface states in spin and charge interconversion in topological-insulator-ferromagnet heterostructures. Our conclusion is further corroborated by studying a series of Y_{3}Fe_{5}O_{12}/(Bi,Sb)_{2}Te_{3} heterostructures. Finally, we use the ferromagnetic resonance linewidth broadening and the inverse Rashba-Edelstein signals to determine the effective interfacial spin mixing conductance and λ_{IREE}. PMID:27563980

  4. Noise signatures of metastable resistivity states in ferromagnetic insulating manganite

    SciTech Connect

    Przybytek, J.; Fink-Finowicki, J.; Puźniak, R.; Markovich, V.; Jung, G.

    2015-07-28

    Pronounced noise signatures enabling one to discriminate metastable resistivity states in La{sub 0.86}Ca{sub 0.14}MnO{sub 3} single crystals have been observed. The normalized noise spectra for metastable resisitivity differ both in shape and magnitude, indicating that the metastable state is associated with transition of the electronic system into another local minimum of the potential landscape. Such scenario is consistent with freezing of the electronic system into a Coulomb glass state.

  5. Hybridization-induced interface states in a topological insulator - ferromagnetic metal bilayer

    NASA Astrophysics Data System (ADS)

    Hsu, Yi-Ting; Jadaun, Priyamvada; Fennie, Craig; Kim, Eun-Ah

    Recent experiments demonstrating large spin-torque in topological insulator(TI)/ferromagnetic metal(FM) bilayer, revealing their potential for spintroics applications raised much excitement. However, there is little understanding on the impact of the bilayer formation on the TI surface state and whether it is possible to represent such bilayer using a simple model. Moreover, due to the large charge-transfer from the FM layer, these Dirac surface states are unlikely to be anywhere near the fermi level to contribute to the observed spin-torque. In order to establish a theoretical starting point, we calculated the band structure of a TI-FM bilayer using density functional theory (DFT) and built a simple effective model that captures the essence of the DFT results. Through this double-pronged approach, we find new surface states we dubbed ``reflection states'' to form close to chemical potential due to level-repulsion between the original Dirac surface states and the energetically close-by FM states with the same momentum. Depending on the coupling strength, the 'reflection' states can carry a large weight of the original surface states and thus inherit not only the spatial localization but also the spin-winding of the original Dirac surface state. This work was supported by the Cornell Center for Materials Research with funding from the NSF MRSEC program (DMR-1120296).

  6. Magnetotransport Methods to Probe Surface States of Topological Insulator Thin Films and Topological Insulator/Ferromagnet (TI/FM) Heterostructures

    NASA Astrophysics Data System (ADS)

    Kumar, Raj

    confirmed by the cos(theta) dependence of field titled MR measurements on the Bi2Se3 thin films. No switching in the AMR or hysteresis behavior in the MR was observed in control experiments performed on non TI materials with superconducting electrodes and metal electrodes on Bi2Se3 TI films. The growth and characterization of Bi2Se3/Bi 2Se3/La0.70Sr0.30MnO3 (TI/FM), a topological insulator/ferromagnet heterostructure is discussed in the last part of the thesis. We have grown Bi2Se3/Bi2Se 3/La0.70Sr0.30MnO3 (TI/FM) heterostructures by the method of pulsed laser deposition. Bi2Se3/La 0.70Sr0.30MnO3 (LSMO) is a strong ferromagnetic material with Tc ˜ 350 K and Bi2Se3 is the most studied topological insulator. XRD and phi scan measurements of Bi2Se3/La 0.70Sr0.30MnO3 (TI/FM) heterostructure showed that epitaxial thin films of Bi2Se3 were grown on the LSMO template. Strong in-plane magnetization was confirmed by magnetometry measurements of the Bi2Se3/LSMO heterostructure. Magnetotransport measurements showed a distorted weak anti-localization effect with hysteretic behavior due to interface induced ferromagnetism in the Bi2Se 3 TI films.

  7. Achieving High-Temperature Ferromagnetic Topological Insulator

    NASA Astrophysics Data System (ADS)

    Katmis, Ferhat

    Topological insulators (TIs) are insulating materials that display conducting surface states protected by time-reversal symmetry, wherein electron spins are locked to their momentum. This unique property opens new opportunities for creating next-generation electronic and spintronic devices, including TI-based quantum computation. Introducing ferromagnetic order into a TI system without compromising its distinctive quantum coherent features could lead to a realization of several predicted novel physical phenomena. In particular, achieving robust long-range magnetic order at the TI surface at specific locations without introducing spin scattering centers could open up new possibilities for devices. Here, we demonstrate topologically enhanced interface magnetism by coupling a ferromagnetic insulator (FMI) to a TI (Bi2Se3); this interfacial ferromagnetism persists up to room temperature, even though the FMI (EuS) is known to order ferromagnetically only at low temperatures (<17 K). The induced magnetism at the interface resulting from the large spin-orbit interaction and spin-momentum locking feature of the TI surface is found to greatly enhance the magnetic ordering (Curie) temperature of the TI/FMI bilayer system. Due to the short range nature of the ferromagnetic exchange interaction, the time-reversal symmetry is broken only near the surface of a TI, while leaving its bulk states unaffected. The topological magneto-electric response originating in such an engineered TI could allow for an efficient manipulation of the magnetization dynamics by an electric field, providing an energy efficient topological control mechanism for future spin-based technologies. Work supported by MIT MRSEC through the MRSEC Program of NSF under award number DMR-0819762, NSF Grant DMR-1207469, the ONR Grant N00014-13-1-0301, and the STC Center for Integrated Quantum Materials under NSF grant DMR-1231319.

  8. Interface induced states at the boundary between a 3D topological insulator Bi2Se3 and a ferromagnetic insulator EuS

    NASA Astrophysics Data System (ADS)

    Eremeev, S. V.; Men`shov, V. N.; Tugushev, V. V.; Chulkov, E. V.

    2015-06-01

    By means of relativistic density functional theory (DFT) calculations we study electron band structure of the topological insulator (TI) Bi2Se3 thin films deposited on the ferromagnetic insulator (FMI) EuS substrate. In the Bi2Se3/EuS heterostructure, the gap opened in the spectrum of the topological state has a hybridization character and is shown to be controlled by the Bi2Se3 film thickness, while magnetic contribution to the gap is negligibly small. We also analyzed the effect of Eu doping on the magnetization of the Bi2Se3 film and demonstrated that the Eu impurity induces magnetic moments on neighboring Se and Bi atoms an order of magnitude larger than the substrate-induced moments. Recent magnetic and magneto-transport measurements in EuS/Bi2Se3 heterostructure are discussed.

  9. Spin pumping in Ferromagnet-Topological Insulator-Ferromagnet Heterostructures

    PubMed Central

    Baker, A. A.; Figueroa, A. I.; Collins-McIntyre, L. J.; van der Laan, G.; Hesjedal, T.

    2015-01-01

    Topological insulators (TIs) are enticing prospects for the future of spintronics due to their large spin-orbit coupling and dissipationless, counter-propagating conduction channels in the surface state. However, a means to interact with and exploit the topological surface state remains elusive. Here, we report a study of spin pumping at the TI-ferromagnet interface, investigating spin transfer dynamics in a spin-valve like structure using element specific time-resolved x-ray magnetic circular dichroism, and ferromagnetic resonance. Gilbert damping increases approximately linearly with increasing TI thickness, indicating efficient behaviour as a spin sink. However, layer-resolved measurements suggest that a dynamic coupling is limited. These results shed new light on the spin dynamics of this novel material class, and suggest great potential for TIs in spintronic devices, through their novel magnetodynamics that persist even up to room temperature. PMID:25601364

  10. Large discrete jumps observed in the transition between Chern states in a ferromagnetic topological insulator

    PubMed Central

    Liu, Minhao; Wang, Wudi; Richardella, Anthony R.; Kandala, Abhinav; Li, Jian; Yazdani, Ali; Samarth, Nitin; Ong, N. Phuan

    2016-01-01

    A striking prediction in topological insulators is the appearance of the quantized Hall resistance when the surface states are magnetized. The surface Dirac states become gapped everywhere on the surface, but chiral edge states remain on the edges. In an applied current, the edge states produce a quantized Hall resistance that equals the Chern number C = ±1 (in natural units), even in zero magnetic field. This quantum anomalous Hall effect was observed by Chang et al. With reversal of the magnetic field, the system is trapped in a metastable state because of magnetic anisotropy. We investigate how the system escapes the metastable state at low temperatures (10 to 200 mK). When the dissipation (measured by the longitudinal resistance) is ultralow, we find that the system escapes by making a few very rapid transitions, as detected by large jumps in the Hall and longitudinal resistances. Using the field at which the initial jump occurs to estimate the escape rate, we find that raising the temperature strongly suppresses the rate. From a detailed map of the resistance versus gate voltage and temperature, we show that dissipation strongly affects the escape rate. We compare the observations with dissipative quantum tunneling predictions. In the ultralow dissipation regime, two temperature scales (T1 ~ 70 mK and T2 ~ 145 mK) exist, between which jumps can be observed. The jumps display a spatial correlation that extends over a large fraction of the sample. PMID:27482539

  11. Large discrete jumps observed in the transition between Chern states in a ferromagnetic topological insulator.

    PubMed

    Liu, Minhao; Wang, Wudi; Richardella, Anthony R; Kandala, Abhinav; Li, Jian; Yazdani, Ali; Samarth, Nitin; Ong, N Phuan

    2016-07-01

    A striking prediction in topological insulators is the appearance of the quantized Hall resistance when the surface states are magnetized. The surface Dirac states become gapped everywhere on the surface, but chiral edge states remain on the edges. In an applied current, the edge states produce a quantized Hall resistance that equals the Chern number C = ±1 (in natural units), even in zero magnetic field. This quantum anomalous Hall effect was observed by Chang et al. With reversal of the magnetic field, the system is trapped in a metastable state because of magnetic anisotropy. We investigate how the system escapes the metastable state at low temperatures (10 to 200 mK). When the dissipation (measured by the longitudinal resistance) is ultralow, we find that the system escapes by making a few very rapid transitions, as detected by large jumps in the Hall and longitudinal resistances. Using the field at which the initial jump occurs to estimate the escape rate, we find that raising the temperature strongly suppresses the rate. From a detailed map of the resistance versus gate voltage and temperature, we show that dissipation strongly affects the escape rate. We compare the observations with dissipative quantum tunneling predictions. In the ultralow dissipation regime, two temperature scales (T 1 ~ 70 mK and T 2 ~ 145 mK) exist, between which jumps can be observed. The jumps display a spatial correlation that extends over a large fraction of the sample. PMID:27482539

  12. Interfacing Topological Insulators with Ferromagnetism

    NASA Astrophysics Data System (ADS)

    Richardella, Anthony

    In topological insulators, the surface states arise from strong spin-orbit coupling while the degeneracy of the Dirac point is protected by time reversal symmetry. Introducing magnetism in proximity to the surface states breaks this symmetry, destroying the non-trivial Berry phase at the Dirac point and leads to a hedgehog spin texture near the newly opened magnetic gap. This symmetry broken phase leads to a host of unusual physics, such as the quantum anomalous Hall (QAH) effect. In this talk, we discuss the growth by molecular beam epitaxy and characterization of such magnetically interfaced and magnetically doped topological insulators. Such materials often suffer from structural defects and interfacial layers, as well as from degradation during device fabrication. In particular, it is shown that Cr doped (Bi1-x,Sbx)2Te3 can exhibit perfect Hall quantization at low temperatures despite these defects. However, the magnetic ordering of this material was found to be quite unusual, displaying a super-paramagnetic like character, perhaps reflecting this disorder. Such observations highlight the surprising behavior of such broken symmetry phases in topological materials. This work was performed in collaboration with A. Kandala, M. Liu, W. Wang, N.P. Ong, C.-X. Liu, and N. Samarth, in addition to the authors of the references cited. This work was supported by funding from ARO/MURI, DARPA and ONR.

  13. Charge transport of graphene ferromagnetic-insulator-superconductor junction with pairing state of broken time reversal symmetry

    NASA Astrophysics Data System (ADS)

    Hajati, Yaser

    2015-04-01

    We investigate the charge transport through a graphene-based ferromagnetic-insulator-superconductor junction with a broken time reversal symmetry (BTRS) of dx2-y2 + is and dx2-y2 + idxy superconductor using the extended Blonder-Tinkham-Klapwijk formalism. Our analysis have shown several charateristics in this junction, providing a useful probe to understand the role of the order parameter symmetry in the superconductivity. We find that the presence of the BTRS (X) state in the superconductor region has a strong effect on the tunneling conductance curves which leads to a decrease in the height of the zero-bias conductance peak (ZBCP). In particular, we show that the magnitude of the superconducting proximity effect depends to a great extent on X and by increasing X, the zero-bias charge conductance oscillations with respect to the rotation angle β are suppressed. In addition, we find that at the maximum rotation angle β = π/4, introducing BTRS in the FIS junction causes oscillatory behavior of the zero-bias charge conductance with the barrier strength (χG) by a period of π and by approaching the X to 1, the amplitude of charge conductance oscillations increases. This behavior is drastically different from none BTRS similar graphene junctions. At last, we suggest an experimental setup for verifying our predicted effects.

  14. Metallic ferromagnetism-insulating charge order transition in doped manganites

    NASA Astrophysics Data System (ADS)

    Phan, Van-Nham; Ninh, Quoc-Huy; Tran, Minh-Tien

    2016-04-01

    We show that an interplay of double exchange and impurity randomness can explain the competition between metal-ferromagnetic and insulating charge ordered states in doped manganites. The double exchange is simplified in the Ising type, whereas the randomness is modeled by the Falicov-Kimball binary distribution. The combined model is considered in a framework of dynamical mean-field theory. Using the Kubo-Greenwood formalism, the transport coefficients are explicitly expressed in terms of single-particle spectral functions. Dividing the system into two sublattices we have pointed out a direct calculation to the checkerboard charge order parameter and the magnetizations. Numerical results show us that the checkerboard charge order can settle inside the ferromagnetic state at low temperature. An insulator-metal transition is also found at the point of the checkerboard charge order-ferromagnetic transition.

  15. Insulating room temperature ferromagnetic SrTiO3

    NASA Astrophysics Data System (ADS)

    Posadas, Agham; Mitra, Chandrima; Lin, Chungwei; Dhamdere, Ajit; Smith, David; Tsoi, Maxim; Demkov, Alex

    2013-03-01

    We report the epitaxial growth of ferromagnetic insulating material based on SrTiO3 using molecular beam epitaxy (MBE). SrTi1-xCoxO3-δ films (x = 0.1 to 0.5) were grown on Si(100) substrates via a buffer layer of four unit cells of undoped SrTiO3. The crystalline structure was characterized by reflection high energy electron diffraction, x-ray diffraction, and cross-section transmission electron microscopy. Robust room-temperature ferromagnetism is confirmed in samples with composition 30-40% Co. We also performed in situx-ray photoelectron spectroscopy of the Sr, Co, Ti, and O core levels to determine stoichiometry and cobalt oxidation state. In all single phase samples, an oxygen vacancy concentration of approximately equal to the amount of Co substitution was measured (compensated doping). In order to elucidate the origin of ferromagnetism, we also performed first-principles calculations of SrTiO3 simultaneously doped with Co and an oxygen vacancy. We find that such a configuration at concentrations of ~ 25% can result in a ferromagnetic insulating state with high spin Co2+. The ability to integrate an insulating ferromagnet on silicon in epitaxial form may potentially be useful for spin filtering and spin wave applications in the field of spintronics.

  16. Superconductivity-induced magnetization depletion in a ferromagnet through an insulator in a ferromagnet-insulator-superconductor hybrid oxide heterostructure.

    PubMed

    Prajapat, C L; Singh, Surendra; Paul, Amitesh; Bhattacharya, D; Singh, M R; Mattauch, S; Ravikumar, G; Basu, S

    2016-05-21

    Coupling between superconducting and ferromagnetic states in hybrid oxide heterostructures is presently a topic of intense research. Such a coupling is due to the leakage of the Cooper pairs into the ferromagnet. However, tunneling of the Cooper pairs though an insulator was never considered plausible. Using depth sensitive polarized neutron reflectivity we demonstrate the coupling between superconductor and magnetic layers in epitaxial La2/3Ca1/3MnO3 (LCMO)/SrTiO3/YBa2Cu3O7-δ (YBCO) hybrid heterostructures, with SrTiO3 as an intervening oxide insulator layer between the ferromagnet and the superconductor. Measurements above and below the superconducting transition temperature (TSC) of YBCO demonstrate a large modulation of magnetization in the ferromagnetic layer below the TSC of YBCO in these heterostructures. This work highlights a unique tunneling phenomenon between the epitaxial layers of an oxide superconductor (YBCO) and a magnetic layer (LCMO) through an insulating layer. Our work would inspire further investigations on the fundamental aspect of a long range order of the triplet spin-pairing in hybrid structures. PMID:27124772

  17. Superconductivity-induced magnetization depletion in a ferromagnet through an insulator in a ferromagnet-insulator-superconductor hybrid oxide heterostructure

    NASA Astrophysics Data System (ADS)

    Prajapat, C. L.; Singh, Surendra; Paul, Amitesh; Bhattacharya, D.; Singh, M. R.; Mattauch, S.; Ravikumar, G.; Basu, S.

    2016-05-01

    Coupling between superconducting and ferromagnetic states in hybrid oxide heterostructures is presently a topic of intense research. Such a coupling is due to the leakage of the Cooper pairs into the ferromagnet. However, tunneling of the Cooper pairs though an insulator was never considered plausible. Using depth sensitive polarized neutron reflectivity we demonstrate the coupling between superconductor and magnetic layers in epitaxial La2/3Ca1/3MnO3 (LCMO)/SrTiO3/YBa2Cu3O7-δ (YBCO) hybrid heterostructures, with SrTiO3 as an intervening oxide insulator layer between the ferromagnet and the superconductor. Measurements above and below the superconducting transition temperature (TSC) of YBCO demonstrate a large modulation of magnetization in the ferromagnetic layer below the TSC of YBCO in these heterostructures. This work highlights a unique tunneling phenomenon between the epitaxial layers of an oxide superconductor (YBCO) and a magnetic layer (LCMO) through an insulating layer. Our work would inspire further investigations on the fundamental aspect of a long range order of the triplet spin-pairing in hybrid structures.

  18. Proximity induced ferromagnetism at the interface between a topological insulator (TI) Bi2Se3 and a ferromagnetic insulator (FI)

    NASA Astrophysics Data System (ADS)

    Wei, Peng; Katmis, Ferhat; Assaf, Badih; Heiman, Don; Jarillo-Herrero, Pablo; Moodera, Jagadeesh

    2012-02-01

    The ferromagnetic phase of the surface states of a TI is predicted to carry many exotic properties, for example quantum anomalous Hall effect, magnetic monopole, and magneto-electric effects etc. In our study, we explore this novel phase utilizing the proximity induced exchange splitting to introduce ferromagnetism close to the surface of the Bi2Se3 film. High quality Bi2Se3 thin films were grown using molecular-beam-epitaxy, and in-situ deposited the ferromagnetic insulator (FI) EuS over this film. Magnetization measurements demonstrated a magnetic moment of more than 7μB per Eu^2+ ion (bulk value), and reaching up to 11μB per Eu^2+ ion for 1nm thick EuS film, showing the unambiguous existence of excess ferromagnetism. The transport studies of these TI/FI bilayers unveiled a clear switching behavior of the magnetoresistance in the Bi2Se3 film. There was significant temperature dependence seen in both MR and the coercivity. Due to the near range nature of the exchange interactions, these extra magnetic moments and the MR results are attributed to come from the induced ferromagnetism at the Bi2Se3 surface.

  19. A high-temperature ferromagnetic topological insulating phase by proximity coupling

    NASA Astrophysics Data System (ADS)

    Katmis, Ferhat; Lauter, Valeria; Nogueira, Flavio S.; Assaf, Badih A.; Jamer, Michelle E.; Wei, Peng; Satpati, Biswarup; Freeland, John W.; Eremin, Ilya; Heiman, Don; Jarillo-Herrero, Pablo; Moodera, Jagadeesh S.

    2016-05-01

    Topological insulators are insulating materials that display conducting surface states protected by time-reversal symmetry, wherein electron spins are locked to their momentum. This unique property opens up new opportunities for creating next-generation electronic, spintronic and quantum computation devices. Introducing ferromagnetic order into a topological insulator system without compromising its distinctive quantum coherent features could lead to the realization of several predicted physical phenomena. In particular, achieving robust long-range magnetic order at the surface of the topological insulator at specific locations without introducing spin-scattering centres could open up new possibilities for devices. Here we use spin-polarized neutron reflectivity experiments to demonstrate topologically enhanced interface magnetism by coupling a ferromagnetic insulator (EuS) to a topological insulator (Bi2Se3) in a bilayer system. This interfacial ferromagnetism persists up to room temperature, even though the ferromagnetic insulator is known to order ferromagnetically only at low temperatures (<17 K). The magnetism induced at the interface resulting from the large spin–orbit interaction and the spin–momentum locking of the topological insulator surface greatly enhances the magnetic ordering (Curie) temperature of this bilayer system. The ferromagnetism extends ~2 nm into the Bi2Se3 from the interface. Owing to the short-range nature of the ferromagnetic exchange interaction, the time-reversal symmetry is broken only near the surface of a topological insulator, while leaving its bulk states unaffected. The topological magneto-electric response originating in such an engineered topological insulator could allow efficient manipulation of the magnetization dynamics by an electric field, providing an energy-efficient topological control mechanism for future spin-based technologies.

  20. A high-temperature ferromagnetic topological insulating phase by proximity coupling.

    PubMed

    Katmis, Ferhat; Lauter, Valeria; Nogueira, Flavio S; Assaf, Badih A; Jamer, Michelle E; Wei, Peng; Satpati, Biswarup; Freeland, John W; Eremin, Ilya; Heiman, Don; Jarillo-Herrero, Pablo; Moodera, Jagadeesh S

    2016-05-26

    Topological insulators are insulating materials that display conducting surface states protected by time-reversal symmetry, wherein electron spins are locked to their momentum. This unique property opens up new opportunities for creating next-generation electronic, spintronic and quantum computation devices. Introducing ferromagnetic order into a topological insulator system without compromising its distinctive quantum coherent features could lead to the realization of several predicted physical phenomena. In particular, achieving robust long-range magnetic order at the surface of the topological insulator at specific locations without introducing spin-scattering centres could open up new possibilities for devices. Here we use spin-polarized neutron reflectivity experiments to demonstrate topologically enhanced interface magnetism by coupling a ferromagnetic insulator (EuS) to a topological insulator (Bi2Se3) in a bilayer system. This interfacial ferromagnetism persists up to room temperature, even though the ferromagnetic insulator is known to order ferromagnetically only at low temperatures (<17 K). The magnetism induced at the interface resulting from the large spin-orbit interaction and the spin-momentum locking of the topological insulator surface greatly enhances the magnetic ordering (Curie) temperature of this bilayer system. The ferromagnetism extends ~2 nm into the Bi2Se3 from the interface. Owing to the short-range nature of the ferromagnetic exchange interaction, the time-reversal symmetry is broken only near the surface of a topological insulator, while leaving its bulk states unaffected. The topological magneto-electric response originating in such an engineered topological insulator could allow efficient manipulation of the magnetization dynamics by an electric field, providing an energy-efficient topological control mechanism for future spin-based technologies. PMID:27225124

  1. Magnetic proximity effect and interlayer exchange coupling of ferromagnetic/topological insulator/ferromagnetic trilayer

    NASA Astrophysics Data System (ADS)

    Li, Mingda; Cui, Wenping; Yu, Jin; Dai, Zuyang; Wang, Zhe; Katmis, Ferhat; Guo, Wanlin; Moodera, Jagadeesh

    2015-01-01

    The magnetic proximity effect between the topological insulator (TI) and ferromagnetic insulator (FMI) is considered to have great potential in spintronics. However, a complete determination of interfacial magnetic structure has been highly challenging. We theoretically investigate the interlayer exchange coupling of two FMIs separated by a TI thin film, and show that the particular electronic states of the TI contributing to the proximity effect can be directly identified through the coupling behavior between two FMIs, together with a tunability of the coupling constant. Such an FMI/TI/FMI structure not only serves as a platform to clarify the magnetic structure of the FMI/TI interface, but also provides insights in designing the magnetic storage devices with ultrafast response.

  2. Probing the Spin Transfer Efficiency at Topological Insulator/Ferromagnetic Insulator Interfaces

    NASA Astrophysics Data System (ADS)

    Wang, Hailong; Kally, James; Lee, Joon Sue; Richardella, Anthony; Kempinger, Susan; Pan, Yu; Kamp, Eric; Samarth, Nitin; Liu, Tao; Chang, Houcheng; Wu, Mingzhong; Reifsnyder-Hickey, Danielle; Mkhoyan, Andre

    The development of next-generation spintronics devices has driven extensive studies of spin-charge conversion through measurement of the inverse spin Hall effect (ISHE) and ferromagnetic resonance (FMR) driven spin pumping of pure spin currents in ferromagnet/non-magnet bilayers. Topological insulators (TIs) such as the Bi-chalcogenides are naturally relevant in this context because the inherent spin-momentum ``locking'' in their surface states promises very efficient spin-charge conversion, although the first experimental studies have involved ferromagnetic metals that provide a shunting current path [e.g. Nature, 511,449 (2014)]. To circumvent the current shunting problem, we are growing and characterizing bilayers of TIs and the ferrimagnetic insulator Y3Fe5O12 (YIG). Here, we report measurements of FMR-driven spin pumping in TI/YIG bilayers, showing robust spin pumping signals at room temperature. Analysis of the ISHE voltages and FMR linewidth broadening show that, as in other studies of spin pumping into TIs [Nano Lett., 15 (10) (2015)], the interface condition presents a critical challenge for enhancing the spin conversion efficiency in these devices. Funded by C-SPIN/SRC/DARPA and ONR.

  3. Quantum Corrections Crossover and Ferromagnetism in Magnetic Topological Insulators

    PubMed Central

    Bao, Lihong; Wang, Weiyi; Meyer, Nicholas; Liu, Yanwen; Zhang, Cheng; Wang, Kai; Ai, Ping; Xiu, Faxian

    2013-01-01

    Revelation of emerging exotic states of topological insulators (TIs) for future quantum computing applications relies on breaking time-reversal symmetry and opening a surface energy gap. Here, we report on the transport response of Bi2Te3 TI thin films in the presence of varying Cr dopants. By tracking the magnetoconductance (MC) in a low doping regime we observed a progressive crossover from weak antilocalization (WAL) to weak localization (WL) as the Cr concentration increases. In a high doping regime, however, increasing Cr concentration yields a monotonically enhanced anomalous Hall effect (AHE) accompanied by an increasing carrier density. Our results demonstrate a possibility of manipulating bulk ferromagnetism and quantum transport in magnetic TI, thus providing an alternative way for experimentally realizing exotic quantum states required by spintronic applications. PMID:23928713

  4. Heat Transport in Graphene Ferromagnet-Insulator-Superconductor Junctions

    NASA Astrophysics Data System (ADS)

    Li, Xiao-Wei

    2011-04-01

    We study heat transport in a graphene ferromagnet-insulator-superconducting junction. It is found that the thermal conductance of the graphene ferromagnet-insulator-superconductor (FIS) junction is an oscillatory function of the barrier strength χ in the thin-barrier limit. The gate potential U0 decreases the amplitude of thermal conductance oscillation. Both the amplitude and phase of the thermal conductance oscillation varies with the exchange energy Eh. The thermal conductance of a graphene FIS junction displays the usual exponential dependence on temperature, reflecting the s-wave symmetry of superconducting graphene.

  5. Weak Delocalization in Graphene on a Ferromagnetic Insulating Film.

    PubMed

    Pietrobon, Luca; Fallarino, Lorenzo; Berger, Andreas; Chuvilin, Andrey; Casanova, Fèlix; Hueso, Luis E

    2015-12-16

    Graphene has been predicted to develop a magnetic moment by proximity effect when placed on a ferromagnetic film, a promise that could open exciting possibilities in the fields of spintronics and magnetic data recording. In this work, the interplay between the magnetoresistance of graphene and the magnetization of an underlying ferromagnetic insulating film is studied in detail. A clear correlation between both magnitudes is observed but through a careful modeling of the magnetization and the weak localization measurements, that such correspondence can be explained by the effects of the magnetic stray fields arising from the ferromagnetic insulator is found. The results emphasize the complexity arising at the interface between magnetic and 2D materials. PMID:26505882

  6. Enhanced Magnetic Proximity Effect at Ferromagnetic Insulator / Magnetic Topological Insulator Interface

    NASA Astrophysics Data System (ADS)

    Li, Mingda; Chang, Cui-Zu; Kirby, Brian; Jamer, Michelle E.; Cui, Wenping; Wu, Lijun; Wei, Peng; Zhu, Yimei; Heiman, Don; Li, Ju; Moodera, Jagadeesh; MIT Team; NIST Team; Northeastern University Collaboration; Boston College Collaboration; Brookhaven National Lab Collaboration

    Magnetic proximity effect at magnetic insulator / topological insulator interface provides a promising approach to realize low-dissipation quantum devices. However, the commonly used magnetic insulators have in-plane anisotropy hence cannot magnetize topological insulator. Here we report an enhancement of proximity exchange coupling in ferromagnetic insulator / magnetic topological insulator EuS / Sb2-xVxTe3 hybrid heterostructure, where proximity effect is enhanced by a factor of 3 through the Vanadium doping. Moreover, an artificial antiferromagnetic-like structure is created between two strong ferromagnets, which may account for the proximity effect enhancement. The interplay between the proximity effect and doping in hybrid heterostructure provides insights into the engineering of magnetic ordering.

  7. Spin transport in normal metal/insulator/topological insulator coupled to ferromagnetic insulator structures

    SciTech Connect

    Kondo, Kenji

    2014-05-07

    In this study, we investigate the spin transport in normal metal (NM)/insulator (I)/topological insulator (TI) coupled to ferromagnetic insulator (FI) structures. In particular, we focus on the barrier thickness dependence of the spin transport inside the bulk gap of the TI with FI. The TI with FI is described by two-dimensional (2D) Dirac Hamiltonian. The energy profile of the insulator is assumed to be a square with barrier height V and thickness d along the transport-direction. This structure behaves as a tunnel device for 2D Dirac electrons. The calculation is performed for the spin conductance with changing the barrier thickness and the components of magnetization of FI layer. It is found that the spin conductance decreases with increasing the barrier thickness. Also, the spin conductance is strongly dependent on the polar angle θ, which is defined as the angle between the axis normal to the FI and the magnetization of FI layer. These results indicate that the structures are promising candidates for novel tunneling magnetoresistance devices.

  8. Spin waves in ferromagnetic insulators coupled via a normal metal

    NASA Astrophysics Data System (ADS)

    Skarsvâg, Hans; Kapelrud, André; Brataas, Arne

    2014-09-01

    Herein, we study spin-wave dispersion and dissipation in a ferromagnetic insulator-normal metal-ferromagnetic insulator system. Long-range dynamic coupling because of spin pumping and spin transfer lead to collective magnetic excitations in the two thin-film ferromagnets. In addition, the dynamic dipolar field contributes to the interlayer coupling. By solving the Landau-Lifshitz-Gilbert-Slonczewski equation for macrospin excitations and the exchange-dipole volume as well as surface spin waves, we compute the effect of the dynamic coupling on the resonance frequencies and linewidths of the various modes. The long-wavelength modes may couple acoustically or optically. In the absence of spin-memory loss in the normal metal, the spin-pumping-induced Gilbert damping enhancement of the acoustic mode vanishes, whereas the optical mode acquires a significant Gilbert damping enhancement, comparable to that of a system attached to a perfect spin sink. The dynamic coupling is reduced for short-wavelength spin waves, and there is no synchronization. For intermediate wavelengths, the coupling can be increased by the dipolar field such that the modes in the two ferromagnetic insulators can couple despite possible small frequency asymmetries. The surface waves induced by an easy-axis surface anisotropy exhibit much greater Gilbert damping enhancement. These modes also may acoustically or optically couple, but they are unaffected by thickness asymmetries.

  9. Superfluid spin transport through easy-plane ferromagnetic insulators.

    PubMed

    Takei, So; Tserkovnyak, Yaroslav

    2014-06-01

    Superfluid spin transport-dissipationless transport of spin-is theoretically studied in a ferromagnetic insulator with easy-plane anisotropy. We consider an open geometry where the spin current is injected into the ferromagnet from one side by a metallic reservoir with a nonequilibrium spin accumulation and ejected into another metallic reservoir located downstream. Spin transport is studied using a combination of magnetoelectric circuit theory, Landau-Lifshitz-Gilbert phenomenology, and microscopic linear-response theory. We discuss how spin superfluidity can be probed in a magnetically mediated negative electron-drag experiment. PMID:24949786

  10. Magnetic proximity effect in the three-dimensional topological insulator/ferromagnetic insulator heterostructure

    NASA Astrophysics Data System (ADS)

    Men'shov, V. N.; Tugushev, V. V.; Eremeev, S. V.; Echenique, P. M.; Chulkov, E. V.

    2013-12-01

    We theoretically study the magnetic proximity effect in the three-dimensional (3D) topological insulator/ferromagnetic insulator (TI/FMI) structures in the context of possibility to manage the Dirac helical state in TI. Within a continual approach based on the k·p Hamiltonian, we predict that, when a 3D TI is brought into contact with a 3D FMI, the ordinary bound state arising at the TI/FMI interface becomes spin polarized due to the orbital mixing at the boundary. Whereas the wave function of FMI decays into the TI bulk on the atomic scale, the induced exchange field, which is proportional to the FMI magnetization, builds up at the scale of the penetration depth of the ordinary interface state. Such an exchange field opens the gap at the Dirac point in the energy spectrum of the topological bound state existing on the TI side of the interface. We estimate the dependence of the gap size on the material parameters of the TI/FMI contact.

  11. Field-effect modulation of anomalous Hall effect in diluted ferromagnetic topological insulator epitaxial films

    NASA Astrophysics Data System (ADS)

    Chang, CuiZu; Liu, MinHao; Zhang, ZuoCheng; Wang, YaYu; He, Ke; Xue, QiKun

    2016-03-01

    High quality chromium (Cr) doped three-dimensional topological insulator (TI) Sb2Te3 films are grown via molecular beam epitaxy on heat-treated insulating SrTiO3 (111) substrates. We report that the Dirac surface states are insensitive to Cr doping, and a perfect robust long-range ferromagnetic order is unveiled in epitaxial Sb2- x Cr x Te3 films. The anomalous Hall effect is modulated by applying a bottom gate, contrary to the ferromagnetism in conventional diluted magnetic semiconductors (DMSs), here the coercivity field is not significantly changed with decreasing carrier density. Carrier-independent ferromagnetism heralds Sb2- x Cr x Te3 films as the base candidate TI material to realize the quantum anomalous Hall (QAH) effect. These results also indicate the potential of controlling anomalous Hall voltage in future TI-based magneto-electronics and spintronics.

  12. Mixed states in ferromagnetic superconductors

    SciTech Connect

    Matsumoto, H.; Teshima, R.; Umezawa, H.; Tachiki, M.

    1983-01-01

    A detailed study of the mixed state of the ferromagnetic rare-earth compounds RRh/sub 4/B/sub 4/, R/sub x/Mo/sub 6/S/sub 8/, and R/sub x/Mo/sub 6/Se/sub 6/ is presented. The saturation effect of the magnetic moments is taken into account. Depending on the parameters, there are many types of phase transitions between the type-II/2, type-II/1, and type-I mixed states and the paramagnetic Meissner state, ferromagnetic Meissner state, spin-periodic Meissner state, and the self-induced vortex state. It is predicted that the magnetization can exhibit a variety of unusual modes.

  13. Topological insulator in junction with ferromagnets: Quantum Hall effects

    NASA Astrophysics Data System (ADS)

    Chudnovskiy, A. L.; Kagalovsky, V.

    2015-06-01

    The ferromagnet-topological insulator-ferromagnet (FM-TI-FM) junction exhibits thermal and electrical quantum Hall effects. The generated Hall voltage and transverse temperature gradient can be controlled by the directions of magnetizations in the FM leads, which inspires the use of FM-TI-FM junctions as electrical and as heat switches in spintronic devices. Thermal and electrical Hall coefficients are calculated as functions of the magnetization directions in ferromagnets and the spin-relaxation time in TI. Both the Hall voltage and the transverse temperature gradient decrease but are not completely suppressed even at very short spin-relaxation times. The Hall coefficients turn out to be independent of the spin-relaxation time for symmetric configuration of FM leads.

  14. Electronic Structure Evolution across the Peierls Metal-Insulator Transition in a Correlated Ferromagnet

    NASA Astrophysics Data System (ADS)

    Bhobe, P. A.; Kumar, A.; Taguchi, M.; Eguchi, R.; Matsunami, M.; Takata, Y.; Nandy, A. K.; Mahadevan, P.; Sarma, D. D.; Neroni, A.; Şaşıoǧlu, E.; Ležaić, M.; Oura, M.; Senba, Y.; Ohashi, H.; Ishizaka, K.; Okawa, M.; Shin, S.; Tamasaku, K.; Kohmura, Y.; Yabashi, M.; Ishikawa, T.; Hasegawa, K.; Isobe, M.; Ueda, Y.; Chainani, A.

    2015-10-01

    Transition metal compounds often undergo spin-charge-orbital ordering due to strong electron-electron correlations. In contrast, low-dimensional materials can exhibit a Peierls transition arising from low-energy electron-phonon-coupling-induced structural instabilities. We study the electronic structure of the tunnel framework compound K2Cr8O16 , which exhibits a temperature-dependent (T -dependent) paramagnetic-to-ferromagnetic-metal transition at TC=180 K and transforms into a ferromagnetic insulator below TMI=95 K . We observe clear T -dependent dynamic valence (charge) fluctuations from above TC to TMI , which effectively get pinned to an average nominal valence of Cr+3.75 (Cr4 +∶Cr3 + states in a 3 ∶1 ratio) in the ferromagnetic-insulating phase. High-resolution laser photoemission shows a T -dependent BCS-type energy gap, with 2 G (0 )˜3.5 (kBTMI)˜35 meV . First-principles band-structure calculations, using the experimentally estimated on-site Coulomb energy of U ˜4 eV , establish the necessity of strong correlations and finite structural distortions for driving the metal-insulator transition. In spite of the strong correlations, the nonintegral occupancy (2.25 d -electrons/Cr ) and the half-metallic ferromagnetism in the t2 g up-spin band favor a low-energy Peierls metal-insulator transition.

  15. Interfacial Coupling-Induced Ferromagnetic Insulator Phase in Manganite Film.

    PubMed

    Zhang, Bangmin; Wu, Lijun; Yin, Wei-Guo; Sun, Cheng-Jun; Yang, Ping; Venkatesan, T; Chen, Jingsheng; Zhu, Yimei; Chow, Gan Moog

    2016-07-13

    Interfaces with subtle differences in atomic and electronic structures in perovskite ABO3 heterostructures often yield intriguingly different properties, yet their exact roles remain elusive. Here, we report an integrated study of unusual transport, magnetic, and structural properties of Pr0.67Sr0.33MnO3 film on SrTiO3 substrate. The variations in the out-of-plane lattice constant and BO6 octahedral rotation across the Pr0.67Sr0.33MnO3/SrTiO3 interface strongly depend on the thickness of the Pr0.67Sr0.33MnO3 film. In the 12 nm film, a new interface-sensitive ferromagnetic polaronic insulator (FI') phase is formed during the cubic-to-tetragonal phase transition of SrTiO3, apparently due to the enhanced electron-phonon interaction and atomic disorder in the film. The transport properties of the FI' phase in the 30 nm film are masked because of the reduced interfacial coupling and smaller interface-to-volume ratio. This work demonstrates how thickness-dependent interfacial coupling leads to the formation of a theoretically predicted ferromagnetic-polaronic insulator, as illustrated in a new phase diagram, that is otherwise ferromagnetic metal (FM) in bulk form. PMID:27276032

  16. Fabrication and magnetic characterization of nanometer-sized ellipses of the ferromagnetic insulator EuS

    NASA Astrophysics Data System (ADS)

    Wolf, M. J.; Sürgers, C.; Fischer, G.; Scherer, T.; Beckmann, D.

    2014-11-01

    The magnetic properties of ferromagnetic elements can be tailored by making use of the shape anisotropy of finite-size systems. One material class of particular interest are ferromagnetic insulators, which can be used as spin filters for spintronics applications. Here we present a way to fabricate nanoscale ellipses of the ferromagnetic insulator europium sulfide (EuS) and investigate their magnetic properties. We observe a distinct influence of the magnetic field orientation on the shape of the magnetization curve. This could be used to separately control the individual magnetic elements of a magnetoresistive device using a ferromagnetic insulator.

  17. Spin pumping from a ferromagnet into a hopping insulator: Role of resonant absorption of magnons

    NASA Astrophysics Data System (ADS)

    Raikh, Mikhail; Zhang, Yue; Pesin, Dmytro

    Motivated by recent experiments on spin pumping from a ferromagnet into organic materials in which the charge transport is due to hopping, we study theoretically the generation and propagation of spin current in a hopping insulator. Unlike metals, the spin polarization at the boundary with ferromagnet is created as a result of magnon absorption within pairs of localized states and it spreads following the current-currying resistor network (although the charge current is absent). We consider a classic resonant mechanism of the ac absorption in insulators and adapt it to the absorption of magnons. A strong enhancement of pumping efficiency is predicted when the Zeeman splitting of the localized states in external magnetic field is equal to the frequency of ferromagnetic resonance. Under this condition the absorption of a magnon takes place within individual sites. This work was supported by the NSF MRSEC program at the University of Utah under Grant No. DMR 1121252 (Z.Y. and M.E.R.) and by the NSF Grant No. DMR 1409089 (D.A.P).

  18. Spin-dependent delay time in ferromagnet/insulator/ferromagnet heterostructures

    SciTech Connect

    Xie, ZhengWei; Zheng Shi, De; Lv, HouXiang

    2014-07-07

    We study theoretically spin-dependent group delay and dwell time in ferromagnet/insulator/ferromagnet (FM/I/FM) heterostructure. The results indicate that, when the electrons with different spin orientations tunnel through the FM/I/FM junction, the spin-up process and the spin-down process are separated on the time scales. As the self-interference delay has the spin-dependent features, the variations of spin-dependent dwell-time and spin-dependent group-delay time with the structure parameters appear different features, especially, in low incident energy range. These different features show up as that the group delay times for the spin-up electrons are always longer than those for spin-down electrons when the barrier height or incident energy increase. In contrast, the dwell times for the spin-up electrons are longer (shorter) than those for spin-down electrons when the barrier heights (the incident energy) are under a certain value. When the barrier heights (the incident energy) exceed a certain value, the dwell times for the spin-up electrons turn out to be shorter (longer) than those for spin-down electrons. In addition, the group delay time and the dwell time for spin-up and down electrons also relies on the comparative direction of magnetization in two FM layers and tends to saturation with the thickness of the barrier.

  19. Magnetization in Intrinsic Topological Insulators Induced by Exchange Interaction with Ferromagnetic Insulator

    NASA Astrophysics Data System (ADS)

    Lauter, Valeria; Katmis, Ferhat; Assaf, Badih; Heiman, Don; Moodera, Jagadeesh

    2015-03-01

    We examine the magnetic proximity-induced symmetry breaking via the exchange interaction in heterostructures of the topological insulator (TI) Bi2Se3 and the ferromagnetic insulator (FMI) EuS. We observed the emergence of a ferromagnetic phase in TI with the excess of magnetic moment at the interface using depth and element sensitive Polarized Neutron Reflectometry (PNR). We find that the magnetization, penetrating into the TI originates through exchange interaction, without structural perturbation at the interface. Due to the different interlayer exchange coupling as well as the properties of the bulk and surface magnetizations, we investigated several different heterostructures after cooling in zero field (ZFC) and in an external magnetic field (FC). The significantly enhanced magnetic properties of the heterostructures as revealed by the PNR studies, as well as the temperature and external magnetic field dependence will be presented. This work was supported by the Scientific User Facilities Division, BES, DOE, NSF ECCS-1402738, DMR-1207469, ONR N00014-13-1-0301.

  20. Manipulating Surface-induced Ferromagnetism in Modulation-doped Topological Insulators

    NASA Astrophysics Data System (ADS)

    Kou, Xufeng; He, Liang; Lang, Murong; Fan, Yabin; Jiang, Ying; Wang, Yong; Xiu, Faxian; Wang, Kang; Device Research Laboratory Team; CenterElectron Microscopy; State Key Laboratory of Silicon Materials Collaboration; ECE Department Collaboration

    2013-03-01

    The manipulation of topological surface states is a key to realize applicable devices of topological insulators. In addition to the direct engineering of time-reversal-symmetry protected surface states, recent work suggests that various physical responses can be obtained from surface helical states by integrating additional ferromagnetism or superconductivity to the original topological order. Here, we report the coexistence and tunability of bulk carrier density-independent and surface-mediated electrically controllable ferromagnetisms in modulation-doped Crx(BiySb1-y)2Te3 epitaxial thin films. We demonstrate for the first time a dramatic enhancement of surface-induced magnetization on TI / Cr-TI bilayer devices. The surface magneto-electric effects can be either enhanced significantly or completely switched-off, by tuning the separation of the surface from the magnetic impurities. The electric-field-modulated ferromagnetism in our modulation-doped TI hetero-structures is fundamentally important for the realization of the quantum anomalous Hall Effect as well as the axion electromagnetic dynamics, and thus provides a new approach for spintronics applications. The authors would also like to acknowledge helpful discussions with Dr. Alexei Fedorov and Dr. Mathew Marcus from the Advanced Light Source at Berkeley.

  1. Persistent Ferromagnetism and Topological Phase Transition at the Interface of a Superconductor and a Topological Insulator

    NASA Astrophysics Data System (ADS)

    Qin, Wei; Zhang, Zhenyu

    2014-12-01

    At the interface of an s -wave superconductor and a three-dimensional topological insulator, Majorana zero modes and Majorana helical states have been proposed to exist respectively around magnetic vortices and geometrical edges. Here we first show that randomly distributed magnetic impurities at such an interface will induce bound states that broaden into impurity bands inside (but near the edges of) the superconducting gap, which remains open unless the impurity concentration is too high. Next we find that an increase in the superconducting gap suppresses both the oscillation magnitude and the period of the Ruderman-Kittel-Kasuya-Yosida interaction between two magnetic impurities. Within a mean-field approximation, the ferromagnetic Curie temperature is found to be essentially independent of the superconducting gap, an intriguing phenomenon due to a compensation effect between the short-range ferromagnetic and long-range antiferromagnetic interactions. The existence of robust superconductivity and persistent ferromagnetism at the interface allows realization of a novel topological phase transition from a nonchiral to a chiral superconducting state at sufficiently low temperatures, providing a new platform for topological quantum computation.

  2. Zero-bias photocurrent in ferromagnetic topological insulator.

    PubMed

    Ogawa, N; Yoshimi, R; Yasuda, K; Tsukazaki, A; Kawasaki, M; Tokura, Y

    2016-01-01

    Magnetic interactions in topological insulators cause essential modifications in the originally mass-less surface states. They offer a mass gap at the Dirac point and/or largely deform the energy dispersion, providing a new path towards exotic physics and applications to realize dissipation-less electronics. The nonequilibrium electron dynamics at these modified Dirac states unveil additional functions, such as highly efficient photon to spin-current conversion. Here we demonstrate the generation of large zero-bias photocurrent in magnetic topological insulator thin films on mid-infrared photoexcitation, pointing to the controllable band asymmetry in the momentum space. The photocurrent spectra with a maximal response to the intra-Dirac-band excitations can be a sensitive measure for the correlation between Dirac electrons and magnetic moments. PMID:27435028

  3. Zero-bias photocurrent in ferromagnetic topological insulator

    PubMed Central

    Ogawa, N.; Yoshimi, R.; Yasuda, K.; Tsukazaki, A.; Kawasaki, M.; Tokura, Y.

    2016-01-01

    Magnetic interactions in topological insulators cause essential modifications in the originally mass-less surface states. They offer a mass gap at the Dirac point and/or largely deform the energy dispersion, providing a new path towards exotic physics and applications to realize dissipation-less electronics. The nonequilibrium electron dynamics at these modified Dirac states unveil additional functions, such as highly efficient photon to spin-current conversion. Here we demonstrate the generation of large zero-bias photocurrent in magnetic topological insulator thin films on mid-infrared photoexcitation, pointing to the controllable band asymmetry in the momentum space. The photocurrent spectra with a maximal response to the intra-Dirac-band excitations can be a sensitive measure for the correlation between Dirac electrons and magnetic moments. PMID:27435028

  4. Josephson effect in spin-singlet superconductor/ferromagnetic insulator/spin-triplet superconductor junctions with helical p-wave states

    NASA Astrophysics Data System (ADS)

    Cheng, Q.; Jin, B.

    2016-01-01

    We study the Josephson effect in spin-singlet superconductor/helical p-wave superconductor junctions with a ferromagnetic barrier using the quasiclassical Green function method. It is found that both sin ϕ-type and cos ϕ-type current-phase relations always exist, irrespective of the gap symmetries in superconductors. The indispensable condition for the sin ϕ-type and cos ϕ-type current is that the magnetization must have a component parallel to the crystallographic a- or b-axis, which is distinct from the case of the p-wave superconductor described by a d -vector with a uniform direction. The relation between the condition and the symmetries of the gap functions is analysed. We investigate in detail the symmetries and the sign reversal of the Josephson current when the magnetization is rotated.

  5. Photoinduced Demagnetization and Insulator-to-Metal Transition in Ferromagnetic Insulating BaFeO3 Thin Films

    NASA Astrophysics Data System (ADS)

    Tsuyama, T.; Chakraverty, S.; Macke, S.; Pontius, N.; Schüßler-Langeheine, C.; Hwang, H. Y.; Tokura, Y.; Wadati, H.

    2016-06-01

    We studied the electronic and magnetic dynamics of ferromagnetic insulating BaFeO3 thin films by using pump-probe time-resolved resonant x-ray reflectivity at the Fe 2 p edge. By changing the excitation density, we found two distinctly different types of demagnetization with a clear threshold behavior. We assigned the demagnetization change from slow (˜150 ps ) to fast (<70 ps ) to a transition into a metallic state induced by laser excitation. These results provide a novel approach for locally tuning magnetic dynamics. In analogy to heat-assisted magnetic recording, metallization can locally tune the susceptibility for magnetic manipulation, allowing one to spatially encode magnetic information.

  6. Photoinduced Demagnetization and Insulator-to-Metal Transition in Ferromagnetic Insulating BaFeO_{3} Thin Films.

    PubMed

    Tsuyama, T; Chakraverty, S; Macke, S; Pontius, N; Schüßler-Langeheine, C; Hwang, H Y; Tokura, Y; Wadati, H

    2016-06-24

    We studied the electronic and magnetic dynamics of ferromagnetic insulating BaFeO_{3} thin films by using pump-probe time-resolved resonant x-ray reflectivity at the Fe 2p edge. By changing the excitation density, we found two distinctly different types of demagnetization with a clear threshold behavior. We assigned the demagnetization change from slow (∼150  ps) to fast (<70  ps) to a transition into a metallic state induced by laser excitation. These results provide a novel approach for locally tuning magnetic dynamics. In analogy to heat-assisted magnetic recording, metallization can locally tune the susceptibility for magnetic manipulation, allowing one to spatially encode magnetic information. PMID:27391735

  7. Spin Hall magnetoresistance in ferromagnetic insulator/normal metal hybrids

    NASA Astrophysics Data System (ADS)

    Althammer, Matthias

    2014-03-01

    Pure spin currents, i.e. the net flow of spin angular momentum without an accompanying charge current, represent a new paradigm for spin transport and spintronics. We have experimentally studied a new type of magnetoresistance effect, which arises from the interaction of charge and spin current flows in ferromagnetic insulator/normal metal hybrid structures. In more detail, we measured the resistance of yttrium iron garnet(YIG)/Pt, YIG/nonferromagnet/Pt, nickel ferrite/Pt, and magnetite/Pt hybrid structures as a function of the magnitude and the orientation of an external magnetic field. The resistance changes observed can be quantitatively traced back to the combined action of spin Hall and inverse spin Hall effect in the Pt metal layer, and are thus termed spin Hall magnetoresistance (SMR) [ 1 , 2 ] . We show that the SMR is qualitatively different from the conventional anisotropic magnetoresistance effect arising in magnetic metals. From the magnetoresistance measurements in YIG/Au/Pt and YIG/Cu/Pt structures and from x-ray magnetic circular dichroism measurements on YIG/Pt heterostructures we exclude a static proximity magnetization in Pt as the origin of the magnetoresistance, in contrast to the mechanism proposed by Huang et al. [ 3 ] . Furthermore, the SMR enables us to quantify the spin Hall angle as a function of temperature in our Pt layers. In addition, we analyze the anomalous Hall type contribution of the SMR to quantify the imaginary part of the spin mixing conductance. Financial support by the DFG via SPP 1538 (project no. GO 944/4) and the Nanoinitiative Munich (NIM) is gratefully acknowledged.

  8. Manipulating surface-related ferromagnetism in modulation-doped topological insulators.

    PubMed

    Kou, Xufeng; He, Liang; Lang, Murong; Fan, Yabin; Wong, Kin; Jiang, Ying; Nie, Tianxiao; Jiang, Wanjun; Upadhyaya, Pramey; Xing, Zhikun; Wang, Yong; Xiu, Faxian; Schwartz, Robert N; Wang, Kang L

    2013-10-01

    A new class of devices based on topological insulators (TI) can be achieved by the direct engineering of the time-reversal-symmetry (TRS) protected surface states. In the meantime, a variety of interesting phenomena are also expected when additional ferromagnetism is introduced to the original topological order. In this Letter, we report the magnetic responses from the magnetically modulation-doped (Bi(z)Sb(1-z))2Te3/Cr(x)(Bi(y)Sb(1-y))2Te3 bilayer films. By electrically tuning the Fermi level across the Dirac point, we show that the top TI surface carriers can effectively mediate the magnetic impurities and generate robust ferromagnetic order. More importantly, such surface magneto-electric effects can be either enhanced or suppressed, depending on the magnetic interaction range inside the TI heterostructures. The manipulation of surface-related ferromagnetism realized in our modulation-doped TI device is important for the realization of TRS-breaking topological physics, and it may also lead to new applications of TI-based multifunctional heterostructures. PMID:24020459

  9. Evidence for ferromagnetic coupling at the doped topological insulator/ferrimagnetic insulator interface

    NASA Astrophysics Data System (ADS)

    Liu, Wenqing; He, Liang; Zhou, Yan; Murata, Koichi; Onbasli, Mehmet C.; Ross, Caroline A.; Jiang, Ying; Wang, Yong; Xu, Yongbing; Zhang, Rong; Wang, Kang. L.

    2016-05-01

    One of the major obstacles of the magnetic topological insulators (TIs) impeding their practical use is the low Curie temperature (Tc). Very recently, we have demonstrated the enhancement of the magnetic ordering in Cr-doped Bi2Se3 by means of proximity to the high-Tc ferrimagnetic insulator (FMI) Y3Fe5O12 and found a large and rapidly decreasing penetration depth of the proximity effect, suggestive of a different carrier propagation process near the TI surface. Here we further present a study of the interfacial magnetic interaction of this TI/FMI heterostrucutre. The synchrotron-based X-ray magnetic circular dichroism (XMCD) technique was used to probe the nature of the exchange coupling of the Bi2-xCrxSe3/Y3Fe5O12 interface. We found that the Bi2-xCrxSe3 grown on Y3Fe5O12(111) predominately contains Cr3+ cations, and the spin direction of the Cr3+ is aligned parallel to that of tetrahedral Fe3+ of the YIG, revealing a ferromagnetic exchange coupling between the Bi2-xCrxSe3 and the Y3Fe5O12.

  10. Large anomalous Hall effect in ferromagnetic insulator-topological insulator heterostructures

    SciTech Connect

    Alegria, L. D.; Petta, J. R.; Ji, H.; Cava, R. J.; Yao, N.; Clarke, J. J.

    2014-08-04

    We demonstrate the van der Waals epitaxy of the topological insulator compound Bi{sub 2}Te{sub 3} on the ferromagnetic insulator Cr{sub 2}Ge{sub 2}Te{sub 6}. The layers are oriented with (001)Bi{sub 2}Te{sub 3}||(001)Cr{sub 2}Ge{sub 2}Te{sub 6} and (110)Bi{sub 2}Te{sub 3}||(100)Cr{sub 2}Ge{sub 2}Te{sub 6}. Cross-sectional transmission electron microscopy indicates the formation of a sharp interface. At low temperatures, bilayers consisting of Bi{sub 2}Te{sub 3} on Cr{sub 2}Ge{sub 2}Te{sub 6} exhibit a large anomalous Hall effect (AHE). Tilted field studies of the AHE indicate that the easy axis lies along the c-axis of the heterostructure, consistent with magnetization measurements in bulk Cr{sub 2}Ge{sub 2}Te{sub 6}. The 61 K Curie temperature of Cr{sub 2}Ge{sub 2}Te{sub 6} and the use of near-stoichiometric materials may lead to the development of spintronic devices based on the AHE.

  11. Controllable fully spin-polarized transport in a ferromagnetically doped topological insulator junction

    SciTech Connect

    Zhou, Benliang; Tang, Dongsheng; Zhou, Guanghui; Zhou, Benhu

    2014-04-21

    We investigate the energy band structure and the spin-dependent transport for a normal/ferromagnetic/normal two-dimension topological insulator (TI) junction. By diagonalizing Hamiltonian for the system, the band structure shows that the edge states on two sides are coupled resulting in a gap opening due to the transverse spatial confinement. Further, the exchange field induced by magnetic impurities can also modulate the band structure with two spin degenerate bands splitting. By using the nonequilibrium Green's function method, the dependence of spin-dependent conductance and spin-polarization on the Fermi energy, the exchange field strength and the ferromagnetic TI (FTI) length are also analyzed, respectively. Interestingly, the degenerate conductance plateaus for spin-up and -down channels are broken, and both the conductances are suppressed by magnetic impurities due to the time-reversal symmetry broken and inelastic scattering. The spin-dependent conductance shows different behaviors when the Fermi energy is tuned into different ranges. Moreover, the conductance can be fully spin polarized by tuning the Fermi energy and the exchange field strength, or by tuning the Fermi energy and the FTI length. Consequently, the junction can transform from a quantum spin Hall state to a quantum anomalous Hall state, which is very important to enable dissipationless charge current for designing perfect spin filter.

  12. Controllable fully spin-polarized transport in a ferromagnetically doped topological insulator junction

    NASA Astrophysics Data System (ADS)

    Zhou, Benliang; Zhou, Benhu; Tang, Dongsheng; Zhou, Guanghui

    2014-04-01

    We investigate the energy band structure and the spin-dependent transport for a normal/ferromagnetic/normal two-dimension topological insulator (TI) junction. By diagonalizing Hamiltonian for the system, the band structure shows that the edge states on two sides are coupled resulting in a gap opening due to the transverse spatial confinement. Further, the exchange field induced by magnetic impurities can also modulate the band structure with two spin degenerate bands splitting. By using the nonequilibrium Green's function method, the dependence of spin-dependent conductance and spin-polarization on the Fermi energy, the exchange field strength and the ferromagnetic TI (FTI) length are also analyzed, respectively. Interestingly, the degenerate conductance plateaus for spin-up and -down channels are broken, and both the conductances are suppressed by magnetic impurities due to the time-reversal symmetry broken and inelastic scattering. The spin-dependent conductance shows different behaviors when the Fermi energy is tuned into different ranges. Moreover, the conductance can be fully spin polarized by tuning the Fermi energy and the exchange field strength, or by tuning the Fermi energy and the FTI length. Consequently, the junction can transform from a quantum spin Hall state to a quantum anomalous Hall state, which is very important to enable dissipationless charge current for designing perfect spin filter.

  13. Pulsed laser deposition of high-quality thin films of the insulating ferromagnet EuS

    SciTech Connect

    Yang, Qi I.; Zhao, Jinfeng; Risbud, Subhash H.; Zhang, Li; Dolev, Merav; Fried, Alexander D.; Marshall, Ann F.; Kapitulnik, Aharon

    2014-02-24

    High-quality thin films of the ferromagnetic insulator europium(II) sulfide (EuS) were fabricated by pulsed laser deposition on Al{sub 2}O{sub 3} (0001) and Si (100) substrates. A single orientation was obtained with the [100] planes parallel to the substrates, with atomic-scale smoothness indicates a near-ideal surface topography. The films exhibit uniform ferromagnetism below 15.9 K, with a substantial component of the magnetization perpendicular to the plane of the films. Optimization of the growth condition also yielded truly insulating films with immeasurably large resistance. This combination of magnetic and electric properties opens the gate for future devices that require a true ferromagnetic insulator.

  14. Resistance noise in epitaxial thin films of ferromagnetic topological insulators

    NASA Astrophysics Data System (ADS)

    Bhattacharyya, Semonti; Kandala, Abhinav; Richardella, Anthony; Islam, Saurav; Samarth, Nitin; Ghosh, Arindam

    2016-02-01

    We report detailed temperature and gate-voltage dependence of 1/f resistance noise in magnetically doped topological insulators (TI) Crx(Bi,Sb)2-xTe3. The noise is remarkably sensitive to the gate voltage, increasing rapidly as the chemical potential is moved towards the charge neutrality point. Unlike in identically prepared (Bi,Sb)2Te3 films, where mobility-fluctuations in the surface states is the dominant mechanism, the noise in the magnetic Crx(Bi,Sb)2-xTe3 originates from transport in the localized band tail of the bulk valence band. A strong increase in noise with decreasing temperature supports this scenario. At higher temperature (≥10 K), we observed large noise peaks at gate voltage-dependent characteristic temperature scales. In line with similar observations in other non-magnetic TI systems, we attribute these peaks to generation-recombination in the Cr-impurity band.

  15. Electronic structure and magnetocrystalline anisotropy of the Bi2Se3 topological insulator/ferromagnet interface

    NASA Astrophysics Data System (ADS)

    Zhang, Jia; Velev, Julian P.; Tsymbal, Evgeny Y.

    Interesting spin-dependent phenomena are expected to emerge when a topological insulator is interfaced with a magnetic material. In this work the magnetic properties of the interface between a topological insulator Bi2Se3 and ferromagnetic metals (FM) fcc (111) Ni and Co are investigated by first-principles calculations. Different interface terminations are considered, and the most stable interface termination is identified to be an interface Ni (Co) atom located atop the hollow site of the interfacial Se monolayer. We find that the proximity effect induces a small magnetic moment on the interface Se atom (0.028 μB for Ni and 0.023 μB for Co). The surface state in Bi2Se3 disappears due to the strong interface hybridization between FM and Bi2Se3 and metal induced gap states appear in the bandgap region of Bi2Se3. We find that both the Bi2Se3/Ni(111) and Bi2Se3/Co(111) interfaces exhibit an in-plane easy axis with the magnetic anisotropy energy of around 2 erg/cm2 per interface. An interesting feature resulting from our calculations is a non-collinear k-dependent spin texture at the interface which may have important consequences for the spin-dependent transport properties, such as the spin transfer torque.

  16. Duality in topological superconductors and topological ferromagnetic insulators in a honeycomb lattice

    NASA Astrophysics Data System (ADS)

    Huang, Shin-Ming; Tsai, Wei-Feng; Chung, Chung-Hou; Mou, Chung-Yu

    2016-02-01

    The ground state of the large Hubbard U limit of a honeycomb lattice near half filling is known to be a singlet d +i d -wave superconductor. It is also known that this d +i d superconductor exhibits a chiral p +i p pairing locally at the Dirac cone, characterized by a 2 Z topological invariant. By constructing a dual transformation, we demonstrate that this 2 Z topological superconductor is equivalent to a collection of two topological ferromagnetic insulators. As a result of the duality, the topology of the electronic structures for a d +i d superconductor is controllable via the change of the chemical potential by tuning the gate voltage. In particular, instead of always being a chiral superconductor, we find that the d +i d superconductor undergoes a topological phase transition from a chiral superconductor to a quasihelical superconductor as the gap amplitude or the chemical potential decreases. The quasihelical superconducting phase is found to be characterized by a topological invariant in the pseudospin charge sector with vanishing both the Chern number and the spin Chern number. We further elucidate the topological phase transition by analyzing the relationship between the topological invariant and the rotation symmetry. Due to the angular momentum carried by the gap function and spin-orbit interactions, we show that by placing d +i d superconductors in proximity to ferromagnets, varieties of chiral superconducting phases characterized by higher Chern numbers can be accessed, providing a platform for hosting large numbers of Majorana modes at edges.

  17. Massive Dirac surface states in topological insulator/magnetic insulator heterostructures

    NASA Astrophysics Data System (ADS)

    Luo, Weidong; Qi, Xiao-Liang

    2013-03-01

    We study the behavior of topological surface states in heterostructures formed by a topological insulator (TI) and a magnetic insulator (MI). Several MIs with compatible magnetic structure and relatively good lattice matching with TIs are identified, and the best candidate material is found to be MnSe, an anti-ferromagnetic insulator. We perform first-principles calculation in Bi2Se3/MnSe superlattices and obtain the surface state bandstructure. The magnetic exchange coupling with MnSe induces a gap of 54 meV at the surface states. In addition we tune the distance between Mn ions and TI surface to study the distance dependence of the exchange coupling. Finally, we study the band bending effect at the Bi2Se3/MnSe interface, and propose possible solutions to avoid band bending. We acknowledge funding support from the Defense Advanced Research Projects Agency (DARPA).

  18. Ferromagnetism in the Mott insulator Ba2NaOsO6

    SciTech Connect

    Erickson, A.S.; Misra, S.; Miller, G.J.; Harrison, W.A.; Kim, J.M.; Fisher, I.R.; /Stanford U., Appl. Phys. Dept. /Stanford U., Geballe Lab.

    2010-01-15

    Results are presented of single crystal structural, thermodynamic, and reflectivity measurements of the double-perovskite Ba{sub 2}NaOsO{sub 6}. These characterize the material as a 5d1 ferromagnetic Mott insulator with an ordered moment of {approx} 0.2 {micro}B per formula unit and T{sub C} = 6.8(3) K. The magnetic entropy associated with this phase transition is close to Rln2, indicating that the quartet groundstate anticipated from consideration of the crystal structure is split, consistent with a scenario in which the ferromagnetism is associated with orbital ordering.

  19. Ferromagnetism on a paramagnetic host background in cobalt-doped Bi2Se3 topological insulator

    NASA Astrophysics Data System (ADS)

    Zhang, Min; Lü, Li; Wei, Zhan-Tao; Yang, Xin-Sheng; Zhao, Yong

    2014-07-01

    Cobalt-doped Bi2Se3 topological insulators have been grown though melt-grown reaction. The Bi2Se3 matrix is diamagnetic and doped sample is a superposition of ferromagnetism (FM) and paramagnetism (PM) behavior at low temperature. The values of MSmol, Hc, and Mr increase as the Co concentration increases. Two possible explanations have been proposed for the origin of ferromagnetism in Co-doped Bi2Se3. One is the magnetic ordering from nanoclusters of Co-Se compound in the crystals, and the other is Ruderman—Kittel—Kasuya—Yosida (RKKY) interaction between magnetic impurities.

  20. Electrically controlled spin polarization and selection in a topological insulator sandwiched between ferromagnetic electrodes

    SciTech Connect

    Guo, Junji; Liao, Wenhu Zhao, Heping; Zhou, Guanghui

    2014-01-14

    We theoretically investigate the electrically controllable spin polarization and selective efficiency of the edge state Dirac electron in a two-dimensional topological insulator (TI) sandwiched between ferromagnetic (FM) electrodes by using the method of Keldysh nonequilibrium Green's function. A nearly full spin polarization of the topological edge state with giant inversion of ∼80% is observed, which is much higher than the value previously reported. Moreover, the selective efficiency for spin-up electrons under the modulation of the parallel configuration of FM electrodes has been demonstrated to be larger than 95% for the first time, while that for spin-down electrons in the antiparallel case is higher than 90% in a wide energy range, owing to the inter-edge spin tunneling induced backscattering and spin dephasing effect. The obtained results may provide a deeper understanding of the TI edge states and a valuable guidance to design spin switch and filter with high on-off speed and selective efficiency based on TIs.

  1. Structural and proximity-induced ferromagnetic properties of topological insulator-magnetic insulator heterostructures

    NASA Astrophysics Data System (ADS)

    Jiang, Zilong; Chang, Cui-Zu; Tang, Chi; Zheng, Jian-Guo; Moodera, Jagadeesh S.; Shi, Jing

    2016-05-01

    The spontaneously broken time reversal symmetry can lead to the formation of an energy gap in the Dirac spectrum of the surface states of a topological insulator (TI) which can consequently give rise to a variety of interesting phenomena potentially useful for spintronics. In this work, we couple a non-magnetic TI to a high Curie temperature TC magnetic insulator to induce strong exchange interaction via the proximity effect. We have successfully grown 5 quintuple layer thick ternary TI (BixSb1-x)2Te3 films on atomically flat yttrium iron garnet (YIG) film with the combination of molecular beam epitaxy and pulsed laser deposition, in which the Fermi level position relative to the Dirac point is varied by controlling the Bi:Sb ratio. The anomalous Hall effect (AHE) and suppressed weak antilocalization (WAL) measured under out of plane magnetic fields reveal that the TI surface in contact with YIG is magnetized. Our high-quality (BixSb1-x)2Te3/Y IG heterostructure provides a tunable system for exploring the quantum anomalous Hall effect (QAHE) at higher temperatures in TI-based spintronic devices.

  2. Evidence of Spin-Injection-Induced Cooper Pair Breaking in Perovskite Ferromagnet-Insulator-Superconductor Heterostructures via Pulsed Current Measurements

    NASA Technical Reports Server (NTRS)

    Yeh, N. C.; Samoilov, A. V.; Veasquez, R. P.; Li, Y.

    1998-01-01

    The effect of spin-polarized currents on the critical current densities of cuprate superconductors is investigated in perovskite ferromagnet-insulator-superconductor heterostructures with a pulsed current technique.

  3. Growth and characterization of insulating ferromagnetic semiconductor (Al,Fe)Sb

    NASA Astrophysics Data System (ADS)

    Anh, Le Duc; Kaneko, Daiki; Hai, Pham Nam; Tanaka, Masaaki

    2015-12-01

    We investigate the crystal structure, transport, and magnetic properties of Fe-doped ferromagnetic semiconductor (Al1-x,Fex)Sb thin films up to x = 14% grown by molecular beam epitaxy. All the samples show p-type conduction at room temperature and insulating behavior at low temperature. The (Al1-x,Fex)Sb thin films with x ≤ 10% maintain the zinc blende crystal structure of the host material AlSb. The (Al1-x,Fex)Sb thin film with x = 10% shows intrinsic ferromagnetism with a Curie temperature (TC) of 40 K. In the (Al1-x,Fex)Sb thin film with x = 14%, a sudden drop of the hole mobility and TC was observed, which may be due to the microscopic phase separation. The observation of ferromagnetism in (Al,Fe)Sb paves the way to realize a spin-filtering tunnel barrier that is compatible with well-established III-V semiconductor devices.

  4. Microwave-induced spin currents in ferromagnetic-insulator|normal-metal bilayer system

    SciTech Connect

    Agrawal, Milan; Serga, Alexander A.; Lauer, Viktor; Papaioannou, Evangelos Th.; Hillebrands, Burkard; Vasyuchka, Vitaliy I.

    2014-09-01

    A microwave technique is employed to simultaneously examine the spin pumping and the spin Seebeck effect processes in a YIG|Pt bilayer system. The experimental results show that for these two processes, the spin current flows in opposite directions. The temporal dynamics of the longitudinal spin Seebeck effect exhibits that the effect depends on the diffusion of bulk thermal-magnons in the thermal gradient in the ferromagnetic-insulator|normal-metal system.

  5. Modulation of pure spin currents with a ferromagnetic insulator

    NASA Astrophysics Data System (ADS)

    Villamor, Estitxu; Isasa, Miren; Vélez, Saül; Bedoya-Pinto, Amilcar; Vavassori, Paolo; Hueso, Luis E.; Bergeret, F. Sebastián; Casanova, Fèlix

    2015-01-01

    We propose and demonstrate spin manipulation by magnetically controlled modulation of pure spin currents in cobalt/copper lateral spin valves, fabricated on top of the magnetic insulator Y3F e5O12 (YIG). The direction of the YIG magnetization can be controlled by a small magnetic field. We observe a clear modulation of the nonlocal resistance as a function of the orientation of the YIG magnetization with respect to the polarization of the spin current. Such a modulation can only be explained by assuming a finite spin-mixing conductance at the Cu/YIG interface, as it follows from the solution of the spin-diffusion equation. These results open a path towards the development of spin logics.

  6. Ferromagnetic states of p-type silicon doped with Mn

    NASA Astrophysics Data System (ADS)

    Yunusov, Z. A.; Yuldashev, Sh. U.; Igamberdiev, Kh. T.; Kwon, Y. H.; Kang, T. W.; Bakhadyrkhanov, M. K.; Isamov, S. B.; Zikrillaev, N. F.

    2014-05-01

    In this work, the ferromagnetic states of Mn-doped p-type silicon samples were investigated. Two different types of ferromagnetic states have been observed in Si (Mn, B). The samples with a relatively high concentration of Mn revealed a ferromagnetic state with a Curie temperature above room temperature, and that ferromagnetism was due to the Mn x B y ferromagnetic clusters. The samples with a moderate concentration of Mn at low temperatures revealed a ferromagnetic state that was mediated by carriers (holes). The samples demonstrated the anomalous Hall effect at temperatures below 100 K and had a negative magneto-resistivity peak at a temperature close to the Curie temperature. The thermal diffusivity measurements demonstrated the existence of a second-order phase transition in the samples with a moderate Mn concentration. The specific heat's critical exponent α = 0.5, determined from the thermal diffusivity measurements, confirmed the long-range nature of the magnetic exchange interaction in these samples.

  7. Predicted ferromagnetic insulator CrO2/TiO2 superlattice with the modified Becke-Johnson potential

    NASA Astrophysics Data System (ADS)

    Guo, San-Dong

    2015-11-01

    The ferromagnetic insulator plays a major role in high-performance spintronic applications. So, it is very necessary to search for better ferromagnetic insulators that are compatible with current semiconductor technology. We investigate the electronic structures and magnetic properties of the \\text{Cr}{{\\text{O}}2}/\\text{Ti}{{\\text{O}}2} superlattice by using Tran and Blaha’s modified Becke and Johnson exchange potential. The calculated results show that the \\text{Cr}{{\\text{O}}2}/\\text{Ti}{{\\text{O}}2} superlattice is a ferromagnetic insulator, which combines ferromagnetic properties of \\text{Cr}{{\\text{O}}2} with insulating properties of \\text{Ti}{{\\text{O}}2} . The ferromagnetic stability is proven by magnetic energy differences for a series of lattice constants a. It is very interesting that both the majority-spin channel and spin non-conservation gaps firstly decrease with increasing lattice constants a, and then increase. However, the minority-spin channel gap firstly decreases, then remains nearly steady. These trends of gaps can be understood by changes of electronic band structures. If the \\text{Cr}{{\\text{O}}2}/\\text{Ti}{{\\text{O}}2} superlattice was experimentally synthesized with ferromagnetic insulator properties, it should be used to design high-performance spintronic devices, achieving new functionality.

  8. Predicted ferromagnetic insulator CrO2/TiO2 superlattice with the modified Becke-Johnson potential

    NASA Astrophysics Data System (ADS)

    Guo, San-Dong

    2014-11-01

    The ferromagnetic insulator plays a major role in high-performance spintronic applications. So, it is very necessary to search for better ferromagnetic insulators that are compatible with current semiconductor technology. We investigate the electronic structures and magnetic properties of the \\text{Cr}{{\\text{O}}2}/\\text{Ti}{{\\text{O}}2} superlattice by using Tran and Blaha’s modified Becke and Johnson exchange potential. The calculated results show that the \\text{Cr}{{\\text{O}}2}/\\text{Ti}{{\\text{O}}2} superlattice is a ferromagnetic insulator, which combines ferromagnetic properties of \\text{Cr}{{\\text{O}}2} with insulating properties of \\text{Ti}{{\\text{O}}2} . The ferromagnetic stability is proven by magnetic energy differences for a series of lattice constants a. It is very interesting that both the majority-spin channel and spin non-conservation gaps firstly decrease with increasing lattice constants a, and then increase. However, the minority-spin channel gap firstly decreases, then remains nearly steady. These trends of gaps can be understood by changes of electronic band structures. If the \\text{Cr}{{\\text{O}}2}/\\text{Ti}{{\\text{O}}2} superlattice was experimentally synthesized with ferromagnetic insulator properties, it should be used to design high-performance spintronic devices, achieving new functionality.

  9. Theory of strain-controlled magnetotransport and stabilization of the ferromagnetic insulating phase in manganite thin films.

    PubMed

    Mukherjee, Anamitra; Cole, William S; Woodward, Patrick; Randeria, Mohit; Trivedi, Nandini

    2013-04-12

    We show that applying strain on half-doped manganites makes it possible to tune the system to the proximity of a metal-insulator transition and thereby generate a colossal magnetoresistance (CMR) response. This phase competition not only allows control of CMR in ferromagnetic metallic manganites but can be used to generate CMR response in otherwise robust insulators at half-doping. Further, from our realistic microscopic model of strain and magnetotransport calculations within the Kubo formalism, we demonstrate a striking result of strain engineering that, under tensile strain, a ferromagnetic charge-ordered insulator, previously inaccessible to experiments, becomes stable. PMID:25167302

  10. Heavily Cr-doped (Bi,Sb)2Te3 as a ferromagnetic insulator with electrically tunable conductivity

    NASA Astrophysics Data System (ADS)

    Ou, Yunbo; Liu, Chang; Zhang, Liguo; Feng, Yang; Jiang, Gaoyuan; Zhao, Dongyang; Zang, Yunyi; Zhang, Qinghua; Gu, Lin; Wang, Yayu; He, Ke; Ma, Xucun; Xue, Qi-Kun

    2016-08-01

    With molecular beam epitaxy we have grown Cry(BixSb1-x)2-yTe3 thin films with homogeneous distribution of Cr dopants and Curie temperature up to 77 K. The films with Cr concentration y ≥ 0.39 are found to be topologically trivial, highly insulating ferromagnets, whose conductivity can be tuned over two orders of magnitude by gate voltage. The ferromagnetic insulators with electrically tunable conductivity can be used to realize the quantum anomalous Hall effect at higher temperature in topological insulator heterostructures and to develop field effect devices for spintronic applications.

  11. Magnetically Controlled Electronic Transport Properties of a Ferromagnetic Junction on the Surface of a Topological Insulator

    NASA Astrophysics Data System (ADS)

    Liu, Zheng-Qin; Wang, Rui-Qiang; Deng, Ming-Xun; Hu, Liang-Bin

    2015-06-01

    We have investigated the transport properties of the Dirac fermions through a ferromagnetic barrier junction on the surface of a strong topological insulator. The current-voltage characteristic curve and the tunneling conductance are calculated theoretically. Two interesting transport features are predicted: observable negative differential conductances and linear conductances tunable from unit to nearly zero. These features can be magnetically manipulated simply by changing the spacial orientation of the magnetization. Our results may contribute to the development of high-speed switching and functional applications or electrically controlled magnetization switching. Supported by National Natural Science Foundation of China under Grant Nos. 11174088, 11175067, 11274124

  12. Nonlocal topological magnetoelectric effect by Coulomb interaction at a topological insulator-ferromagnet interface

    NASA Astrophysics Data System (ADS)

    Rex, Stefan; Nogueira, Flavio S.; Sudbø, Asle

    2016-01-01

    The interface between a topological insulator and a ferromagnetic insulator exhibits an interesting interplay of topological Dirac electrons and magnetism. As has been shown recently, the breaking of time-reversal invariance by magnetic order generates a Chern-Simons term in the action, that in turn leads to a Berry phase and a magnetoelectric effect of topological origin. Here, we consider the system in the presence of a long-range Coulomb interaction between the Dirac electrons, and find that the magnetoelectric effect of the fluctuating electric field becomes nonlocal. We derive a Landau-Lifshitz equation for the fluctuation-induced magnetization dynamics and the Euler-Lagrange equation of the Coulomb field by explicit one-loop calculations. Via the Coulomb interaction, divergences in the in-plane magnetization affect the magnetization dynamics over large distances in a topologically protected way.

  13. Proximity-Driven Enhanced Magnetic Order at Ferromagnetic-Insulator-Magnetic-Topological-Insulator Interface

    NASA Astrophysics Data System (ADS)

    Li, Mingda; Chang, Cui-Zu; Kirby, Brian. J.; Jamer, Michelle E.; Cui, Wenping; Wu, Lijun; Wei, Peng; Zhu, Yimei; Heiman, Don; Li, Ju; Moodera, Jagadeesh S.

    2015-08-01

    Magnetic exchange driven proximity effect at a magnetic-insulator-topological-insulator (MI-TI) interface provides a rich playground for novel phenomena as well as a way to realize low energy dissipation quantum devices. Here we report a dramatic enhancement of proximity exchange coupling in the MI/magnetic-TI EuS /Sb2 -xVx Te3 hybrid heterostructure, where V doping is used to drive the TI (Sb2 Te3 ) magnetic. We observe an artificial antiferromagneticlike structure near the MI-TI interface, which may account for the enhanced proximity coupling. The interplay between the proximity effect and doping in a hybrid heterostructure provides insights into the engineering of magnetic ordering.

  14. Unconventional Fermi surface in an insulating state

    SciTech Connect

    Harrison, Neil; Tan, B. S.; Hsu, Y. -T.; Zeng, B.; Hatnean, M. Ciomaga; Zhu, Z.; Hartstein, M.; Kiourlappou, M.; Srivastava, A.; Johannes, M. D.; Murphy, T. P.; Park, J. -H.; Balicas, L.; Lonzarich, G. G.; Balakrishnan, G.; Sebastian, Suchitra E.

    2015-07-17

    Insulators occur in more than one guise; a recent finding was a class of topological insulators, which host a conducting surface juxtaposed with an insulating bulk. Here, we report the observation of an unusual insulating state with an electrically insulating bulk that simultaneously yields bulk quantum oscillations with characteristics of an unconventional Fermi liquid. We present quantum oscillation measurements of magnetic torque in high-purity single crystals of the Kondo insulator SmB6, which reveal quantum oscillation frequencies characteristic of a large three-dimensional conduction electron Fermi surface similar to the metallic rare earth hexaborides such as PrB6 and LaB6. As a result, the quantum oscillation amplitude strongly increases at low temperatures, appearing strikingly at variance with conventional metallic behavior.

  15. Ultra Low Energy Switching of Ferromagnet with Perpendicular Anisotropy on Topological Insulator by Voltage Controlled Magnetic Anisotropy

    NASA Astrophysics Data System (ADS)

    Ghosh, Bahniman; Pramanik, Tanmoy; Dey, Rik; Roy, Urmimala; Register, Leonard; Banerjee, Sanjay

    2015-03-01

    We propose and demonstrate, through simulation, an ultra low energy memory device on a topological insulator thin film. The device consists of a thin layer of Fe deposited on the surface of a topological insulator, Bi2Se3. The top surface of Fe is covered with MgO so that the ferromagnetic layer has perpendicular anisotropy. Current is passed on the surface of the topological insulator which switches the magnetization of the Fe ferromagnet through strong exchange interaction, between electrons contributing to the surface current on the Bi2Se3 and the d electrons in the ferromagnet, and spin transfer torque due to shunting of current through the ferromagnet. Voltage controlled magnetic anisotropy enables ultra low energy switching. Our micromagnetic simulations, predict switching time of the order of 2.4 ns and switching energy of the order of 0.16 fJ for a ferromagnetic bit with thermal stability of 90 kBT. The proposed structure combines the advantages of both large spin torque from topological insulators and those of perpendicular anisotropy materials. This work is supported by NRI SWAN and NSF NASCENT Center.

  16. Growth and characterization of insulating ferromagnetic semiconductor (Al,Fe)Sb

    SciTech Connect

    Anh, Le Duc Kaneko, Daiki; Tanaka, Masaaki; Hai, Pham Nam

    2015-12-07

    We investigate the crystal structure, transport, and magnetic properties of Fe-doped ferromagnetic semiconductor (Al{sub 1−x},Fe{sub x})Sb thin films up to x = 14% grown by molecular beam epitaxy. All the samples show p-type conduction at room temperature and insulating behavior at low temperature. The (Al{sub 1−x},Fe{sub x})Sb thin films with x ≤ 10% maintain the zinc blende crystal structure of the host material AlSb. The (Al{sub 1−x},Fe{sub x})Sb thin film with x = 10% shows intrinsic ferromagnetism with a Curie temperature (T{sub C}) of 40 K. In the (Al{sub 1−x},Fe{sub x})Sb thin film with x = 14%, a sudden drop of the hole mobility and T{sub C} was observed, which may be due to the microscopic phase separation. The observation of ferromagnetism in (Al,Fe)Sb paves the way to realize a spin-filtering tunnel barrier that is compatible with well-established III-V semiconductor devices.

  17. Spin pumping through a topological insulator probed by x-ray detected ferromagnetic resonance

    NASA Astrophysics Data System (ADS)

    Figueroa, A. I.; Baker, A. A.; Collins-McIntyre, L. J.; Hesjedal, T.; van der Laan, G.

    2016-02-01

    In the field of spintronics, the generation of a pure spin current (without macroscopic charge flow) through spin pumping of a ferromagnetic (FM) layer opens up the perspective of a new generation of dissipation-less devices. Microwave driven ferromagnetic resonance (FMR) can generate a pure spin current that enters adjacent layers, allowing for both magnetization reversal (through spin-transfer torque) and to probe spin coherence in non-magnetic materials. However, standard FMR is unable to probe multilayer dynamics directly, since the measurement averages over the contributions from the whole system. The synchrotron radiation-based technique of x-ray detected FMR (XFMR) offers an elegant solution to this drawback, giving access to element-, site-, and layer-specific dynamical measurements in heterostructures. In this work, we show how XFMR has provided unique information to understand spin pumping and spin transfer torque effects through a topological insulator (TI) layer in a pseudo-spin valve heterostructure. We demonstrate that TIs function as efficient spin sinks, while also allowing a limited dynamic coupling between ferromagnetic layers. These results shed new light on the spin dynamics of this novel class of materials, and suggest future directions for the development of room temperature TI-based spintronics.

  18. Electrical control of the ferromagnetism in Sb2-xCrxTe3 magnetic topological insulators

    NASA Astrophysics Data System (ADS)

    Zhang, Zuocheng; Feng, Xiao; Guo, Minghua; Chang, Cuicu; Zhang, Jinsong; Li, Kang; Wang, Lili; Chen, Xi; He, Ke; Xue, Qikun; Ma, Xucun; Wang, Yayu; Tsinghua University Team; Institute of Physics, Chinese Academy of Sciences Collaboration

    2013-03-01

    The spin helical Dirac fermions living on the surface of three-dimensional topological insulators (TIs) provide a platform for exploring the coupling between the charge and spin degrees of freedom. In particular, breaking the time reversal symmetry in TIs is expected to create exotic topological magnetoelectric effects. To realize these phenomena and apply them in TI-based spintronic devices, it is desirable to achieve in situ manipulation of the magnetism in TIs via an electrical field. In this talk we present the fabrication and transport studies of Cr doped Sb2Te3 magnetic TI thin films. By applying a gate voltage in a field effect transistor device, we can control the coercive force and Curie temperature. The ferromagnetic order is found to be enhanced when more hole-type carriers are injected into the sample. This trend suggests the itinerant bulk holes in TIs can mediate ferromagnetic ordering of local moments in a similar manner as that in the diluted magnetic semiconductors. The electrical control of the ferromagnetism in TIs demonstrated here paves the road for realizing the TI-based devices.

  19. Proximity-driven enhanced magnetic order at ferromagnetic-insulator-magnetic-topological-insulator interface

    SciTech Connect

    Li, Mingda; Zhu, Yimei; Chang, Cui -Zu; Kirby, B. J.; Jamer, Michelle E.; Cui, Wenping; Wu, Lijun; Wei, Peng; Heiman, Don; Li, Ju; Moodera, Jagadeesh S.; Katmis, Ferhat

    2015-08-17

    Magnetic exchange driven proximity effect at a magnetic-insulator–topological-insulator (MI-TI) interface provides a rich playground for novel phenomena as well as a way to realize low energy dissipation quantum devices. In this study, we report a dramatic enhancement of proximity exchange coupling in the MI/magnetic-TI EuS/Sb2–xVxTe3 hybrid heterostructure, where V doping is used to drive the TI (Sb2Te3) magnetic. We observe an artificial antiferromagneticlike structure near the MI-TI interface, which may account for the enhanced proximity coupling. The interplay between the proximity effect and doping in a hybrid heterostructure provides insights into the engineering of magnetic ordering.

  20. Proximity-driven enhanced magnetic order at ferromagnetic-insulator-magnetic-topological-insulator interface

    DOE PAGESBeta

    Li, Mingda; Zhu, Yimei; Chang, Cui -Zu; Kirby, B. J.; Jamer, Michelle E.; Cui, Wenping; Wu, Lijun; Wei, Peng; Heiman, Don; Li, Ju; et al

    2015-08-17

    Magnetic exchange driven proximity effect at a magnetic-insulator–topological-insulator (MI-TI) interface provides a rich playground for novel phenomena as well as a way to realize low energy dissipation quantum devices. In this study, we report a dramatic enhancement of proximity exchange coupling in the MI/magnetic-TI EuS/Sb2–xVxTe3 hybrid heterostructure, where V doping is used to drive the TI (Sb2Te3) magnetic. We observe an artificial antiferromagneticlike structure near the MI-TI interface, which may account for the enhanced proximity coupling. The interplay between the proximity effect and doping in a hybrid heterostructure provides insights into the engineering of magnetic ordering.

  1. Spin thermoelectric efficiency across a normal-metal/ferromagnetic-insulator interface

    NASA Astrophysics Data System (ADS)

    Yan, Yonghong; Wu, Haifei; Jiang, Feng

    2016-08-01

    We investigate the spin and heat transport across a normal-metal/ferromagnetic-insulator (NM/FI) interface based on the s-d exchange model. Under a certain temperature gradient, the heat current carried by magnons partly flows into the metal, and is partly converted to spin power corresponding to spin current. We find that when the magnon dispersion of the FI (such as yttrium iron garnet) is quadratic, the conversion efficiency of heat current to spin power is about ηs ∼ 0.2ηC with ηC being the Carnot efficiency. The corresponding spin thermopower is roughly Ss ∼ 110 μ V / K . The efficiency and the spin thermopower can be enhanced by opening a gap via, for example, introducing a magnetic field. Effects of temperature in the presence of a gap and dimensionality are also discussed briefly.

  2. Magnetic-field-modulated resonant tunneling in ferromagnetic-insulator-nonmagnetic junctions.

    PubMed

    Song, Yang; Dery, Hanan

    2014-07-25

    We present a theory for resonance-tunneling magnetoresistance (MR) in ferromagnetic-insulator-nonmagnetic junctions. The theory sheds light on many of the recent electrical spin injection experiments, suggesting that this MR effect rather than spin accumulation in the nonmagnetic channel corresponds to the electrically detected signal. We quantify the dependence of the tunnel current on the magnetic field by quantum rate equations derived from the Anderson impurity model, with the important addition of impurity spin interactions. Considering the on-site Coulomb correlation, the MR effect is caused by competition between the field, spin interactions, and coupling to the magnetic lead. By extending the theory, we present a basis for operation of novel nanometer-size memories. PMID:25105652

  3. Driving and detecting ferromagnetic resonance in insulators with the spin Hall effect.

    SciTech Connect

    Sklenar, Joseph; Zhang, Wei; Jungfleisch, Matthias B.; Jiang, Wanjun; Chang, Houchen; Pearson, John E.; Wu, Mingzhong; Ketterson, John B.; Hoffmann, Axel

    2015-11-06

    We demonstrate the generation and detection of spin-torque ferromagnetic resonance in Pt/Y3Fe5O12 (YIG) bilayers. A unique attribute of this system is that the spin Hall effect lies at the heart of both the generation and detection processes and no charge current is passing through the insulating magnetic layer. When the YIG undergoes resonance, a dc voltage is detected longitudinally along the Pt that can be described by two components. One is the mixing of the spin Hall magnetoresistance with the microwave current. The other results from spin pumping into the Pt being converted to a dc current through the inverse spin Hall effect. The voltage is measured with applied magnetic field directions that range in-plane to nearly perpendicular. We find that for magnetic fields that are mostly out-of-plane, an imaginary component of the spin mixing conductance is required to model our data.

  4. Thermal spin current and spin accumulation at ferromagnetic insulator/nonmagnetic metal interface

    NASA Astrophysics Data System (ADS)

    Shen, Y. H.; Wang, X. S.; Wang, X. R.

    2016-07-01

    Spin current injection and spin accumulation near a ferromagnetic insulator (FI)/nonmagnetic metal (NM) bilayer film under a thermal gradient is investigated theoretically. By using the Fermi golden rule and the Boltzmann equations, we find that FI and NM can exchange spins via interfacial electron-magnon scattering because of the imbalance between magnon emission and absorption caused by either the deviation of the magnon number from the equilibrium Bose-Einstein distribution or the difference in magnon temperature and electron temperature. A temperature gradient in FI and/or a temperature difference across the FI/NM interface generates a spin current which carries angular momenta parallel to the magnetization of FI from the hotter side to the colder one. Interestingly, the spin current induced by a temperature gradient in NM is negligibly small due to the nonmagnetic nature of the nonequilibrium electron distributions. The results agree well with all existing experiments.

  5. Magnetotransport in metal/insulating-ferromagnet heterostructures: Spin Hall magnetoresistance or magnetic proximity effect

    NASA Astrophysics Data System (ADS)

    Zhou, X.; Ma, L.; Shi, Z.; Fan, W. J.; Zheng, Jian-Guo; Evans, R. F. L.; Zhou, S. M.

    2015-08-01

    We study the anomalous Hall-like effect (AHLE) and the effective anisotropic magnetoresistance (EAMR) in antiferromagnetic γ -IrMn3/Y3Fe5O12(YIG ) and Pt/YIG heterostructures. For γ -IrMn3/YIG , the EAMR and the AHLE resistivity change sign with temperature due to the competition between the spin Hall magnetoresistance (SMR) and the magnetic proximity effect (MPE) induced by the interfacial antiferromagnetic uncompensated magnetic moment. In contrast, for Pt/YIG, the AHLE resistivity changes sign with temperature whereas no sign change is observed in the EAMR. This is because the MPE and the SMR play a dominant role in the AHLE and the EAMR, respectively. As different types of galvanomagnetic properties, the AHLE and the EAMR have proved vital in disentangling the MPE and the SMR in metal/insulating-ferromagnet heterostructures.

  6. Tunneling conductance in topological insulator ferromagnet/d+id mixed wave superconductor junctions

    NASA Astrophysics Data System (ADS)

    Li, Hong; Yang, Xinjian; Jia, Cuiping

    2014-05-01

    The tunneling conductance in a topological insulator (TI) ferromagnet/d+id mixed wave superconductor (FM/d+id S) junction is studied based on the Blonder-Tinkham-Klapwijk (BTK) theory. It is demonstrated that the conductance strongly depends on the magnetic gap, the superconducting pair symmetry orientation, and the magnitude of the ratio of Δ2/Δ1. Here Δ1(Δ2) is the absolute pair potential of d(d) component. For a large magnetic gap, the tunneling spectrum is characterized by a dip structure. When α=0.25π, the conductance dip appears at eV=±Δ2. Thus, the d+id pairing symmetry as well as the magnitude of the gap can be determined from the experiment of scanning tunneling spectroscopy.

  7. Carrier-mediated ferromagnetism in the magnetic topological insulator Cr-doped (Sb,Bi)2Te3

    PubMed Central

    Ye, Mao; Li, Wei; Zhu, Siyuan; Takeda, Yukiharu; Saitoh, Yuji; Wang, Jiajia; Pan, Hong; Nurmamat, Munisa; Sumida, Kazuki; Ji, Fuhao; Liu, Zhen; Yang, Haifeng; Liu, Zhengtai; Shen, Dawei; Kimura, Akio; Qiao, Shan; Xie, Xiaoming

    2015-01-01

    Magnetically doped topological insulators, possessing an energy gap created at the Dirac point through time-reversal-symmetry breaking, are predicted to exhibit exotic phenomena including the quantized anomalous Hall effect and a dissipationless transport, which facilitate the development of low-power-consumption devices using electron spins. Although several candidates of magnetically doped topological insulators were demonstrated to show long-range magnetic order, the realization of the quantized anomalous Hall effect is so far restricted to the Cr-doped (Sb,Bi)2Te3 system at extremely low temperature; however, the microscopic origin of its ferromagnetism is poorly understood. Here we present an element-resolved study for Cr-doped (Sb,Bi)2Te3 using X-ray magnetic circular dichroism to unambiguously show that the long-range magnetic order is mediated by the p-hole carriers of the host lattice, and the interaction between the Sb(Te) p and Cr d states is crucial. Our results are important for material engineering in realizing the quantized anomalous Hall effect at higher temperatures. PMID:26582485

  8. Carrier-mediated ferromagnetism in the magnetic topological insulator Cr-doped (Sb,Bi)2Te3

    NASA Astrophysics Data System (ADS)

    Ye, Mao; Li, Wei; Zhu, Siyuan; Takeda, Yukiharu; Saitoh, Yuji; Wang, Jiajia; Pan, Hong; Nurmamat, Munisa; Sumida, Kazuki; Ji, Fuhao; Liu, Zhen; Yang, Haifeng; Liu, Zhengtai; Shen, Dawei; Kimura, Akio; Qiao, Shan; Xie, Xiaoming

    2015-11-01

    Magnetically doped topological insulators, possessing an energy gap created at the Dirac point through time-reversal-symmetry breaking, are predicted to exhibit exotic phenomena including the quantized anomalous Hall effect and a dissipationless transport, which facilitate the development of low-power-consumption devices using electron spins. Although several candidates of magnetically doped topological insulators were demonstrated to show long-range magnetic order, the realization of the quantized anomalous Hall effect is so far restricted to the Cr-doped (Sb,Bi)2Te3 system at extremely low temperature; however, the microscopic origin of its ferromagnetism is poorly understood. Here we present an element-resolved study for Cr-doped (Sb,Bi)2Te3 using X-ray magnetic circular dichroism to unambiguously show that the long-range magnetic order is mediated by the p-hole carriers of the host lattice, and the interaction between the Sb(Te) p and Cr d states is crucial. Our results are important for material engineering in realizing the quantized anomalous Hall effect at higher temperatures.

  9. Carrier-mediated ferromagnetism in the magnetic topological insulator Cr-doped (Sb,Bi)2Te3.

    PubMed

    Ye, Mao; Li, Wei; Zhu, Siyuan; Takeda, Yukiharu; Saitoh, Yuji; Wang, Jiajia; Pan, Hong; Nurmamat, Munisa; Sumida, Kazuki; Ji, Fuhao; Liu, Zhen; Yang, Haifeng; Liu, Zhengtai; Shen, Dawei; Kimura, Akio; Qiao, Shan; Xie, Xiaoming

    2015-01-01

    Magnetically doped topological insulators, possessing an energy gap created at the Dirac point through time-reversal-symmetry breaking, are predicted to exhibit exotic phenomena including the quantized anomalous Hall effect and a dissipationless transport, which facilitate the development of low-power-consumption devices using electron spins. Although several candidates of magnetically doped topological insulators were demonstrated to show long-range magnetic order, the realization of the quantized anomalous Hall effect is so far restricted to the Cr-doped (Sb,Bi)2Te3 system at extremely low temperature; however, the microscopic origin of its ferromagnetism is poorly understood. Here we present an element-resolved study for Cr-doped (Sb,Bi)2Te3 using X-ray magnetic circular dichroism to unambiguously show that the long-range magnetic order is mediated by the p-hole carriers of the host lattice, and the interaction between the Sb(Te) p and Cr d states is crucial. Our results are important for material engineering in realizing the quantized anomalous Hall effect at higher temperatures. PMID:26582485

  10. Half-metallic ferromagnetism on surfaces of insulating and antiferromagnetic LaFeO3 thin films

    NASA Astrophysics Data System (ADS)

    Mishra, Rohan; Kim, Young-Min; He, Qian; Kim, Seong-Keun; Chang, Seohyoung; Bhattacharya, Anand; Pantelides, Sokrates T.; Borisevich, Albina

    The surfaces of perovskite transition metal oxides having correlated electrons show novel electronic and magnetic phenomena. In this work, we combine scanning transmission electron microscopy imaging and electron energy loss spectroscopy (EELS) with density functional theory (DFT) calculations to study the surface of (LaFeO3)m /(SrFeO3)n heterostructure thin films. Using EELS, we observe a reduction in the oxidation state of Fe on moving from the bulk to the surface over a length of ~5 unit cells. Simultaneously acquired STEM images allow us to map the associated changes in their structure, such as cation displacements and changes in oxygen polyhedral tilts. DFT calculations coupled with the STEM results show that by reducing the surface layer of a LaFeO3 film such that the surface is terminated with FeO4 tetrahedra instead of the FeO6 octahedra as present in the bulk, it is possible to stabilize an exotic phase where the surface layer displays a half-metallic ferromagnetic behavior, while the bulk remains antiferromagnetic and insulating, similar to the class of topological insulators. The calculations also predict that the magnetism and conductivity at the surface can be controlled by the partial pressure of oxygen.

  11. Coupling of Crystal Structure and Magnetism in the Layered, Ferromagnetic Insulator CrI 3

    DOE PAGESBeta

    McGuire, Michael A.; Dixit, Hemant; Cooper, Valentino R.; Sales, Brian C.

    2014-12-23

    Here, we examine the crystallographic and magnetic properties of single crystals of CrI3, an easily cleavable, layered and insulating ferromagnet with a Curie temperature of 61 K. Our X-ray diffraction studies reveal a first-order crystallographic phase transition occurring near 210–220 K upon warming, with significant thermal hysteresis. The low-temperature structure is rhombohedral (Rmore » $$\\bar{3}$$, BiI3-type) and the high-temperature structure is monoclinic (C2/m, AlCl3-type). Evidence for coupling between the crystallographic and magnetic degrees of freedom in CrI3 was found; we observed an anomaly in the interlayer spacing at the Curie temperature and an anomaly in the magnetic susceptibility at the structural transition. First-principles calculations reveal the importance of proper treatment of the long-ranged interlayer forces, and van der Waals density functional theory does an excellent job of predicting the crystal structures and their relative stability. Our calculations suggest that the ferromagnetic order found in the bulk material may persist into monolayer form, suggesting that CrI3 and other chromium trihalides may be promising materials for spintronic and magnetoelectronic research.« less

  12. One-dimensional edge state transport in a topological Kondo insulator

    NASA Astrophysics Data System (ADS)

    Nakajima, Yasuyuki; Syers, Paul; Wang, Xiangfeng; Wang, Renxiong; Paglione, Johnpierre

    2016-03-01

    Topological insulators, with metallic boundary states protected against time-reversal-invariant perturbations, are a promising avenue for realizing exotic quantum states of matter, including various excitations of collective modes predicted in particle physics, such as Majorana fermions and axions. According to theoretical predictions, a topological insulating state can emerge from not only a weakly interacting system with strong spin-orbit coupling, but also in insulators driven by strong electron correlations. The Kondo insulator compound SmB6 is an ideal candidate for realizing this exotic state of matter, with hybridization between itinerant conduction electrons and localized f-electrons driving an insulating gap and metallic surface states at low temperatures. Here we exploit the existence of surface ferromagnetism in SmB6 to investigate the topological nature of metallic surface states by studying magnetotransport properties at very low temperatures. We find evidence of one-dimensional surface transport with a quantized conductance value of e2/h originating from the chiral edge channels of ferromagnetic domain walls, providing strong evidence that topologically non-trivial surface states exist in SmB6.

  13. Anomalous Hall and spin Hall conductivities in three-dimensional ferromagnetic topological insulator/normal insulator heterostructures

    NASA Astrophysics Data System (ADS)

    Men'shov, Vladimir N.; Tugushev, Victor V.; Chulkov, Evgueni V.

    2016-05-01

    In this letter we theoretically demonstrate how an interface perturbation and size effect can be used to manipulate the transport properties of semiconductor heterostructures composed of a thin film of a three-dimensional topological insulator (TI) doped with magnetic impurities and sandwiched between topologically normal insulators. In the framework of a continual scheme, we argue that electron states of the TI film are strongly dominated by its thickness and magnetization as well as by an interface potential whose variation can lead to the modification of topological properties of the heterostructure. This opens diverse possibilities to efficiently tune intrinsic Hall conductivity in the system. We calculate a phase diagram of the heterostructure, which demonstrates a series of quantum transitions between distinct regimes of conductivity. We derive the anomalous Hall conductivity and the spin Hall conductivity dependences on the chemical potential. Applicability conditions of the used approach are also discussed.

  14. Nonlocal edge state transport in topological insulators

    NASA Astrophysics Data System (ADS)

    Protogenov, Alexander P.; Verbus, Valery A.; Chulkov, Evgueni V.

    2013-11-01

    We use the N-terminal scheme for studying the edge-state transport in two-dimensional topological insulators. We find the universal nonlocal response in the ballistic transport approach. This macroscopic exhibition of the topological order offers different areas for applications.

  15. Direct measurement of proximity-induced magnetism at the interface between a topological insulator and a ferromagnet

    PubMed Central

    Lee, Changmin; Katmis, Ferhat; Jarillo-Herrero, Pablo; Moodera, Jagadeesh S.; Gedik, Nuh

    2016-01-01

    When a topological insulator (TI) is in contact with a ferromagnet, both time-reversal and inversion symmetries are broken at the interface. An energy gap is formed at the TI surface, and its electrons gain a net magnetic moment through short-range exchange interactions. Magnetic TIs can host various exotic quantum phenomena, such as massive Dirac fermions, Majorana fermions, the quantum anomalous Hall effect and chiral edge currents along the domain boundaries. However, selective measurement of induced magnetism at the buried interface has remained a challenge. Using magnetic second-harmonic generation, we directly probe both the in-plane and out-of-plane magnetizations induced at the interface between the ferromagnetic insulator (FMI) EuS and the three-dimensional TI Bi2Se3. Our findings not only allow characterizing magnetism at the TI–FMI interface but also lay the groundwork for imaging magnetic domains and domain boundaries at the magnetic TI surfaces. PMID:27344976

  16. Direct measurement of proximity-induced magnetism at the interface between a topological insulator and a ferromagnet.

    PubMed

    Lee, Changmin; Katmis, Ferhat; Jarillo-Herrero, Pablo; Moodera, Jagadeesh S; Gedik, Nuh

    2016-01-01

    When a topological insulator (TI) is in contact with a ferromagnet, both time-reversal and inversion symmetries are broken at the interface. An energy gap is formed at the TI surface, and its electrons gain a net magnetic moment through short-range exchange interactions. Magnetic TIs can host various exotic quantum phenomena, such as massive Dirac fermions, Majorana fermions, the quantum anomalous Hall effect and chiral edge currents along the domain boundaries. However, selective measurement of induced magnetism at the buried interface has remained a challenge. Using magnetic second-harmonic generation, we directly probe both the in-plane and out-of-plane magnetizations induced at the interface between the ferromagnetic insulator (FMI) EuS and the three-dimensional TI Bi2Se3. Our findings not only allow characterizing magnetism at the TI-FMI interface but also lay the groundwork for imaging magnetic domains and domain boundaries at the magnetic TI surfaces. PMID:27344976

  17. Direct measurement of proximity-induced magnetism at the interface between a topological insulator and a ferromagnet

    NASA Astrophysics Data System (ADS)

    Lee, Changmin; Katmis, Ferhat; Jarillo-Herrero, Pablo; Moodera, Jagadeesh S.; Gedik, Nuh

    2016-06-01

    When a topological insulator (TI) is in contact with a ferromagnet, both time-reversal and inversion symmetries are broken at the interface. An energy gap is formed at the TI surface, and its electrons gain a net magnetic moment through short-range exchange interactions. Magnetic TIs can host various exotic quantum phenomena, such as massive Dirac fermions, Majorana fermions, the quantum anomalous Hall effect and chiral edge currents along the domain boundaries. However, selective measurement of induced magnetism at the buried interface has remained a challenge. Using magnetic second-harmonic generation, we directly probe both the in-plane and out-of-plane magnetizations induced at the interface between the ferromagnetic insulator (FMI) EuS and the three-dimensional TI Bi2Se3. Our findings not only allow characterizing magnetism at the TI-FMI interface but also lay the groundwork for imaging magnetic domains and domain boundaries at the magnetic TI surfaces.

  18. Bound States of a Ferromagnetic Wire in a Superconductor

    NASA Astrophysics Data System (ADS)

    Sau, Jay D.; Brydon, P. M. R.

    2015-09-01

    We consider the problem of bound states in strongly anisotropic ferromagnetic impurities in a superconductor, motivated by recent experiments that claim to observe Majorana modes at the ends of ferromagnetic wires on a superconducting substrate [S. Nadj-Perge et al., Science 346, 602 (2014)]. Generalizing the successful theory of bound states of spherically symmetric impurities, we consider a wirelike potential using both analytical and numerical approaches. We find that away from the ends of the wire the bound states form bands with pronounced van Hove singularities, giving rise to subgap peaks in the local density of states. For sufficiently strong magnetization of the wire, we show that this process generically produces a sharp peak at zero energy in the local density of states near the ends of the wire. This zero-energy peak has qualitative similarities to the claimed signature of a Majorana mode observed in the aforementioned experiment.

  19. Bound States of a Ferromagnetic Wire in a Superconductor.

    PubMed

    Sau, Jay D; Brydon, P M R

    2015-09-18

    We consider the problem of bound states in strongly anisotropic ferromagnetic impurities in a superconductor, motivated by recent experiments that claim to observe Majorana modes at the ends of ferromagnetic wires on a superconducting substrate [S. Nadj-Perge et al., Science 346, 602 (2014)]. Generalizing the successful theory of bound states of spherically symmetric impurities, we consider a wirelike potential using both analytical and numerical approaches. We find that away from the ends of the wire the bound states form bands with pronounced van Hove singularities, giving rise to subgap peaks in the local density of states. For sufficiently strong magnetization of the wire, we show that this process generically produces a sharp peak at zero energy in the local density of states near the ends of the wire. This zero-energy peak has qualitative similarities to the claimed signature of a Majorana mode observed in the aforementioned experiment. PMID:26431011

  20. Enhancing ferromagnetic resonance absorption for very thin insulating magnetic films with spin plasmonics

    SciTech Connect

    Chui, S. T.

    2015-05-14

    We consider enhancing the ferromagnetic resonance (FMR) absorption of very thin insulating magnetic films by placing it on top of a dielectric. We find that the signal is enhanced by at least an order of magnitude due to a new nonreciprocal interface resonance that is a mixture of the magnetic surface plasmon mode and a wave guide mode. This resonance occurs over a wide range of thicknesses of the dielectric that is still much less than the wavelength and is made possible by the negative magnetic susceptibility of the magnetic layer. The line width of absorption is reduced by an order of magnitude less than the Gilbert damping parameter. At some frequency, the group velocity of this resonance is negative. Experimentally, very thin yttrium iron garnet (YIG) films are grown on a Gadolinium Gallium Garnet (GGG) substrate which can be considered the dielectric. Our model applies to experiments performed in the YIG/GGG system. Indeed, our picture resolves the disagreement on the magnitude of the spin diffusion lengths obtained with the FMR and the Brillouin scattering techniques. It also provides for a way to make new adaptive thin film miniaturized photonic nonreciprocal devices with low loss.

  1. Voltage-Controlled Magnetic Anisotropy in Heavy Metal/Ferromagnet/Insulator-Based Structures

    NASA Astrophysics Data System (ADS)

    Li, Xiang

    Electric-field assisted writing of magnetic memory that exploits the voltage-controlled magnetic anisotropy (VCMA) effect offers a great potential for high density and low power applications. Magnetoelectric Random Access Memory (MeRAM) has been investigated due to its lower switching current, compared with traditional current-controlled devices utilizing spin transfer torque (STT) or spin-orbit torque (SOT) for magnetization switching. It is of great promise to integrate MeRAM into the advanced CMOS back-end-of-line (BEOL) processes for on-chip embedded applications, and enable non-volatile electronic systems with low static power dissipation and instant-on operation capability. In this thesis, different heavy metal|ferromagnet|insulator-based structures are grown by magnetron sputtering to improve the VCMA effect over the traditional Ta|CoFeB|MgO-based structures. We also established an accurate measurement technique for VCMA characterization. An improved thermal annealing stability of VCMA over 400°C is achieved in Mo|CoFeB|MgO-based structures. In addition, we observed a weak CoFeB thickness dependence of both VCMA coefficient and interfacial perpendicular magnetic anisotropy (PMA) in both Ta|CoFeB|MgO and Mo|CoFeB|MgO-based structures.

  2. Observation of large low field magnetoresistance in ramp-edge tunneling junctions based on doped manganite ferromagnetic electrodes and a SrTiO{sub 3} insulator

    SciTech Connect

    Kwon, C.; Jia, Q.X.; Fan, Y.; Hundley, M.F.; Reagor, D.W.; Hawley, M.E.; Peterson, D.E.

    1998-07-01

    The authors report the fabrication of ferromagnet-insulator-ferromagnet junction devices using a ramp-edge geometry based on (La{sub 0.7}Sr{sub 0.3})MnO{sub 3} ferromagnetic electrodes and a SrTiO{sub 3} insulator. The multilayer thin films were deposited using pulsed laser deposition and the devices were patterned using photolithography and ion milling. As expected from the spin-dependent tunneling, the junction magnetoresistance depends on the relative orientation of the magnetization in the electrodes. The maximum junction magnetoresistance (JMR) of 30% is observed below 300 Oe at low temperatures (T < 100 K).

  3. Breaking time reversal symmetry, quantum anomalous Hall state and dissipationless chiral conduction in topological insulators

    NASA Astrophysics Data System (ADS)

    Moodera, Jagadeesh

    Breaking time reversal symmetry (TRS) in a topological insulator (TI) with ferromagnetic perturbation can lead to many exotic quantum phenomena exhibited by Dirac surface states including the quantum anomalous Hall (QAH) effect and dissipationless quantized Hall transport. The realization of the QAH effect in realistic materials requires ferromagnetic insulating materials and topologically non-trivial electronic band structures. In a TI, the ferromagnetic order and TRS breaking is achievable by conventional way, through doping with a magnetic element, or by ferromagnetic proximity coupling. Our experimental studies by both approaches will be discussed. In doped TI van Vleck ferromagnetism was observed. The proximity induced magnetism at the interface was stable, beyond the expected temperature range. We shall describe in a hard ferromagnetic TI system a robust QAH state and dissipationless edge current flow is achieved,1,2 a major step towards dissipationless electronic applications with no external fields, making such devices more amenable for metrology and spintronics applications. Our study of the gate and temperature dependences of local and nonlocal magnetoresistance, may elucidate the causes of the dissipative edge channels and the need for very low temperature to observe QAH. In close collaboration with: CuiZu Chang,2,3 Ferhat Katmis, 1 . 2 , 3 Peng Wei. 1 , 2 , 3 ; From Nuclear Eng. Dept. MIT, M. Li, J. Li; From Penn State U, W-W. Zhao, D. Y. Kim, C-x. Liu, J. K. Jain, M. H. W. Chan; From Oakridge National Lab, V. Lauter; From Northeastern U., B. A. Assaf, M. E. Jamer, D. Heiman; From Argonne Lab, J. W. Freeland; From Ruhr-Universitaet Bochum (Germany), F. S. Nogueira, I. Eremin; From Saha Institute of Nuclear Physics (India), B. Satpati. Work supported by NSF Grant DMR-1207469, the ONR Grant N00014-13-1-0301, and the STC Center for Integrated Quantum Materials under NSF Grant DMR-1231319.

  4. Ferromagnetic Ground States in Face-Centered Cubic Hubbard Clusters.

    PubMed

    Souza, T X R; Macedo, C A

    2016-01-01

    In this study, the ground state energies of face-centered cubic Hubbard clusters are analyzed using the Lanczos method. Examination of the ground state energy as a function of the number of particle per site n showed an energy minimum for face-centered cubic structures. This energy minimum decreased in n with increasing coulombic interaction parameter U. We found that the ground state energy had a minimum at n = 0.6, when U = 3W, where W denotes the non-interacting energy bandwidth and the face-centered cubic structure was ferromagnetic. These results, when compared with the properties of nickel, shows strong similarity with other finite temperature analyses in the literature and supports the Hirsh's conjecture that the interatomic direct exchange interaction dominates in driving the system into a ferromagnetic phase. PMID:27583653

  5. Corner states in three dimensional topological insulators

    NASA Astrophysics Data System (ADS)

    Medhi, Amal; Banerjee, Abhimanyu; Shenoy, Vijay B.

    2013-03-01

    Localized electronic states appear at the corner formed by the intersection of two surfaces of a topological insulator. By constructing a 3D cubic lattice tight binding model that captures both topological and trivial phases, we study how the corner state evolve as the system changes from trivial to topological. We report the dispersion of the corner states and comment on novel physics and applications. AM acknowledges support from CPDF programme at IISc, Bangalore. VBS thanks DST (Ramanujan grant) and DAE (SRC grant) for generous support.

  6. Strongly enhanced spin current in topological insulator/ferromagnetic metal heterostructures by spin pumping

    SciTech Connect

    Wu, C. N.; Hung, H. Y.; Lin, H. Y.; Lin, P. H.; Kwo, J. E-mail: raynien@phys.nthu.edu.tw; Lin, Y. H.; Fanchiang, Y. T.; Hong, M. E-mail: raynien@phys.nthu.edu.tw; Lin, J. G.; Lee, S. F.

    2015-05-07

    Spin pumping effect in Bi{sub 2}Se{sub 3}/Fe{sub 3}Si and Fe/Bi{sub 2}Te{sub 3} heterostructures was studied. High quality films of Bi{sub 2}Se{sub 3}(001) on ferromagnetic Fe{sub 3}Si(111) layer and Fe(111) films on Bi{sub 2}Te{sub 3}(001) layer were grown epitaxially by molecular beam epitaxy. Using a microwave cavity source, large voltages due to the Inverse Spin Hall Effect (V{sub ISHE}) were detected in Bi{sub 2}Se{sub 3}(001)/Fe{sub 3}Si(111) bi-layer at room temperature. V{sub ISHE} of up to 63.4 ± 4.0 μV at 100 mW microwave power (P{sub MW}) was observed. In addition, Fe(111)/Bi{sub 2}Te{sub 3}(001) bi-layer also showed a large V{sub ISHE} of 3.0 ± 0.1 μV at P{sub MW} of 25 mW. V{sub ISHE} of both structures showed microwave linear power dependence in accordance with the theoretical model of spin pumping. The spin Hall angle was calculated to be 0.0053 ± 0.002 in Bi{sub 2}Se{sub 3} and was estimated to be 0.0068 ± 0.003 in Bi{sub 2}Te{sub 3}. The charge current density (J{sub c}) of Bi{sub 2}Se{sub 3}/Fe{sub 3}Si and Fe/Bi{sub 2}Te{sub 3} structures are comparable and are about 2–5 times higher than the Fe{sub 3}Si/normal metal and Fe{sub 3}Si/GaAs results. The significant enhancement of spin current in topological insulator/ferromagnetic metal (TI/FM) and FM/TI bilayers is attributed to strong spin-orbit coupling inherent of TIs and demonstrates the high potential of exploiting TI-based structures for spintronic applications.

  7. From helical state to chiral state in ferromagnetic bilayer graphene

    NASA Astrophysics Data System (ADS)

    Xu, Lei; Zhou, Yuan; Zhang, Jun

    2015-06-01

    We explore topological phases in biased ferromagnetic bilayer graphene, formed by bilayer graphene subjected to an external ferromagnetic exchange field, under a magnetic field. The most likely way to obtain a variety of distinct broken symmetry topological phases is proposed by means of ferromagnetic exchange field. Both spin-filtered quantum Hall and quantum spin Hall (QSH) phases are found. Edge modes in this QSH phase carry charge, spin and valley currents. When both time reversal and inversion symmetries are broken, the QSH phase remains robust against weak disorder. Moreover, topological phase transition from helical phase to chiral phase can be driven by simply tuning bias voltage or Fermi energy. A few possible experimental realizations are also discussed.

  8. Itinerant ferromagnetism to insulating spin glass in SrRu{sub 1-x}Cu{sub x}O{sub 3} (0 {<=} x {<=} 0.3)

    SciTech Connect

    Mangalam, R.V.K.; Sundaresan, A.

    2009-03-05

    The ferromagnetic metallic oxide, SrRuO{sub 3} (T{sub C} {approx} 165 K) undergoes structural, magnetic and metal-insulator transitions upon substitution of Cu at the Ru-site. For x = 0.2 in SrRu{sub 1-x}Cu{sub x}O{sub 3}, the structure becomes a tetragonal with the space group I4/mcm and there is a signature of both ferromagnetic (T{sub C} = 65 K) and antiferromagnetic (T{sub N} = 32 K) ordering due to possible magnetic phase separation. The antiferromagnetism arises due to short range ordering of Cu- and Ru-moments. Jahn-Teller distortion of (Ru,Cu)-O{sub 6} octahedra indicates that the copper ions are in 2+ oxidation state with t{sub 2g}{sup 6}e{sub g}{sup 3} electronic configuration. For x {>=} 0.1, narrowing of Ru-4d bandwidth by the substitution of Cu ions results in semiconducting behavior. For x = 0.3, the ac and dc susceptibility measurements indicate a spin glass behavior. The origin of spin glass behavior has been attributed to competing ferromagnetic and antiferromagnetic interactions.

  9. Damping of confined modes in a ferromagnetic thin insulating film: angular momentum transfer across a nanoscale field-defined interface.

    PubMed

    Adur, Rohan; Du, Chunhui; Wang, Hailong; Manuilov, Sergei A; Bhallamudi, Vidya P; Zhang, Chi; Pelekhov, Denis V; Yang, Fengyuan; Hammel, P Chris

    2014-10-24

    We observe a dependence of the damping of a confined mode of precessing ferromagnetic magnetization on the size of the mode. The micron-scale mode is created within an extended, unpatterned yttrium iron garnet film by means of the intense local dipolar field of a micromagnetic tip. We find that the damping of the confined mode scales like the surface-to-volume ratio of the mode, indicating an interfacial damping effect (similar to spin pumping) due to the transfer of angular momentum from the confined mode to the spin sink of ferromagnetic material in the surrounding film. Though unexpected for insulating systems, the measured intralayer spin-mixing conductance g_↑↓=5.3×10(19)  m(-2) demonstrates efficient intralayer angular momentum transfer. PMID:25379927

  10. Direct measurement of proximity-induced magnetism at the interface between a topological insulator and a ferromagnet

    DOE PAGESBeta

    Lee, Changmin; Katmis, Ferhat; Jarillo-Herrero, Pablo; Moodera, Jagadeesh S.; Gedik, Nuh

    2016-06-27

    When a topological insulator (TI) is in contact with a ferromagnet, both time-reversal and inversion symmetries are broken at the interface. An energy gap is formed at the TI surface, and its electrons gain a net magnetic moment through short-range exchange interactions. Magnetic TIs can host various exotic quantum phenomena, such as massive Dirac fermions, Majorana fermions, the quantum anomalous Hall effect and chiral edge currents along the domain boundaries. However, selective measurement of induced magnetism at the buried interface has remained a challenge. Using magnetic second-harmonic generation, we directly probe both the in-plane and out-of-plane magnetizations induced at themore » interface between the ferromagnetic insulator (FMI) EuS and the three-dimensional TI Bi2Se3. Furthermore, our findings not only allow characterizing magnetism at the TI–FMI interface but also lay the groundwork for imaging magnetic domains and domain boundaries at the magnetic TI surfaces.« less

  11. Reentrant Insulating State in Ultrathin Manganite Films

    SciTech Connect

    Bell, Christopher

    2011-08-11

    The transport and magnetic properties of La{sub 0.7}Sr{sub 0.3}MnO{sub 3} thin-films grown by pulsed laser deposition on (LaAlO{sub 3}){sub 0.3}(SrAl{sub 0.5}Ta{sub 0.5}O{sub 3}){sub 0.7} single crystal substrates have been investigated. A systematic series with various thicknesses of La{sub 0.7}Sr{sub 0.3}MnO{sub 3} was used to establish a phase diagram - which showed a clear difference compared to films grown on SrTiO{sub 3} substrates, highlighting the importance of film thickness and substrate strain. At 8 unit cells, the boundary between the metallic and insulating ground states, a second abrupt metal-insulator transition was observed at low temperatures, which could be tuned with by magnetic field, and is interpreted as a signature of electronic phase separation.

  12. Hopf insulators and their topologically protected surface states

    NASA Astrophysics Data System (ADS)

    Wang, Sheng-Tao; Deng, Dong-Ling; Shen, Chao; Duan, Lu-Ming

    2014-03-01

    Three-dimensional (3D) topological insulators in general need to be protected by certain kinds of symmetries other than the presumed U(1) charge conservation. A peculiar exception is the Hopf insulators which are 3D topological insulators characterized by an integer Hopf index. To demonstrate the existence and physical relevance of the Hopf insulators, we construct a class of tight-binding model Hamiltonians which realize all kinds of Hopf insulators with arbitrary integer Hopf index. These Hopf insulator phases have topologically protected surface states and we numerically demonstrate the robustness of these topologically protected states under general random perturbations without any symmetry other than the U(1) charge conservation that is implicit in all kinds of topological insulators. NBR-PC (973 Program) 2011CBA00300 (2011CBA00302), the DARPA OLE program, the IARPA MUSIQC program, the ARO and the AFOSR MURI program.

  13. Self-consistent model of spin accumulation magnetoresistance in ferromagnet/insulator/semiconductor tunnel junctions

    NASA Astrophysics Data System (ADS)

    Appelbaum, Ian; Tinkey, Holly N.; Li, Pengke

    2014-12-01

    Spin accumulation in a paramagnetic semiconductor due to voltage-biased current tunneling from a polarized ferromagnet is experimentally manifest as a small additional spin-dependent resistance. We describe a rigorous model incorporating the necessary self-consistency between electrochemical potential splitting, spin-dependent injection current, and applied voltage that can be used to simulate this so-called "3 T " signal as a function of temperature, doping, ferromagnet bulk spin polarization, tunnel barrier features and conduction nonlinearity, and junction voltage bias.

  14. Optical Control of Ferromagnetism in a Magnetically-Doped Topological Insulator

    NASA Astrophysics Data System (ADS)

    Yeats, Andrew L.; Mintun, Peter J.; Pan, Yu; Richardella, Anthony; Samarth, Nitin; Awschalom, David D.

    Many proposed experiments involving topological insulators (TIs) require spatial control over time-reversal symmetry and chemical potential. We demonstrate micron-scale optical control of both magnetization and chemical potential in thin films of Cr-doped (Bi,Sb)2Te3. By optically modulating the coercivity of the films, we write and erase arbitrary spatial configurations of their magnetization, which we then image with Kerr microscopy. Additionally, by optically manipulating a space charge layer in the underlying SrTiO3 substrates, we can control the local chemical potential of the films. This allows us to write and erase p- n junctions in the films, which we image with photocurrent microscopy. Both effects persist for > 16 hours. We will present systematic Kerr microscopy, photocurrent microscopy, and electrical transport studies of these materials and various electronic and magnetic structures patterned on them. We will discuss the prospects for using these optical phenomena to study and control the unique physics of TIs, such as edge-state transport in the quantum anomalous Hall regime. This work is supported by ONR, AFOSR-MURI, ARO, and NSF.

  15. Interaction-enhanced magnetically ordered insulating state at the edge of a two-dimensional topological insulator

    NASA Astrophysics Data System (ADS)

    Kharitonov, Maxim

    2012-10-01

    We develop a theory of the correlated magnetically ordered insulating state at the edge of a two-dimensional topological insulator. We demonstrate that the gapped spin-polarized state, induced by the application of the magnetic field B, is naturally facilitated by electron interactions, which drive the critical easy-plane ferromagnetic correlations in the helical liquid. As the key manifestation, the gap Δ in the spectrum of collective excitations, which carry both spin and charge, is enhanced and exhibits a scaling dependence Δ∝B1/(2-K), controlled by the Luttinger liquid parameter K. This scaling dependence could be probed through the activation behavior G˜(e2/h)exp(-Δ/T) of the longitudinal conductance of a Hall-bar device at lower temperatures, providing a straightforward way to extract the parameter K experimentally. Our findings thus suggest that the signatures of the interaction-driven quantum criticality of the helical liquid could be revealed already in a standard Hall-bar measurement.

  16. Local magnetoresistance through Si and its bias voltage dependence in ferromagnet/MgO/silicon-on-insulator lateral spin valves

    SciTech Connect

    Saito, Y. Tanamoto, T.; Ishikawa, M.; Sugiyama, H.; Inokuchi, T.; Hamaya, K.; Tezuka, N.

    2014-05-07

    Local magnetoresistance (MR) through silicon (Si) and its bias voltage (V{sub bias}) (bias current (I{sub bias})) dependence in ferromagnet (FM)/MgO/silicon-on-insulator lateral spin valves are investigated. From the experimental measurements, we find that the local-MR through Si increases with increasing V{sub bias}. This anomalous increase of local-MR as a function of V{sub bias} can be understood by considering the standard drift-diffusion theory improved by taking into account the difference in the interface resistances and first order quantum effect between FM/MgO/Si (source) and Si/MgO/FM (drain) interfaces. The interface resistance dependence on experimentally obtained local-MR ratios also agrees with the improved standard spin diffusion theory. These results indicate that experimentally observed local-MR is certainly related to the spin signal through the Si bulk band.

  17. Coupling of Crystal Structure and Magnetism in the Layered, Ferromagnetic Insulator CrI 3

    SciTech Connect

    McGuire, Michael A.; Dixit, Hemant; Cooper, Valentino R.; Sales, Brian C.

    2014-12-23

    Here, we examine the crystallographic and magnetic properties of single crystals of CrI3, an easily cleavable, layered and insulating ferromagnet with a Curie temperature of 61 K. Our X-ray diffraction studies reveal a first-order crystallographic phase transition occurring near 210–220 K upon warming, with significant thermal hysteresis. The low-temperature structure is rhombohedral (R$\\bar{3}$, BiI3-type) and the high-temperature structure is monoclinic (C2/m, AlCl3-type). Evidence for coupling between the crystallographic and magnetic degrees of freedom in CrI3 was found; we observed an anomaly in the interlayer spacing at the Curie temperature and an anomaly in the magnetic susceptibility at the structural transition. First-principles calculations reveal the importance of proper treatment of the long-ranged interlayer forces, and van der Waals density functional theory does an excellent job of predicting the crystal structures and their relative stability. Our calculations suggest that the ferromagnetic order found in the bulk material may persist into monolayer form, suggesting that CrI3 and other chromium trihalides may be promising materials for spintronic and magnetoelectronic research.

  18. State-of-the-Art Review of Retrofitted Wall Insulation.

    SciTech Connect

    Tsongas, George A.

    1985-09-01

    It appeared worthwhile to review the state of the art of retrofitting wall insulation in residences and to summarize its status as an energy conservation measure. The characteristics of the available insulating materials and approaches have been summarized, including their thermal characteristics. The influence of a variety of factors on the thermal performance of wall insulation as well as on the overall building's energy use has been assessed: insulation density, mean temperature, and moisture content; setting and shrinkage; incomplete filling of wall cavities; air convection within the insulation; south wall solar heating; as well as effects of added wall insulation on infiltration heat loss, on use of lowered indoor temperatures, and on decreased overall building energy use due to the reduction of the outdoor balance point temperature (i.e., length of the heating season). Numerous side effects that may result when retrofitting have also been discussed, including moisture damage, fire hazards, corrosion, health hazards, and indoor air pollution. Other concerns have also been addressed, such as quality control and consumer protection, including the need for infrared thermographic inspection programs and a possible derating of the R-value of insulations to account for the substantial influence of typical void areas. The fact that some insulation types seem preferable to others has been noted. The potential market for retrofitting wall insulation has been assessed, and the advisability of utility wall insulation retrofitting programs has been discussed. Finally, recommendations for further study have been presented. 70 refs., 7 figs., 3 tabs.

  19. Observability of surface Andreev bound states in a topological insulator in proximity to an s-wave superconductor.

    PubMed

    Snelder, M; Golubov, A A; Asano, Y; Brinkman, A

    2015-08-12

    To guide experimental work on the search for Majorana zero-energy modes, we calculate the superconducting pairing symmetry of a three-dimensional topological insulator in combination with an s-wave superconductor. We show how the pairing symmetry changes across different topological regimes. We demonstrate that a dominant p-wave pairing relation is not sufficient to realise a Majorana zero-energy mode useful for quantum computation. Our main result is the relation between odd-frequency pairing and Majorana zero energy modes by using Green functions techniques in three-dimensional topological insulators in the so-called Majorana regime. We discuss thereafter how the pairing relations in the different regimes can be observed in the tunneling conductance of an s-wave proximised three-dimensional topological insulator. We discuss the necessity to incorporate a ferromagnetic insulator to localise the zero-energy bound state to the interface as a Majorana mode. PMID:26189576

  20. Heavy fermions. Unconventional Fermi surface in an insulating state.

    PubMed

    Tan, B S; Hsu, Y-T; Zeng, B; Hatnean, M Ciomaga; Harrison, N; Zhu, Z; Hartstein, M; Kiourlappou, M; Srivastava, A; Johannes, M D; Murphy, T P; Park, J-H; Balicas, L; Lonzarich, G G; Balakrishnan, G; Sebastian, Suchitra E

    2015-07-17

    Insulators occur in more than one guise; a recent finding was a class of topological insulators, which host a conducting surface juxtaposed with an insulating bulk. Here, we report the observation of an unusual insulating state with an electrically insulating bulk that simultaneously yields bulk quantum oscillations with characteristics of an unconventional Fermi liquid. We present quantum oscillation measurements of magnetic torque in high-purity single crystals of the Kondo insulator SmB6, which reveal quantum oscillation frequencies characteristic of a large three-dimensional conduction electron Fermi surface similar to the metallic rare earth hexaborides such as PrB6 and LaB6. The quantum oscillation amplitude strongly increases at low temperatures, appearing strikingly at variance with conventional metallic behavior. PMID:26138105

  1. Resonant valence bond states in zinc vacancies induce the ferromagnetism of ZnO

    NASA Astrophysics Data System (ADS)

    Sun, Shih-Jye

    2016-05-01

    A theoretical model was proposed to investigate the mechanism of ferromagnetism in ZnO as well as to simulate the experimental result that the ferromagnetism can be enhanced by UV irradiation as UV photon energy is equivalent to the band gap. In the model, the spin moments arise from the trapped electrons in oxygen vacancy states and coexist with the itinerant electrons which reside in zinc vacancy states and fall into resonant valence bond states. Charge exchange between the conduction band of ZnO and both vacancy states makes electrons on both vacancy states delocalized and results in a decrease of the ferromagnetism as well.

  2. Transition states of magnetization reversal in ferromagnetic nanorings

    NASA Astrophysics Data System (ADS)

    Chaves-O'Flynn, Gabriel; Kent, Andrew; Stein, Daniel

    2008-03-01

    Thin ferromagnetic rings are of interest for fundamental studies of magnetization reversal, in part, because they are a rare example of a geometry for which an analytical solution for the rate of thermally induced switching has been determined [1]. The theoretical model predicts the transition state to be either a global magnetization rotation of constant azimuthal angle or a localized fluctuation, denoted the instanton saddle. Numerically we have confirmed that for a range of values of external magnetic field and ring size the instanton saddle is energetically favored [2]. The model takes the annular width to be small compared to the mean radius of the annulus; in which case the main contribution to the magnetization energy comes from the surface magnetostatic energy. We present numerical micromagnetic calculations of the activation energy for thermally induced magnetization reversal for the two different transition states for the case when the annular width is equal in magnitude to the mean radius of the ring. Results of the total and surface magnetostatic energies are compared for different ring sizes. [1] K. Martens, D.L. Stein, A.D. Kent, PRB 73, 054413 (2006) [2] G.D. Chaves-O'Flynn, K. Xiao, D.L. Stein, A. D. Kent, arXiv:0710.2546 (2007)

  3. Ferromagnetism through Cr doping in topological insulator Bi2Te3

    NASA Astrophysics Data System (ADS)

    Maurya, V. K.; Patnaik, S.

    2014-04-01

    In this paper we report the preparation of single crystals of Bi2Te3 and Cr0.05(Bi2Te3)0.95 by modified Bridgman method. XRD, EDX and SEM analysis of these crystals have been done for the confirmation of the purity of the samples. Resistivity measurement from 2 K to 300 K has been done by Vander-Pauw method. We observed 20% increase in resistivity after Cr doping. Hall measurement has been done at 2 K for both the samples and we found that after Cr doping charge carrier density has increased but mobility is decreased by a factor of ˜7.9. We also provide evidence for ferromagnetism with Cr doping onto Bi2Te3.

  4. Crystallinity of tellurium capping and epitaxy of ferromagnetic topological insulator films on SrTiO3

    PubMed Central

    Park, Jihwey; Soh, Yeong-Ah; Aeppli, Gabriel; Feng, Xiao; Ou, Yunbo; He, Ke; Xue, Qi-Kun

    2015-01-01

    Thin films of topological insulators are often capped with an insulating layer since topological insulators are known to be fragile to degradation. However, capping can hinder the observation of novel transport properties of the surface states. To understand the influence of capping on the surface states, it is crucial to understand the crystal structure and the atomic arrangement at the interfaces. Here, we use x-ray diffraction to establish the crystal structure of magnetic topological insulator Cr-doped (Bi,Sb)2Te3 (CBST) films grown on SrTiO3 (1 1 1) substrates with and without a Te capping layer. We find that both the film and capping layer are single crystal and that the crystal quality of the film is independent of the presence of the capping layer, but that x-rays cause sublimation of the CBST film, which is prevented by the capping layer. Our findings show that the different transport properties of capped films cannot be attributed to a lower crystal quality but to a more subtle effect such as a different electronic structure at the interface with the capping layer. Our results on the crystal structure and atomic arrangements of the topological heterostructure will enable modelling the electronic structure and design of topological heterostructures. PMID:26123202

  5. Crystallinity of tellurium capping and epitaxy of ferromagnetic topological insulator films on SrTiO3.

    PubMed

    Park, Jihwey; Soh, Yeong-Ah; Aeppli, Gabriel; Feng, Xiao; Ou, Yunbo; He, Ke; Xue, Qi-Kun

    2015-01-01

    Thin films of topological insulators are often capped with an insulating layer since topological insulators are known to be fragile to degradation. However, capping can hinder the observation of novel transport properties of the surface states. To understand the influence of capping on the surface states, it is crucial to understand the crystal structure and the atomic arrangement at the interfaces. Here, we use x-ray diffraction to establish the crystal structure of magnetic topological insulator Cr-doped (Bi,Sb)2Te3 (CBST) films grown on SrTiO3 (1 1 1) substrates with and without a Te capping layer. We find that both the film and capping layer are single crystal and that the crystal quality of the film is independent of the presence of the capping layer, but that x-rays cause sublimation of the CBST film, which is prevented by the capping layer. Our findings show that the different transport properties of capped films cannot be attributed to a lower crystal quality but to a more subtle effect such as a different electronic structure at the interface with the capping layer. Our results on the crystal structure and atomic arrangements of the topological heterostructure will enable modelling the electronic structure and design of topological heterostructures. PMID:26123202

  6. Crystallinity of tellurium capping and epitaxy of ferromagnetic topological insulator films on SrTiO3

    NASA Astrophysics Data System (ADS)

    Park, Jihwey; Soh, Yeong-Ah; Aeppli, Gabriel; Feng, Xiao; Ou, Yunbo; He, Ke; Xue, Qi-Kun

    2015-06-01

    Thin films of topological insulators are often capped with an insulating layer since topological insulators are known to be fragile to degradation. However, capping can hinder the observation of novel transport properties of the surface states. To understand the influence of capping on the surface states, it is crucial to understand the crystal structure and the atomic arrangement at the interfaces. Here, we use x-ray diffraction to establish the crystal structure of magnetic topological insulator Cr-doped (Bi,Sb)2Te3 (CBST) films grown on SrTiO3 (1 1 1) substrates with and without a Te capping layer. We find that both the film and capping layer are single crystal and that the crystal quality of the film is independent of the presence of the capping layer, but that x-rays cause sublimation of the CBST film, which is prevented by the capping layer. Our findings show that the different transport properties of capped films cannot be attributed to a lower crystal quality but to a more subtle effect such as a different electronic structure at the interface with the capping layer. Our results on the crystal structure and atomic arrangements of the topological heterostructure will enable modelling the electronic structure and design of topological heterostructures.

  7. Nature of Pressure-induced Insulating States in Simple Metals

    NASA Astrophysics Data System (ADS)

    Naumov, Ivan; Hemley, Russell

    As experimentally established, all the alkali metals and heavy alkaline earth metals (Ca, Sr and Ba) become progressively less conductive on compression, at least up to some critical limit over a broad pressure range. Of these metals, Li and Na clearly undergo pressure-induced metal-insulator transitions, which may also be called reverse Mott transitions. Here, using group theory arguments and first-principles calculations, we show that such transitions can be understood in terms of band representations introduced by Zak. The valence bands in the insulating states are described by simple and composite band representations constructed from localized Wannier functions centered on points unoccupied by atoms. The character of the Wannier functions is closely related to the degree of s-p(-d) hybridization and reflects multi-center chemical bonding in these insulating states. The conditions under which an insulating state is allowed for structures having an integer number of atoms per primitive unit cell as well as re-entrant (i.e., metal-insulator-metal) transition sequences are detailed, resulting in predictions of semimetallic phases with flat surface states. The general principles developed are tested and applied to the alkali and alkaline earth metals, including elements where high-pressure insulating phases have been identified or reported (e.g., Li, Na, and Ca). This research was supported by EFree, an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science, Basic Energy Sciences under Award DESC0001057.

  8. Cobaltites: Emergence of magnetism and metallicity from a non-magnetic, insulating state

    NASA Astrophysics Data System (ADS)

    Phelan, Daniel Patrick

    In cobalt oxides, the energy splitting between different spin-states of Co3+ ions can be quite small, which means that more than one spin-state can simultaneously co-exist in the same compound and that transitions between different spin-state can occur. This makes understanding the magnetic coupling between cobalt sites rather complex. Such is the case for pure and hole-doped LaCoO3. In its ground state, LaCoO3 is a non-magnetic insulator. The lack of a magnetic moment, is due to the fact that the ground spin-state of Co3+ ions is a low-spin, S=0, state. However, since a spin-state that has a net spin is on the order of 100 K higher in energy than the ground spin-state, a magnetic moment appears as the temperature is increased, and the system behaves as a paramagnet above 100 K. The higher-energy spin-state is either an intermediate-spin (S=1) state of a high-spin (S=2) state - an issue that has been debated for quite some time. When holes are chemically doped into the system, as in La1- xSrxCoO3 (LSCO), the non-magnetic, insulating ground state evolves into a ferromagnetic, metallic state. This evolution is complicated because it occurs due to the convoluted effects of Co4+ ions being doped into the system and the fact that the ground spin-state of Co3+ ions changes as a function of the hole concentration. In this dissertation, the magnetic transitions in pure and hole-doped LaCoO3 are investigated by neutron scattering techniques. In the pure compound, it is shown that thermally excited spins have both fluctuating ferromagnetic and antiferro-magnetic spin-correlations, which is suggested to result from a dynamic orbital ordering of the occupied e. g orbitals of the intermediate-spin state. It is also shown that the thermally excited spin-state is split in energy by 0.6 meV. In the hole-doped compound, LSCO, it is shown that the evolution into a metallic ferromagnet occurs by the percolation of isotropic ferromagnetic droplets. It is also shown that incommensurate

  9. Hubbard models with nearly flat bands: Ground-state ferromagnetism driven by kinetic energy

    NASA Astrophysics Data System (ADS)

    Müller, Patrick; Richter, Johannes; Derzhko, Oleg

    2016-04-01

    We consider the standard repulsive Hubbard model with a flat lowest-energy band for two one-dimensional lattices (diamond chain and ladder) as well as for a two-dimensional lattice (bilayer) at half filling of the flat band. The considered models do not fall in the class of Mielke-Tasaki flat-band ferromagnets, since they do not obey the connectivity conditions. However, the ground-state ferromagnetism can emerge, if the flat band becomes dispersive. To study this kinetic-energy-driven ferromagnetism we use perturbation theory and exact diagonalization of finite lattices. We find as a typical scenario that small and moderate dispersion may lead to a ferromagnetic ground state for sufficiently large on-site Hubbard repulsion U >Uc , where Uc increases monotonically with the acquired bandwidth. However, we also observe for some specific parameter cases, that (i) ferromagnetism appears at already very small Uc, (ii) ferromagnetism does not show up at all, (iii) the critical on-site repulsion Uc is a nonmonotonic function of the bandwidth, or that (iv) a critical bandwidth is needed to open the window for ground-state ferromagnetism.

  10. Spin-to-charge conversion in lateral and vertical topological-insulator/ferromagnet heterostructures with microwave-driven precessing magnetization

    NASA Astrophysics Data System (ADS)

    Mahfouzi, Farzad; Nagaosa, Naoto; Nikolić, Branislav K.

    2014-09-01

    Using the charge-conserving Floquet-Green function approach to open quantum systems driven by an external time-periodic potential, we analyze how spin current pumped by the precessing magnetization of a ferromagnetic (F) layer is injected laterally into the interface with strong spin-orbit coupling (SOC) and converted into charge current flowing in the same direction. In the case of a metallic interface with the Rashba SOC used in recent experiments [J. C. R. Sánchez, L. Vila, G. Desfonds, S. Gambarelli, J. P. Attané, J. M. De Teresa, C. Magén, and A. Fert, Nat. Commun. 4, 2944 (2013), 10.1038/ncomms3944], both spin ISα and charge I current flow within the interface where I /ISα≃ 2-8% (depending on the precession cone angle), while for a F/topological-insulator (F/TI) interface employed in related experiments [Y. Shiomi, K. Nomura, Y. Kajiwara, K. Eto, M. Novak, K. Segawa, Y. Ando, and E. Saitoh, arXiv:1312.7091] the conversion efficiency is greatly enhanced (I /ISα≃ 40-60%) due to perfect spin-momentum locking on the surface of a TI. The spin-to-charge conversion occurs also when spin current is pumped vertically through the F/TI interface with smaller efficiency (I /ISα˜0.001%), but with the charge current signal being sensitive to whether the Dirac fermions at the interface are massive or massless.

  11. Imaging Dirac-mass disorder from magnetic dopant atoms in the ferromagnetic topological insulator Crx(Bi0.1Sb0.9)2-xTe3.

    PubMed

    Lee, Inhee; Kim, Chung Koo; Lee, Jinho; Billinge, Simon J L; Zhong, Ruidan; Schneeloch, John A; Liu, Tiansheng; Valla, Tonica; Tranquada, John M; Gu, Genda; Davis, J C Séamus

    2015-02-01

    To achieve and use the most exotic electronic phenomena predicted for the surface states of 3D topological insulators (TIs), it is necessary to open a "Dirac-mass gap" in their spectrum by breaking time-reversal symmetry. Use of magnetic dopant atoms to generate a ferromagnetic state is the most widely applied approach. However, it is unknown how the spatial arrangements of the magnetic dopant atoms influence the Dirac-mass gap at the atomic scale or, conversely, whether the ferromagnetic interactions between dopant atoms are influenced by the topological surface states. Here we image the locations of the magnetic (Cr) dopant atoms in the ferromagnetic TI Cr0.08(Bi0.1Sb0.9)1.92Te3. Simultaneous visualization of the Dirac-mass gap Δ(r) reveals its intense disorder, which we demonstrate is directly related to fluctuations in n(r), the Cr atom areal density in the termination layer. We find the relationship of surface-state Fermi wavevectors to the anisotropic structure of Δ(r) not inconsistent with predictions for surface ferromagnetism mediated by those states. Moreover, despite the intense Dirac-mass disorder, the anticipated relationship [Formula: see text] is confirmed throughout and exhibits an electron-dopant interaction energy J* = 145 meV·nm(2). These observations reveal how magnetic dopant atoms actually generate the TI mass gap locally and that, to achieve the novel physics expected of time-reversal symmetry breaking TI materials, control of the resulting Dirac-mass gap disorder will be essential. PMID:25605947

  12. Imaging Dirac-mass disorder from magnetic dopant atoms in the ferromagnetic topological insulator Crx(Bi0.1Sb0.9)2-xTe3

    DOE PAGESBeta

    Lee, Inhee; Kim, Chung Koo; Lee, Jinho; Billinge, Simon J. L.; Zhong, Ruidan D.; Schneeloch, John A.; Liu, Tiansheng S.; Valla, Tonica; Tranquada, John M.; Gu, Genda D.; et al

    2015-01-20

    To achieve and use the most exotic electronic phenomena predicted for the surface states of 3D topological insulators (TIs), it is necessary to open a “Dirac-mass gap” in their spectrum by breaking time-reversal symmetry. Use of magnetic dopant atoms to generate a ferromagnetic state is the most widely applied approach. However, it is unknown how the spatial arrangements of the magnetic dopant atoms influence the Dirac-mass gap at the atomic scale or, conversely, whether the ferromagnetic interactions between dopant atoms are influenced by the topological surface states. Here we image the locations of the magnetic (Cr) dopant atoms in themore » ferromagnetic TI Cr₀.₀₈(Bi₀.₁Sb₀.₉)₁.₉₂Te₃. Simultaneous visualization of the Dirac-mass gap Δ(r) reveals its intense disorder, which we demonstrate is directly related to fluctuations in n(r), the Cr atom areal density in the termination layer. We find the relationship of surface-state Fermi wavevectors to the anisotropic structure of Δ(r) not inconsistent with predictions for surface ferromagnetism mediated by those states. Moreover, despite the intense Dirac-mass disorder, the anticipated relationship Δ(r)∝n(r) is confirmed throughout and exhibits an electron–dopant interaction energy J* = 145 meV·nm². In addition, these observations reveal how magnetic dopant atoms actually generate the TI mass gap locally and that, to achieve the novel physics expected of time-reversal symmetry breaking TI materials, control of the resulting Dirac-mass gap disorder will be essential.« less

  13. In-surface confinement of topological insulator nanowire surface states

    NASA Astrophysics Data System (ADS)

    Chen, Fan W.; Jauregui, Luis A.; Tan, Yaohua; Manfra, Michael; Klimeck, Gerhard; Chen, Yong P.; Kubis, Tillmann

    2015-09-01

    The bandstructures of [110] and [001] Bi2Te3 nanowires are solved with the atomistic 20 band tight binding functionality of NEMO5. The theoretical results reveal: The popular assumption that all topological insulator (TI) wire surfaces are equivalent is inappropriate. The Fermi velocity of chemically distinct wire surfaces differs significantly which creates an effective in-surface confinement potential. As a result, topological insulator surface states prefer specific surfaces. Therefore, experiments have to be designed carefully not to probe surfaces unfavorable to the surface states (low density of states) and thereby be insensitive to the TI-effects.

  14. In-surface confinement of topological insulator nanowire surface states

    SciTech Connect

    Chen, Fan W.; Jauregui, Luis A.; Tan, Yaohua; Manfra, Michael; Klimeck, Gerhard; Chen, Yong P.; Kubis, Tillmann

    2015-09-21

    The bandstructures of [110] and [001] Bi{sub 2}Te{sub 3} nanowires are solved with the atomistic 20 band tight binding functionality of NEMO5. The theoretical results reveal: The popular assumption that all topological insulator (TI) wire surfaces are equivalent is inappropriate. The Fermi velocity of chemically distinct wire surfaces differs significantly which creates an effective in-surface confinement potential. As a result, topological insulator surface states prefer specific surfaces. Therefore, experiments have to be designed carefully not to probe surfaces unfavorable to the surface states (low density of states) and thereby be insensitive to the TI-effects.

  15. Scanning Tunneling Spectroscopy Investigations of Surface States in Three Dimensional Topological Insulators and Topological Crystalline Insulators

    NASA Astrophysics Data System (ADS)

    Okada, Yoshinori

    2014-03-01

    Three dimensional topological insulators (TIs) are a new class of material possessing topologically protected spin-polarized Dirac fermions on their surface. This new material has gathered much attention because of its great potential for realizing novel phenomena that are important for both fundamentals and applications. 3D topological insulators have been extensively probed by surface sensitive tools such as ARPES and spectroscopic imaging scanning tunneling microscopy (STM). In this talk, we will especially focus on STM measurements of Pb1-xSnxSe. This material belongs to a recently discovered new category of topological insulators called topological crystalline insulators (TCIs). In TCIs, topology and crystal symmetry intertwine to create surface states with a unique set of characteristics different from conventional 3D TIs. We have discovered broken mirror symmetry driven states that coexist with massless Dirac electrons in different regions of momentum space. Our findings experimentally demonstrate the unique tunability of surface Dirac electrons which is promising for the future realization of novel electronic states within TCIs. Current address: WPI-AIMR Tohoku University, Japan.

  16. Edge-states ferromagnetism of WS{sub 2} nanosheets

    SciTech Connect

    Huo, Nengjie; Li, Yan; Kang, Jun; Li, Renxiong; Xia, Qinglin; Li, Jingbo

    2014-05-19

    The multilayer WS{sub 2} nanosheets prepared from WO{sub 3} nanowires exhibit strong ferromagnetic behavior with saturation magnetization (M{sub S}) of 0.0058 emu/g and coercive field (H{sub C}) of 92 Oe at room temperature. By decreasing the temperature down to 3 K the H{sub c} is increased up to 1115 Oe, revealing the existence of long-range magnetic ordering. Density functional theory spin-polarized calculations predict that strong ferromagnetic moments in WS{sub 2} nanosheets are attributed to the zigzag edge sulphur S and tungsten W atoms. Our findings also suggest that the WS{sub 2} nanosheets with a high density of edge spins could be used to fabricate spintronics devices, which are circuits utilizing the spin of the electron to process and store information.

  17. Twofold spin-triplet pairing states and tunneling conductance in ferromagnet/ferromagnet/iron pnictide superconductor heterojunctions

    SciTech Connect

    Yang, X.; Tao, Y.C.; Dong, Z.C.; Hu, J.G.

    2013-06-15

    By applying an extended eight-component Bogoliubov–de Gennes equation, we study theoretically the tunneling conductance in clean ferromagnet/ferromagnet/iron pnictide superconductor (FM/FM/iron-based SC) heterojunctions. Under the condition of noncollinear magnetizations, twofold novel Andreev reflections exist due to the existence of two bands in the SC, in which the incident electron and the two Andreev-reflected holes, belonging to the same spin subband, form twofold spin-triplet pairing states near the FM/iron-based SC interface. It is shown that the conversions of the conductance not only between the zero-bias peak and valley at zero energy but also between the peaks and dips at two gap energies are strongly dependent on both the interband coupling strength in the SC and the spin polarization in the FM. The qualitative differences from tunneling into a conventional s-wave SC are also presented, which may help with experimentally probing and identifying the antiphase s-wave pairing symmetry in the iron-based SC. -- Highlights: •An eight-component Bogoliubov–de Gennes (BDG) equation. •Twofold novel ARs and twofold usual ARs. •Conversions of conductance between the zero-bias peak and valley at zero energy. •Conversions of conductance between peaks and dips at two gap energies. •The importance of the interband coupling strength in the SC.

  18. Exchange bias and coercivity for ferromagnets coupled to the domain state and spin glass state

    NASA Astrophysics Data System (ADS)

    Zhan, Xiaozhi; Mao, Zhongquan; Chen, Xi

    2016-05-01

    The exchange bias (EB) effect for systems with a ferromagnetic (FM) layer coupled to bond-diluted pinning layers has been investigated by Monte Carlo simulations. Two bond dilution concentrations are chosen to obtain two kinds of pinning layers: the antiferromagnetic domain state (DS) and the spin glass (SG) state. It is found that when coupled to the more disordered SG state, the ferromagnet shows enhanced EB with higher coercivity due to larger amounts of both frozen and reversible spins at the pinning interface. Spin configurations of the FM/DS interface layer reveal that reversible spins are mostly found in domain boundaries and small domains, while most spins in large domains maintain antiferromagnetic coupling and contribute to the EB effect. The coercivity is linear to the amount of interface reversible spins, but with different slopes in the temperature ranges above or below the blocking temperature t B. This bimodal temperature-dependent coercivity indicates a sudden change in macroscopic interface coupling at the temperature t B.

  19. Insulation.

    ERIC Educational Resources Information Center

    Rhea, Dennis

    This instructional unit is one of 10 developed by students on various energy-related areas that deals specifically with insulation. Its objective is for the student to be able to determine insulation needs of new or existing structures, select type to use, use installation techniques, calculate costs, and apply safety factors. Some topics covered…

  20. Final Report: Stability and Novel Properties of Magnetic Materials and Ferromagnet / Insulator Interfaces

    SciTech Connect

    Voyles, Paul

    2013-07-24

    We report investigations of the synthesis, structure, and properties of new materials for spintronic applications integrated onto silicon substrates. Our primary focus is materials with very high, negative, intrinsic spin polarization of the density of states at the Fermi level. We have developed a new synthesis method for Fe3O4 thin films through selective oxidation of Fe, resulting in smooth, low-defect density films. We have synthesized Fe4N films and shown that they preferentially oxidize to Fe3O4. When integrated into magnetic tunnel junctions consisting of Fe4N / AlOx / Fe, oxidation at the Fe4N / AlOx interface creates Fe3O4, leading to negative tunneling magnetoresistance (TMR). Oxidation of Fe in nominally symmetric CoFe / AlOx / CoFe also produces Fe3O4 and negative TMR under selected oxidation conditions.

  1. Metal-insulator transition near a superconducting state

    NASA Astrophysics Data System (ADS)

    Kaveh, M.; Mott, N. F.

    1992-03-01

    We show that when the metal-insulation transition occurs near a superconducting state it results in a different critical behavior from that of amorphous metals or uncompensated doped semiconductors. This difference results from the enhancement of the effective electron-electron interaction caused by fluctuations to the superconducting state. This explains the recent experiments of Micklitz and co-workers on amorphous superconducting mixtures Ga-Ar and Bi-Kr.

  2. Ferromagnetic superexchange in insulating Cr2MoO6 by controlling orbital hybridization

    DOE PAGESBeta

    Zhu, M.; Do, D.; Dela Cruz, Clarina R.; Dun, Zhiling; Cheng, J. -G.; Goto, H.; Uwatoko, Yoshiya; Zou, T.; Zhou, Haidon D.; Mahanti, Subhendra D.; et al

    2015-09-11

    We report the magnetic and electronic structures of the newly synthesized inverse-trirutile compound Cr2MoO6. Despite the same crystal symmetry and similar bond-lengths and bond-angles to Cr2TeO6, Cr2MoO6 possesses a magnetic structure of the Cr2MoO6 type, different from that seen in Cr2TeO6. Ab-initio electronic structure calculations show that the sign and strength of the Cr-O-Cr exchange coupling is strongly influenced by the hybridization between Mo 4d and O 2p orbitals. This result further substantiates our recently proposed mechanism for tuning the exchange interaction between two magnetic atoms by modifying the electronic states of the non-magnetic atoms in the exchange path throughmore » orbital hybridization. This approach is fundamentally different from the conventional methods of controlling the exchange interaction by either carrier injection or through structural distortions.« less

  3. Bloch Walls and Macroscopic String States in Bethe's Solution of the Heisenberg Ferromagnetic Linear Chain

    NASA Astrophysics Data System (ADS)

    Dhar, Abhishek; Sriram Shastry, B.

    2000-09-01

    We present a calculation of the lowest excited states of the Heisenberg ferromagnet in 1D for any wave vector. These turn out to be string solutions of Bethe's equations with a macroscopic number of particles in them. They are identified as generalized quantum Bloch wall states, and a simple physical picture is provided for the same.

  4. Ferromagnetism and topological surface states of manganese doped Bi{sub 2}Te{sub 3}: Insights from density-functional calculations

    SciTech Connect

    Li, Yuanchang; Zou, Xiaolong; Li, Jia; Zhou, Gang

    2014-03-28

    Based on first-principles calculations, the electronic, magnetic, and topological characters of manganese (Mn) doped topological insulator Bi{sub 2}Te{sub 3} were investigated. The Mn substitutionally doped Bi{sub 2}Te{sub 3}, where Mn atoms tend to be uniformly distributed, was shown to be p-type ferromagnetic, arising from hole-mediated Ruderman-Kittel-Kasuya-Yosida interaction. Mn doping leads to an intrinsic band splitting at Γ point, which is substantially different from that of nonmagnetic dopant. The topological surface state of Bi{sub 2}Te{sub 3} is indeed gapped by Mn doping; however, the bulk conductance limits the appearance of an insulating state. Moreover, the n-type doping behavior of Bi{sub 2}Te{sub 3} is derived from Mn entering into the van der Waals gap of Bi{sub 2}Te{sub 3}.

  5. Stability of composite fermion states in Chern insulators

    NASA Astrophysics Data System (ADS)

    Potasz, Pawel; Jaworowski, Blazej

    We analyze an existence of composite fermion (CF) states in fractional Chern insulators (FCI) using exact diagonalization. The consider Chern insulator models for spinless fermions exhibit a signature of CF states at 2/5 and 3/7 filling factors. Evidences of fractional quantum Hall type phases for a region in a parameter space with larger energy gap are shown by looking at momenta of the n-fold degenerate ground state, spectral flow, quasihole excitation spectrum, and entanglement spectra. We analyze stability of phases as a function of model parameters showing strong correlation with flatness of Berry curvature. The authors acknowledges partial financial support from the sources Granted for science development in the years 2013-2016, Grant No. IP2012 007372.

  6. The ground state of a spin-1 anti-ferromagnetic atomic condensate for Heisenberg limited metrology

    NASA Astrophysics Data System (ADS)

    Wu, Ling-Na; You, Li

    2016-05-01

    The ground state of a spin-1 atomic condensate with anti-ferromagnetic interaction can be applied to quantum metrology approaching the Heisenberg limit. Unlike a ferromagnetic condensate state where individual atomic spins are aligned in the same direction, atoms in an anti-ferromagnetic ground state condensate exist as spin singlet pairs, whose inherent correlation promises metrological precisions beyond the standard quantum limit (SQL) for uncorrelated atoms. The degree of improvement over the SQL is measured by quantum Fisher information (QFI), whose dependence on the ratio of linear Zeeman shift p to spin-dependent atomic interaction c is studied. At a typical value of p = 0 . 4 c corresponding to a magnetic field of 28 . 6 μ G with c = h × 50 Hz (for 23 Na atom condensate in the F = 1 state at a typical density of ~1014cm-3), the scaled QFI can reach ~ 0 . 48 N , which is close to the limits of N for NooN state, or 0 . 5 N for twin-Fock state. We hope our work will stimulate experimental efforts towards reaching the anti-ferromagnetic condensate ground state at extremely low magnetic fields.

  7. Signature of magnetic-dependent gapless odd frequency states at superconductor/ferromagnet interfaces.

    PubMed

    Di Bernardo, A; Diesch, S; Gu, Y; Linder, J; Divitini, G; Ducati, C; Scheer, E; Blamire, M G; Robinson, J W A

    2015-01-01

    The theory of superconductivity developed by Bardeen, Cooper and Schrieffer (BCS) explains the stabilization of electron pairs into a spin-singlet, even frequency, state by the formation of an energy gap within which the density of states is zero. At a superconductor interface with an inhomogeneous ferromagnet, a gapless odd frequency superconducting state is predicted, in which the Cooper pairs are in a spin-triplet state. Although indirect evidence for such a state has been obtained, the gap structure and pairing symmetry have not so far been determined. Here we report scanning tunnelling spectroscopy of Nb superconducting films proximity coupled to epitaxial Ho. These measurements reveal pronounced changes to the Nb subgap superconducting density of states on driving the Ho through a metamagnetic transition from a helical antiferromagnetic to a homogeneous ferromagnetic state for which a BCS-like gap is recovered. The results prove odd frequency spin-triplet superconductivity at superconductor/inhomogeneous magnet interfaces. PMID:26329811

  8. Signature of magnetic-dependent gapless odd frequency states at superconductor/ferromagnet interfaces

    PubMed Central

    Di Bernardo, A.; Diesch, S.; Gu, Y.; Linder, J.; Divitini, G.; Ducati, C.; Scheer, E.; Blamire, M.G.; Robinson, J.W.A.

    2015-01-01

    The theory of superconductivity developed by Bardeen, Cooper and Schrieffer (BCS) explains the stabilization of electron pairs into a spin-singlet, even frequency, state by the formation of an energy gap within which the density of states is zero. At a superconductor interface with an inhomogeneous ferromagnet, a gapless odd frequency superconducting state is predicted, in which the Cooper pairs are in a spin-triplet state. Although indirect evidence for such a state has been obtained, the gap structure and pairing symmetry have not so far been determined. Here we report scanning tunnelling spectroscopy of Nb superconducting films proximity coupled to epitaxial Ho. These measurements reveal pronounced changes to the Nb subgap superconducting density of states on driving the Ho through a metamagnetic transition from a helical antiferromagnetic to a homogeneous ferromagnetic state for which a BCS-like gap is recovered. The results prove odd frequency spin-triplet superconductivity at superconductor/inhomogeneous magnet interfaces. PMID:26329811

  9. Surface state photoelectrons in topological insulators: Green's function approach.

    PubMed

    Schmeltzer, D; Saxena, A

    2015-12-01

    We compute the photoemission intensity and polarization for the surface states in topological insulators. Due to the chirality and linear energy dispersion the effective electron-photon coupling is normalized by the tunneling amplitude (τ) into the vacuum. We investigate a chiral Dirac Hamiltonian for different cases: helical, Zeeman and warping, allowing us to study spin textures. Using the Green's function formalism we obtain exact results for the emitted photoelectrons to second order in the laser field. The number of emitted photoelectrons is sensitive to the laser coherent state intensity whereas the photoelectron polarization is sensitive to the surface topology of electronic states and incoming photon polarization. PMID:26565417

  10. Gate control of ferromagnetic insulating phase in lightly-doped La0.875Sr0.125MnO3-δ film

    NASA Astrophysics Data System (ADS)

    Kuang, H.; Wang, J.; Hu, F. X.; Zhao, Y. Y.; Liu, Y.; Wu, R. R.; Sun, J. R.; Shen, B. G.

    2016-02-01

    The electric field effect on the lightly doped La0.875Sr0.125MnO3-δ (LSMO) thin film in electric double-layer transistors was investigated by measuring transport properties of the film under various gate voltages. It was found that the positive gate bias leads to an increase of the charge-orbital ordering (COO) transition temperature and a decrease of the Curie temperature TC, indicating the suppression of ferromagnetic metal (FMM) phases and preference of COO/ferromagnetic insulator (FMI) with the hole depletion by gate bias. Such different electric field effects can be ascribed to the weakening of the ferromagnetic interaction and enhancement of Jahn-Teller (JT) distortion caused by the transformation of JT inactive Mn4+-ions to JT active Mn3+-ions. Moreover, a step-like increase in the high temperature region of the ρ-T curve, which is related to the transition of cooperative JT distortion, was found to develop with increasing the positive bias, indicating that the cooperative JT distorted phase is stabilized by the depletion of holes in LSMO film. These results demonstrate that the modulation of holes via electric field strongly affects the balance between energy gains of different interactions and thus produce different effects on the competing FMI, FMM, and cooperative JT distorted phases in LSMO film.

  11. Emergence of magnetic topological states in topological insulators doped with magnetic impurities

    NASA Astrophysics Data System (ADS)

    Tran, Minh-Tien; Nguyen, Hong-Son; Le, Duc-Anh

    2016-04-01

    Emergence of the topological invariant and the magnetic moment in topological insulators doped with magnetic impurities is studied based on a mutual cooperation between the spin-orbit coupling of electrons and the spin exchange of these electrons with magnetic impurity moments. The mutual cooperation is realized based on the Kane-Mele model in the presence of magnetic impurities. The topological invariants and the spontaneous magnetization are self-consistently determined within the dynamical mean-field theory. We find different magnetic topological phase transitions, depending on the electron filling. At half filling an antiferromagnetic topological insulator, which exhibits the quantum spin Hall effect, exists in the phase region between the paramagnetic topological insulator and the trivially topological antiferromagnetic insulator. At quarter and three-quarter fillings, a ferromagnetic topological insulator, which exhibits the quantum anomalous Hall effect, occurs in the strong spin-exchange regime.

  12. Metal-to-insulator switching in quantum anomalous Hall states

    DOE PAGESBeta

    Kou, Xufeng; Pan, Lei; Wang, Jing; Fan, Yabin; Choi, Eun Sang; Lee, Wei -Li; Nie, Tianxiao; Murata, Koichi; Shao, Qiming; Zhang, Shou -Cheng; et al

    2015-10-07

    After decades of searching for the dissipationless transport in the absence of any external magnetic field, quantum anomalous Hall effect (QAHE) was recently achieved in magnetic topological insulator films. However, the universal phase diagram of QAHE and its relation with quantum Hall effect (QHE) remain to be investigated. Here, we report the experimental observation of the giant longitudinal resistance peak and zero Hall conductance plateau at the coercive field in the six quintuple-layer (Cr0.12Bi0.26Sb0.62)2Te3 film, and demonstrate the metal-to-insulator switching between two opposite QAHE plateau states up to 0.3 K. Moreover, the universal QAHE phase diagram is confirmed through themore » angle-dependent measurements. Our results address that the quantum phase transitions in both QAHE and QHE regimes are in the same universality class, yet the microscopic details are different. Additionally, the realization of the QAHE insulating state unveils new ways to explore quantum phase-related physics and applications.« less

  13. Metal-to-insulator switching in quantum anomalous Hall states

    PubMed Central

    Kou, Xufeng; Pan, Lei; Wang, Jing; Fan, Yabin; Choi, Eun Sang; Lee, Wei-Li; Nie, Tianxiao; Murata, Koichi; Shao, Qiming; Zhang, Shou-Cheng; Wang, Kang L.

    2015-01-01

    After decades of searching for the dissipationless transport in the absence of any external magnetic field, quantum anomalous Hall effect (QAHE) was recently achieved in magnetic topological insulator films. However, the universal phase diagram of QAHE and its relation with quantum Hall effect (QHE) remain to be investigated. Here, we report the experimental observation of the giant longitudinal resistance peak and zero Hall conductance plateau at the coercive field in the six quintuple-layer (Cr0.12Bi0.26Sb0.62)2Te3 film, and demonstrate the metal-to-insulator switching between two opposite QAHE plateau states up to 0.3 K. Moreover, the universal QAHE phase diagram is confirmed through the angle-dependent measurements. Our results address that the quantum phase transitions in both QAHE and QHE regimes are in the same universality class, yet the microscopic details are different. In addition, the realization of the QAHE insulating state unveils new ways to explore quantum phase-related physics and applications. PMID:26442609

  14. Metal-to-insulator switching in quantum anomalous Hall states

    SciTech Connect

    Kou, Xufeng; Pan, Lei; Wang, Jing; Fan, Yabin; Choi, Eun Sang; Lee, Wei -Li; Nie, Tianxiao; Murata, Koichi; Shao, Qiming; Zhang, Shou -Cheng; Wang, Kang L.

    2015-10-07

    After decades of searching for the dissipationless transport in the absence of any external magnetic field, quantum anomalous Hall effect (QAHE) was recently achieved in magnetic topological insulator films. However, the universal phase diagram of QAHE and its relation with quantum Hall effect (QHE) remain to be investigated. Here, we report the experimental observation of the giant longitudinal resistance peak and zero Hall conductance plateau at the coercive field in the six quintuple-layer (Cr0.12Bi0.26Sb0.62)2Te3 film, and demonstrate the metal-to-insulator switching between two opposite QAHE plateau states up to 0.3 K. Moreover, the universal QAHE phase diagram is confirmed through the angle-dependent measurements. Our results address that the quantum phase transitions in both QAHE and QHE regimes are in the same universality class, yet the microscopic details are different. Additionally, the realization of the QAHE insulating state unveils new ways to explore quantum phase-related physics and applications.

  15. Metal-to-insulator switching in quantum anomalous Hall states

    NASA Astrophysics Data System (ADS)

    Kou, Xufeng; Pan, Lei; Wang, Jing; Fan, Yabin; Choi, Eun Sang; Lee, Wei-Li; Nie, Tianxiao; Murata, Koichi; Shao, Qiming; Zhang, Shou-Cheng; Wang, Kang L.

    2015-10-01

    After decades of searching for the dissipationless transport in the absence of any external magnetic field, quantum anomalous Hall effect (QAHE) was recently achieved in magnetic topological insulator films. However, the universal phase diagram of QAHE and its relation with quantum Hall effect (QHE) remain to be investigated. Here, we report the experimental observation of the giant longitudinal resistance peak and zero Hall conductance plateau at the coercive field in the six quintuple-layer (Cr0.12Bi0.26Sb0.62)2Te3 film, and demonstrate the metal-to-insulator switching between two opposite QAHE plateau states up to 0.3 K. Moreover, the universal QAHE phase diagram is confirmed through the angle-dependent measurements. Our results address that the quantum phase transitions in both QAHE and QHE regimes are in the same universality class, yet the microscopic details are different. In addition, the realization of the QAHE insulating state unveils new ways to explore quantum phase-related physics and applications.

  16. Metal-to-insulator switching in quantum anomalous Hall states

    NASA Astrophysics Data System (ADS)

    Pan, Lei; Kou, Xufeng; Wang, Jing; Fan, Yabin; Choi, Eun Sang; Shao, Qiming; Zhang, Shou Cheng; Wang, Kang Lung

    Quantum anomalous Hall effect (QAHE) was recently achieved in magnetic topological insulator films as a form of dissipationless transport without external magnetic field. However, the universal phase diagram of QAHE and its relation with quantum Hall effect (QHE) remain to be investigated. Here, we report the experimental observation of the giant longitudinal resistance peak and zero Hall conductance plateau at the coercive field in the six quintuple-layer (Cr0.12Bi0.26Sb0.62)2 Te3 film, and demonstrate the metal-to-insulator switching between two opposite QAHE plateau states up to 0.3 K. The universal QAHE phase diagram is further confirmed through the angle-dependent measurements. Our results address that the quantum phase transitions in both QAHE and QHE regimes are in the same universality class, yet the microscopic details are different.

  17. Quantum tunneling between Chern states in a Topological Insulator

    NASA Astrophysics Data System (ADS)

    Liu, Minhao; Wang, Wudi; Richardella, Anthony R.; Kandala, Abhinav; Li, Jian; Yazdani, Ali; Samarth, Nitin; Ong, N. P.

    The tunneling of a macroscopic object through a barrier is a quintessentially quantum phenomenon important in field theory, low-temperature physics and quantum computing. Progress has been achieved in experiments on Josephson junctions, molecular magnets, and domain wall dynamics. However, a key feature - rapid expansion of the true vacuum triggered by a tunneling event is virtually unexplored. Here we report the detection of large jumps in the Hall resistance Ryx in a magnetized topological insulator which result from tunneling out of a metastable topological state. In the TI, the conducting electrons are confined to surface Dirac states. When magnetized, the TI enters the quantum anomalous Hall insulator state in which Ryx is strictly quantized. If the magnetic field is reversed, the sample is trapped in a metastable state. We find that, below 145 mK, Ryx exhibits abrupt jumps as large as one quantum unit on time-scales under 1 ms. If the temperature is raised, the escape rate is suppressed consistent with tunneling in the presence of dissipation. The jumps involve expansion of the thermodynamically stable state bubble over macroscopic lengths, but dissipation limits the final size. The results uncover novel effects of dissipation on macroscopic tunneling. We acknowledge support from DARPA SPAWAR (N66001-11-1-4110) and the Gordon and Betty Moore Foundations (GBMF4539).

  18. Phase states of a 2D easy-plane ferromagnet with strong inclined anisotropy

    SciTech Connect

    Fridman, Yu. A. Klevets, F. N.; Gorelikov, G. A.; Meleshko, A. G.

    2012-12-15

    We investigate the spin states of a 2D film exhibiting easy-axis anisotropy and a strong single-ion inclined anisotropy whose axis forms a certain angle with the normal to the film surface. Such a system may have an angular ferromagnetic phase, a spatially inhomogeneous state, and a quadrupole phase, whose realization depends substantially on the inclined anisotropy and the orientation of the wavevector in the film plane.

  19. Topological interface states in multiscale spoof-insulator-spoof waveguides.

    PubMed

    Meng, Yan; Xiang, Hong; Zhang, Ruo-Yang; Wu, Xiaoxiao; Han, Dezhuan; Chan, C T; Wen, Weijia

    2016-08-15

    The spoof-insulator-spoof (SIS) structure can serve as a waveguide for spoof surface plasmon polaritons (spoof SPPs). If a periodic geometry modulation in the wavelength scale is introduced to the SIS waveguide, this multiscale SIS (MSIS) waveguide possesses band gaps for spoof SPPs analogous to the band gaps in a photonic crystal. Inspired by the topological interface states found in photonic crystals, we construct an interface by connecting two MSIS waveguides with different topological properties (inverted Zak phases of bulk bands). The topological interface states in the MSIS waveguides are observed experimentally. The measured decay lengths of the interface states agree excellently with the numerical results. These localized interface states may find potential applications in miniaturized microwave devices. PMID:27519066

  20. Gapped Surface States in a Strong-Topological-Insulator Material

    NASA Astrophysics Data System (ADS)

    Weber, A. P.; Gibson, Q. D.; Ji, Huiwen; Caruso, A. N.; Fedorov, A. V.; Cava, R. J.; Valla, T.

    2015-06-01

    A three-dimensional strong-topological insulator or semimetal hosts topological surface states which are often said to be gapless so long as time-reversal symmetry is preserved. This narrative can be mistaken when surface state degeneracies occur away from time-reversal-invariant momenta. The mirror invariance of the system then becomes essential in protecting the existence of a surface Fermi surface. Here we show that such a case exists in the strong-topological-semimetal Bi4Se3. Angle-resolved photoemission spectroscopy and ab initio calculations reveal partial gapping of surface bands on the Bi2Se3 termination of Bi4Se3(111 ), where an 85 meV gap along Γ ¯K ¯ closes to zero toward the mirror-invariant Γ ¯M ¯ azimuth. The gap opening is attributed to an interband spin-orbit interaction that mixes states of opposite spin helicity.

  1. Fractional charge and spin states in topological insulator constrictions

    NASA Astrophysics Data System (ADS)

    Klinovaja, Jelena; Loss, Daniel

    2015-09-01

    We theoretically investigate the properties of two-dimensional topological insulator constrictions both in the integer and fractional regimes. In the presence of a perpendicular magnetic field, the constriction functions as a spin filter with near-perfect efficiency and can be switched by electric fields only. Domain walls between different topological phases can be created in the constriction as an interface between tunneling, magnetic fields, charge density wave, or electron-electron interaction dominated regions. These domain walls host non-Abelian bound states with fractional charge and spin and result in degenerate ground states with parafermions. If a proximity gap is induced bound states give rise to an exotic Josephson current with 8 π periodicity.

  2. Enhanced ferromagnetic and metal insulator transition in Sm0.55Sr0.45MnO3 thin films: Role of oxygen vacancy induced quenched disorder

    NASA Astrophysics Data System (ADS)

    Srivastava, M. K.; Siwach, P. K.; Kaur, A.; Singh, H. K.

    2010-11-01

    Effect of quenched disorder (QD) caused by oxygen vacancy (OV) and substrate induced inhomogeneous compressive strain, on the magnetic and transport properties of oriented polycrystalline Sm0.55Sr0.45MnO3 thin films is investigated. QD is related intimately to the ordering/disordering of the OVs and controls the paramagnetic-ferromagnetic/insulator-metal transition. OV ordered films show enhanced TC/TIM˜165 K, which is depressed by oxygen annealing. OV disordering realized by quenching reduces TC/TIM. The first order IM transition observed in SSMO single crystals is transformed into nonhysteretic and continuous one in the OV ordered films. QD appears to be diluted by OV disorder/annihilation and results in stronger carrier localization.

  3. Tunneling density of states as a function of thickness in superconductor/ strong ferromagnet bilayers

    SciTech Connect

    Reymond, S.

    2010-04-29

    We have made an experimental study of the tunneling density of states (DOS) in strong ferromagnetic thin films (CoFe) in proximity with a thick superconducting film (Nb) as a function of d{sub F}, the ferromagnetic thickness. Remarkably, we find that as d{sub F} increases, the superconducting DOS exhibits a scaling behavior in which the deviations from the normal-state conductance have a universal shape that decreases exponentially in amplitude with characteristic length d* {approx} 0.4 nm. We do not see oscillations in the DOS as a function of d{sub F}, as expected from predictions based on the Usadel equations, although an oscillation in T{sub c}(d{sub F}) has been seen in the same materials.

  4. Ferromagnetism of magnetically doped topological insulators in Cr{sub x}Bi{sub 2−x}Te{sub 3} thin films

    SciTech Connect

    Ni, Y.; Zhang, Z. Hadimani, R. L.; Tuttle, G.; Jiles, D. C.; Nlebedim, I. C.

    2015-05-07

    We investigated the effect of magnetic doping on magnetic and transport properties of Bi{sub 2}Te{sub 3} thin films. Cr{sub x}Bi{sub 2−x}Te{sub 3} thin films with x = 0.03, 0.14, and 0.29 were grown epitaxially on mica substrate with low surface roughness (∼0.4 nm). It is found that Cr is an electron acceptor in Bi{sub 2}Te{sub 3} and increases the magnetization of Cr{sub x}Bi{sub 2−x}Te{sub 3}. When x = 0.14 and 0.29, ferromagnetism appears in Cr{sub x}Bi{sub 2−x}Te{sub 3} thin films, where anomalous Hall effect and weak localization of magnetoconductance were observed. The Curie temperature, coercivity, and remnant Hall resistance of thin films increase with increasing Cr concentration. The Arrott-Noakes plot demonstrates that the critical mechanism of the ferromagnetism can be described better with 3D-Heisenberg model than with mean field model. Our work may benefit for the practical applications of magnetic topological insulators in spintronics and magnetoelectric devices.

  5. Frequency mixer having ferromagnetic film

    DOEpatents

    Khitun, Alexander; Roshchin, Igor V.; Galatsis, Kosmas; Bao, Mingqiang; Wang, Kang L.

    2016-03-29

    A frequency conversion device, which may include a radiofrequency (RF) mixer device, includes a substrate and a ferromagnetic film disposed over a surface of the substrate. An insulator is disposed over the ferromagnetic film and at least one microstrip antenna is disposed over the insulator. The ferromagnetic film provides a non-linear response to the frequency conversion device. The frequency conversion device may be used for signal mixing and amplification. The frequency conversion device may also be used in data encryption applications.

  6. Photoinduced Chern insulating states in semi-Dirac materials

    NASA Astrophysics Data System (ADS)

    Saha, Kush

    2016-08-01

    Two-dimensional (2D) semi-Dirac materials are characterized by a quadratic dispersion in one direction and a linear dispersion along the orthogonal direction. We study the topological phase transition in such 2D systems in the presence of an electromagnetic field. We show that a Chern insulating state emerges in a semi-Dirac system with two gapless Dirac nodes in the presence of light. In particular, we show that the intensity of a circularly polarized light can be used as a knob to generate topological states with nonzero Chern number. In addition, for fixed intensity and frequency of the light, a semi-Dirac system with two gapped Dirac nodes with trivial band topology can reveal the topological transition as a function of polarization of the light.

  7. Coexistence of metallic and insulating-like states in graphene.

    PubMed

    Wu, Fang; Huang, Jing; Li, Qunxiang; Deng, Kaiming; Kan, Erjun

    2015-01-01

    Since graphene has been taken as the potential host material for next-generation electric devices, coexistence of high carrier mobility and an energy gap has the determining role in its real applications. However, in conventional methods of band-gap engineering, the energy gap and carrier mobility in graphene are seemed to be the two terminals of a seesaw, which limit its rapid development in electronic devices. Here we demonstrated the realization of insulating-like state in graphene without breaking Dirac cone. Using first-principles calculations, we found that ferroelectric substrate not only well reserves the Dirac fermions, but also induces pseudo-gap states in graphene. Calculated transport results clearly revealed that electrons cannot move along the ferroelectric direction. Thus, our work established a new concept of opening an energy gap in graphene without reducing the high mobility of carriers, which is a step towards manufacturing graphene-based devices. PMID:25754862

  8. Broken lattice-symmetry influence on electronic anisotropy and interface exchange-coupling-induced ferromagnetic state in TI thin films and heterostructures

    NASA Astrophysics Data System (ADS)

    Moodera, Jagadeesh

    2014-03-01

    Inducing an exchange gap locally on the Dirac surface states of a topological insulator (TI) is ideal for observing the predicted unique features such as the quantized topological magnetoelectric effect, half-integer quantized Hall effect, as well as to con?ne Majorana fermions. Our work experimentally demonstrated the proximity-induced interface ferromagnetism in a heterostructure combining a ferromagnetic insulator EuS layer with Bi2Se3, without introducing defects. An exchange gap was observed to be induced on the surface of the TI. Extensive magnetic and magneto-transport (magnetoresistance and anomalous Hall effect) investigation of the heterostructures, including synchrotron interfacial (XAS and XMCD measurements) studies have shown the emergence of a ferromagnetic phase in TI, which is a step forward to unveiling the above exotic properties. Also, to understand the intrinsic properties of TI it is necessary to correlate structure with the exotic electronic properties as well as interaction with other materials. Molecular beam epitaxy (MBE) ideally allows us to engineer the system whereas using synchrotron and electron diffraction based experimental techniques helps us to investigate with atomic resolution. We will elucidate our studies on well-defined TI films and heterostructure, and the role of imperfections on the symmetry of the material that leads to internal atomic ordering by the decoration of the defects. Charge transport and mobility are seen to relate with film growth strain and relaxation as well as display strong directional dependence on the defect geometry. Work done in collaboration with Peng Wei, Ferhat Katmis and others. NSF and ONR grants supported this work.

  9. Thermoelectric transport of edge/surface states of topological insulators

    NASA Astrophysics Data System (ADS)

    Murakami, Shuichi; Takahashi, Ryuji

    2011-03-01

    In my talk we theoretically study thermoelectric properties of topological insulators (TI), where novel properties of edge/surface states are expected to appear. As compared to the number of bulk states, the edge/surface states are very few; we therefore consider a narrow ribbon for 2D and a thin slab for 3D TI to make the edge/surface-state transport larger. By considering edge/surface and bulk transport together, we calculate the charge and heat conductivity, and Seebeck coefficient. We find that in 2D TI the bulk and edge transport compete each other in the thermoelectric transport. By lowering temperature, the thermoelectric figure of merit ZT has a minimum, corresponding to the bulk-to-edge crossover, and then increases again at low temperature where the edge state dominates. The crossover is estimated to be at around 5K-10K for 10nm-width ribbon. We also discuss surface state transport for 3D TI as well.

  10. Optical Properties of a Vibrationally Modulated Solid State Mott Insulator

    PubMed Central

    Kaiser, S.; Clark, S. R.; Nicoletti, D.; Cotugno, G.; Tobey, R. I.; Dean, N.; Lupi, S.; Okamoto, H.; Hasegawa, T.; Jaksch, D.; Cavalleri, A.

    2014-01-01

    Optical pulses at THz and mid-infrared frequencies tuned to specific vibrational resonances modulate the lattice along chosen normal mode coordinates. In this way, solids can be switched between competing electronic phases and new states are created. Here, we use vibrational modulation to make electronic interactions (Hubbard-U) in Mott-insulator time dependent. Mid-infrared optical pulses excite localized molecular vibrations in ET-F2TCNQ, a prototypical one-dimensional Mott-insulator. A broadband ultrafast probe interrogates the resulting optical spectrum between THz and visible frequencies. A red-shifted charge-transfer resonance is observed, consistent with a time-averaged reduction of the electronic correlation strength U. Secondly, a sideband manifold inside of the Mott-gap appears, resulting from a periodically modulated U. The response is compared to computations based on a quantum-modulated dynamic Hubbard model. Heuristic fitting suggests asymmetric holon-doublon coupling to the molecules and that electron double-occupancies strongly squeeze the vibrational mode. PMID:24448171

  11. Ferromagnetic cluster glass state induced by non-magnetic ions in a paramagnetic host

    NASA Astrophysics Data System (ADS)

    Yamamoto, Takafumi D.; Okazaki, Ryuji; Taniguchi, Hiroki; Terasaki, Ichiro

    A paramagnetic metal CaRuO3 has been known to show unique impurity effects, where a magnetic ordering is induced by a partial substitution of transition metal ions for Ru. Since this phenomenon occurs regardless of the magnetism of the substituted ions, it must reflect a magnetic instability of this ruthenate. Understanding such physical properties is one of intriguing issues in condensed matter physics. In this talk, we report an unconventional magnetic state induced by substituting non-magnetic Sc3+ ions. We find that the static magnetic susceptibilities of all Sc-substituted samples show ferromagnetic-like features below 40 K, while the Curie-Weiss temperature dramatically changes with increasing x. This inconsistency is a sign of non-uniform magnetic system. We propose a phenomenological model and show that the static magnetic properties can be described as a volume average of a paramagnetic component originated from Ru4+ ions and a ferromagnetic one driven by Sc substitution [T. D. Yamamoto et al., JPSJ 84, 014708 (2015).]. Furthermore our dynamic magnetic measurements reveal a ferromagnetic cluster glass state embedded in the paramagnetic and metallic host of CaRuO3.

  12. Stabilization of Ferromagnetic States by Electron Doping in ZnO-Based Diluted Magnetic Semiconductors

    NASA Astrophysics Data System (ADS)

    Sato, Kazunori; Katayama-Yoshida, Hiroshi

    2001-03-01

    In order to investigate functionality of ZnO as a diluted magnetic semiconductor (DMS), we had studied the magnetism in ZnO doped with 3d transition metal atoms (TM) and showed that it was also a candidate for a new functional magnetic material [1]. In this paper, we develop our previous work and give detailed materials design with ZnO-based DMS based on ab initio calculations. The electronic structure of a TM-doped ZnO was calculated within the local density approximation by the Korringa-Kohn-Rostoker method combined with the coherent potential approximation. Total energies of Zn_1-xTM^\\uparrow_xO and Zn_1-xTM^\\uparrow_x/2TM^downarrow_x/2O, where up and down arrows mean the directions of respective atomic magnetic moments, were compared and appearance of the ferromagnetism was discussed. Effects of carrier doping to these systems were also considered. It was found that their magnetic states were controllable by changing the carrier density. In particular, ferromagnetic states were stabilized by electron doping in the case of Fe, Co or Ni doped ZnO. From the point of practical applications, it is favorable feature to realize high Curie temperature ferromagnet, because n-type ZnO is easily available. [1] K. Sato and H. Katayama-Yoshida, Jpn. J. Appl. Phys. 39 (2000) L555.

  13. Dynamical splayed ferromagnetic ground state in the quantum spin ice Yb(2)Sn(2)O(7).

    PubMed

    Yaouanc, A; Dalmas de Réotier, P; Bonville, P; Hodges, J A; Glazkov, V; Keller, L; Sikolenko, V; Bartkowiak, M; Amato, A; Baines, C; King, P J C; Gubbens, P C M; Forget, A

    2013-03-22

    From magnetic, specific heat, (170)Yb Mössbauer effect, neutron diffraction, and muon spin relaxation measurements on polycrystalline Yb(2)Sn(2)O(7), we show that below the first order transition at 0.15 K all of the Yb(3+) ions are long-range magnetically ordered and each has a moment of 1.1 μ(B) which lies at ≃ 10° to a common fourfold cubic axis. The four sublattice moments have four different directions away from this axis and are therefore noncoplanar. We term this arrangement splayed ferromagnetism. This ground state has a dynamical component with a fluctuation rate in the megahertz range. The net ferromagnetic exchange interaction has an anisotropy that favors the local threefold axis. We discuss our results in terms of the phase diagram proposed by Savary and Balents [Phys. Rev. Lett. 108, 037202 (2012)] for a pyrochlore lattice of Kramers 1/2 effective spins. PMID:25166842

  14. Steady-state thermal gradient induced by pulsed laser excitation in a ferromagnetic layer

    NASA Astrophysics Data System (ADS)

    Shihab, S.; Thevenard, L.; Lemaître, A.; Duquesne, J.-Y.; Gourdon, C.

    2016-04-01

    In all-optical pump-probe experiments on ferromagnetic layers, the determination of the temperature under the pump laser spot is crucial for a quantitative modeling of the magnetization dynamics. We present here a method to quantify this thermal gradient, exemplified on a (Ga, Mn)(As, P) ferromagnetic semiconductor layer on a GaAs substrate. To estimate the local steady-state temperature, we use the coercive field as a thermometer. The probe records the hysteresis cycle spatially across the hot spot, using the magnetic linear birefringence/dichroism of the sample. Our results are analyzed using the heat diffusion equation with two fitting parameters, the thermal conductivity of the layer/substrate sample and the thermal resistance between the substrate and the thermostat. This opens the way to a quantitative modeling of laser pulse-triggered magnetization dynamics in the presence of transient temperature effects.

  15. Ferromagnetism in ruthenate perovskites

    NASA Astrophysics Data System (ADS)

    Dang, Hung T.; Mravlje, Jernej; Millis, Andrew J.

    2014-03-01

    In apparent contrast to the usual rule that stronger correlations favor magnetism and other forms of order, while weaker correlations lead to Fermi liquid metals, it has been experimentally established that CaRuO3, a more correlated material, is a paramagnetic metal with a Fermi liquid ground state while SrRuO3, which is less strongly correlated, is ferromagnetic below a Curie temperature of 160K. We present density functional plus dynamical mean field theory calculations which resolve this conundrum. We show that in these materials ferromagnetism occurs naturally for cubic perovskite systems at moderate correlations but is suppressed both by proximity to the Mott insulating phase and by increasing the amplitude of a GdFeO3 distortion. These factors are strongly related to the differences between Ca and Sr ruthenates and are used as the keys to solve the problem. Placement of the ruthenate materials on the metal-insulator phase diagram and comparison to previous works on the Ruddlesden-Popper materials are also discussed. Supported by the Basic Energy Sciences Program of the US Department of Energy under grant DOE ER046169 and the Columbia-Ecole Polytechnique Alliance program.

  16. Two-dimensional topological insulator edge state backscattering by dephasing

    NASA Astrophysics Data System (ADS)

    Essert, Sven; Krueckl, Viktor; Richter, Klaus

    2015-11-01

    To understand the seemingly absent temperature dependence in the conductance of two-dimensional topological insulator edge states, we perform a numerical study which identifies the quantitative influence of the combined effect of dephasing and elastic scattering in charge puddles close to the edges. We show that this mechanism may be responsible for the experimental signatures in HgTe/CdTe quantum wells if the puddles in the samples are large and weakly coupled to the sample edges. We propose experiments on artificial puddles which allow one to verify this hypothesis and to extract the real dephasing time scale using our predictions. In addition, we present a method to include the effect of dephasing in wave-packet-time-evolution algorithms.

  17. Spatial characters of metallic surface states of topological insulators

    NASA Astrophysics Data System (ADS)

    Han, Jinhee; Lee, Hyungjun; Choi, Hyoung Joon

    2011-03-01

    We study the electronic structure of metallic surface states in Bi 2 Se 3 , Bi 2 Te 3 , and Sb 2 Te 3 using an ab-initio pseudopotential density-functional method. We implemented the spin-orbit interaction into the SIESTA in a form of additional fully non-local projectors. For surface states on (001) surface, we used a supercell containing 10 quintuple layers. We obtained bulk and surface electronic structures of topological insulators Bi 2 Se 3 , Bi 2 Te 3 , and Sb 2 Te 3 , which are close to previous theoretical results and consistent with Dirac-cone band dispersions measured by angle-resolved photoemission spectroscopy. Then, we analyzed the wavefunctions of the metallic surface states near the Fermi level to find out spatial distributions of the surface-state wavefunctions, which turn out to be localized in the surface region with a typical spread of about 2 quintuple layers, and the shapes of the wavefunctions around Bi (or Sb) atoms close to the surface. This work was supported by the NRF of Korea (Grant No. 2009-0081204) and KISTI Supercomputing Center (Project No. KSC-2008-S02-0004).

  18. Fano effect and Andreev bound states in a hybrid superconductor-ferromagnetic nanostructure

    NASA Astrophysics Data System (ADS)

    Siqueira, E. C.; Orellana, P. A.; Cestari, R. C.; Figueira, M. S.; Cabrera, G. G.

    2015-10-01

    In this work, it is considered a hybrid nanostructure composed by a quantum dot coupled to two ferromagnetic leads and a superconductor lead. It is shown that the zero-bias transmittance for the co-tunneling between the ferromagnetic leads presents Fano anti-resonances due to the destructive interference between the two spin channels mixing by the relative orientation of the magnetizations in the leads. When the superconductor is coupled to the system, electron-hole correlations between different spin states lead to a resonance in the place of the dip appearing in the transmittance. Such an effect is accompanied by two Fano anti-resonances explained by a "leakage" of conduction channels from the co-tunneling to the Andreev transport. In the non-equilibrium regime, correlations within the quantum dot introduce a dependence of the resonance condition on the finite bias applied to the ferromagnetic leads. However, it is still possible to observe signatures of the same interference effect in the electrical current.

  19. Ferromagnetic/Superconducting Multilayers

    NASA Astrophysics Data System (ADS)

    Bader, S. D.

    1998-03-01

    Although it is well known that magnetism influences superconductivity, the converse issue has been less well explored. Recent theoretical predictions for ferromagnetic/ superconducting/ ferromagnetic trilayers exhibiting interlayer magnetic coupling in the normal state indicate that the coupling should be suppressed below the superconducting transition temperature.(C.A. R. Sá de Melo, Phys. Rev. Lett. 79), 1933 (1997); O. Sipr, B.L. Györffy, J. Phys. Cond. Matt. 7, 5239 (1995). To realize such a situation, a requirement (when the magnetic layers are thick) is that the superconducting layer thickness must simultaneously be less than the range over which the magnetic interlayer coupling decays, but greater than the superconducting coherence length. This introduces serious materials constraints. The present work describes initial explorations of three sputtered multilayer systems in an attempt to observe coupling of the ferromagnetic layers across a superconducting spacer:((a) J.E. Mattson, R.M. Osgood III, C.D. Potter, C.H. Sowers, and S.D. Bader, J. Vac. Sci. Technol. A 15), 1774 (1997); (b) J.E. Mattson, C.D. Potter, M.J. Conover, C.H. Sowers, and S.D. Bader, Phys. Rev. B 55, 70 (1997), and (c) R.M. Osgood III, J.E. Pearson, C.H. Sowers, and S.D. Bader, submitted (1997). (a) Ni/Nb, (b) Fe_4N/NbN, and (c) GdN/NbN. In these systems we have retained thinner superconducting layers than had been achieved previously, but interlayer magnetic coupling is not observed even in the normal state. For Ni/Nb the interfacial Ni loses its moment, which also reduces the superconducting pair-breaking. GdN is an insulating ferromagnet, so itinerancy is sacrificed, and, probably as a result of this, no coupling is observed. Each system gives rise to interesting and anisotropic superconducting properties. Thus, although the goal remains elusive, our search highlights the challenges and opportunities.

  20. Detection of magnetic state in a nanoscale ferromagnetic ring by using ballistic semiconductor two-dimensional electron gas

    NASA Astrophysics Data System (ADS)

    Hara, Masahiro; Shibata, Junya; Kimura, Takashi; Otani, Yoshichika

    2006-02-01

    We have developed a method of measuring magnetization process in a ferromagnetic ring by analyzing a characteristic response of a semiconductor two-dimensional electron gas (2DEG) lying beneath the ring. A 2DEG microcross structure is formed underneath a ferromagnetic ring to detect the position of paired domain walls of the onion state. The variation of the bend resistance due to the rotation of the paired domain walls is quantitatively reproduced by a semiclassical billiard model.

  1. Thermodynamic signatures of edge states in Topological Insulators

    NASA Astrophysics Data System (ADS)

    Quelle, Anton; Cobanera, Emilio; Morais Smith, Cristinae

    Topological insulators are states of matter distinguished by the presence of symmetry protected metallic boundary modes. These edge modes have been characterised in terms of transport and spectroscopic measurements, but a thermodynamic description has been lacking. The challenge arises because in conventional thermodynamics the potentials are required to scale linearly with extensive variables like volume, which does not allow for a general treatment of boundary effects. In this paper, we overcome this challenge with Hill thermodynamics. In this extension of the thermodynamic formalism, the grand potential is split into an extensive, conventional contribution, and the subdivision potential, which is the central construct of Hill's theory. For topologically non-trivial electronic matter, the subdivision potential captures measurable contributions to the density of states and the heat capacity: it is the thermodynamic manifestation of the topological edge structure. Furthermore, the subdivision potential reveals phase transitions of the edge even when they are not manifested in the bulk, thus opening a variety of new possibilities for investigating, manipulating, and characterizing topological quantum matter solely in terms of equilibrium boundary physics.

  2. Quantum filter of spin polarized states: Metal–dielectric–ferromagnetic/semiconductor device

    SciTech Connect

    Makarov, Vladimir I.; Khmelinskii, Igor

    2014-02-01

    Highlights: • Development of a new spintronics device. • Development of quantum spin polarized filters. • Development of theory of quantum spin polarized filter. - Abstract: Recently we proposed a model for the Quantum Spin-Polarized State Filter (QSPSF). The magnetic moments are transported selectively in this model, detached from the electric charge carriers. Thus, transfer of a spin-polarized state between two conductors was predicted in a system of two levels coupled by exchange interaction. The strength of the exchange interaction between the two conductive layers depends on the thickness of the dielectric layer separating them. External magnetic fields modulate spin-polarized state transfer, due to Zeeman level shift. Therefore, a linearly growing magnetic field generates a series of current peaks in a nearby coil. Thus, our spin-state filter should contain as least three nanolayers: (1) conductive or ferromagnetic; (2) dielectric; and (3) conductive or semiconductive. The spectrum of spin-polarized states generated by the filter device consists of a series of resonance peaks. In a simple case the number of lines equals S, the total spin angular momentum of discrete states in one of the coupled nanolayers. Presently we report spin-polarized state transport in metal–dielectric–ferromagnetic (MDF) and metal–dielectric–semiconductor (MDS) three-layer sandwich devices. The exchange-resonance spectra in such devices are quite specific, differing also from spectra observed earlier in other three-layer devices. The theoretical model is used to interpret the available experimental results. A detailed ab initio analysis of the magnetic-field dependence of the output magnetic moment averaged over the surface of the device was carried out. The model predicts the resonance structure of the signal, although at its present accuracy it cannot predict the positions of the spectral peaks.

  3. Realization of insulating state and superconductivity in the Rashba semiconductor BiTeCl

    NASA Astrophysics Data System (ADS)

    Ying, Jian-Jun; Struzhkin, Viktor V.; Cao, Zi-Yu; Goncharov, Alexander F.; Mao, Ho-Kwang; Chen, Fei; Chen, Xian-Hui; Gavriliuk, Alexander G.; Chen, Xiao-Jia

    2016-03-01

    Measurements of the resistivity, Hall coefficient, and Raman spectroscopy are performed on a Rashba semiconductor BiTeCl single crystal at high pressures up to 50 GPa. We find that applying pressure first induces a theoretically predicted insulating state, followed by a superconducting phase with an insulating normal state. Upon heavy compression, another different superconducting phase is entered into with a metallic normal state. A domelike evolution of the superconducting transition temperature with pressure is obtained with a crossover from the electron to hole carriers across the boundary of the two superconducting phases. These findings imply the possible realization of a topological state of the insulating and superconducting phases in this material.

  4. Steady-state and transient results on insulation materials

    SciTech Connect

    Graves, R.S.; Yarbrough, D.W.; McElroy, D.L.; Fine, H.A.

    1991-01-01

    The Unguarded Thin-Heater Apparatus (UTHA, ASTM C 1114) was used to determine the thermal conductivity (k), specific heat (C), and thermal diffusivity ({alpha}) of selected building materials from 24 to 50{degree}C. Steady-state and transient measurements yielded data on four types of material: gypsum wall board containing 0, 15, and 30 wt % wax; calcium silicate insulations with densities ({rho}) of 307, 444, and 605 kg/m{sup 3}; three wood products: southern yellow pine flooring (575 kg/m{sup 3}), Douglas fir plywood (501 kg/m{sup 3}), and white spruce flooring (452 kg/m{sup 3}); and two cellular plastic foams: extruded polystyrene (30 kg/m{sup 3}) blown with HCFC-142b and polyisocyanurate rigid board (30.2 kg/m{sup 3}) blown with CFC-11. The extruded polystyrene was measured several times after production (25 days, 45 days, 74 days, 131 days, and 227 days). The UTHA is an absolute technique that yields k with an uncertainty of less than {plus minus}2% as determined by modeling, by determinate error analyses, and by use of Standard Reference Materials SRM-1450b and SRM-1451. 37 refs., 5 figs., 10 tabs.

  5. Brane parity orders in the insulating state of Hubbard ladders

    NASA Astrophysics Data System (ADS)

    Degli Esposti Boschi, Cristian; Montorsi, Arianna; Roncaglia, Marco

    2016-08-01

    The Mott insulating state of the Hubbard model at half filling could be depicted as a spin liquid of singly occupied sites with holon-doublon quantum fluctuations localized in pairs. In one dimension the behavior is captured by a finite value of the charge parity string correlator, which fails to remain finite when generalized to higher dimensions. We recover a definition of parity brane correlator which may remain nonvanishing in the presence of interchain coupling, by assigning an appropriate fractional phase to the parity breaking fluctuations. In the case of Hubbard ladders at half filling, we find that the charge parity brane is nonzero at any repulsive value of interaction. The spin-parity brane instead becomes nonvanishing in the even-leg case, in correspondence to the onset of the spin gapped D-Mott phase, which is absent in the odd-leg case. The behavior of the parity correlators is also analyzed by means of a numerical DMRG analysis of the one- and two-leg ladder.

  6. Imaging Dirac-mass disorder from magnetic dopant atoms in the ferromagnetic topological insulator Crx(Bi0.1Sb0.9)2-xTe3

    SciTech Connect

    Lee, Inhee; Kim, Chung Koo; Lee, Jinho; Billinge, Simon J. L.; Zhong, Ruidan D.; Schneeloch, John A.; Liu, Tiansheng S.; Valla, Tonica; Tranquada, John M.; Gu, Genda D.; Davis, J. C. Séamus

    2015-01-20

    To achieve and use the most exotic electronic phenomena predicted for the surface states of 3D topological insulators (TIs), it is necessary to open a “Dirac-mass gap” in their spectrum by breaking time-reversal symmetry. Use of magnetic dopant atoms to generate a ferromagnetic state is the most widely applied approach. However, it is unknown how the spatial arrangements of the magnetic dopant atoms influence the Dirac-mass gap at the atomic scale or, conversely, whether the ferromagnetic interactions between dopant atoms are influenced by the topological surface states. Here we image the locations of the magnetic (Cr) dopant atoms in the ferromagnetic TI Cr₀.₀₈(Bi₀.₁Sb₀.₉)₁.₉₂Te₃. Simultaneous visualization of the Dirac-mass gap Δ(r) reveals its intense disorder, which we demonstrate is directly related to fluctuations in n(r), the Cr atom areal density in the termination layer. We find the relationship of surface-state Fermi wavevectors to the anisotropic structure of Δ(r) not inconsistent with predictions for surface ferromagnetism mediated by those states. Moreover, despite the intense Dirac-mass disorder, the anticipated relationship Δ(r)∝n(r) is confirmed throughout and exhibits an electron–dopant interaction energy J* = 145 meV·nm². In addition, these observations reveal how magnetic dopant atoms actually generate the TI mass gap locally and that, to achieve the novel physics expected of time-reversal symmetry breaking TI materials, control of the resulting Dirac-mass gap disorder will be essential.

  7. Pressure induced novel-phenomena in Mott insulator Ca2RuO4

    NASA Astrophysics Data System (ADS)

    Yamauchi, Yohei; Nakamura, Fumihiko; Sakaki, Mariko; Takemoto, Tetsuo; Suzuki, Takashi; Alireza, Patricia L.; Maeno, Yoshiteru

    2010-12-01

    In order to explore unconventional superconductivity, we have studied pressure effect on a 4d-electron Mott insulator Ca2RuO4. Pressurisation to Ca2RuO4 above 0.5 GPa transforms it from a Mott insulator to a metal with a ferromagnetic ground state. The itinerancy of the ferromagnetic state at 2 GPa is quantitatively evidenced by the magnetisation process at 2 K. Moreover, the pressure phase diagram of this system suggests the existence of a ferromagnetic quantum phase transition at ˜10 GPa.

  8. Realization of tunable Dirac cone and insulating bulk states in topological insulators (Bi1−xSbx)2Te3

    PubMed Central

    Niu, Chengwang; Dai, Ying; Zhu, Yingtao; Ma, Yandong; Yu, Lin; Han, Shenghao; Huang, Baibiao

    2012-01-01

    The bulk-insulating topological insulators with tunable surface states are necessary for applications in spintronics and quantum computation. Here we present theoretical evidence for modulating the topological surface states and achieving the insulating bulk states in solid-solution (Bi1−xSbx)2Te3. Our results reveal that the band inversion occurs in (Bi1−xSbx)2Te3, indicating the non-triviality across the entire composition range, and the Dirac point moves upwards till it lies within the bulk energy gap accompanying the increase of Sb concentration x. In addition, with increasing x, the formation of prominent native defects becomes much more difficult, resulting in the truly insulating bulk. The solid-solution system is a promising way of tuning the properties of topological insulators and designing novel topologically insulating devices. PMID:23240080

  9. Symmetry-selected spin-split hybrid states in C60/ferromagnetic interfaces

    NASA Astrophysics Data System (ADS)

    Li, Dongzhe; Barreteau, Cyrille; Kawahara, Seiji Leo; Lagoute, Jérôme; Chacon, Cyril; Girard, Yann; Rousset, Sylvie; Repain, Vincent; Smogunov, Alexander

    2016-02-01

    The understanding of orbital hybridization and spin polarization at the organic-ferromagnetic interface is essential in the search for efficient hybrid spintronic devices. Here, using first-principles calculations, we report a systematic study of spin-split hybrid states of C60 deposited on various ferromagnetic surfaces: bcc-Cr(001), bcc-Fe(001), bcc-Co(001), fcc-Co(001), and hcp-Co(0001). We show that the adsorption geometry of the molecule with respect to the surface crystallographic orientation of the magnetic substrate as well as the strength of the interaction play a crucial role in the spin polarization of the hybrid orbitals. We find that a large spin polarization in vacuum above the buckyball can only be achieved if the molecule is adsorbed upon a bcc-(001) surface by its pentagonal ring. Therefore, bcc-Cr(001), bcc-Fe(001), and bcc-Co(001) are the optimal candidates. Spin-polarized scanning tunneling spectroscopy measurements on single C60 adsorbed on Cr(001) and Co/Pt(111) also confirm that the symmetry both of the substrate and of the molecular conformation has a strong influence on the induced spin polarization. Our finding may give valuable insights for further engineering of spin filtering devices through single molecular orbitals.

  10. Non-ferromagnetic overburden casing

    SciTech Connect

    Vinegar, Harold J.; Harris, Christopher Kelvin; Mason, Stanley Leroy

    2010-09-14

    Systems, methods, and heaters for treating a subsurface formation are described herein. At least one system for electrically insulating an overburden portion of a heater wellbore is described. The system may include a heater wellbore located in a subsurface formation and an electrically insulating casing located in the overburden portion of the heater wellbore. The casing may include at least one non-ferromagnetic material such that ferromagnetic effects are inhibited in the casing.

  11. Ground state study of the thin ferromagnetic nano-islands for artificial spin ice arrays

    SciTech Connect

    Vieira Júnior, D. S.; Leonel, S. A. Dias, R. A. Toscano, D. Coura, P. Z. Sato, F.

    2014-09-07

    In this work, we used numerical simulations to study the magnetic ground state of the thin elongated (elliptical) ferromagnetic nano-islands made of Permalloy. In these systems, the effects of demagnetization of dipolar source generate a strong magnetic anisotropy due to particle shape, defining two fundamental magnetic ground state configurations—vortex or type C. To describe the system, we considered a model Hamiltonian in which the magnetic moments interact through exchange and dipolar potentials. We studied the competition between the vortex states and aligned states—type C—as a function of the shape of each elliptical nano-islands and constructed a phase diagram vortex—type C state. Our results show that it is possible to obtain the elongated nano-islands in the C-state with aspect ratios less than 2, which is interesting from the technological point of view because it will be possible to use smaller islands in spin ice arrays. Generally, the experimental spin ice arrangements are made with quite elongated particles with aspect ratio approximately 3 to ensure the C-state.

  12. Non-Equilibrium Superconductivity and Magnetic Pair Breaking in Perovskite Half-Metallic Ferromagnet-Insulator-Superconductor (F-I-S) Heterostructures

    NASA Technical Reports Server (NTRS)

    Fu, C.-C.; Yeh, N.-C.; Samoilov, A. V.; Vakili, K.; Li, Y.; Vasquez, R. P.

    1999-01-01

    The effect of spin-polarized quasiparticle currents on the critical current density (J-c) of cuprate superconductors is studied in perovskite F-I-S heterostructures as a function of insulator thickness and of underlying magnetic materials. A pulsed current technique is employed to minimize extraneous Joule heating on the superconductor. At temperatures near T-c, F-I-S samples with insulator thicknesses\\1e2nm show precipitous decrease in J_c as current injection (I_m) is increased. In contrast, J_c in a controlled sample with a substituted non-magnetic material (N-I-S) exhibit no dependence on I_m. Similarly, a F-I-S sample with a 10 mn insulating barrier also show little J_c effect versus I_m. At low temperatures with I_m = 0, significant suppression of J-c is observed only in the thin barrier F-I-S samples, although T_c and the normal-state resistivity of all samples are comparable. These phenomena can be attributed to the Cooper pair breaking induced by externally-injected and internally-reflected spin-polarized quasiparticle currents. We estimate an order of magnitude range for the spin diffusion length of 100 nm to 100\\ mum.

  13. Interplay between chemical state, electric properties, and ferromagnetism in Fe-doped ZnO films

    SciTech Connect

    Chen, G.; Peng, J. J.; Song, C.; Zeng, F.; Pan, F.

    2013-03-14

    Valence state of Fe ions plays an important role in the physical properties of Fe doped ZnO films. Here, a series of Zn{sub 1-x}Fe{sub x}O films with different Fe concentrations (x = 0, 2.3, 5.4, 7.1, and 9.3 at. %) were prepared to investigate their structural, piezoelectric, ferroelectric, bipolar resistive switching properties, and electrical-control of ferromagnetism at room temperature. The structure characterizations indicate that the chemical state of Fe ions substituting Zn{sup 2+} site changes from Fe{sup 3+} to Fe{sup 2+} with the increase of Fe dopant concentration. We found enhanced piezoelectric and ferroelectric properties in Zn{sub 0.977}Fe{sub 0.023}O films with more Fe{sup 3+} due to the smaller Fe{sup 3+} ionic size in comparison with Zn{sup 2+} while the increase of Fe{sup 2+} concentration by a larger amount of Fe dopant results in the worse ferroelectric and piezoelectric performance. All Pt/Zn{sub 1-x}Fe{sub x}O/Pt devices show bipolar resistive switching properties. Especially, devices with lower Fe dopant concentration exhibit better endurance properties due to their higher crystalline quality. The variation of oxygen vacancies during resistive switching provides an opportunity to tune ferromagnetism of Fe-doped ZnO films, giving rise to the integration of charge and spin into a simple Pt/Zn{sub 1-x}Fe{sub x}O/Pt devices. The multifunctional properties of Fe-doped ZnO films are promising for communication systems and information storage devices.

  14. Electromagnetic absorption and Kerr effect in quantum Hall ferromagnetic states of bilayer graphene

    NASA Astrophysics Data System (ADS)

    Côté, R.; Barrette, Manuel; Bouffard, Élie

    2015-09-01

    In a quantizing magnetic field, the chiral two-dimensional electron gas in Landau level N =0 of bilayer graphene goes through a series of phase transitions at integer filling factors ν ∈[-3 ,3 ] when the strength of an electric field applied perpendicularly to the layers is increased. At filling factor ν =3 , the electron gas can be described by a simple two-level system where layer and spin degrees of freedom are frozen. The gas then behaves as an orbital quantum Hall ferromagnet. A Coulomb-induced Dzyaloshinskii-Moriya term in the orbital pseudospin Hamiltonian is responsible for a series of transitions first to a Wigner crystal state and then to a spiral state as the electric field is increased. Both states have a nontrivial orbital pseudospin texture. In this work, we study how the phase diagram at ν =3 is modified by an electric field applied in the plane of the layers and then derive several experimental signatures of the uniform and nonuniform states in the phase diagram. In addition to the transport gap, we study the electromagnetic absorption and the Kerr rotation due to the excitations of the orbital pseudospin-wave modes in the broken-symmetry states.

  15. Control of ferromagnetism at room temperature in (Ti,Co)O2-δ via chemical doping of electron carriers

    NASA Astrophysics Data System (ADS)

    Yamada, Y.; Fukumura, T.; Ueno, K.; Kawasaki, M.

    2011-12-01

    Ferromagnetism at room temperature in (Ti,Co)O2 - δ was controlled by changing its electron density via chemical doping, where the oxygen vacancy δ served as an electron donor. With increasing the electron density, the ferromagnetic anomalous Hall effect and magnetization emerged from a paramagnetic state, while undergoing an insulator to metal transition. This result supports that the electron carriers mediated the ferromagnetic exchange interaction and is consistent with the electric field effect study on the ferromagnetism recently reported [Y. Yamada, K. Ueno, T. Fukumura, H. T. Yuan, H. Shimotani, Y. Iwasa, L. Gu, S. Tsukimoto, Y. Ikuhara, and M. Kawasaki, Science 332, 1065 (2011)].

  16. Surface-State Spin Textures and Mirror Chern Numbers in Topological Kondo Insulators

    NASA Astrophysics Data System (ADS)

    Legner, Markus; Rüegg, Andreas; Sigrist, Manfred

    2015-10-01

    The recent discovery of topological Kondo insulators has triggered renewed interest in the well-known Kondo insulator samarium hexaboride, which is hypothesized to belong to this family. In this Letter, we study the spin texture of the topologically protected surface states in such a topological Kondo insulator. In particular, we derive close relationships between (i) the form of the hybridization matrix at certain high-symmetry points, (ii) the mirror Chern numbers of the system, and (iii) the observable spin texture of the topological surface states. In this way, a robust classification of topological Kondo insulators and their surface-state spin texture is achieved. We underpin our findings with numerical calculations of several simplified and realistic models for systems like samarium hexaboride.

  17. Surface-State Spin Textures and Mirror Chern Numbers in Topological Kondo Insulators.

    PubMed

    Legner, Markus; Rüegg, Andreas; Sigrist, Manfred

    2015-10-01

    The recent discovery of topological Kondo insulators has triggered renewed interest in the well-known Kondo insulator samarium hexaboride, which is hypothesized to belong to this family. In this Letter, we study the spin texture of the topologically protected surface states in such a topological Kondo insulator. In particular, we derive close relationships between (i) the form of the hybridization matrix at certain high-symmetry points, (ii) the mirror Chern numbers of the system, and (iii) the observable spin texture of the topological surface states. In this way, a robust classification of topological Kondo insulators and their surface-state spin texture is achieved. We underpin our findings with numerical calculations of several simplified and realistic models for systems like samarium hexaboride. PMID:26550740

  18. Evolution of the local superconducting density of states in ErRh{sub 4}B{sub 4} close to the ferromagnetic transition.

    SciTech Connect

    Crespo, V.; Rodrigo, J. G.; Suderow, H.; Vieira, S.; Hinks, D.; Schuller, I. K.; Materials Science Division; Univ. Autonoma de Madrid; Univ. of California at San Diego

    2009-01-01

    We present local tunneling spectroscopy experiments in the superconducting and ferromagnetic phases of the reentrant superconductor ErRh4B4. The tunneling conductance curves jump from showing normal to superconducting features within a few mK close to the ferromagnetic transition temperature, with a clear hysteretic behavior. Within the ferromagnetic phase, we do not detect any superconducting correlations. Within the superconducting phase we find a peculiar V-shaped density of states at low energies, which is produced by the magnetically modulated phase that coexists with superconductivity just before ferromagnetism sets in.

  19. High spin polarization and the origin of unique ferromagnetic ground state in CuFeSb

    NASA Astrophysics Data System (ADS)

    Sirohi, Anshu; Singh, Chandan K.; Thakur, Gohil S.; Saha, Preetha; Gayen, Sirshendu; Gaurav, Abhishek; Jyotsna, Shubhra; Haque, Zeba; Gupta, L. C.; Kabir, Mukul; Ganguli, Ashok K.; Sheet, Goutam

    2016-06-01

    CuFeSb is isostructural to the ferro-pnictide and chalcogenide superconductors and it is one of the few materials in the family that are known to stabilize in a ferromagnetic ground state. Majority of the members of this family are either superconductors or antiferromagnets. Therefore, CuFeSb may be used as an ideal source of spin polarized current in spin-transport devices involving pnictide and the chalcogenide superconductors. However, for that the Fermi surface of CuFeSb needs to be sufficiently spin polarized. In this paper we report direct measurement of transport spin polarization in CuFeSb by spin-resolved Andreev reflection spectroscopy. From a number of measurements using multiple superconducting tips we found that the intrinsic transport spin polarization in CuFeSb is high (˜47%). In order to understand the unique ground state of CuFeSb and the origin of large spin polarization at the Fermi level, we have evaluated the spin-polarized band structure of CuFeSb through first principles calculations. Apart from supporting the observed 47% transport spin polarization, such calculations also indicate that the Sb-Fe-Sb angles and the height of Sb from the Fe plane are strikingly different for CuFeSb than the equivalent parameters in other members of the same family thereby explaining the origin of the unique ground state of CuFeSb.

  20. Quantum Hall states stabilized in semi-magnetic bilayers of topological insulators.

    PubMed

    Yoshimi, R; Yasuda, K; Tsukazaki, A; Takahashi, K S; Nagaosa, N; Kawasaki, M; Tokura, Y

    2015-01-01

    By breaking the time-reversal symmetry in three-dimensional topological insulators with the introduction of spontaneous magnetization or application of magnetic field, the surface states become gapped, leading to quantum anomalous Hall effect or quantum Hall effect, when the chemical potential locates inside the gap. Further breaking of inversion symmetry is possible by employing magnetic topological insulator heterostructures that host non-degenerate top and bottom surface states. Here we demonstrate the tailored-material approach for the realization of robust quantum Hall states in the bilayer system, in which the cooperative or cancelling combination of the anomalous and ordinary Hall responses from the respective magnetic and non-magnetic layers is exemplified. The appearance of quantum Hall states at filling factor 0 and +1 can be understood by the relationship of energy band diagrams for the two independent surface states. The designable heterostructures of magnetic topological insulator may explore a new arena for intriguing topological transport and functionality. PMID:26497065

  1. Quantum Hall states stabilized in semi-magnetic bilayers of topological insulators

    NASA Astrophysics Data System (ADS)

    Yoshimi, R.; Yasuda, K.; Tsukazaki, A.; Takahashi, K. S.; Nagaosa, N.; Kawasaki, M.; Tokura, Y.

    2015-10-01

    By breaking the time-reversal symmetry in three-dimensional topological insulators with the introduction of spontaneous magnetization or application of magnetic field, the surface states become gapped, leading to quantum anomalous Hall effect or quantum Hall effect, when the chemical potential locates inside the gap. Further breaking of inversion symmetry is possible by employing magnetic topological insulator heterostructures that host non-degenerate top and bottom surface states. Here we demonstrate the tailored-material approach for the realization of robust quantum Hall states in the bilayer system, in which the cooperative or cancelling combination of the anomalous and ordinary Hall responses from the respective magnetic and non-magnetic layers is exemplified. The appearance of quantum Hall states at filling factor 0 and +1 can be understood by the relationship of energy band diagrams for the two independent surface states. The designable heterostructures of magnetic topological insulator may explore a new arena for intriguing topological transport and functionality.

  2. Ferromagnetic superexchange in insulating Cr2MoO6 by controlling orbital hybridization

    SciTech Connect

    Zhu, M.; Do, D.; Dela Cruz, Clarina R.; Dun, Zhiling; Cheng, J. -G.; Goto, H.; Uwatoko, Yoshiya; Zou, T.; Zhou, Haidon D.; Mahanti, Subhendra D.; Ke, Xianglin

    2015-09-11

    We report the magnetic and electronic structures of the newly synthesized inverse-trirutile compound Cr2MoO6. Despite the same crystal symmetry and similar bond-lengths and bond-angles to Cr2TeO6, Cr2MoO6 possesses a magnetic structure of the Cr2MoO6 type, different from that seen in Cr2TeO6. Ab-initio electronic structure calculations show that the sign and strength of the Cr-O-Cr exchange coupling is strongly influenced by the hybridization between Mo 4d and O 2p orbitals. This result further substantiates our recently proposed mechanism for tuning the exchange interaction between two magnetic atoms by modifying the electronic states of the non-magnetic atoms in the exchange path through orbital hybridization. This approach is fundamentally different from the conventional methods of controlling the exchange interaction by either carrier injection or through structural distortions.

  3. Edge states and integer quantum Hall effect in topological insulator thin films.

    PubMed

    Zhang, Song-Bo; Lu, Hai-Zhou; Shen, Shun-Qing

    2015-01-01

    The integer quantum Hall effect is a topological state of quantum matter in two dimensions, and has recently been observed in three-dimensional topological insulator thin films. Here we study the Landau levels and edge states of surface Dirac fermions in topological insulators under strong magnetic field. We examine the formation of the quantum plateaux of the Hall conductance and find two different patterns, in one pattern the filling number covers all integers while only odd integers in the other. We focus on the quantum plateau closest to zero energy and demonstrate the breakdown of the quantum spin Hall effect resulting from structure inversion asymmetry. The phase diagrams of the quantum Hall states are presented as functions of magnetic field, gate voltage and chemical potential. This work establishes an intuitive picture of the edge states to understand the integer quantum Hall effect for Dirac electrons in topological insulator thin films. PMID:26304795

  4. Metal-insulator transitions of bulk and domain-wall states in pyrochlore iridates

    NASA Astrophysics Data System (ADS)

    Ueda, Kentaro

    A family of pyrochlore iridates R2Ir2O7 offers an ideal platform to explore intriguing phases such as topological Mott insulator and Weyl semimetal. Here we report transport and spectroscopic studies on the metal-insulator transition (MIT) induced by the modulations of effective electron correlation and magnetic structures, which is finely tuned by external pressure, chemical substitutions (R = Nd1-x Prx and SmyNd1-y) , and magnetic field. A reentrant insulator-metal-insulator transition is observed near the paramagnetic insulator-metal phase boundary reminiscent of a first-order Mott transition for R = SmyNd1-y compounds (y~0.8). The metallic states on the magnetic domain walls (DWs), which are observed for R = Nd in real space as well as in transport properties, is simultaneously turned into the insulating one. These findings imply that the DW electronic state is intimately linked to the bulk states. For the mixed R = Nd1-x Prx compounds, the divergent behavior of resistivity with antiferromagnetic order is significantly suppressed by applying a magnetic field along [001] direction. It is attributed to the phase transition from the antiferromagnetic insulating state to the novel Weyl (semi-)metal state accompanied by the change of magnetic structure. The present study combined with experiment and theory suggests that there are abundant exotic phases with physical parameters such as electron correlation and Ir-5 d magnetic order pattern. Work performed in collaboration with J. Fujioka, B.-J. Yang, C. Terakura, N. Nagaosa, Y. Tokura (University of Tokyo, RIKEN CEMS), J. Shiogai, A. Tsukazaki, S. Nakamura, S. Awaji (Tohoku University). 1This work was supported by JSPS FIRST Program and Grant-in-Aid for Scientific Research (Grants No. 80609488 and No. 24224009).

  5. Shot noise in the edge states of two-dimensional topological insulators

    NASA Astrophysics Data System (ADS)

    Aseev, P. P.; Nagaev, K. E.

    2016-07-01

    We calculate the resistance and shot noise in the edge states of a two-dimensional topological insulator that result from the exchange of electrons between these states and conducting puddles in the bulk of the insulator. The two limiting cases where the energy relaxation is either absent or very strong are considered. A finite time of spin relaxation in the puddles is introduced phenomenologically. Depending on this time and on the strength of coupling between the edge states and the puddles, the Fano factor F =SI/2 e I ranges from 0 to 1/3, which is in an agreement with the available experimental data.

  6. Dirac electron states formed at the heterointerface between a topological insulator and a conventional semiconductor.

    PubMed

    Yoshimi, R; Tsukazaki, A; Kikutake, K; Checkelsky, J G; Takahashi, K S; Kawasaki, M; Tokura, Y

    2014-03-01

    Topological insulators are a class of semiconductor exhibiting charge-gapped insulating behaviour in the bulk, but hosting a spin-polarized massless Dirac electron state at the surface. The presence of a topologically protected helical edge channel has been verified for the vacuum-facing surface of several topological insulators by means of angle-resolved photoemission spectroscopy and scanning tunnelling microscopy. By performing tunnelling spectroscopy on heterojunction devices composed of p-type topological insulator (Bi1−xSbx)2Te3 and n-type conventional semiconductor InP, we report the observation of such states at the solid-state interface. Under an applied magnetic field, we observe a resonance in the tunnelling conductance through the heterojunction due to the formation of Landau levels of two-dimensional Dirac electrons at the interface. Moreover, resonant tunnelling spectroscopy reveals a systematic dependence of the Fermi velocity and Dirac point energy on the composition x. The successful formation of robust non-trivial edge channels at a solid-state interface is an essential step towards functional junctions based on topological insulators. PMID:24553653

  7. Nature of the insulating ground state of the 5d postperovskite CaIrO3

    DOE PAGESBeta

    Kim, Sun -Woo; Liu, Chen; Kim, Hyun -Jung; Lee, Jun -Ho; Yao, Yongxin; Ho, Kai -Ming; Cho, Jun -Hyung

    2015-08-26

    In this study, the insulating ground state of the 5d transition metal oxide CaIrO3 has been classified as a Mott-type insulator. Based on a systematic density functional theory (DFT) study with local, semilocal, and hybrid exchange-correlation functionals, we reveal that the Ir t2g states exhibit large splittings and one-dimensional electronic states along the c axis due to a tetragonal crystal field. Our hybrid DFT calculation adequately describes the antiferromagnetic (AFM) order along the c direction via a superexchange interaction between Ir4+ spins. Furthermore, the spin-orbit coupling (SOC) hybridizes the t2g states to open an insulating gap. These results indicate thatmore » CaIrO3 can be represented as a spin-orbit Slater insulator, driven by the interplay between a long-range AFM order and the SOC. Such a Slater mechanism for the gap formation is also demonstrated by the DFT + dynamical mean field theory calculation, where the metal-insulator transition and the paramagnetic to AFM phase transition are concomitant with each other.« less

  8. Characterizing Featureless Mott Insulating State by Quasiparticle Interferences - A DMFT Prospect

    NASA Astrophysics Data System (ADS)

    Mukherjee, Shantanu; Lee, Wei-Cheng

    In this talk we discuss the quasiparticle interferences (QPIs) of a Mott insulator using a T-matrix formalism implemented with the dynamical mean-field theory (T-DMFT). In the Mott insulating state, the DMFT predicts a singularity in the real part of electron self energy s (w) at low frequencies, which completely washes out the QPI at small bias voltage. However, the QPI patterns produced by the non-interacting Fermi surfaces can appear at a critical bias voltage in Mott insulating state. The existence of this non-zero critical bias voltage is a direct consequence of the singular behavior of Re[s (w)] /sim n/w with n behaving as the 'order parameter' of Mott insulating state. We propose that this reentry of non-interacting QPI patterns could serve as an experimental signature of Mott insulating state, and the 'order parameter' can be experimentally measured W.C.L acknowledges financial support from start up fund from Binghamton University.

  9. Insulating States in the Integer Quantum Hall Regime

    NASA Astrophysics Data System (ADS)

    Knighton, Talbot; Serafin, Alessandro; Wu, Zhe; Tarquini, Vinicio; Xia, J. F.; Sullivan, Neil; Pfeiffer, Loren; West, Ken; Huang, Jian

    Quantum Hall measurements are performed at temperatures 20-300 mK in high quality p-type GaAs quantum well systems having mobility μ = 4 ×106 cm2/V .s for density 5 ×1010 cm-2. We report a series of insulating phases appearing at or near integer filling factors ν >= 1 . The DC resistance demonstrates a maximum of 25M Ω, much larger than the quantum resistance h /e2 , with threshold transport behavior at low currents around 10 pA at low temperatures. The threshold diminishes upon heating up to 200 mK, consistent with a finite temperature melting of bubble phases or Wigner crystal. Additionally, these peaks have a complex electrical impedance for AC signals, with large phase shifts down to 1Hz. In this regime, the ac impedance of the two chiral edges show distinct correlated characteristics. NSF DMR-1410302.

  10. Steady-State Thermal Performance Evaluation of Steel-Framed Wall Assembly with Local Foam Insulation

    SciTech Connect

    Kosny, Jan; Biswas, Kaushik; Childs, Phillip W

    2010-01-01

    During January and May, 2009, two configurations of steel-framed walls constructed with conventional 2 4 steel studs insulated with R-19 ~14cm. (5.5-in. thick) and R-13 ~9cm. (3.5-in. thick) fiberglass insulation batts were tested in the Oak Ridge National Laboratory (ORNL) guarded hot-box using ASTM C1363 test procedure. The first test wall used conventional 2 4 steel studs insulated with 2.5-cm. (1-in.) thick foam profiles, called stud snugglers. These stud snugglers converted the 2 4 wall assembly into a 2 6 assembly allowing application of R-19 fiberglass insulation. The second wall tested for comparison was a conventional 2 4 steel stud wall using R-13 insulation batts. Further, numerical simulations were performed in order to evaluate the steady-state thermal performance of various wood- and steel-framed wall assemblies. The effects of adding the stud-snugglers to the wood and steel studs were also investigated numerically. Different combinations of insulation and framing factor were used in the simulations.

  11. Origin of Transitions between Metallic and Insulating States in Simple Metals

    NASA Astrophysics Data System (ADS)

    Naumov, Ivan I.; Hemley, Russell J.

    2015-04-01

    Unifying principles that underlie recently discovered transitions between metallic and insulating states in elemental solids under pressure are developed. Using group theory arguments and first-principles calculations, we show that the electronic properties of the phases involved in these transitions are controlled by symmetry principles. The valence bands in these systems are described by simple and composite band representations constructed from localized Wannier functions centered on points unoccupied by atoms, and which are not necessarily all symmetrical. The character of the Wannier functions is closely related to the degree of s -p (-d ) hybridization and reflects multicenter chemical bonding in these insulating states. The conditions under which an insulating state is allowed for structures having an integer number of atoms per primitive unit cell as well as reentrant (i.e., metal-insulator-metal) transition sequences are detailed, resulting in predictions of behavior such as phases having band-contact lines. The general principles developed are tested and applied to the alkali and alkaline earth metals, including elements where high-pressure insulating phases have been reported (e.g., Li, Na, and Ca).

  12. Emergence of superconductivity from the dynamically heterogeneous insulating state in La2-xSrxCuO4

    NASA Astrophysics Data System (ADS)

    Shi, Xiaoyan; Logvenov, G.; Bollinger, A. T.; Božović, I.; Panagopoulos, C.; Popović, Dragana

    2013-01-01

    A central issue for copper oxides is the nature of the insulating ground state at low carrier densities and the emergence of high-temperature superconductivity from that state with doping. Even though this superconductor-insulator transition (SIT) is a zero-temperature transition, measurements are not usually carried out at low temperatures. Here we use magnetoresistance to probe both the insulating state at very low temperatures and the presence of superconducting fluctuations in La2-xSrxCuO4 films, for doping levels that range from the insulator to the superconductor (x  =  0.03-0.08). We observe that the charge glass behaviour, characteristic of the insulating state, is suppressed with doping, but it coexists with superconducting fluctuations that emerge already on the insulating side of the SIT. The unexpected quenching of the superconducting fluctuations by the competing charge order at low temperatures provides a new perspective on the mechanism for the SIT.

  13. Emergence of superconductivity from the dynamically heterogeneous insulating state in La(2-x)Sr(x)CuO4.

    PubMed

    Shi, Xiaoyan; Logvenov, G; Bollinger, A T; Božović, I; Panagopoulos, C; Popović, Dragana

    2013-01-01

    A central issue for copper oxides is the nature of the insulating ground state at low carrier densities and the emergence of high-temperature superconductivity from that state with doping. Even though this superconductor-insulator transition (SIT) is a zero-temperature transition, measurements are not usually carried out at low temperatures. Here we use magnetoresistance to probe both the insulating state at very low temperatures and the presence of superconducting fluctuations in La(2-x)Sr(x)CuO(4) films, for doping levels that range from the insulator to the superconductor (x  =  0.03-0.08). We observe that the charge glass behaviour, characteristic of the insulating state, is suppressed with doping, but it coexists with superconducting fluctuations that emerge already on the insulating side of the SIT. The unexpected quenching of the superconducting fluctuations by the competing charge order at low temperatures provides a new perspective on the mechanism for the SIT. PMID:23160270

  14. Surface plasmon resonance phenomenon of the insulating state polyaniline

    SciTech Connect

    Umiati, Ngurah Ayu Ketut; Triyana, Kuwat; Kamsul

    2015-04-16

    Surface Plasmon Resonance (SPR) phenomenon of the insulating polyaniline (PANI) is has been observed. Surface Plasmon (SP) is the traveled electromagnetic wave that passes through the interface of dielectric metal and excited by attenuated total reflection (ATR) method in Kretschmannn configuration (Au-PANI prism). The resonance condition is observed through the angle of SPR in such condition that SP wave is coupled by the evanescent constant of laser beam. In this research, the laser beam was generated by He–Ne and its wavelength (λ) was 632,8 nm. SPR curve is obtained through observation of incidence angles of the laser beam in prism. SPR phenomenon at the boundary between Au – PANI layer has showed by reflection dip when the laser beam passes through the prism. In this early study, the observation was carried out through simulation Winspall 3.02 software and preliminary compared with some experimental data reported in other referred literatures. The results shows that the optimum layer of Au and polyaniline are 50 and 1,5 nm thick respectively. Our own near future experimental work would be further performed and reported elsewhere.

  15. Energy barriers for bit-encoding states based on 360° domain walls in ultrathin ferromagnetic nanorings

    NASA Astrophysics Data System (ADS)

    Muratov, C. B.; Osipov, V. V.; Vanden-Eijnden, E.

    2015-05-01

    A numerical thermal stability study of the bit-encoding states in a proposed multi-level magnetic storage element based on an ultrathin ferromagnetic nanoring is presented. The material parameters and the ring dimensions for which there are five distinct metastable magnetization configurations separated by energy barriers exceeding 50kBT at room temperature are identified. The results are obtained, using the string method for the study of rare events to locate the transition states separating the metastable states and to identify the most likely thermally activated pathways.

  16. Ferromagnetic resonance in thin films submitted to multiaxial stress state: application of the uniaxial equivalent stress concept and experimental validation

    NASA Astrophysics Data System (ADS)

    Gueye, M.; Zighem, F.; Belmeguenai, M.; Gabor, M.; Tiusan, C.; Faurie, D.

    2016-07-01

    In this paper a unique expression of the anisotropy field induced by any multiaxial stress state in a magnetic thin film and probed by ferromagnetic resonance is derived. This analytical development has been made using the uniaxial equivalent stress concept, for which correspondances between definitions given by different authors in the literature is found. The proposed model for the anisotropy field has been applied to \\text{C}{{\\text{o}}2}\\text{FeAl} thin films (25 nm) stressed both by piezoelectric actuation (non-equi-biaxial) or by bending tests (uniaxial) and measured with a broadband ferromagnetic resonance technique. The overall exprimental data can be easily plotted on a unique graph from which the magnetostriction coefficient has been estimated.

  17. Studies on Ferromagnetic and Photoluminescence Properties of ITO and Cu-Doped ITO Nanoparticles Synthesized by Solid State Reaction

    NASA Astrophysics Data System (ADS)

    Babu, S. Harinath; Kaleemulla, S.; Rao, N. Madhusudhana; Krishnamoorthi, C.

    2016-07-01

    Cubic structured indium-tin-oxide (ITO) and copper-doped ITO nanoparticles were synthesized by solid state reaction. The structure, morphology, chemical, magnetic, and photoluminescence properties of the synthesized nanoparticles were studied by x-ray diffraction, field emission scanning electron microscopy, x-ray photoelectron spectroscopy, vibrating sample magnetometry, and photoluminescence spectrophotometry, respectively. Magnetic studies confirmed that the ITO nanoparticles were ferromagnetic at room temperature (300 K) and at 100 K, and it was believed that the observed ferromagnetism may be due to oxygen vacancies and defects present in the system. No hysteresis loop was observed in copper-doped ITO nanoparticles at room temperature and 100 K. The ITO and Cu-doped ITO nanoparticles exhibited two broad emission peaks in the visible region of the electromagnetic spectrum.

  18. Magnetic States in Ensemble of Ferromagnetic Nanoparticles in Cu-Mn-Al Alloy.

    PubMed

    Konoplyuk, S M; Kozlova, L E; Kokorin, V V; Perekos, A O; Kolomiets, O V

    2016-12-01

    Two Cu-Mn-Al samples of different compositions were studied: one exhibiting martensitic transformation, another without structural transition. X-ray diffraction and magnetic measurements demonstrate that different magnetic behaviors of alloys originate from different concentrations and sizes of ferromagnetic nanoparticles, which appear after solid solution decomposition.Estimation of magnetic moments of ferromagnetic nanoparticles from magnetization curves was performed using Langevin function and compared to those obtained from X-ray examination. Granular systems are known to show giant magnetoresistance. Therefore, magnetoresistance of Cu-Mn-Al melt-spun ribbons after different aging times was measured. The study has shown that increase in the concentration of Mn atoms and time of aging in Cu-Mn-Al alloy leads to an increase in the amount of precipitated phase appearing as ferromagnetic nanoparticles. PMID:26762264

  19. Quantum hall ferromagnets

    NASA Astrophysics Data System (ADS)

    Kumar, Akshay

    We study several quantum phases that are related to the quantum Hall effect. Our initial focus is on a pair of quantum Hall ferromagnets where the quantum Hall ordering occurs simultaneously with a spontaneous breaking of an internal symmetry associated with a semiconductor valley index. In our first example ---AlAs heterostructures--- we study domain wall structure, role of random-field disorder and dipole moment physics. Then in the second example ---Si(111)--- we show that symmetry breaking near several integer filling fractions involves a combination of selection by thermal fluctuations known as "order by disorder" and a selection by the energetics of Skyrme lattices induced by moving away from the commensurate fillings, a mechanism we term "order by doping". We also study ground state of such systems near filling factor one in the absence of valley Zeeman energy. We show that even though the lowest energy charged excitations are charge one skyrmions, the lowest energy skyrmion lattice has charge > 1 per unit cell. We then broaden our discussion to include lattice systems having multiple Chern number bands. We find analogs of quantum Hall ferromagnets in the menagerie of fractional Chern insulator phases. Unlike in the AlAs system, here the domain walls come naturally with gapped electronic excitations. We close with a result involving only topology: we show that ABC stacked multilayer graphene placed on boron nitride substrate has flat bands with non-zero local Berry curvature but zero Chern number. This allows access to an interaction dominated system with a non-trivial quantum distance metric but without the extra complication of a non-zero Chern number.

  20. Superconducting- and Insulating-Ground States in La2CuO4 Structural Isomers

    NASA Astrophysics Data System (ADS)

    Krockenberger, Yoshiharu; Eleazer, Bennett; Irie, Hiroshi; Yamamoto, Hideki

    2014-11-01

    La2CuO4, with the K2NiF4-structure, is known as the parent compound of hole-doped cuprate superconductors where the Cu ions are octahedrally coordinated and subject to the Jahn-Teller effect. While an octahedral coordination is known for stabilizing an antiferromagnetic and insulating state, we synthesized additional coordination geometries of copper in La2CuO4 where Cu is either square-planar or pyramidal coordinated by a state-of-the-art thin-film growth method. The degeneracy of the Jahn-Teller effect together with rearrangements of the crystal field are expected to have an influence on the electronic states. Indeed, we observed a metallic and superconducting state for La2CuO4 having the Nd2CuO4-structure with square-planar coordinated copper while octahedral and pyramidal coordinations are in favor of the insulating state.

  1. Oxygen-isotope effect of the paramagnetic-insulating to ferromagnetic-metallic transition in La1-xCaxMnO3

    NASA Astrophysics Data System (ADS)

    Franck, J. P.; Isaac, I.; Chen, Weimin; Chrzanowski, J.; Irwin, J. C.

    1998-09-01

    The oxygen-isotope effect of the ferromagnetic transition in La1-xCaxMnO3 was investigated from x=20% to x=43%. This is the range of the conducting ferromagnetic phase. We find that αO=-Δ ln Tc/Δ ln m decreases from 0.36 to 0.14 with increasing Ca concentration. A large value of αO=0.83 was found for x=20%, it is possibly connected with excess oxygen content. The isotope effect decreases with increasing tolerance factor, pointing to the importance of double exchange. The isotope effect at 35% Ca is independent of magnetic field.

  2. Emergence of a Chern-insulating state from a semi-Dirac dispersion

    NASA Astrophysics Data System (ADS)

    Huang, Huaqing; Liu, Zhirong; Zhang, Hongbin; Duan, Wenhui; Vanderbilt, David

    By combining first-principles calculations with Wannier-based tight-binding modeling, we demonstrate that a TiO2/VO2 heterostructure that was previously proposed as a prototypical semi-Dirac system becomes a Chern insulator (quantum anomalous Hall insulator) in the presence of spin-orbit coupling. We show that this occurs only when the semi-Dirac structure is of a special type that can be formed by the merging of three conventional Dirac points. Our results reveal how the nontrivial topology with nonzero Chern number emerges naturally from this kind of semi-Dirac structure, establishing a general scenario that provides a new route to the formation of Chern-insulating states in practical materials systems.

  3. Robust topological surface state in Kondo insulator SmB{sub 6} thin films

    SciTech Connect

    Yong, Jie Jiang, Yeping; Zhang, Xiaohang; Greene, Richard L.; Usanmaz, Demet; Curtarolo, Stefano; Li, Linze; Pan, Xiaoqing; Shin, Jongmoon; Takeuchi, Ichiro

    2014-12-01

    Fabrication of smooth thin films of topological insulators with true insulating bulk are extremely important for utilizing their novel properties in quantum and spintronic devices. Here, we report the growth of crystalline thin films of SmB{sub 6}, a topological Kondo insulator with true insulating bulk, by co-sputtering both SmB{sub 6} and B targets. X-ray diffraction, Raman spectroscopy, and transmission electron microscopy indicate films that are polycrystalline with a (001) preferred orientation. When cooling down, resistivity ρ shows an increase around 50 K and saturation below 10 K, consistent with the opening of the hybridization gap and surface dominated transport, respectively. The ratio ρ{sub 2K}/ρ{sub 300K} is only about two, much smaller than that of bulk, which indicates a much larger surface-to-bulk ratio. Point contact spectroscopy using a superconductor tip on SmB{sub 6} films shows both a Kondo Fano resonance and Andeev reflection, indicating an insulating Kondo lattice with metallic surface states.

  4. Thermoelectric detection of ferromagnetic resonance of a nanoscale ferromagnet.

    PubMed

    Bakker, F L; Flipse, J; Slachter, A; Wagenaar, D; van Wees, B J

    2012-04-20

    We present thermoelectric measurements of the heat dissipated due to ferromagnetic resonance of a Permalloy strip. A microwave magnetic field, produced by an on-chip coplanar strip waveguide, is used to drive the magnetization precession. The generated heat is detected via Seebeck measurements on a thermocouple connected to the ferromagnet. The observed resonance peak shape is in agreement with the Landau-Lifshitz-Gilbert equation and is compared with thermoelectric finite-element modeling. Unlike other methods, this technique is not restricted to electrically conductive media and is therefore also applicable to for instance ferromagnetic insulators. PMID:22680756

  5. Amperean Pairing at the Surface of Topological Insulators

    NASA Astrophysics Data System (ADS)

    Kargarian, Mehdi; Efimkin, Dmitry K.; Galitski, Victor

    2016-08-01

    The surface of a 3D topological insulator is described by a helical electron state with the electron's spin and momentum locked together. We show that in the presence of ferromagnetic fluctuations the surface of a topological insulator is unstable towards a superconducting state with unusual pairing, dubbed Amperean pairing. The key idea is that the dynamical fluctuations of a ferromagnetic layer deposited on the surface of a topological insulator couple to the electrons as gauge fields. The transverse components of the magnetic gauge fields are unscreened and can mediate an effective interaction between electrons. There is an attractive interaction between electrons with momenta in the same direction which makes the pairing to be of Amperean type. We show that this attractive interaction leads to a p -wave pairing instability of the Fermi surface in the Cooper channel.

  6. Amperean Pairing at the Surface of Topological Insulators.

    PubMed

    Kargarian, Mehdi; Efimkin, Dmitry K; Galitski, Victor

    2016-08-12

    The surface of a 3D topological insulator is described by a helical electron state with the electron's spin and momentum locked together. We show that in the presence of ferromagnetic fluctuations the surface of a topological insulator is unstable towards a superconducting state with unusual pairing, dubbed Amperean pairing. The key idea is that the dynamical fluctuations of a ferromagnetic layer deposited on the surface of a topological insulator couple to the electrons as gauge fields. The transverse components of the magnetic gauge fields are unscreened and can mediate an effective interaction between electrons. There is an attractive interaction between electrons with momenta in the same direction which makes the pairing to be of Amperean type. We show that this attractive interaction leads to a p-wave pairing instability of the Fermi surface in the Cooper channel. PMID:27563988

  7. Thickness Dependence of the Quantum Anomalous Hall Effect in Magnetic Topological Insulator Films.

    PubMed

    Feng, Xiao; Feng, Yang; Wang, Jing; Ou, Yunbo; Hao, Zhenqi; Liu, Chang; Zhang, Zuocheng; Zhang, Liguo; Lin, Chaojing; Liao, Jian; Li, Yongqing; Wang, Li-Li; Ji, Shuai-Hua; Chen, Xi; Ma, Xucun; Zhang, Shou-Cheng; Wang, Yayu; He, Ke; Xue, Qi-Kun

    2016-08-01

    The evolution of the quantum anomalous Hall effect with the thickness of Cr-doped (Bi,Sb)2 Te3 magnetic topological insulator films is studied, revealing how the effect is caused by the interplay of the surface states, band-bending, and ferromagnetic exchange energy. Homogeneity in ferromagnetism is found to be the key to high-temperature quantum anomalous Hall material. PMID:27166762

  8. Mott Insulating Ground State on a Triangular Surface Lattice

    SciTech Connect

    Weitering, H.; Shi, X.; Weitering, H.; Johnson, P.; Chen, J.; DiNardo, N.; DiNardo, N.; Kempa, K.

    1997-02-01

    Momentum-resolved direct and inverse photoemission spectra of the K/Si(111)-({radical}(3){times}{radical}(3))R30{degree}-B interface reveals the presence of strongly localized surface states. The K overlayer remains nonmetallic up to the saturation coverage. This system most likely presents the first experimental realization of a frustrated spin 1/2 Heisenberg antiferromagnet on a two-dimensional triangular lattice. {copyright} {ital 1997} {ital The American Physical Society}

  9. Skyrmion-induced bound states on the surface of three-dimensional topological insulators

    NASA Astrophysics Data System (ADS)

    Andrikopoulos, Dimitrios; Sorée, Bart; De Boeck, Jo

    2016-05-01

    The interaction between the surface of a 3D topological insulator and a skyrmion/anti-skyrmion structure is studied in order to investigate the possibility of electron confinement due to the skyrmion presence. Both hedgehog (Néel) and vortex (Bloch) skyrmions are considered. For the hedgehog skyrmion, the in-plane components cannot be disregarded and their interaction with the surface state of the topological insulator (TI) has to be taken into account. A semi-classical description of the skyrmion chiral angle is obtained using the variational principle. It is shown that both the hedgehog and the vortex skyrmion can induce bound states on the surface of the TI. However, the number and the properties of these states depend strongly on the skyrmion type and the skyrmion topological number NSk. The probability densities of the bound electrons are also derived where it is shown that they are localized within the skyrmion region.

  10. Universal scaling for the spin-electricity conversion on surface states of topological insulators

    NASA Astrophysics Data System (ADS)

    Yamamoto, K. T.; Shiomi, Y.; Segawa, Kouji; Ando, Yoichi; Saitoh, E.

    2016-07-01

    We have investigated spin-electricity conversion on surface states of bulk-insulating topological insulator (TI) materials using a spin-pumping technique. The sample structure is Ni-Fe ∣Cu ∣TI trilayers, in which magnetic proximity effects on the TI surfaces are negligibly small owing to the inserted Cu layer. Voltage signals produced by the spin-electricity conversion are clearly observed and are enhanced with decreasing temperature, in line with the dominant surface transport at lower temperatures. The efficiency of the spin-electricity conversion is greater for TI samples with a higher resistivity of bulk states and longer mean free path of surface states, consistent with the surface spin-electricity conversion.

  11. Decoupling of the Antiferromagnetic and Insulating States in Tb doped Sr2 IrO4

    NASA Astrophysics Data System (ADS)

    Zheng, H.; Wang, J. C.; Ye, F.; Aswartham, S.; Terzic, J.; Yuan, S. J.; Haskel, D.; Choi, Y.; Chikara, S.; Schlottmann, P.; Custelcean, R.; Cao, G.

    We report results of a comprehensive study of single-crystal Sr2Ir1-xTbxO4 (0 <=x <=0.03). This study found that mere 3% (x =0.03) tetravalent Tb4+ substituting for Ir4+ (rather than Sr2+) completely suppresses the long-range collinear AFM transition but retains the insulating state. Tb doping effectively changes the relative strength of the SOI and the tetragonal CEF and enhances the Hund's rule coupling that competes with the SOI, and destabilizes the AFM state. We observe unconventional correlation between the AFM and insulating states in which the magnetic transition plays no critical role in the formation of the charge gap in the iridate. This work was supported by NSF through Grant DMR-1265162.

  12. Ferromagnetic ground state for a hypothetical iron-based extended metal atom chain.

    PubMed

    Szarek, Paweł; Wegner, Wojciech; Grochala, Wojciech

    2016-03-01

    Theoretical calculations for the first tri-iron-based extended metal atom chain (EMAC) molecule are reported. The studied triple-high-spin (S = 6) complex exhibits ferromagnetic ordering (according to Ising and spin-projection approximations), which renders it unique among all previously prepared and theoretically calculated EMAC compounds. This ordering originates from the prevailing ferromagnetic nearest-neighbor interactions, while the magnetic superexchange between terminal Fe(2+) sites is weaker and antiferromagnetic. Calculations indicate that this linear chain system based on a tri-iron core shows potential for the development of spin-frustrated behavior, which could be achieved through rational modification of the equatorial and axial ligands. PMID:26910724

  13. Gapless quantum excitations from an icelike splayed ferromagnetic ground state in stoichiometric Yb2Ti2O7

    NASA Astrophysics Data System (ADS)

    Gaudet, J.; Ross, K. A.; Kermarrec, E.; Butch, N. P.; Ehlers, G.; Dabkowska, H. A.; Gaulin, B. D.

    2016-02-01

    The ground state of the quantum spin ice candidate magnet Yb2Ti2O7 is known to be sensitive to weak disorder at the ˜1 % level which occurs in single crystals grown from the melt. Powders produced by solid state synthesis tend to be stoichiometric and display large and sharp heat capacity anomalies at relatively high temperatures, TC˜0.26 K. We have carried out neutron elastic and inelastic measurements on well characterized and equilibrated stoichiometric powder samples of Yb2Ti2O7 which show resolution-limited Bragg peaks to appear at low temperatures, but whose onset correlates with temperatures much higher than TC. The corresponding magnetic structure is best described as an icelike splayed ferromagnet. The spin dynamics in Yb2Ti2O7 are shown to be gapless on an energy scale <0.09 meV at all temperatures and organized into a continuum of scattering with vestiges of highly overdamped ferromagnetic spin waves present. These excitations differ greatly from conventional spin waves predicted for Yb2Ti2O7 's mean field ordered state, but appear robust to weak disorder as they are largely consistent with those displayed by nonstoichiometric crushed single crystals and single crystals, as well as by powder samples of Yb2Ti2O7 's sister quantum magnet Yb2Sn2O7 .

  14. Gapless quantum excitations from an icelike splayed ferromagnetic ground state in stoichiometric Yb2Ti2O7

    DOE PAGESBeta

    Gaudet, J.; Ross, K. A.; Kermarrec, E.; Butch, N. P.; Ehlers, G.; Dabkowska, H. A.; Gaulin, B. D.

    2016-02-03

    We know the ground state of the quantum spin ice candidate magnet Yb2Ti2O7 to be sensitive to weak disorder at the similar to 1% level which occurs in single crystals grown from the melt. Powders produced by solid state synthesis tend to be stoichiometric and display large and sharp heat capacity anomalies at relatively high temperatures, T-C similar to 0.26 K. We have carried out neutron elastic and inelastic measurements on well characterized and equilibrated stoichiometric powder samples of Yb2Ti2O7 which show resolution-limited Bragg peaks to appear at low temperatures, but whose onset correlates with temperatures much higher than T-C.more » The corresponding magnetic structure is best described as an icelike splayed ferromagnet. In the spin dynamics of Yb2Ti2O7 we see the gapless on an energy scale <0.09 meV at all temperatures and organized into a continuum of scattering with vestiges of highly overdamped ferromagnetic spin waves present. These excitations differ greatly from conventional spin waves predicted for Yb2Ti2O7's mean field ordered state, but appear robust to weak disorder as they are largely consistent with those displayed by nonstoichiometric crushed single crystals and single crystals, as well as by powder samples of Yb2Ti2O7's sister quantum magnet Yb2Ti2O7.« less

  15. Ferromagnetic nanorings

    NASA Astrophysics Data System (ADS)

    Vaz, C. A. F.; Hayward, T. J.; Llandro, J.; Schackert, F.; Morecroft, D.; Bland, J. A. C.; Kläui, M.; Laufenberg, M.; Backes, D.; Rüdiger, U.; Castaño, F. J.; Ross, C. A.; Heyderman, L. J.; Nolting, F.; Locatelli, A.; Faini, G.; Cherifi, S.; Wernsdorfer, W.

    2007-06-01

    Ferromagnetic metal rings of nanometre range widths and thicknesses exhibit fundamentally new spin states, switching behaviour and spin dynamics, which can be precisely controlled via geometry, material composition and applied field. Following the discovery of the 'onion state', which mediates the switching to and between vortex states, a range of fascinating phenomena has been found in these structures. In this overview of our work on ring elements, we first show how the geometric parameters of ring elements determine the exact equilibrium spin configuration of the domain walls of rings in the onion state, and we show how such behaviour can be understood as the result of the competition between the exchange and magnetostatic energy terms. Electron transport provides an extremely sensitive probe of the presence, spatial location and motion of domain walls, which determine the magnetic state in individual rings, while magneto-optical measurements with high spatial resolution can be used to probe the switching behaviour of ring structures with very high sensitivity. We illustrate how the ring geometry has been used for the study of a wide variety of magnetic phenomena, including the displacement of domain walls by electric currents, magnetoresistance, the strength of the pinning potential introduced by nanometre size constrictions, the effect of thermal excitations on the equilibrium state and the stochastic nature of switching events.

  16. Observation of Floquet-Bloch states on the surface of a topological insulator.

    PubMed

    Wang, Y H; Steinberg, H; Jarillo-Herrero, P; Gedik, N

    2013-10-25

    The unique electronic properties of the surface electrons in a topological insulator are protected by time-reversal symmetry. Circularly polarized light naturally breaks time-reversal symmetry, which may lead to an exotic surface quantum Hall state. Using time- and angle-resolved photoemission spectroscopy, we show that an intense ultrashort midinfrared pulse with energy below the bulk band gap hybridizes with the surface Dirac fermions of a topological insulator to form Floquet-Bloch bands. These photon-dressed surface bands exhibit polarization-dependent band gaps at avoided crossings. Circularly polarized photons induce an additional gap at the Dirac point, which is a signature of broken time-reversal symmetry on the surface. These observations establish the Floquet-Bloch bands in solids and pave the way for optical manipulation of topological quantum states of matter. PMID:24159040

  17. Andreev bound states and current-phase relations in three-dimensional topological insulators

    NASA Astrophysics Data System (ADS)

    Snelder, M.; Veldhorst, M.; Golubov, A. A.; Brinkman, A.

    2013-03-01

    To guide the search for the Majorana fermion, we theoretically study superconductor/topological-insulator/superconductor (S/TI/S) junctions in an experimentally relevant regime. We find that the striking features present in these systems, including the doubled periodicity of the Andreev bound states (ABSs) due to tunneling via Majorana states, can still be present at high electron densities. We show that via the inclusion of magnetic layers, this 4π periodic ABS can still be observed in three-dimensional (3D) topological insulators, where finite angle incidence usually results in the opening of a gap at zero energy and hence results in a 2π periodic ABS. Furthermore, we study the Josephson-junction characteristics and find that the gap size can be controlled and decreased by tuning the magnetization direction and amplitude. These findings pave the way for designing experiments on S/3DTI/S junctions.

  18. Disorder-induced inhomogeneities of the superconducting state close to the superconductor-insulator transition.

    SciTech Connect

    Sacepe, B.; Chapelier, C.; Baturina, T. I.; Vinokur, V. M.; Baklanov, M. R.; Sanquer, M.; Materials Science Division; CEA, INAC; Inst. Semiconductor Physics; IMEC

    2008-01-01

    Scanning tunneling spectroscopy at very low temperatures on homogeneously disordered superconducting titanium nitride thin films reveals strong spatial inhomogeneities of the superconducting gap {Delta} in the density of states. Upon increasing disorder, we observe suppression of the superconducting critical temperature T{sub c} towards zero, enhancement of spatial fluctuations in {Delta}, and growth of the {Delta}/T{sub c} ratio. These findings suggest that local superconductivity survives across the disorder-driven superconductor-insulator transition.

  19. Surface state photoelectrons in topological insulators: Green’s function approach

    NASA Astrophysics Data System (ADS)

    Schmeltzer, D.; Saxena, A.

    2015-12-01

    We compute the photoemission intensity and polarization for the surface states in topological insulators. Due to the chirality and linear energy dispersion the effective electron-photon coupling is normalized by the tunneling amplitude (τ) into the vacuum. We investigate a chiral Dirac Hamiltonian for different cases: helical, Zeeman and warping, allowing us to study spin textures. Using the Green’s function formalism we obtain exact results for the emitted photoelectrons to second order in the laser field. The number of emitted photoelectrons is sensitive to the laser coherent state intensity whereas the photoelectron polarization is sensitive to the surface topology of electronic states and incoming photon polarization.

  20. The electrical behavior of GaAs-insulator interfaces - A discrete energy interface state model

    NASA Technical Reports Server (NTRS)

    Kazior, T. E.; Lagowski, J.; Gatos, H. C.

    1983-01-01

    The relationship between the electrical behavior of GaAs Metal Insulator Semiconductor (MIS) structures and the high density discrete energy interface states (0.7 and 0.9 eV below the conduction band) was investigated utilizing photo- and thermal emission from the interface states in conjunction with capacitance measurements. It was found that all essential features of the anomalous behavior of GaAs MIS structures, such as the frequency dispersion and the C-V hysteresis, can be explained on the basis of nonequilibrium charging and discharging of the high density discrete energy interface states.

  1. On the photon-drag effect of photocurrent of surface states of topological insulators

    NASA Astrophysics Data System (ADS)

    Lee, Hyun C.

    2016-05-01

    The photocurrent of surface states of topological insulator due to photon-drag effect is computed, being based on pure Dirac model of surface states. The scattering by disorder is taken into account to provide a relaxation mechanism for the photocurrent. The Keldysh-Schwinger formalism has been employed for the systematic calculation of photocurrent. The helicity dependent photocurrent of sizable magnitude transverse to the in-plane photon momentum is found, which is consistent with experimental data. Other helicity independent photocurrents with various polarization states are also calculated.

  2. Ferromagnetism and manipulation of topological surface states in Bi2Se3 family by 2p light elements

    NASA Astrophysics Data System (ADS)

    Niu, Chengwang; Dai, Ying; Zhang, Zhenkui; Ma, Yandong; Huang, Baibiao

    2012-06-01

    The manipulation effects by doping of 2p light elements X (X = B, C, and N) on topological surface states in V2VI3 (V = Bi and Sb, VI = Se and Te) are systemically explored. Our results unveil that X doping at anion sites can induce magnetic moments and gap opening at the Dirac point. To have a stable magnetic ground state, the dopant 2p states must be sufficiently localized, which closely depends on the X-V bond lengths. The incorporation of 2p dopants paves a promising way of tuning the properties of topological insulators and may find applications in spintronics.

  3. Insulating state in tetralayers reveals an even–odd interaction effect in multilayer graphene

    PubMed Central

    Grushina, Anya L.; Ki, Dong-Keun; Koshino, Mikito; Nicolet, Aurelien A. L.; Faugeras, Clément; McCann, Edward; Potemski, Marek; Morpurgo, Alberto F.

    2015-01-01

    Close to charge neutrality, the electronic properties of graphene and its multilayers are sensitive to electron–electron interactions. In bilayers, for instance, interactions are predicted to open a gap between valence and conduction bands, turning the system into an insulator. In mono and (Bernal-stacked) trilayers, which remain conducting at low temperature, interactions do not have equally drastic consequences. It is expected that interaction effects become weaker for thicker multilayers, whose behaviour should converge to that of graphite. Here we show that this expectation does not correspond to reality by revealing the occurrence of an insulating state close to charge neutrality in Bernal-stacked tetralayer graphene. The phenomenology—incompatible with the behaviour expected from the single-particle band structure—resembles that observed in bilayers, but the insulating state in tetralayers is visible at higher temperature. We explain our findings, and the systematic even–odd effect of interactions in Bernal-stacked layers of different thickness that emerges from experiments, in terms of a generalization of the interaction-driven, symmetry-broken states proposed for bilayers. PMID:25732058

  4. Spin-patterned plasmonics: towards optical access to topological-insulator surface states.

    PubMed

    Spektor, Grisha; David, Asaf; Bartal, Guy; Orenstein, Meir; Hayat, Alex

    2015-12-14

    Topological insulators (TI) are new phases of matter with topologically protected surface states (SS) possessing novel physical properties such as spin-momentum locking. Coupling optical angular momentum to the SS is of interest for both fundamental understanding and applications in future spintronic devices. However, due to the nanoscale thickness of the surface states, the light matter interaction is dominated by the bulk. Here we propose and experimentally demonstrate a plasmonic cavity enabling both nanoscale light confinement and control of surface plasmon-polariton (SPP) spin angular momentum (AM)--towards coupling to topological-insulator SS. The resulting SPP field components within the cavity are arranged in a chess-board-like pattern. Each chess-board square exhibits approximately a uniform circular polarization (spin AM) of the local in-plane field interleaved by out-of-plane field vortices (orbital AM). As the first step, we demonstrate the predicted pattern experimentally by near-field measurements on a gold-air interface, with excellent agreement to our theory. Our results pave the way towards efficient optical access to topological-insulator surface states using plasmonics. PMID:26699065

  5. Photoconductivity oscillations in surface state of three-dimensional topological insulator subjected to a magnetic field

    SciTech Connect

    Shao, J. M.; Yao, J. D.; Yang, G. W.

    2015-05-21

    We describe a theoretical study of the terahertz (THz) radiation field-induced dc transport response of the surface state of a 3D topological insulator that has been subjected to a perpendicular magnetic field. Using the Landau–Floquet state and linear response theory, we obtain the photoconductivity characteristics for various types of polarized THz field. The longitudinal photoconductivity shows a clear oscillatory dependence on ω/ω{sub B}, where ω{sub B}=v{sub F}√(2eB/ℏ). This oscillation occurs because of the oscillatory structure of the Landau density of states and occurs in agreement with the photon-assisted transitions between the different Landau levels. The THz field's polarization has a major influence on the photoconductivity. A linear transverse polarization will lead to the most obvious oscillation, while the circular polarization is next to it, but the longitudinal polarization has no influence. We also discuss the broadening effect on the impurity potential and its influence. The findings with regard to the interactions between topological insulators and THz fields actually open a path toward the development of THz device applications of topological insulators.

  6. Robustness of topological order and formation of quantum well states in topological insulators exposed to ambient environment

    PubMed Central

    Chen, Chaoyu; He, Shaolong; Weng, Hongming; Zhang, Wentao; Zhao, Lin; Liu, Haiyun; Jia, Xiaowen; Mou, Daixiang; Liu, Shanyu; He, Junfeng; Peng, Yingying; Feng, Ya; Xie, Zhuojin; Liu, Guodong; Dong, Xiaoli; Zhang, Jun; Wang, Xiaoyang; Peng, Qinjun; Wang, Zhimin; Zhang, Shenjin; Yang, Feng; Chen, Chuangtian; Xu, Zuyan; Dai, Xi; Fang, Zhong; Zhou, X. J.

    2012-01-01

    The physical property investigation (like transport measurements) and ultimate application of the topological insulators usually involve surfaces that are exposed to ambient environment (1 atm and room temperature). One critical issue is how the topological surface state will behave under such ambient conditions. We report high resolution angle-resolved photoemission measurements to directly probe the surface state of the prototypical topological insulators, Bi2Se3 and Bi2Te3, upon exposing to various environments. We find that the topological order is robust even when the surface is exposed to air at room temperature. However, the surface state is strongly modified after such an exposure. Particularly, we have observed the formation of two-dimensional quantum well states near the exposed surface of the topological insulators. These findings provide key information in understanding the surface properties of the topological insulators under ambient environment and in engineering the topological surface state for applications. PMID:22355146

  7. Direct observation of spin-resolved full and empty electron states in ferromagnetic surfaces

    SciTech Connect

    Berti, G. Calloni, A.; Brambilla, A.; Bussetti, G.; Duò, L.; Ciccacci, F.

    2014-07-15

    We present a versatile apparatus for the study of ferromagnetic surfaces, which combines spin-polarized photoemission and inverse photoemission spectroscopies. Samples can be grown by molecular beam epitaxy and analyzed in situ. Spin-resolved photoemission spectroscopy analysis is done with a hemispherical electron analyzer coupled to a 25 kV-Mott detector. Inverse photoemission spectroscopy experiments are performed with GaAs crystals as spin-polarized electron sources and a UV bandpass photon detector. As an example, measurements on the oxygen passivated Fe(100)-p(1×1)O surface are presented.

  8. Thermodynamically self-consistent non-stochastic micromagnetic model for the ferromagnetic state

    SciTech Connect

    Dvornik, Mykola Vansteenkiste, Arne; Van Waeyenberge, Bartel

    2014-10-20

    In this work, a self-consistent thermodynamic approach to micromagnetism is presented. The magnetic degrees of freedom are modeled using the Landau-Lifshitz-Baryakhtar theory, which separates the different contributions to the magnetic damping, and thereby allows them to be coupled to the electron and phonon systems in a self-consistent way. We show that this model can quantitatively reproduce ultrafast magnetization dynamics in Nickel suggesting that in ferromagnetic metals the ultrafast angular momentum transfer happens via the relativistic spin-electron scattering.

  9. Stable non-Fermi-liquid phase of itinerant spin-orbit coupled ferromagnets

    NASA Astrophysics Data System (ADS)

    Bahri, Yasaman; Potter, Andrew C.

    2015-07-01

    Direct (nongradient) coupling between a gapless bosonic field and a Fermi surface results in the destruction of Landau quasiparticles and a breakdown of Fermi liquid theory. Such a non-Fermi-liquid phase arises in spin-orbit coupled ferromagnets with spontaneously broken continuous symmetries due to strong coupling between rotational Goldstone modes and itinerant electrons. These systems provide an experimentally accessible context for studying non-Fermi-liquid physics. Possible examples include low-density Rashba coupled electron gases, which have a natural tendency towards spontaneous ferromagnetism, or topological insulator surface states with proximity-induced ferromagnetism. Crucially, unlike the related case of a spontaneous nematic distortion of the Fermi surface, for which controlled field theory calculations predict that the non-Fermi-liquid regime will be masked by a superconducting dome, we show that the non-Fermi-liquid phase in spin-orbit coupled ferromagnets is stable.

  10. Stable non-Fermi liquid phase of itinerant spin-orbit coupled ferromagnets

    NASA Astrophysics Data System (ADS)

    Bahri, Yasaman; Potter, Andrew

    2015-03-01

    Direct coupling between gapless bosons and a Fermi surface results in the destruction of Landau quasiparticles and a breakdown of Fermi liquid theory. Such a non-Fermi liquid phase arises in spin-orbit coupled ferromagnets with spontaneously broken continuous symmetries due to strong coupling between rotational Goldstone modes and itinerant electrons. These systems provide an experimentally accessible context for studying non-Fermi liquid physics. Possible examples include low-density Rashba coupled electron gases, which have a natural tendency towards spontaneous ferromagnetism, or topological insulator surface states with proximity-induced ferromagnetism. Crucially, unlike the related case of a spontaneous nematic distortion of the Fermi surface, for which the non-Fermi liquid regime is expected to be masked by a superconducting dome, we show that the non-Fermi liquid phase in spin-orbit coupled ferromagnets is stable.

  11. Quantum Transport of Spin-helical Dirac Fermion Topological Surface States in Topological Insulators

    NASA Astrophysics Data System (ADS)

    Chen, Yong P.

    Three-dimensional (3D) topological insulators (TI) are a novel class of electronic materials with topologically-nontrivial band structure such that the bulk is gapped and insulating yet the surface has topologically protected gapless conducting states. Such ``topological surface states'' (TSS) give helically spin polarized Dirac fermions, and offer a promising platform to realize various other novel physics such as topological magnetoelectric effects and Majorana fermions. However, it is often challenging to unambiguously access and study the transport properties of TSS in many practical TI materials due to non-negligible bulk conducting states. I will discuss our recent experiments on high-quality ``intrinsic'' TIs with insulating bulk and surface-dominated conduction that allow us to reveal a number of characteristic transport properties of spin-helical Dirac fermion topological surface states. We have observed, for example, a thickness-independent and surface-dominated conductance (even at room temperature) in exfoliated TI thin films and well-developed ``half-integer'' Dirac fermion quantum Hall effect (QHE) arising from TSS (observed up to 40K); fully-tunable ``two-species'' Dirac fermion QHE and other intriguing states in dual gated devices where both top and bottom surfaces can be independently controlled; current-induced helical spin-polarization detected by spin sensitive transport measurements using magnetic electrodes; and in TI nanoribbons, Shubnikov-de Hass (SdH) oscillations showing gate-tunable Berry phase and ultra-relativistic Dirac mass; and a ``half-integer'' Aharonov-Bohm effect (ABE) unique to the circumferentially quantized spin helical Dirac fermion surface state modes (sub-bands), with a gate-tunable conductance oscillation and alternation between the ``half-integer'' ABE and regular ABE periodic in fermi momentum. Such TIs and related devices may enable promising future applications in spintronics, thermoelectrics and various topological

  12. Mapping a fractional quantum Hall state to a fractional Chern insulator

    NASA Astrophysics Data System (ADS)

    Zhang, Yinhan; Shi, Junren

    2016-04-01

    We establish a variational principle for properly mapping a fractional quantum Hall state to a fractional Chern insulator (FCI). We find that the mapping has a gauge freedom which could generate a class of FCI ground-state wave functions appropriate for different forms of interactions. Therefore, the gauge should be fixed by a variational principle that minimizes the interaction energy of the FCI model. For a soft and isotropic electron-electron interaction, the principle leads to a gauge coinciding with that for maximally localized two-dimensional projected Wannier functions of a Landau level.

  13. Surface states in a 3D topological insulator: The role of hexagonal warping and curvature

    SciTech Connect

    Repin, E. V.; Burmistrov, I. S.

    2015-09-15

    We explore a combined effect of hexagonal warping and a finite effective mass on both the tunneling density of electronic surface states and the structure of Landau levels of 3D topological insulators. We find the increasing warping to transform the square-root van Hove singularity into a logarithmic one. For moderate warping, an additional logarithmic singularity and a jump in the tunneling density of surface states appear. By combining the perturbation theory and the WKB approximation, we calculate the Landau levels in the presence of hexagonal warping. We predict that due to the degeneracy removal, the evolution of Landau levels in the magnetic field is drastically modified.

  14. Local electronic state in the half-metallic ferromagnet CrO2 investigated by site-selective 53Cr NMR measurements

    NASA Astrophysics Data System (ADS)

    Takeda, Hikaru; Shimizu, Yasuhiro; Kobayashi, Yoshiaki; Itoh, Masayuki; Jin-no, Takaaki; Isobe, Masahiko; Ueda, Yutaka; Yoshida, Sho; Muraoka, Yuji; Yokoya, Takayoshi

    2016-06-01

    We have made 53Cr NMR measurements on polycrystalline and single-crystalline film samples to study the local electronic state of a half-metallic ferromagnet CrO2 which has the rutile structure with one crystallographically equivalent Cr site. We observe two kinds of 53Cr NMR spectra in the ferromagnetic state of both the samples. An analysis on the hyperfine field shows the presence of two Cr sites with different orbital occupancies, suggesting that a local orbital order takes place with breaking the local symmetry in the rutile structure. This may be ascribed to the negative charge transfer between chromium and oxygen ions.

  15. CO Oxidation Facilitated by Robust Surface States on Au-Covered Topological Insulators

    SciTech Connect

    Chen, Hua; Zhu, Wenguang; Xiao, Di; Zhang, Zhenyu

    2011-01-01

    Surface states the electronic states emerging as a solid material terminates at a surface are usually vulnerable to contaminations and defects. The robust topological surface state(s) (TSS) on the three-dimensional topological insulators provide a perfect platform for exploiting surface states in less stringent environments. Employing first-principles density functional theory calculations, we demonstrate that the TSS can play a vital role in facilitating surface reactions by serving as an effective electron bath. We use CO oxidation on gold-covered Bi2Se3 as a prototype example, and show that the robust TSS can significantly enhance the adsorption energy of both CO and O2 molecules, by promoting different directions of electron transfer. The concept of TSS as an electron bath may lead to new design principles beyond the conventional d-band theory of heterogeneous catalysis.

  16. Characterization of Surface Chemical States of a Thick Insulator: Chemical State Imaging on MgO Surface

    NASA Astrophysics Data System (ADS)

    Yi, Yeonjin; Cho, Sangwan; Noh, Myungkeun; Whang, Chung-Nam; Jeong, Kwangho; Shin, Hyun-Joon

    2005-02-01

    We report a surface characterization tool that can be effectively used to investigate the chemical state and subtle radiation damage on a thick insulator surface. It has been used to examine the MgO surface of a plasma display panel (PDP) consisting of a stack of insulator layers of approximately 51 μm thickness on a 2-mm-thick glass plate. The scanning photoelectron microscopy (SPEM) image of the insulating MgO surface was obtained by using the difference in Au 4f peak shift due to the surface charging at each pixel, where a Au adlayer of approximately 15 {\\AA} thickness was formed on the surface to overcome the serious charging shift of the peak position and the spectral deterioration in the photoelectron spectra. The observed contrast in the SPEM image reveals the chemical modification of the underlying MgO surface induced by the plasma discharge damage. The chemical state analysis of the MgO surface was carried out by comparing the Mg 2p, C 1s and O 1s photoemission spectra collected at each pixel of the SPEM image. We assigned four suboxide phases, MgO, MgCO3, Mg(OH)2 and Mg1+, on the initial MgO surface, where the Mg(OH)2 and Mg1+ phases vanished rapidly as the discharge-induced surface damage began.

  17. Exchange Coupling Nanophase Fe-Pd Ferromagnets Through Solid State Transformation

    SciTech Connect

    Shugart, Kathleen N.; Ludtka, Gerard Michael; Ludtka, Gail Mackiewicz-; Soffa, William A

    2011-01-01

    This study continues previous work on off stoichiometric Fe-Pd alloys using a combined reaction strategy during thermomechanical processing [1,2]. Severe plastic deformation of the initial disordered fcc gamma phase ( ), followed by heat treatment in the two phase field produces a nano-composite ferromagnet comprised of soft alpha-Fe/ferrite in a high-anisotropy L10 FePd matrix. The length scale and morphology of the transformation products have been characterized using x-ray diffraction, and scanning and transmission electron microscopy. The transformed microstructures exhibit strong texture retention similar to the stoichiometric alloy suggesting a massive ordering mode. The alloy has shown a proclivity to exchange couple at a length scale not in agreement with proposed theories of exchange coupling [3,4]. The magnetic properties were measured using standard vibrating sample magnetometry (VSM). This research has been supported by the National Science Foundation (NSF-DMR).

  18. Ground state of underdoped cuprates in vicinity of superconductor-to-insulator transition

    DOE PAGESBeta

    Wu, Jie; Bollinger, Anthony T.; Sun, Yujie; Božović, Ivan

    2016-08-15

    When an insulating underdoped cuprate is doped beyond a critical concentration (xc), high-temperature superconductivity emerges. We have synthesized a series of La2–xSrxCuO4 (LSCO) samples using the combinatorial spread technique that allows us to traverse the superconductor-to-insulator transition (SIT) in extremely fine doping steps, Δx≈0.00008. We have measured the Hall resistivity (ρH) as a function of temperature down to 300 mK in magnetic fields up to 9 T. At very low temperatures, ρH shows an erratic behavior, jumps and fluctuations exceeding 100%, hysteresis, and memory effects, indicating that the insulating ground state is a charge-cluster glass (CCG). Furthermore, based on themore » phase diagram depicted in our experiment, we propose a unified picture to account for the anomalous electric transport in the vicinity of the SIT, suggesting that the CCG is in fact a disordered and glassy version of the charge density wave.« less

  19. Steady-state heat transfer in He II through porous superconducting cable insulation

    SciTech Connect

    Baudouy, B.J.P.; Juster, F.P.; Meuris, C.; Vieillard, L.

    1996-12-31

    The LHC program includes the study of thermal behavior of the superconducting cables wound in the dipole magnet cooled by superfluid helium (He II). Insulation of these superconducting cables forms the major thermal shield hindering the He II cooling. This is particularly a problem in magnets which are subjected to thermal loads. To investigate He II heat transfer processes an experimental model has been realized which creates a one-dimensional heat transfer in such media. Insulation is generally realized by wrapping around the superconducting cable a combination of different kind of Kapton{reg_sign} tapes, fiber-glass impregnated by epoxy resin or Kevlar{reg_sign} fiber tapes. Steady-state heat transfer in He II through these multi-layer porous slabs has been analyzed. Experimental results for a range of heat flux show the existence of different thermal regimes related to He II. It is shown that the parameters of importance are a global geometrical factor which could be considered as an equivalent {open_quotes}permeability{close_quotes} related to He II heat transfer, the transfer function f(T) of He II and the thermal conductivity of the slab. The authors present and analyze results for different insulations as a function of the temperature.

  20. Kohn's theory of the insulating state: a quantum-chemistry viewpoint.

    PubMed

    Resta, Raffaele

    2006-03-14

    The qualitative difference between insulators and conductors not only manifests itself in the excitation spectra but also--according to Kohn's theory [Phys. Rev. 133, A171 (1964)]--in a different organization of the electrons in their ground state: the wave function is localized in insulators and delocalized in conductors. Such localization, however, is hidden in a rather subtle way in the many-body wave function. The theory has been substantially revisited and extended in modern times, invariably within a periodic-boundary-condition framework, i.e., ideally addressing an infinite condensed system. Here we show how the localization/delocalization of the many-body wave function shows up when considering either three-dimensional clusters of increasing size or quasi-one-dimensional systems (linear polymers, nanotubes, and nanowires) of increasing length, within the ordinary "open" boundary conditions adopted for finite systems. We also show that the theory, when specialized to uncorrelated wave functions, has a very close relationship with Boy's theory of localization [Rev. Mod. Phys. 32, 296 (1960)]: the Boys orbitals in the bulk of the sample behave in a qualitatively different way in insulating versus conducting cases. PMID:16542065

  1. Manipulating the ferromagnetism in narrow-bandwidth Pr1-xCaxMnO3 (0 ≤ x ≤ 0.6) by means of the Mn-Ru t2g ferromagnetic super-exchanges

    NASA Astrophysics Data System (ADS)

    Wang, Y. L.; Liu, M. F.; Xie, Y. L.; Yan, Z. B.; Dong, S.; Liu, J.-M.

    2015-09-01

    The concurrent ferromagnetic and metal-insulator transitions via the double-exchange route and electronic phase separation scenario represent the core ingredients of the physics of manganites. In this work, a Ca2+ and Ru4+ co-substitution of Pr3+ and Mn3+ in narrow-bandwidth and insulating PrMnO3, namely, Pr1-xCaxMn1-xRuxO3 (PCMRO, x ≤ 0.6), is carried out in order to investigate an alternative approach to effectively manipulate the ferromagnetism of PrMnO3-based manganites. It is revealed that PCMRO over the whole substitution range is homogeneous solid solution with increased lattice distortion. The preference of Ru4+ valence state and the absence of Mn4+ valence state disable the Mn3+-Mn4+ eg-orbital double-exchange, and the random occupation of Ru4+ in the lattice excludes the charge ordering and electronic phase separation. While all these consequences should favor antiferromagnetic insulating states, nevertheless, a high-temperature ferromagnetic transition is triggered by the co-substitution and the magnetization can reach up to ˜1.0 μB/f.u. at x ˜ 0.2-0.3, much bigger than the moment (<0.1 μB/f.u.) of Pr1-xCaxMnO3 in the weak ferromagnetic insulator state. It is suggested that this strong ferromagnetism is substantially ascribed to the Mn3+-Ru4+ t2g-orbital ferromagnetic super-exchange, and a simple geometric network illustration of the magnetism and electrical transport is presented.

  2. Charge density stabilised local electron spin pair states in insulating polymers

    SciTech Connect

    Serra, S.; Dissado, L. A.

    2014-12-14

    A model is presented that addresses the energy stability of localized electron states in insulating polymers with respect to delocalized free electron-like states at variable charge densities. The model was derived using an effective Hamiltonian for the total energy of electrons trapped in large polarons and spin-paired bipolarons, which includes the electrostatic interaction between charges that occurs when the charge density exceeds the infinite dilution limit. The phase diagram of the various electronic states with respect to the charge density is derived using parameters determined from experimental data for polyethylene, and it is found that a phase transition from excess charge in the form of stable polarons to a stable state of bipolarons with charge = 2 and spin number S = 0 is predicted for a charge density between 0.2 C/m{sup 3} and ∼2 C/m{sup 3}. This transition is consistent with a change from low mobility charge transport to charge transport in the form of pulses with a mobility orders of magnitude higher that has been observed in several insulating polymers.

  3. Thermoelectric properties of silicene in the topological- and band-insulator states

    NASA Astrophysics Data System (ADS)

    Wierzbicki, Michał; Barnaś, Józef; Swirkowicz, Renata

    2015-04-01

    Thermoelectric effects in silicene nanoribbons are analyzed theoretically and numerically. The main focus is on the influence of topological edge states and transition between the topological-insulator and conventional gap-insulator states on the thermoelectric properties, and especially on the spin-related thermoelectric effects. The model includes a staggered exchange field and also an external electric field normal to the atomic plane. Both fields separately open a gap in the edge states and therefore lead to a nonzero thermopower in the vicinity of the gap edges. Interplay of both fields leads to a spin-dependent gap, and thus gives rise to a spin thermopower in addition to the conventional Seebeck effect. The role of the Coulomb interaction taken in the form of the Hubbard term in the mean-field approximation is also analyzed. This interaction leads to antiparallel configuration of the edge magnetic moments in the ground state. It is shown that the Coulomb interaction significantly modifies topological properties of the materials, and thus also their transport and thermoelectric properties.

  4. Exotic topological states near a quantum metal-insulator transition in pyrochlore iridates

    NASA Astrophysics Data System (ADS)

    Tian, Zhaoming

    Pyrochlore iridates have attracted great interest as prime candidates that may host topologically nontrivial states, spin ice ordering and quantum spin liquid states, in particular through the interplay between different degrees of freedom, such as local moments and mobile electrons. Based on our extensive study using our high quality single crystals, we will discuss such examples, i.e. chiral spin liquid in a quadratic band touching state, Weyl semimetallic state and chiral domain wall transport nearby a quantum insulator-semimetal transition in pyrochlore iridates. This work is based on the collaboration with Nakatsuji Satoru, Kohama Yoshimitsu, Tomita Takahiro, Kindo Koichi, Jun J. Ishikawa, Balents Leon, Ishizuka Hiroaki, Timothy H. Hsieh. ZM. Tian was supported by JSPS Postdoctoral Fellowship (No.P1402).

  5. Novel magnetic states in insulating d4 oxides with strong spin-orbit coupling

    NASA Astrophysics Data System (ADS)

    Svoboda, Christopher; Trivedi, Nandini

    2015-03-01

    The comparable energy scales in 4 d and 5 d transition metal oxides, arising from Coulomb correlations, spin-orbit coupling and bandwidth, can lead to new phases and phenomena. With this motivation we examine an ion with d4 electron configuration in the t2 g sector separated from the other states by crystal field splitting. Upon including spin-orbit coupling, the completely filled j = 3 / 2 manifold is nonmagnetic but with a nonzero magnetic susceptibility. Upon introducing hopping between two d4 atoms, we find novel entangled ferromagnetism generated by the superexchange interaction in a significant part of the phase diagram. We further present results for the temperature dependent susceptibility calculated using exact diagonalization to illustrate this novel magnetic behavior and the role Hund's coupling plays in producing these phases. We make predictions for resonant X-ray scattering and magnetic measurements in pyrochlore osmates. We acknowledge the support of the CEM, and NSF MRSEC, under Grant DMR-1420451.

  6. High-spin multiplicities in ferromagnetic ground states of supramolecular halide complexes based on the gadolinium chloride

    NASA Astrophysics Data System (ADS)

    Paduani, C.

    2016-03-01

    Calculations using density functional theory are performed to study supramolecular assemblage of high spin halide complexes based on the gadolinium chloride. With the addition of Cl atoms to both Gd and B in number that exceeds their formal valence by 1 the calculated vertical detachment energy increases to 6.08 and 5.57 eV in GdCl4 and BCl4, respectively, indicating superhalogen behavior. By using BCl4 and GdCl4 clusters as building blocks to decorate the Gd atom the vertical detachment energy increases to 7.12 and 7.70 eV in the anionic clusters Gd(BCl4)4- and Gd(GdCl4)4-, respectively, which is indicative of hyperhalogen behavior. High spin multiplicities in the ferromagnetic state are observed for these clusters indicating therein outstanding paramagnetic response.

  7. Unidirectional Spin-Dependent Molecule-Ferromagnet Hybridized States Anisotropy in Cobalt Phthalocyanine Based Magnetic Tunnel Junctions

    NASA Astrophysics Data System (ADS)

    Barraud, Clément; Bouzehouane, Karim; Deranlot, Cyrile; Fusil, Stéphane; Jabbar, Hashim; Arabski, Jacek; Rakshit, Rajib; Kim, Dong-Jik; Kieber, Christophe; Boukari, Samy; Bowen, Martin; Beaurepaire, Eric; Seneor, Pierre; Mattana, Richard; Petroff, Frédéric

    2015-05-01

    Organic or molecular spintronics is a rising field of research at the frontier between condensed matter physics and chemistry. It aims to mix spin physics and the richness of chemistry towards designing new properties for spin electronics devices through engineering at the molecular scale. Beyond the expectation of a long spin lifetime, molecules can be also used to tailor the spin polarization of the injected current through the spin-dependent hybridization between molecules and ferromagnetic electrodes. In this Letter, we provide direct evidence of a hybrid interface spin polarization reversal due to the differing hybridization between phthalocyanine molecules and each cobalt electrode in Co /CoPc /Co magnetic tunnel junctions. Tunnel magnetoresistance and anisotropic tunnel magnetoresistance experiments show that interfacial hybridized electronic states have a unidirectional anisotropy that can be controlled by an electric field and that spin hybridization at the bottom and top interfaces differ, leading to an inverse tunnel magnetoresistance.

  8. Fe valence states and ferromagnetism occurring in reduced anatase Ti0.97Fe0.03O2-δ

    NASA Astrophysics Data System (ADS)

    Lee, Hi Min; Kim, Chul Sung

    2007-05-01

    Fe-doped anatase TiO2 polycrystalline films and powders have been prepared by the sol-gel method. Air-annealed film shows paramagnetic behavior at room temperature. However, when the film is further annealed in a vacuum, the ferromagnetic properties are strongly enhanced with the magnetic moment of 0.42μB/Fe at 5kOe. Mössbauer spectrum of air-annealed film at 295K shows a single doublet of Fe3+. On the other hand, the absorption spectrum after vacuum annealing exhibits two doublets, in which one is the same component with air-annealed case and the other is a new doublet corresponding to Fe2+ state. The temperature dependence of absorption linewidth and quadrupole splitting for the Fe2+ doublet indicates that Fe2+ ions are not paramagnetic and their magnetic interaction exists also at 295K.

  9. Magnetic gating of a 2D topological insulator.

    PubMed

    Dang, Xiaoqian; Burton, J D; Tsymbal, Evgeny Y

    2016-09-28

    Deterministic control of transport properties through manipulation of spin states is one of the paradigms of spintronics. Topological insulators offer a new playground for exploring interesting spin-dependent phenomena. Here, we consider a ferromagnetic 'gate' representing a magnetic adatom coupled to the topologically protected edge state of a two-dimensional (2D) topological insulator to modulate the electron transmission of the edge state. Due to the locked spin and wave vector of the transport electrons the transmission across the magnetic gate depends on the mutual orientation of the adatom magnetic moment and the current. If the Fermi energy matches an exchange-split bound state of the adatom, the electron transmission can be blocked due to the full back scattering of the incident wave. This antiresonance behavior is controlled by the adatom magnetic moment orientation so that the transmission of the edge state can be changed from 1 to 0. Expanding this consideration to a ferromagnetic gate representing a 1D chain of atoms shows a possibility to control the spin-dependent current of a strip of a 2D topological insulator by magnetization orientation of the ferromagnetic gate. PMID:27437829

  10. Magnetic gating of a 2D topological insulator

    NASA Astrophysics Data System (ADS)

    Dang, Xiaoqian; Burton, J. D.; Tsymbal, Evgeny Y.

    2016-09-01

    Deterministic control of transport properties through manipulation of spin states is one of the paradigms of spintronics. Topological insulators offer a new playground for exploring interesting spin-dependent phenomena. Here, we consider a ferromagnetic ‘gate’ representing a magnetic adatom coupled to the topologically protected edge state of a two-dimensional (2D) topological insulator to modulate the electron transmission of the edge state. Due to the locked spin and wave vector of the transport electrons the transmission across the magnetic gate depends on the mutual orientation of the adatom magnetic moment and the current. If the Fermi energy matches an exchange-split bound state of the adatom, the electron transmission can be blocked due to the full back scattering of the incident wave. This antiresonance behavior is controlled by the adatom magnetic moment orientation so that the transmission of the edge state can be changed from 1 to 0. Expanding this consideration to a ferromagnetic gate representing a 1D chain of atoms shows a possibility to control the spin-dependent current of a strip of a 2D topological insulator by magnetization orientation of the ferromagnetic gate.

  11. Record surface state mobility and quantum Hall effect in topological insulator thin films via interface engineering

    NASA Astrophysics Data System (ADS)

    Koirala, Nikesh; Brahlek, Matthew; Salehi, Maryam; Wu, Liang; Dai, Jixia; Waugh, Justin; Nummy, Thomas; Han, Myung-Geun; Moon, Jisoo; Zhu, Yimei; Dessau, Daniel; Wu, Weida; Armitage, N. Peter; Oh, Seongshik

    Thin films of topological insulators (TIs) with conduction dominated by high mobility topological surface state (TSS) channel have been difficult to achieve due to increased material defects, thus making it difficult to probe TIs in quantum regime. Here by utilizing a structurally matched buffer layer based on In2Se3, we have achieved Bi2Se3 films with low defect density resulting in `order of magnitude' improvement in mobilities and carrier densities. This has led to TSS dominated transport and first observation of quantum Hall effect in Bi2Se3.

  12. A strong ferroelectric ferromagnet created by means of spin-lattice coupling.

    SciTech Connect

    Lee, J. H.; Fang, L.; Vlahos, E.; Ke, X.; Jung, Y.W.; Fitting Kourkaoutis, L.; Kim, J. W.; Ryan, P.; Heeg, T.; Roeckerath, M.; Goian, V.; Bernhagen, M.; Uecker, R.; Hammel, P.C.; Rabe, K. M.; Kamba, S.; Schubert, J.; Freeland, J.W.; Muller, D.A.; Fennie, C.J.; Schiffer, P.; Gopalan, V.; Johnston-Halperin, E.; Schlom, D. G.

    2010-08-19

    Ferroelectric ferromagnets are exceedingly rare, fundamentally interesting multiferroic materials that could give rise to new technologies in which the low power and high speed of field-effect electronics are combined with the permanence and routability of voltage-controlled ferromagnetism. Furthermore, the properties of the few compounds that simultaneously exhibit these phenomena are insignificant in comparison with those of useful ferroelectrics or ferromagnets: their spontaneous polarizations or magnetizations are smaller by a factor of 1,000 or more. The same holds for magnetic- or electric-field-induced multiferroics. Owing to the weak properties of single-phase multiferroics, composite and multilayer approaches involving strain-coupled piezoelectric and magnetostrictive components are the closest to application today. Recently, however, a new route to ferroelectric ferromagnets was proposed by which magnetically ordered insulators that are neither ferroelectric nor ferromagnetic are transformed into ferroelectric ferromagnets using a single control parameter, strain. The system targeted, EuTiO{sub 3}, was predicted to exhibit strong ferromagnetism (spontaneous magnetization, {approx}7 Bohr magnetons per Eu) and strong ferroelectricity (spontaneous polarization, {approx}10 {micro}C cm{sup -2}) simultaneously under large biaxial compressive strain. These values are orders of magnitude higher than those of any known ferroelectric ferromagnet and rival the best materials that are solely ferroelectric or ferromagnetic. Hindered by the absence of an appropriate substrate to provide the desired compression we turned to tensile strain. Here we show both experimentally and theoretically the emergence of a multiferroic state under biaxial tension with the unexpected benefit that even lower strains are required, thereby allowing thicker high-quality crystalline films. This realization of a strong ferromagnetic ferroelectric points the way to high

  13. Angle-resolved Photoemission Spectroscopy Study on the Surface States of the Correlated Topological Insulator YbB6

    PubMed Central

    Xia, M.; Jiang, J.; Ye, Z. R.; Wang, Y. H.; Zhang, Y.; Chen, S. D.; Niu, X. H.; Xu, D. F.; Chen, F.; Chen, X. H.; Xie, B. P.; Zhang, T.; Feng, D. L.

    2014-01-01

    YbB6 is recently predicted to be a moderately correlated topological insulator, which provides a playground to explore the interplay between correlation and topological properties. With angle-resolved photoemission spectroscopy, we directly observed almost linearly dispersive bands around the time-reversal invariant momenta and with negligible kz dependence, consistent with odd number of surface states crossing the Fermi level in a Z2 topological insulator. Circular dichroism photoemission spectra suggest that these in-gap states possess chirality of orbital angular momentum, which is related to the chiral spin texture, further indicative of their topological nature. The observed insulating gap of YbB6 is about 100 meV, larger than that found by theoretical calculations. Our results present strong evidence that YbB6 is a correlated topological insulator and provide a foundation for further studies of this promising material. PMID:25102781

  14. Observation of topological surface state quantum Hall effect in an intrinsic three-dimensional topological insulator

    NASA Astrophysics Data System (ADS)

    Xu, Yang; Miotkowski, Ireneusz; Chen, Yong; Yong P. Chen's Group in Purdue Team; Chih-Kang Shih's Group in UT-Austin Collaboration; M. Zahid Hasan's Group in Princeton Collaboration

    2015-03-01

    A three-dimensional (3D) topological insulator (TI) is a novel quantum matter with a gapped insulating bulk yet a conducting surface hosting topologically-protected gapless surface states of Dirac fermions. One of the most distinct electronic transport signatures predicted for such topological surface states (TSS) is a half-integer quantum Hall effect (QHE) in a magnetic field. We have observed well-developed QHE arising from TSS in an intrinsic TI of BiSbTeSe2. Our samples can exhibit surface dominated conduction even close to room temperature, while the bulk conduction is negligible. At low temperatures and high perpendicular magnetic fields, the Hall conductance shows well quantized integer plateaux in exfoliated flake devices on SiO2 / Si substrates, where the top and bottom surface each contributing a half integer e2 / h Hall conductance, accompanied by vanishing longitudinal resistance. We have also studied dual-gated devices where both the top and bottom surfaces can be independently gated. Such intrinsic 3D TI materials exhibiting no measurable bulk conduction and well-developed surface state QHE pave the way for further applications of topological quantum electronics. DARPA MESO program (Grant N66001-11-1-4107).

  15. Intrinsic conduction through topological surface states of insulating Bi2Te3 epitaxial thin films.

    PubMed

    Hoefer, Katharina; Becker, Christoph; Rata, Diana; Swanson, Jesse; Thalmeier, Peter; Tjeng, L H

    2014-10-21

    Topological insulators represent a novel state of matter with surface charge carriers having a massless Dirac dispersion and locked helical spin polarization. Many exciting experiments have been proposed by theory, yet their execution has been hampered by the extrinsic conductivity associated with the unavoidable presence of defects in Bi2Te3 and Bi2Se3 bulk single crystals, as well as impurities on their surfaces. Here we present the preparation of Bi2Te3 thin films that are insulating in the bulk and the four-point probe measurement of the conductivity of the Dirac states on surfaces that are intrinsically clean. The total amount of charge carriers in the experiment is of the order of 10(12) cm(-2) only, and mobilities up to 4,600 cm(2)/Vs have been observed. These values are achieved by carrying out the preparation, structural characterization, angle-resolved and X-ray photoemission analysis, and temperature-dependent four-point probe conductivity measurement all in situ under ultra-high-vacuum conditions. This experimental approach opens the way to prepare devices that can exploit the intrinsic topological properties of the Dirac surface states. PMID:25294928

  16. Breakdown of the Kondo insulating state in SmB6 by introducing Sm vacancies

    NASA Astrophysics Data System (ADS)

    Valentine, Michael E.; Koohpayeh, Seyed; Phelan, W. Adam; McQueen, Tyrel M.; Rosa, Priscila F. S.; Fisk, Zachary; Drichko, Natalia

    2016-08-01

    We explore the stability of the hybridization gap in SmB6 to the presence of a small number of Sm vacancies typical for this material, and demonstrate the extreme fragility of the Kondo insulating state. For the most stoichiometric sample we detect the hybridization gap which appears below 50 K as a depressed electronic Raman intensity below about 30 meV. The spectral weight that shifts to higher frequencies on the opening of the hybridization gap forms two electronic maxima at 100 and 41 meV. We assign these maxima to the excitations between hybridized 4 f -5 d bands using recent band structure calculations. Below 30 K, in-gap exciton modes with long lifetimes protected by the hybridization gap develop at 16-18 meV. With the increase of the number of Sm vacancies the exciton features broaden, evidencing a decrease in the lifetime due to the presence of electronic states in the gap. At a concentration of Sm vacancies of only about 1% the in-gap exciton is completely quenched, and the hybridization gap is not fully opened. We suggest that only the most stoichiometric SmB6 samples possess a bulk gap necessary for the topological Kondo insulator state.

  17. Dual evidence of surface Dirac states in thin cylindrical topological insulator Bi₂Te₃ nanowires.

    PubMed

    Tian, Mingliang; Ning, Wei; Qu, Zhe; Du, Haifeng; Wang, Jian; Zhang, Yuheng

    2013-01-01

    How the surface state (SS) develops and how the spin transport in a curved cylindrical topological insulator nanowire have attracted theoretical attention recently. However, experimental confirmation for the SS in such a real modeling system still remains insufficient. Here we carried out a systematic comparative study on the cylindrical single-crystal Bi₂Te₃ nanowires of various diameters, and report unambiguously dual evidence for the Dirac SS. Both the predicted anomalous Aharonov-Bohm (AB) quantum oscillations with a period of h/e in H(//) and the 1/2-shifted Shubnikov-de Haas (SdH) oscillations (i.e., γ = -1/2) in H(⊥) were indentified below 1.4 K. In addition, Altshuler-Aronov-Spivak (AAS)-like oscillations with a period of h/2e and ordinary SdH oscillations with γ = 0 were also resolved. These data provide clear evidence of coexistence of the nontrivial topological Dirac state and trivial electron state on the surface of topological insulator nanowire. PMID:23390572

  18. Interpreting current-induced spin polarization in topological insulator surface states

    NASA Astrophysics Data System (ADS)

    Li, Pengke; Appelbaum, Ian

    2016-06-01

    Several recent experiments on three-dimensional topological insulators claim to observe a large charge current-induced nonequilibrium ensemble spin polarization of electrons in the helical surface state. We present a comprehensive criticism of such claims, using both theory and experiment: First, we clarify the interpretation of quantities extracted from these measurements by deriving standard expressions from a Boltzmann transport equation approach in the relaxation-time approximation at zero and finite temperature to emphasize our assertion that, despite high in-plane spin projection, obtainable current-induced ensemble spin polarization is minuscule. Second, we use a simple experiment to demonstrate that magnetic field-dependent open-circuit voltage hysteresis (identical to those attributed to current-induced spin polarization in topological insulator surface states) can be generated in analogous devices where current is driven through thin films of a topologically trivial metal. This result ipso facto discredits the naive interpretation of previous experiments with TIs, which were used to claim observation of helicity, i.e., spin-momentum locking in the topologically protected surface state.

  19. Intrinsic conduction through topological surface states of insulating Bi2Te3 epitaxial thin films

    PubMed Central

    Hoefer, Katharina; Becker, Christoph; Rata, Diana; Swanson, Jesse; Thalmeier, Peter; Tjeng, L. H.

    2014-01-01

    Topological insulators represent a novel state of matter with surface charge carriers having a massless Dirac dispersion and locked helical spin polarization. Many exciting experiments have been proposed by theory, yet their execution has been hampered by the extrinsic conductivity associated with the unavoidable presence of defects in Bi2Te3 and Bi2Se3 bulk single crystals, as well as impurities on their surfaces. Here we present the preparation of Bi2Te3 thin films that are insulating in the bulk and the four-point probe measurement of the conductivity of the Dirac states on surfaces that are intrinsically clean. The total amount of charge carriers in the experiment is of the order of 1012 cm−2 only, and mobilities up to 4,600 cm2/Vs have been observed. These values are achieved by carrying out the preparation, structural characterization, angle-resolved and X-ray photoemission analysis, and temperature-dependent four-point probe conductivity measurement all in situ under ultra-high-vacuum conditions. This experimental approach opens the way to prepare devices that can exploit the intrinsic topological properties of the Dirac surface states. PMID:25294928

  20. Self-dual quantum electrodynamics as boundary state of the three-dimensional bosonic topological insulator

    NASA Astrophysics Data System (ADS)

    Xu, Cenke; You, Yi-Zhuang

    2015-12-01

    Inspired by recent developments in constructing novel Dirac liquid boundary states of a three-dimensional (3D) topological insulator, we propose one possible two-dimensional boundary state of a 3D bosonic symmetry protected topological state with U (1) e⋊Z2T×U (1) s symmetry. This boundary theory is described by a (2 +1 ) -dimensional quantum electrodynamics (QED3) with two flavors of Dirac fermions (Nf=2 ) coupled with a noncompact U(1) gauge field, L =∑j=12ψ¯jγμ(∂μ-i aμ) ψj-i Aμsψi¯γμτij zψj+i/2 π ɛμ ν ρaμ∂νAρe , where aμ is the internal noncompact U(1) gauge field, and Aμs and Aμe are two external gauge fields that couple to U (1) s and U (1) e global symmetries, respectively. We demonstrate that this theory has a "self-dual" structure, which is a fermionic analog of the self-duality of the noncompact CP1 theory with easy plane anisotropy. Under the self-duality, the boundary action takes exactly the same form except for an exchange between Aμs and Aμe. The self-duality may still hold after we break one of the U(1) symmetries (which makes the system a bosonic topological insulator), with some subtleties that will be discussed.

  1. Duo gating on a 3D topological insulator - independent tuning of both topological surface states

    NASA Astrophysics Data System (ADS)

    Li, Chuan; de Ronde, Bob; Snelder, Marieke; Stehno, Martin; Huang, Yingkai; Golden, Mark; Brinkman, Alexander; ICE Team; IOP Collaboration

    ABSTRACT: Topological insulators are associated with a trove of exciting physics, such as the ability to host robust anyons, Majorana Bound States, which can be used for quantum computation. For future Majorana devices it is desirable to have the Fermi energy tuned as close as possible to the Dirac point of the topological surface state. Based on previous work on gating BSTS, we report the experimental progress towards gate-tuning of the top and bottom topological surface states of BiSbTeSe2 crystal flakes. When the Fermi level is moved across the Dirac point conduction is shown to change from electron dominated transport to hole dominated transport independently for either surface. In the high magnetic field, one can tune the system precisely between the different landau levels of both surfaces, thus a full gating map of the possible landau levels combination is established. In addition, we provide a simple capacitance model to explain the general hysteresis behaviors in topological insulator systems.

  2. Dual evidence of surface Dirac states in thin cylindrical topological insulator Bi2Te3 nanowires

    PubMed Central

    Tian, Mingliang; Ning, Wei; Qu, Zhe; Du, Haifeng; Wang, Jian; Zhang, Yuheng

    2013-01-01

    How the surface state (SS) develops and how the spin transport in a curved cylindrical topological insulator nanowire have attracted theoretical attention recently. However, experimental confirmation for the SS in such a real modeling system still remains insufficient. Here we carried out a systematic comparative study on the cylindrical single-crystal Bi2Te3 nanowires of various diameters, and report unambiguously dual evidence for the Dirac SS. Both the predicted anomalous Aharonov-Bohm (AB) quantum oscillations with a period of h/e in H// and the 1/2-shifted Shubnikov-de Haas (SdH) oscillations (i.e., γ = −1/2) in H⊥ were indentified below 1.4 K. In addition, Altshuler-Aronov-Spivak (AAS)-like oscillations with a period of h/2e and ordinary SdH oscillations with γ = 0 were also resolved. These data provide clear evidence of coexistence of the nontrivial topological Dirac state and trivial electron state on the surface of topological insulator nanowire. PMID:23390572

  3. Selective scattering between Floquet-Bloch and Volkov states in a topological insulator

    NASA Astrophysics Data System (ADS)

    Mahmood, Fahad; Chan, Ching-Kit; Alpichshev, Zhanybek; Gardner, Dillon; Lee, Young; Lee, Patrick A.; Gedik, Nuh

    2016-04-01

    The coherent optical manipulation of solids is emerging as a promising way to engineer novel quantum states of matter. The strong time-periodic potential of intense laser light can be used to generate hybrid photon-electron states. Interaction of light with Bloch states leads to Floquet-Bloch states, which are essential in realizing new photo-induced quantum phases. Similarly, dressing of free-electron states near the surface of a solid generates Volkov states, which are used to study nonlinear optics in atoms and semiconductors. The interaction of these two dynamic states with each other remains an open experimental problem. Here we use time- and angle-resolved photoemission spectroscopy (Tr-ARPES) to selectively study the transition between these two states on the surface of the topological insulator Bi2Se3. We find that the coupling between the two strongly depends on the electron momentum, providing a route to enhance or inhibit it. Moreover, by controlling the light polarization we can negate Volkov states to generate pure Floquet-Bloch states. This work establishes a systematic path for the coherent manipulation of solids via light-matter interaction.

  4. Selective scattering between Floquet-Bloch and Volkov states in a topological insulator

    NASA Astrophysics Data System (ADS)

    Mahmood, Fahad; Chan, Ching-Kit; Alpichshev, Zhanybek; Gardner, Dillon; Lee, Young; Lee, Patrick; Gedik, Nuh

    The coherent optical manipulation of solids is emerging as a promising way to engineer novel quantum states of matter. The strong time periodic potential of intense laser light can be used to generate hybrid photon-electron states. Interaction of light with Bloch states leads to Floquet-Bloch states which are essential in realizing new photo-induced quantum phases. Similarly, dressing of free electron states near the surface of a solid generates Volkov states which are used to study non-linear optics in atoms and semiconductors. The interaction of these two dynamic states with each other remains an open experimental problem. Here we use Time and Angle Resolved Photoemission Spectroscopy (Tr-ARPES) to selectively study the transition between these two states on the surface of the topological insulator Bi2Se3. We find that the coupling between the two strongly depends on the electron momentum, providing a route to enhance or inhibit it. Moreover, by controlling the light polarization we can negate Volkov states in order to generate pure Floquet-Bloch states. This work establishes a systematic path for the coherent manipulation of solids via light-matter interaction.

  5. A Ferromagnetic Salicylaldoximate/Azide Mn(II)2Mn(III)6 Cluster with an S = 17 Ground State and a Single-Molecule-Magnet Response.

    PubMed

    Vicente, R; El Fallah, M S; Casanovas, B; Font-Bardia, M; Escuer, A

    2016-06-20

    One new Mn(II)2Mn(III)6 cluster exhibiting an S = 17 spin ground state and single-molecule-magnet properties has been designed linking Mn(III)3-salicylaldoximate triangles and tetracoordinated Mn(II) cations by means of end-on azido bridges. The ferromagnetic coupling has been rationalized as a function of their structural parameters. PMID:27227379

  6. Laser spectroscopic probing of coexisting superfluid and insulating states of an atomic Bose-Hubbard system

    NASA Astrophysics Data System (ADS)

    Kato, Shinya; Inaba, Kensuke; Sugawa, Seiji; Shibata, Kosuke; Yamamoto, Ryuta; Yamashita, Makoto; Takahashi, Yoshiro

    2016-04-01

    A system of ultracold atoms in an optical lattice has been regarded as an ideal quantum simulator for a Hubbard model with extremely high controllability of the system parameters. While making use of the controllability, a comprehensive measurement across the weakly to strongly interacting regimes in the Hubbard model to discuss the quantum many-body state is still limited. Here we observe a great change in the excitation energy spectra across the two regimes in an atomic Bose-Hubbard system by using a spectroscopic technique, which can resolve the site occupancy in the lattice. By quantitatively comparing the observed spectra and numerical simulations based on sum rule relations and a binary fluid treatment under a finite temperature Gutzwiller approximation, we show that the spectra reflect the coexistence of a delocalized superfluid state and a localized insulating state across the two regimes.

  7. Laser spectroscopic probing of coexisting superfluid and insulating states of an atomic Bose–Hubbard system

    PubMed Central

    Kato, Shinya; Inaba, Kensuke; Sugawa, Seiji; Shibata, Kosuke; Yamamoto, Ryuta; Yamashita, Makoto; Takahashi, Yoshiro

    2016-01-01

    A system of ultracold atoms in an optical lattice has been regarded as an ideal quantum simulator for a Hubbard model with extremely high controllability of the system parameters. While making use of the controllability, a comprehensive measurement across the weakly to strongly interacting regimes in the Hubbard model to discuss the quantum many-body state is still limited. Here we observe a great change in the excitation energy spectra across the two regimes in an atomic Bose–Hubbard system by using a spectroscopic technique, which can resolve the site occupancy in the lattice. By quantitatively comparing the observed spectra and numerical simulations based on sum rule relations and a binary fluid treatment under a finite temperature Gutzwiller approximation, we show that the spectra reflect the coexistence of a delocalized superfluid state and a localized insulating state across the two regimes. PMID:27094083

  8. Large Fermi Surface of Heavy Electrons at the Border of Mott Insulating State in NiS2

    DOE PAGESBeta

    Friedemann, S.; Chang, H.; Gamża, M. B.; Reiss, P.; Chen, X.; Alireza, P.; Coniglio, W. A.; Graf, D.; Tozer, S.; Grosche, F. M.

    2016-05-12

    One early triumph of quantum physics is the explanation why some materials are metallic whereas others are insulating. While a treatment based on single electron states is correct for most materials this approach can fail spectacularly, when the electrostatic repulsion between electrons causes strong correlations. Not only can these favor new and subtle forms of matter, such as magnetism or superconductivity, they can even cause the electrons in a half-filled energy band to lock into position, producing a correlated, or Mott insulator. The transition into the Mott insulating state raises important fundamental questions. Foremost among these is the fate ofmore » the electronic Fermi surface and the associated charge carrier mass, as the Mott transition is approached. We report the first direct observation of the Fermi surface on the metallic side of a Mott insulating transition by high pressure quantum oscillatory measurements in NiS2. We find our results point at a large Fermi surface consistent with Luttinger's theorem and a strongly enhanced quasiparticle effective mass. These two findings are in line with central tenets of the Brinkman-Rice picture of the correlated metal near the Mott insulating state and rule out alternative scenarios in which the carrier concentration vanishes continuously at the metal-insulator transition.« less

  9. Large Fermi Surface of Heavy Electrons at the Border of Mott Insulating State in NiS2

    NASA Astrophysics Data System (ADS)

    Friedemann, S.; Chang, H.; Gamża, M. B.; Reiss, P.; Chen, X.; Alireza, P.; Coniglio, W. A.; Graf, D.; Tozer, S.; Grosche, F. M.

    2016-05-01

    One early triumph of quantum physics is the explanation why some materials are metallic whereas others are insulating. While a treatment based on single electron states is correct for most materials this approach can fail spectacularly, when the electrostatic repulsion between electrons causes strong correlations. Not only can these favor new and subtle forms of matter, such as magnetism or superconductivity, they can even cause the electrons in a half-filled energy band to lock into position, producing a correlated, or Mott insulator. The transition into the Mott insulating state raises important fundamental questions. Foremost among these is the fate of the electronic Fermi surface and the associated charge carrier mass, as the Mott transition is approached. We report the first direct observation of the Fermi surface on the metallic side of a Mott insulating transition by high pressure quantum oscillatory measurements in NiS2. Our results point at a large Fermi surface consistent with Luttinger’s theorem and a strongly enhanced quasiparticle effective mass. These two findings are in line with central tenets of the Brinkman-Rice picture of the correlated metal near the Mott insulating state and rule out alternative scenarios in which the carrier concentration vanishes continuously at the metal-insulator transition.

  10. Large Fermi Surface of Heavy Electrons at the Border of Mott Insulating State in NiS2

    PubMed Central

    Friedemann, S.; Chang, H.; Gamża, M. B.; Reiss, P.; Chen, X.; Alireza, P.; Coniglio, W. A.; Graf, D.; Tozer, S.; Grosche, F. M.

    2016-01-01

    One early triumph of quantum physics is the explanation why some materials are metallic whereas others are insulating. While a treatment based on single electron states is correct for most materials this approach can fail spectacularly, when the electrostatic repulsion between electrons causes strong correlations. Not only can these favor new and subtle forms of matter, such as magnetism or superconductivity, they can even cause the electrons in a half-filled energy band to lock into position, producing a correlated, or Mott insulator. The transition into the Mott insulating state raises important fundamental questions. Foremost among these is the fate of the electronic Fermi surface and the associated charge carrier mass, as the Mott transition is approached. We report the first direct observation of the Fermi surface on the metallic side of a Mott insulating transition by high pressure quantum oscillatory measurements in NiS2. Our results point at a large Fermi surface consistent with Luttinger’s theorem and a strongly enhanced quasiparticle effective mass. These two findings are in line with central tenets of the Brinkman-Rice picture of the correlated metal near the Mott insulating state and rule out alternative scenarios in which the carrier concentration vanishes continuously at the metal-insulator transition. PMID:27174799

  11. Large Fermi Surface of Heavy Electrons at the Border of Mott Insulating State in NiS2.

    PubMed

    Friedemann, S; Chang, H; Gamża, M B; Reiss, P; Chen, X; Alireza, P; Coniglio, W A; Graf, D; Tozer, S; Grosche, F M

    2016-01-01

    One early triumph of quantum physics is the explanation why some materials are metallic whereas others are insulating. While a treatment based on single electron states is correct for most materials this approach can fail spectacularly, when the electrostatic repulsion between electrons causes strong correlations. Not only can these favor new and subtle forms of matter, such as magnetism or superconductivity, they can even cause the electrons in a half-filled energy band to lock into position, producing a correlated, or Mott insulator. The transition into the Mott insulating state raises important fundamental questions. Foremost among these is the fate of the electronic Fermi surface and the associated charge carrier mass, as the Mott transition is approached. We report the first direct observation of the Fermi surface on the metallic side of a Mott insulating transition by high pressure quantum oscillatory measurements in NiS2. Our results point at a large Fermi surface consistent with Luttinger's theorem and a strongly enhanced quasiparticle effective mass. These two findings are in line with central tenets of the Brinkman-Rice picture of the correlated metal near the Mott insulating state and rule out alternative scenarios in which the carrier concentration vanishes continuously at the metal-insulator transition. PMID:27174799

  12. Volume-wise destruction of the antiferromagnetic Mott insulating state through quantum tuning

    DOE PAGESBeta

    B. A. Frandsen; Liu, L.; Cheung, S. C.; Guguchia, Z.; Khasanov, R.; Morenzoni, E.; Munsie, T. J.S.; Hallas, A. M.; Wilson, M. N.; Cai, Y.; et al

    2016-08-17

    RENiO3 (RE=rare-earth element) and V2O3 are archetypal Mott insulator systems. When tuned by chemical substitution (RENiO3) or pressure (V2O3), they exhibit a quantum phase transition (QPT) between an antiferromagnetic Mott insulating state and a paramagnetic metallic state. Because novel physics often appears near a Mott QPT, the details of this transition, such as whether it is first or second order, are important. Here, we demonstrate through muon spin relaxation/rotation (μSR) experiments that the QPT in RENiO3 and V2O3 is first order: the magnetically ordered volume fraction decreases to zero at the QPT, resulting in a broad region of intrinsic phasemore » separation, while the ordered magnetic moment retains its full value until it is suddenly destroyed at the QPT. These findings bring to light a surprising universality of the pressure-driven Mott transition, revealing the importance of phase separation and calling for further investigation into the nature of quantum fluctuations underlying the transition.« less

  13. Volume-wise destruction of the antiferromagnetic Mott insulating state through quantum tuning

    PubMed Central

    Frandsen, Benjamin A.; Liu, Lian; Cheung, Sky C.; Guguchia, Zurab; Khasanov, Rustem; Morenzoni, Elvezio; Munsie, Timothy J. S.; Hallas, Alannah M.; Wilson, Murray N.; Cai, Yipeng; Luke, Graeme M.; Chen, Bijuan; Li, Wenmin; Jin, Changqing; Ding, Cui; Guo, Shengli; Ning, Fanlong; Ito, Takashi U.; Higemoto, Wataru; Billinge, Simon J. L.; Sakamoto, Shoya; Fujimori, Atsushi; Murakami, Taito; Kageyama, Hiroshi; Alonso, Jose Antonio; Kotliar, Gabriel; Imada, Masatoshi; Uemura, Yasutomo J.

    2016-01-01

    RENiO3 (RE=rare-earth element) and V2O3 are archetypal Mott insulator systems. When tuned by chemical substitution (RENiO3) or pressure (V2O3), they exhibit a quantum phase transition (QPT) between an antiferromagnetic Mott insulating state and a paramagnetic metallic state. Because novel physics often appears near a Mott QPT, the details of this transition, such as whether it is first or second order, are important. Here, we demonstrate through muon spin relaxation/rotation (μSR) experiments that the QPT in RENiO3 and V2O3 is first order: the magnetically ordered volume fraction decreases to zero at the QPT, resulting in a broad region of intrinsic phase separation, while the ordered magnetic moment retains its full value until it is suddenly destroyed at the QPT. These findings bring to light a surprising universality of the pressure-driven Mott transition, revealing the importance of phase separation and calling for further investigation into the nature of quantum fluctuations underlying the transition. PMID:27531192

  14. Mott insulating states and quantum phase transitions of correlated SU(2 N ) Dirac fermions

    NASA Astrophysics Data System (ADS)

    Zhou, Zhichao; Wang, Da; Meng, Zi Yang; Wang, Yu; Wu, Congjun

    2016-06-01

    The interplay between charge and spin degrees of freedom in strongly correlated fermionic systems, in particular of Dirac fermions, is a long-standing problem in condensed matter physics. We investigate the competing orders in the half-filled SU (2 N ) Hubbard model on a honeycomb lattice, which can be accurately realized in optical lattices with ultracold large-spin alkaline-earth fermions. Employing large-scale projector determinant quantum Monte Carlo simulations, we have explored quantum phase transitions from the gapless Dirac semimetals to the gapped Mott insulating phases in the SU(4) and SU(6) cases. Both of these Mott insulating states are found to be columnar valence bond solid (cVBS) and to be absent of the antiferromagnetic Néel ordering and the loop current ordering. Inside the cVBS phases, the dimer ordering is enhanced by increasing fermion components and behaves nonmonotonically as the interaction strength increases. Although the transitions generally should be of first order due to a cubic invariance possessed by the cVBS order, the coupling to gapless Dirac fermions can soften the transitions to second order through a nonanalytic term in the free energy. Our simulations provide important guidance for the experimental explorations of novel states of matter with ultracold alkaline-earth fermions.

  15. Volume-wise destruction of the antiferromagnetic Mott insulating state through quantum tuning.

    PubMed

    Frandsen, Benjamin A; Liu, Lian; Cheung, Sky C; Guguchia, Zurab; Khasanov, Rustem; Morenzoni, Elvezio; Munsie, Timothy J S; Hallas, Alannah M; Wilson, Murray N; Cai, Yipeng; Luke, Graeme M; Chen, Bijuan; Li, Wenmin; Jin, Changqing; Ding, Cui; Guo, Shengli; Ning, Fanlong; Ito, Takashi U; Higemoto, Wataru; Billinge, Simon J L; Sakamoto, Shoya; Fujimori, Atsushi; Murakami, Taito; Kageyama, Hiroshi; Alonso, Jose Antonio; Kotliar, Gabriel; Imada, Masatoshi; Uemura, Yasutomo J

    2016-01-01

    RENiO3 (RE=rare-earth element) and V2O3 are archetypal Mott insulator systems. When tuned by chemical substitution (RENiO3) or pressure (V2O3), they exhibit a quantum phase transition (QPT) between an antiferromagnetic Mott insulating state and a paramagnetic metallic state. Because novel physics often appears near a Mott QPT, the details of this transition, such as whether it is first or second order, are important. Here, we demonstrate through muon spin relaxation/rotation (μSR) experiments that the QPT in RENiO3 and V2O3 is first order: the magnetically ordered volume fraction decreases to zero at the QPT, resulting in a broad region of intrinsic phase separation, while the ordered magnetic moment retains its full value until it is suddenly destroyed at the QPT. These findings bring to light a surprising universality of the pressure-driven Mott transition, revealing the importance of phase separation and calling for further investigation into the nature of quantum fluctuations underlying the transition. PMID:27531192

  16. The zero-field glassy ground state and field-induced ferromagnetic transition in (La₀.₄Pr₀.₆)₁.₂Sr₁.₈Mn₂O₇.

    PubMed

    Tackett, R; Lawes, G; Suryanarayanan, R; Apostu, M; Revcolevschi, A

    2011-04-20

    We have investigated glassy magnetic freezing in(La₀.₄Pr₀.₆)₁.₂Sr₁.₈Mn₂O₇ single crystals together with the field-induced transition to a metastable ferromagnetic phase using ac magnetic susceptibility and heat capacity measurements. The magnetization measurements show evidence for the development of a zero-field glassy ground state below 45 K along with a hysteretic, field-induced change in susceptibility associated with the transition to the ferromagnetic phase above 5 T. The heat capacity develops a clear peak at higher temperatures with the application of large magnetic fields, consistent with the development of a ferromagnetic order, while at low temperatures the Sommerfeld coefficient is monotonically reduced by an applied field, suggesting suppression of spin fluctuations. The heat capacity shows hysteretic behaviour, accompanied by a sharp decrease at a critical field, when held at fixed temperature, which does not recover on reducing the field back to zero. These measurements suggest that the zero-field ground state for (La₀.₄Pr₀.₆)₁.₂Sr₁.₈Mn₂O₇ consists of frozen disordered spin clusters, which develop into a metastable ferromagnetic state in modest magnetic fields. PMID:21460425

  17. The Magnetic Equation of State of the Random Dipolar Coupled Ising Ferromagnet LITHIUM(1)TERBIUM(P)YTTRIUM(1 - FLUORINE(4)

    NASA Astrophysics Data System (ADS)

    Brierley, Steven Kenneth

    The magnetic equation of state for the randomly diluted, dipolar coupled Ising ferromagnet LiTb(,p)Y(,1 -p)F(,4) has been measured using a sensitive SQUID magnetometer for two concentrations of the magnetic ion: p = 0.90, corresponding to a weakly diluted system; and p = 0.63, a relatively strongly diluted system. In order to determine the nature of the critical behavior in such random ferromagnets, the data have been compared to three different theoretical expressions for the equation of state. The first one is that for a pure dipolar coupled Ising ferromagnet. The remaining two expressions are for random dipolar Ising ferromagnets: a random universal form with concentration independent parameters, and a random nonasymptotic form with explicitly concentration dependent parameters. For both concentrations it has been found that the random nonasymptotic equation of state gives the best description of the critical behavior. These results are consistent with previous alternating current susceptibility measurements on the same crystals and with numerical calculations which predicted that the random universal (concentration independent) behavior would not be clearly observable in experimentally accessible temperature ranges. An analysis has also been carried out of the smearing of the phase transition near T(,c). It has been found that the rounding can be accounted for by assuming a distribution of critical temperatures. It is suggested that the origin of such a distribution of T(,c)'s is a macroscopic concentration gradient across the sample arising from changes in the melt composition during crystal growth.

  18. Topological surface states in Kondo insulator SmB6 via planar tunneling spectroscopy*

    NASA Astrophysics Data System (ADS)

    Park, Wan Kyu; Sun, Lunan; Noddings, Alex; Greene, Laura; Kim, Dae-Jeong; Fisk, Zachary

    Samarium hexaboride (SmB6) belongs to a class of materials called Kondo insulators in which the hybridization between itinerant electrons and local moments leads to an emergent state of matter. With inherently large spin-orbit coupling along with strong correlation, SmB6 has been recently predicted to be topological meaning that topologically robust conducting states should exist at its surfaces. Although extensive investigations have provided growing evidence for the existence of such states, corroborative spectroscopic evidences are still lacking unlike in the weakly correlated counterparts. We adopt planar tunneling spectroscopy to unveil their detailed nature and behavior utilizing its inherently high energy resolution. Measurements of tunneling conductance on two different crystal surfaces (001) and (011) reveal the expected linear density of states for two and one Dirac cones, respectively. Moreover, it is found that these topological states cease to be protected well before they merge into the bulk states at the gap edges. Microscopic modeling of the tunneling processes accounting for the interaction with spin excitons as predicted by a recent theory provide consistent explanations for all the observed features, corroborating the proposed picture on the incompletely protected surface states in SmB6

  19. Skyrmion-induced bound states on the surface of 3D Topological Insulators

    NASA Astrophysics Data System (ADS)

    Andrikopoulos, Dimitrios; Soree, Bart

    In this work, we study the interaction between the surface state of a 3D Topological Insulator and a skyrmion magnetic texture. The skyrmion texture couples to the spin of the surface state electron with strength ΔS. Vortex and hedgehog skyrmion and anti-skyrmion structures are considered and their interaction is compared. Due to the vortex structure, the interaction of the in-plane components can be neglected and a step function is used to describe the skyrmion magnetization profile. In the hedgehog case, it is shown that the in-plane components cannot be disregarded and thus a realistic description for the skyrmion is required. Working in the micromagnetic framework, we derive a macrospin description for the skyrmion using the variational principle and then numerically solve for the bound states. It is shown that the existense and properties of these states as a function of skyrmion size, strongly depend on the skyrmion type. Both vortex and hedgehog skyrmions or anti-skyrmions can induce bound states with energies | E | < ΔS . For the hedgehog skyrmion case however, bound state appearance depends on the chirality. Finally, the probability densities in these states are computed and it is demonstrated that the electrons are localized throughout the skyrmion region. Also affiliated with imec, Belgium.

  20. Effective Hamiltonian for surface states of topological insulator thin films with hexagonal warping

    NASA Astrophysics Data System (ADS)

    Siu, Zhuo Bin; Tan, Seng Ghee; Jalil, Mansoor B. A.

    2016-05-01

    The effective Hamiltonian of the surface states on semi-infinite slabs of the topological insulators (TI) Bi2Te3 and Bi2Se3 require the addition of a cubic momentum hexagonal warping term on top of the usual Dirac fermion Hamiltonian in order to reproduce the experimentally measured constant energy contours at intermediate values of Fermi energy. In this work, we derive the effective Hamiltonian for the surface states of a Bi2Se3 thin film incorporating the corresponding hexagonal warping terms. We then calculate the dispersion relation of the effective Hamiltonian and show that the hexagonal warping leads distorts the equal energy contours from the circular cross sections of the Dirac cones.

  1. The surface-state of the topological insulator Bi2Se3 revealed by cyclotron resonance

    SciTech Connect

    Mcdonald, Ross D; Ayala - Valenzuela, Oscar E; Altarawneh, Moaz M; Analytis, James G

    2011-01-14

    Transport measurements of topological insulators are dominated by the conductivity of the bulk, leading to substantial difficulties in resolving the properties of the surface. To this end, we use high magnetic field, rf- and microwave-spectroscopy to selectively couple to the surface conductivity of Bi2Se3 at high frequency. In the frequency range of a few GHz we observe a crossover from quantum oscillations indicative of a small 3D Fermi surface, to cyclotron resonance indicative of a 2D surface state. By probing the conductivity at reduced skin depths, we have observed a 2D cyclotron resonance from a material whose bulk Fermi-surface is 3D. The frequency-magnetic field scaling of this resonance is inconsistent with the bulk effective mass, but more consistent with the dispersion and band filling of a Dirac-like surface state as observed by ARPES, with substantial manybody renormalization.

  2. Atomic diffusion in the surface state of Mott insulator NiS2

    NASA Astrophysics Data System (ADS)

    Clark, C.; Friedemann, S.

    2016-02-01

    We present resistivity measurements of Mott insulator NiS2 single crystals after heat treatment. We find a strong increase of the low temperature resistivity that relaxes back towards the pristine behaviour over several days with a time constant of 45 h at room temperature. The low temperature resistivity has previously been shown to be dominated by surface conduction (Thio and Bennett, 1994 [1]). Consequently, the changes induced by heat treatment are attributed to changes of surface states. Our results suggest the creation of vacancies in the surface that are refilled from the bulk via atomic diffusion. We estimate a diffusion constant of D ≈ 1 × 10-10 m2 s-1 at room temperature. We identify sulphur vacancies as the most likely to form via oxidisation of sulphur forming volatile SO2 during heat treatment. Our results point towards these sulphur vacancies to be the source of surface state localisation in NiS2.

  3. Non-magnetic compensation in ferromagnetic Ga1-xMnxAs and Ga1-xMnxP synthesized by ion implantation and pulsed-laser melting

    SciTech Connect

    Scarpulla, M.A.; Stone, P.R.; Sharp, I.D.; Haller, E.E.; Dubon, O.D.; Beeman, J.W.; Yu, K.M.

    2008-02-05

    The electronic and magnetic effects of intentional compensation with non-magnetic donors are investigated in the ferromagnetic semiconductors Ga1-xMnxAs and Ga1-xMnxP synthesized using ion implantation and pulsed-laser melting (II-PLM). It is demonstrated that compensation with non-magnetic donors and MnI have similarqualitative effects on materials properties. With compensation TC decreases, resistivity increases, and stronger magnetoresistance and anomalous Hall effect attributed to skew scattering are observed. Ga1-xMnxAs can be controllably compensated with Te through a metal-insulator transition through which the magnetic and electrical properties vary continuously. The resistivity of insulating Ga1-xMnxAs:Te can be described by thermal activation to the mobility edge and simply-activated hopping transport. Ga1-xMnxP doped with S is insulating at all compositions but shows decreasing TC with compensation. The existence of a ferromagnetic insulating state in Ga1-xMnxAs:Te and Ga1-xMnxP:S having TCs of the same order as the uncompensated materials demonstrates that localized holes are effective at mediating ferromagnetism in ferromagnetic semiconductors through the percolation of ferromagnetic 'puddles' which at low temperatures.

  4. Nature of the insulating ground state of the 5d postperovskite CaIrO3

    SciTech Connect

    Kim, Sun -Woo; Liu, Chen; Kim, Hyun -Jung; Lee, Jun -Ho; Yao, Yongxin; Ho, Kai -Ming; Cho, Jun -Hyung

    2015-08-26

    In this study, the insulating ground state of the 5d transition metal oxide CaIrO3 has been classified as a Mott-type insulator. Based on a systematic density functional theory (DFT) study with local, semilocal, and hybrid exchange-correlation functionals, we reveal that the Ir t2g states exhibit large splittings and one-dimensional electronic states along the c axis due to a tetragonal crystal field. Our hybrid DFT calculation adequately describes the antiferromagnetic (AFM) order along the c direction via a superexchange interaction between Ir4+ spins. Furthermore, the spin-orbit coupling (SOC) hybridizes the t2g states to open an insulating gap. These results indicate that CaIrO3 can be represented as a spin-orbit Slater insulator, driven by the interplay between a long-range AFM order and the SOC. Such a Slater mechanism for the gap formation is also demonstrated by the DFT + dynamical mean field theory calculation, where the metal-insulator transition and the paramagnetic to AFM phase transition are concomitant with each other.

  5. Band structure and spin texture of Bi2Se3 3 d ferromagnetic metal interface

    NASA Astrophysics Data System (ADS)

    Zhang, Jia; Velev, Julian P.; Dang, Xiaoqian; Tsymbal, Evgeny Y.

    2016-07-01

    The spin-helical surface states in a three-dimensional topological insulator (TI), such as Bi2Se3 , are predicted to have superior efficiency in converting charge current into spin polarization. This property is said to be responsible for the giant spin-orbit torques observed in ferromagnetic metal/TI structures. In this work, using first-principles and model tight-binding calculations, we investigate the interface between the topological insulator Bi2Se3 and 3 d -transition ferromagnetic metals Ni and Co. We find that the difference in the work functions of the topological insulator and the ferromagnetic metals shift the topological surface states down about 0.5 eV below the Fermi energy where the hybridization of these surface states with the metal bands destroys their helical spin structure. The band alignment of Bi2Se3 and Ni (Co) places the Fermi energy far in the conduction band of bulk Bi2Se3 , where the spin of the carriers is aligned with the magnetization in the metal. Our results indicate that the topological surface states are unlikely to be responsible for the huge spin-orbit torque effect observed experimentally in these systems.

  6. ARPES study of the surface states and their aging in a topological insulator, Bi2Se3

    NASA Astrophysics Data System (ADS)

    Maiti, Kalobaran

    Topological insulators possess time reversal symmetry protected metallic surface states over the insulating bulk, where these surface states are expected to be immune to weak disorder, chemical passivation of the surface or temperature change. However, significant discrepancy from such behavior has been found experimentally in various materials. We studied the detailed electronic structure and its aging of a topological insulator, Bi2Se3 employing high resolution photoemission spectroscopy. Both the band structure results and high resolution angle resolved photoemission data reveal significantly different surface electronic structure for different surface terminations. Furthermore, oxygen impurity on Se terminated surface exhibits an electron doping scenario, while oxygen on Bi terminated surface corresponds to a hole doping scenario. The intensity of the Dirac states reduces with aging indicating fragility of the topological order due to surface impurities.

  7. A hierarchy of ``meson'' bound state excitations in the 1D ferromagnetic Ising chain CoNb2O6

    NASA Astrophysics Data System (ADS)

    Morris, Christopher; Koopayeh, Seyed; Ghosh, Anirban; Tchernyshyov, Oleg; McQueen, Tyrel M.; Armitage, N. Peter; Valdés Aguilar, Rolando; Krizan, Jason; Cava, Robert J.

    2014-03-01

    The quantum magnet CoNb2O6 was recently demonstrated to be an excellent realization of the one-dimensional ferromagnetic Ising spin chain. Low energy spin-flip excitations in the chains were recently observed via inelastic neutron scattering.[2] The energy spectrum of these excitations was shown to have a interesting energy scaling governed by symmetries of the E8 exceptional Lie group. Here, time-domain terahertz spectroscopy (TDTS) is used to investigate these optically active spin flip excitations in CoNb2O6. A series of nine spin flip bound states is observed, whose energies can be modeled exceedingly well by the Airy function solutions to a 1D Schrödinger equation. Additionally, a novel bound state of excitations on neighboring chains is observed just below the onset of a two particle continuum. Work supported by The Institute of Quantum Matter under DOE grant DE-FG02-08ER46544 and by the Gordon and Betty Moore Foundation through Grant GBMF2628.

  8. Six-state, three-level, six-fold ferromagnetic wire system

    NASA Astrophysics Data System (ADS)

    Blachowicz, T.; Ehrmann, A.

    2013-04-01

    Six stable states at remanence were identified in iron wire samples of 6-fold spatial symmetry using micromagnetic simulations and the finite element method. Onion and domain-wall magnetic states were tailored by sample shape and guided by an applied magnetic field with a fixed in-plane direction. Different directions of externally applied magnetic fields revealed a tendency for stability or nonstability of the considered states.

  9. Protective capping of topological surface states of intrinsically insulating Bi2Te3

    NASA Astrophysics Data System (ADS)

    Hoefer, Katharina; Becker, Christoph; Wirth, Steffen; Hao Tjeng, Liu

    2015-09-01

    We have identified epitaxially grown elemental Te as a capping material that is suited to protect the topological surface states of intrinsically insulating Bi2Te3. By using angle-resolved photoemission, we were able to show that the Te overlayer leaves the dispersive bands of the surface states intact and that it does not alter the chemical potential of the Bi2Te3 thin film. From in-situ four-point contact measurements, we observed that the conductivity of the capped film is still mainly determined by the metallic surface states and that the contribution of the capping layer is minor. Moreover, the Te overlayer can be annealed away in vacuum to produce a clean Bi2Te3 surface in its pristine state even after the exposure of the capped film to air. Our findings will facilitate well-defined and reliable ex-situ experiments on the properties of Bi2Te3 surface states with nontrivial topology.

  10. Anomalous quantum Hall effect induced by disorder in topological insulators

    NASA Astrophysics Data System (ADS)

    Raymond, Laurent; Verga, Alberto D.; Demion, Arnaud

    2015-08-01

    We investigate a transition between a two-dimensional topological insulator conduction state, characterized by a conductance G =2 (in fundamental units e2/h ) and a Chern insulator with G =1 , induced by polarized magnetic impurities. Two kinds of coupling, ferromagnetic and antiferromagnetic, are considered with the electron and hole subbands. We demonstrate that for strong disorder, a phase G =1 exists even for ferromagnetic order, in contrast with the prediction of the mean field approximation. This result is supported by direct numerical computations using Landauer transport formula, and by analytical calculations of the chemical potential and mass renormalization as a function of the disorder strength, in the self-consistent Born approximation. The transition is related to the suppression of one of the spin conduction channels, for strong enough disorder, by selective spin scattering and localization.

  11. Disorder-induced Room Temperature Ferromagnetism in Glassy Chromites

    PubMed Central

    Araujo, C. Moyses; Nagar, Sandeep; Ramzan, Muhammad; Shukla, R.; Jayakumar, O. D.; Tyagi, A. K.; Liu, Yi-Sheng; Chen, Jeng-Lung; Glans, Per-Anders; Chang, Chinglin; Blomqvist, Andreas; Lizárraga, Raquel; Holmström, Erik; Belova, Lyubov; Guo, Jinghua; Ahuja, Rajeev; Rao, K. V.

    2014-01-01

    We report an unusual robust ferromagnetic order above room temperature upon amorphization of perovskite [YCrO3] in pulsed laser deposited thin films. This is contrary to the usual expected formation of a spin glass magnetic state in the resulting disordered structure. To understand the underlying physics of this phenomenon, we combine advanced spectroscopic techniques and first-principles calculations. We find that the observed order-disorder transformation is accompanied by an insulator-metal transition arising from a wide distribution of Cr-O-Cr bond angles and the consequent metallization through free carriers. Similar results also found in YbCrO3-films suggest that the observed phenomenon is more general and should, in principle, apply to a wider range of oxide systems. The ability to tailor ferromagnetic order above room temperature in oxide materials opens up many possibilities for novel technological applications of this counter intuitive effect. PMID:24732685

  12. Record Surface State Mobility and Quantum Hall Effect in Topological Insulator Thin Films via Interface Engineering.

    PubMed

    Koirala, Nikesh; Brahlek, Matthew; Salehi, Maryam; Wu, Liang; Dai, Jixia; Waugh, Justin; Nummy, Thomas; Han, Myung-Geun; Moon, Jisoo; Zhu, Yimei; Dessau, Daniel; Wu, Weida; Armitage, N Peter; Oh, Seongshik

    2015-12-01

    Material defects remain as the main bottleneck to the progress of topological insulators (TIs). In particular, efforts to achieve thin TI samples with dominant surface transport have always led to increased defects and degraded mobilities, thus making it difficult to probe the quantum regime of the topological surface states. Here, by utilizing a novel buffer layer scheme composed of an In2Se3/(Bi0.5In0.5)2Se3 heterostructure, we introduce a quantum generation of Bi2Se3 films with an order of magnitude enhanced mobilities than before. This scheme has led to the first observation of the quantum Hall effect in Bi2Se3. PMID:26583739

  13. A solid state gas density monitor for gas insulated substation equipment

    SciTech Connect

    Saaski, E.W.; Hartl, J.C.

    1982-07-01

    A solid state gas density monitor has been developed for SF/sub 6/ service in gas-insulated substation equipment that is based on principles and construction techniques similar to ionization-type fire alarms. The monitor has a resolution of .7kPa (.1 psi) at 20/sup 0/C and, depending on ion chamber voltage and geometry, exhibits a linearity of /sup +//sub -/1 to /sup +//sub -/4. kPa (/sup +//sub -/.15 to /sup +//sub -/.6 psi) at 20/sup 0/C over the density range 0.20 to 0.66 MPa (30 to 95 psia) at 20/sup 0/C. The monitor incorporates two level contact closure actuated by gas density falling below one of two pre-set alarm limits, and an LCD read-out for visual monitoring and instrument calibration. The display presents density in terms of equivalent psia at 68/sup 0/F. (20/sup 0/C.).

  14. Klein tunneling of helical edge states in narrow strips of a two-dimensional topological insulator

    NASA Astrophysics Data System (ADS)

    Takagaki, Y.

    2016-01-01

    The quantum transmission of helical edge states across a square potential barrier is numerically investigated in narrow channels of a two-dimensional topological insulator. Although the transmission probability in general decreases when a potential offset is introduced in the middle of the channels, the transmission remains almost perfect regardless of the amplitude and length of the potential offset when the hybridization energy gap is closed by tuning the off-diagonal spin-orbit terms in the effective four-band Hamiltonian. The approximate absence of scattering resembling the Klein tunneling, where the transmission is unimpeded as an electron propagates relativistically as a hole in the barrier without decay, improves further when an interference condition is satisfied within the barrier. The dependence of the residual reflection on the Fermi level reveals anomalous characteristics in the Klein tunneling regime.

  15. Record surface state mobility and quantum Hall effect in topological insulator thin films via interface engineering

    DOE PAGESBeta

    Koirala, Nikesh; Han, Myung -Geun; Brahlek, Matthew; Salehi, Maryam; Wu, Liang; Dai, Jixia; Waugh, Justin; Nummy, Thomas; Moon, Jisoo; Zhu, Yimei; et al

    2015-11-19

    Material defects remain as the main bottleneck to the progress of topological insulators (TIs). In particular, efforts to achieve thin TI samples with dominant surface transport have always led to increased defects and degraded mobilities, thus making it difficult to probe the quantum regime of the topological surface states. Here, by utilizing a novel buffer layer scheme composed of an In2Se3/(Bi0.5In0.5)2Se3 heterostructure, we introduce a quantum generation of Bi2Se3 films with an order of magnitude enhanced mobilities than before. Furthermore, this scheme has led to the first observation of the quantum Hall effect in Bi2Se3.

  16. Record surface state mobility and quantum Hall effect in topological insulator thin films via interface engineering

    SciTech Connect

    Koirala, Nikesh; Han, Myung -Geun; Brahlek, Matthew; Salehi, Maryam; Wu, Liang; Dai, Jixia; Waugh, Justin; Nummy, Thomas; Moon, Jisoo; Zhu, Yimei; Dessau, Daniel; Wu, Weida; Armitage, N. Peter; Oh, Seongshik

    2015-11-19

    Material defects remain as the main bottleneck to the progress of topological insulators (TIs). In particular, efforts to achieve thin TI samples with dominant surface transport have always led to increased defects and degraded mobilities, thus making it difficult to probe the quantum regime of the topological surface states. Here, by utilizing a novel buffer layer scheme composed of an In2Se3/(Bi0.5In0.5)2Se3 heterostructure, we introduce a quantum generation of Bi2Se3 films with an order of magnitude enhanced mobilities than before. Furthermore, this scheme has led to the first observation of the quantum Hall effect in Bi2Se3.

  17. Interfacial thermal conductance across metal-insulator/semiconductor interfaces due to surface states

    NASA Astrophysics Data System (ADS)

    Lu, Tingyu; Zhou, Jun; Nakayama, Tsuneyoshi; Yang, Ronggui; Li, Baowen

    2016-02-01

    We point out that the effective channel for the interfacial thermal conductance, the inverse of Kapitza resistance, of metal-insulator/semiconductor interfaces is governed by the electron-phonon interaction mediated by the surface states allowed in a thin region near the interface. Our detailed calculations demonstrate that the interfacial thermal conductance across Pb/Pt/Al/Au-diamond interfaces are only slightly different among these metals, and reproduce well the experimental results of the interfacial thermal conductance across metal-diamond interfaces observed by Stoner et al. [Phys. Rev. Lett. 68, 1563 (1992), 10.1103/PhysRevLett.68.1563] and most recently by Hohensee et al. [Nat. Commun. 6, 6578 (2015), 10.1038/ncomms7578].

  18. Instability of insulating states in optical lattices due to collective phonon excitations

    NASA Astrophysics Data System (ADS)

    Yukalov, V. I.; Ziegler, K.

    2015-02-01

    The effect of collective phonon excitations on the properties of cold atoms in optical lattices is investigated. These phonon excitations are collective excitations, whose appearance is caused by intersite atomic interactions correlating the atoms, and they do not arise without such interactions. These collective excitations should not be confused with lattice vibrations produced by an external force. No such force is assumed. But the considered phonons are purely self-organized collective excitations, characterizing atomic oscillations around lattice sites, due to intersite atomic interactions. It is shown that these excitations can essentially influence the possibility of atoms' being localized. The states that would be insulating in the absence of phonon excitations can become delocalized when these excitations are taken into account. This concerns long-range as well as local atomic interactions. To characterize the region of stability, the Lindemann criterion is used.

  19. Stationary states of extended nonlinear Schrödinger equation with a source

    NASA Astrophysics Data System (ADS)

    Borich, M. A.; Smagin, V. V.; Tankeev, A. P.

    2007-02-01

    Structure of nonlinear stationary states of the extended nonlinear Schrödinger equation (ENSE) with a source has been analyzed with allowance for both third-order and nonlinearity dispersion. A new class of particular solutions (solitary waves) of the ENSe has been obtained. The scenario of the destruction of these states under the effect of an external perturbation has been investigated analytically and numerically. The results obtained can be used to interpret experimental data on the weakly nonlinear dynamics of the magnetostatic envelope in heterophase ferromagnet-insulator-metal, metal-insulator-ferromagnet-insulator-metal, and other similar structures and upon the simulation of nonlinear processes in optical systems.

  20. A General Theorem Relating the Bulk Topological Number to Edge States in Two-dimensional Insulators

    SciTech Connect

    Qi, Xiao-Liang; Wu, Yong-Shi; Zhang, Shou-Cheng; /Stanford U., Phys. Dept. /Tsinghua U., Beijing

    2010-01-15

    We prove a general theorem on the relation between the bulk topological quantum number and the edge states in two dimensional insulators. It is shown that whenever there is a topological order in bulk, characterized by a non-vanishing Chern number, even if it is defined for a non-conserved quantity such as spin in the case of the spin Hall effect, one can always infer the existence of gapless edge states under certain twisted boundary conditions that allow tunneling between edges. This relation is robust against disorder and interactions, and it provides a unified topological classification of both the quantum (charge) Hall effect and the quantum spin Hall effect. In addition, it reconciles the apparent conflict between the stability of bulk topological order and the instability of gapless edge states in systems with open boundaries (as known happening in the spin Hall case). The consequences of time reversal invariance for bulk topological order and edge state dynamics are further studied in the present framework.

  1. Plasma-Wave Terahertz Detection Mediated by Topological Insulators Surface States.

    PubMed

    Viti, Leonardo; Coquillat, Dominique; Politano, Antonio; Kokh, Konstantin A; Aliev, Ziya S; Babanly, Mahammad B; Tereshchenko, Oleg E; Knap, Wojciech; Chulkov, Evgueni V; Vitiello, Miriam S

    2016-01-13

    Topological insulators (TIs) represent a novel quantum state of matter, characterized by edge or surface-states, showing up on the topological character of the bulk wave functions. Allowing electrons to move along their surface, but not through their inside, they emerged as an intriguing material platform for the exploration of exotic physical phenomena, somehow resembling the graphene Dirac-cone physics, as well as for exciting applications in optoelectronics, spintronics, nanoscience, low-power electronics, and quantum computing. Investigation of topological surface states (TSS) is conventionally hindered by the fact that in most of experimental conditions the TSS properties are mixed up with those of bulk-states. Here, we activate, probe, and exploit the collective electronic excitation of TSS in the Dirac cone. By engineering Bi2Te(3-x)Sex stoichiometry, and by gating the surface of nanoscale field-effect-transistors, exploiting thin flakes of Bi2Te2.2Se0.8 or Bi2Se3, we provide the first demonstration of room-temperature terahertz (THz) detection mediated by overdamped plasma-wave oscillations on the "activated" TSS of a Bi2Te2.2Se0.8 flake. The reported detection performances allow a realistic exploitation of TSS for large-area, fast imaging, promising superb impacts on THz photonics. PMID:26678677

  2. Generic helical edge states due to Rashba spin-orbit coupling in a topological insulator

    NASA Astrophysics Data System (ADS)

    Ortiz, Laura; Molina, Rafael A.; Platero, Gloria; Lunde, Anders Mathias

    2016-05-01

    We study the helical edge states of a two-dimensional topological insulator without axial spin symmetry due to the Rashba spin-orbit interaction. Lack of axial spin symmetry can lead to so-called generic helical edge states, which have energy-dependent spin orientation. This opens the possibility of inelastic backscattering and thereby nonquantized transport. Here we find analytically the new dispersion relations and the energy dependent spin orientation of the generic helical edge states in the presence of Rashba spin-orbit coupling within the Bernevig-Hughes-Zhang model, for both a single isolated edge and for a finite width ribbon. In the single-edge case, we analytically quantify the energy dependence of the spin orientation, which turns out to be weak for a realistic HgTe quantum well. Nevertheless, finite size effects combined with Rashba spin-orbit coupling result in two avoided crossings in the energy dispersions, where the spin orientation variation of the edge states is very significantly increased for realistic parameters. Finally, our analytical results are found to compare well to a numerical tight-binding regularization of the model.

  3. Nanoscale manipulation of the Mott insulating state coupled to charge order in 1T-TaS2

    NASA Astrophysics Data System (ADS)

    Cho, Doohee; Cheon, Sangmo; Kim, Ki-Seok; Lee, Sung-Hoon; Cho, Yong-Heum; Cheong, Sang-Wook; Yeom, Han Woong

    2016-01-01

    The controllability over strongly correlated electronic states promises unique electronic devices. A recent example is an optically induced ultrafast switching device based on the transition between the correlated Mott insulating state and a metallic state of a transition metal dichalcogenide 1T-TaS2. However, the electronic switching has been challenging and the nature of the transition has been veiled. Here we demonstrate the nanoscale electronic manipulation of the Mott state of 1T-TaS2. The voltage pulse from a scanning tunnelling microscope switches the insulating phase locally into a metallic phase with irregularly textured domain walls in the charge density wave order inherent to this Mott state. The metallic state is revealed as a correlated phase, which is induced by the moderate reduction of electron correlation due to the charge density wave decoherence.

  4. Nanoscale manipulation of the Mott insulating state coupled to charge order in 1T-TaS2

    PubMed Central

    Cho, Doohee; Cheon, Sangmo; Kim, Ki-Seok; Lee, Sung-Hoon; Cho, Yong-Heum; Cheong, Sang-Wook; Yeom, Han Woong

    2016-01-01

    The controllability over strongly correlated electronic states promises unique electronic devices. A recent example is an optically induced ultrafast switching device based on the transition between the correlated Mott insulating state and a metallic state of a transition metal dichalcogenide 1T-TaS2. However, the electronic switching has been challenging and the nature of the transition has been veiled. Here we demonstrate the nanoscale electronic manipulation of the Mott state of 1T-TaS2. The voltage pulse from a scanning tunnelling microscope switches the insulating phase locally into a metallic phase with irregularly textured domain walls in the charge density wave order inherent to this Mott state. The metallic state is revealed as a correlated phase, which is induced by the moderate reduction of electron correlation due to the charge density wave decoherence. PMID:26795073

  5. Superconductor-ferromagnet bilayer under external drive: The role of vortex-antivortex matter

    NASA Astrophysics Data System (ADS)

    Frota, D. A.; Chaves, A.; Ferreira, W. P.; Farias, G. A.; Milošević, M. V.

    2016-03-01

    Using advanced Ginzburg-Landau simulations, we study the superconducting state of a thin superconducting film under a ferromagnetic layer, separated by an insulating oxide, in applied external magnetic field and electric current. The taken uniaxial ferromagnet is organized into a series of parallel domains with alternating polarization of out-of-plane magnetization, sufficiently strong to induce vortex-antivortex pairs in the underlying superconductor in absence of other magnetic field. We show the organization of such vortex-antivortex matter into rich configurations, some of which are not matching the periodicity of the ferromagnetic film. The variety of possible configurations is enhanced by applied homogeneous magnetic field, where additional vortices in the superconductor may lower the energy of the system by either annihilating the present antivortices under negative ferromagnetic domains or by lowering their own energy after positioning under positive ferromagnetic domains. As a consequence, both the vortex-antivortex reordering in increasing external field and the evolution of the energy of the system are highly nontrivial. Finally, we reveal the very interesting effects of applied dc electric current on the vortex-antivortex configurations, since resulting Lorentzian force has opposite direction for vortices and antivortices, while direction of the applied current with respect to ferromagnetic domains is of crucial importance for the interaction of the applied and the Meissner current, as well as the consequent vortex-antivortex dynamics—both of which are reflected in the anisotropic critical current of the system.

  6. In-situ XMCD evaluation of ferromagnetic state at FeRh thin film surface induced by 1 keV Ar ion beam irradiation and annealing

    NASA Astrophysics Data System (ADS)

    Matsui, T.; Aikoh, K.; Sakamaki, M.; Amemiya, K.; Iwase, A.

    2015-12-01

    Surface ferromagnetic state of FeRh thin films irradiated with 1 keV Ar ion-beam has been investigated by using soft X-ray Magnetic Circular Dichroism (XMCD). It was revealed that the Fe atoms of the samples were strongly spin-polarized after Ar ion-beam irradiation. Due to its small penetration depth, 1 keV Ar ion-beam irradiation can modify the magnetic state at subsurface of the samples. In accordance with the XMCD sum rule analysis, the main component of the irradiation induced ferromagnetism at the FeRh film surface was to be effective spin magnetic moment, and not to be orbital moment. We also confirmed that the surface ferromagnetic state could be produced by thermal annealing of the excessively ion irradiated paramagnetic subsurface of the FeRh thin films. This novel magnetic modification technique by using ion irradiation and subsequent annealing can be a potential tool to control the surface magnetic state of FeRh thin films.

  7. Interaction between the intrinsic edge state and the helical boundary state of topological insulator phase in bilayer graphene

    NASA Astrophysics Data System (ADS)

    Lü, Xiaoling; Jiang, Liwei; Zheng, Yisong

    2016-04-01

    Graphene has intrinsic edge states localized at zigzag edge or lattice defect. Helical boundary states can also be established in such a two-dimensional carbon material at the boundary of topological insulator (TI) phase realized by the extrinsic Rashba spin-orbital coupling (SOC) in gated bilayer graphene. We theoretically investigate the interaction between these two kinds of edge (boundary) states when they coexist in a bilayer graphene. We find that this interaction gives rise to some very interesting results. In a zigzag edged nanoribbon of bilayer graphene, it is possible that the TI helical state does not localize at the TI phase boundary. Instead it moves to the nanoribbon edge even though the SOC is absent therein. In a bulk lattice of bilayer graphene embedded with two line defects, the numbers of helical state subbands at the two line defects are not equal to each other. In such a case, the backscattering lacking is still forbidden since the Kramers pairs are valley polarized.

  8. Interplay between Mn-acceptor state and Dirac surface states in Mn-doped Bi2Se3 topological insulator

    NASA Astrophysics Data System (ADS)

    Mahani, M. R.; Pertsova, A.; Islam, M. Fhokrul; Canali, C. M.

    2014-11-01

    We investigate the properties of a single substitutional Mn impurity and its associated acceptor state on the (111) surface of Bi2Se3 topological insulator. Combining ab initio calculations with microscopic tight-binding modeling, we identify the effects of inversion symmetry and time-reversal-symmetry breaking on the electronic states in the vicinity of the Dirac point. In agreement with experiments, we find evidence that the Mn ion is in the +2 valence state and introduces an acceptor in the bulk band gap. The Mn acceptor has predominantly p character and is localized mainly around the Mn impurity and its nearest-neighbor Se atoms. Its electronic structure and spin-polarization are determined by the hybridization between the Mn d levels and the p levels of surrounding Se atoms, which is strongly affected by electronic correlations at the Mn site. The opening of the gap at the Dirac point depends crucially on the quasiresonant coupling and the strong real-space overlap between the spin-chiral surface states and the midgap spin-polarized Mn-acceptor states.

  9. Decoupling of the antiferromagnetic and insulating states in Tb-doped Sr2IrO4

    DOE PAGESBeta

    Wang, J. C.; Aswartham, S.; Ye, Feng; Terzic, J.; Zheng, H.; Haskel, Daniel; Chikara, Shalinee; Choi, Yong; Schlottmann, P.; Custelcean, Radu; et al

    2015-12-08

    Sr2IrO4 is a spin-orbit coupled insulator with an antiferromagnetic (AFM) transition at TN = 240 K. We report results of a comprehensive study of single-crystal Sr2Ir1-xTbxO4 (0≤x≤0.03). This study found that mere 3% (x=0.03) tetravalent Tb4+(4f7) substituting for Ir4+ (rather than Sr2+) completely suppresses the long-range collinear AFM transition but retains the insulating state, leading to a phase diagram featuring a decoupling of magnetic interactions and charge gap. The insulating state at x = 0.03 is characterized by an unusually large specific heat at low temperatures and an incommensurate magnetic state having magnetic peaks at (0.95, 0, 0) and (0,more » 0.95, 0) in the neutron diffraction, suggesting a spiral or spin density wave order. It is apparent that Tb doping effectively changes the relative strength of the SOI and the tetragonal CEF and enhances the Hund’s rule coupling that competes with the SOI, and destabilizes the AFM state. However, the disappearance of the AFM accompanies no metallic state chiefly because an energy level mismatch for the Ir and Tb sites weakens charge carrier hopping and renders a persistent insulating state. Furthermore, this work highlights an unconventional correlation between the AFM and insulating states in which the magnetic transition plays no critical role in the formation of the charge gap in the iridate.« less

  10. Decoupling of the antiferromagnetic and insulating states in Tb-doped S r2Ir O4

    NASA Astrophysics Data System (ADS)

    Wang, J. C.; Aswartham, S.; Ye, Feng; Terzic, J.; Zheng, H.; Haskel, Daniel; Chikara, Shalinee; Choi, Yong; Schlottmann, P.; Custelcean, Radu; Yuan, S. J.; Cao, G.

    2015-12-01

    S r2Ir O4 is a spin-orbit-coupled insulator with an antiferromagnetic (AFM) transition at TN=240 K . We report results of a comprehensive study of single-crystal S r2I r1 -xT bxO4(0 ≤x ≤0.03 ) . This study found that a mere 3% (x =0.03 ) of tetravalent T b4 +(4 f7 ) substituting for I r4 + (rather than S r2 + ) completely suppresses the long-range collinear AFM transition but retains the insulating state, leading to a phase diagram featuring a decoupling of the magnetic interactions and charge gap. The insulating state at x =0.03 is characterized by an unusually large specific heat at low temperatures and an incommensurate magnetic state having magnetic peaks at (0.95,0,0) and (0,0.95,0) in the neutron diffraction, suggesting a spiral or spin-density-wave order. It is apparent that Tb doping effectively changes the relative strength of the spin-orbit interaction (SOI) and the tetragonal crystal electric field and enhances the Hund's rule coupling that competes with the SOI, and destabilizes the AFM state. However, the disappearance of the AFM is accompanied by no metallic state chiefly because an energy level mismatch for the Ir and Tb sites weakens charge carrier hopping and causes a persistent insulating state. This work highlights an unconventional correlation between the AFM and insulating states in which the magnetic transition plays no critical role in the formation of the charge gap in the iridate.

  11. Thermoelectric spin transport through ferromagnetic heterostructures

    NASA Astrophysics Data System (ADS)

    Bender, Scott A.

    2015-10-01

    We study how spin is transferred by ferromagnetic dynamics in a charge insulator in response to a thermoelectric bias, which is established by supplying heat and/or spin accumulation via normal leads. At zero temperature, magnetic anisotropies pin the macroscopic order, which necessitates a finite threshold bias to induce a spin current in a steady state of unpinned dynamics. At finite temperatures, however, thermal spin waves provide a spin transport channel in response to a linear thermoelectric bias. The theoretical description is rooted in the Landau-Lifshitz-Gilbert phenomenology both for the macroscopic dynamics of the magnetic order and quantum kinetics of thermal magnons. In this paper we connect the classical and quantum aspects of the underlying magnetic dynamics and spin transport, as well as provide a unified view of the exchange mediated bias of spin See-beck physics of the magnetic interface and bulk.

  12. State-of-the-Art Highly Insulating Window Frames - Research and Market Review

    SciTech Connect

    Gustavsen, Arild; Jelle, Bjorn Petter; Arasteh, Dariush; Kohler, Christian

    2007-01-01

    This document reports the findings of a market and research review related to state-of-the-art highly insulating window frames. The market review focuses on window frames that satisfy the Passivhaus requirements (window U-value less or equal to 0.8 W/m{sup 2}K ), while other examples are also given in order to show the variety of materials and solutions that may be used for constructing window frames with a low thermal transmittance (U-value). The market search shows that several combinations of materials are used in order to obtain window frames with a low U-value. The most common insulating material seems to be Polyurethane (PUR), which is used together with most of the common structural materials such as wood, aluminum, and PVC. The frame research review also shows examples of window frames developed in order to increase the energy efficiency of the frames and the glazings which the frames are to be used together with. The authors find that two main tracks are used in searching for better solutions. The first one is to minimize the heat losses through the frame itself. The result is that conductive materials are replaced by highly thermal insulating materials and air cavities. The other option is to reduce the window frame area to a minimum, which is done by focusing on the net energy gain by the entire window (frame, spacer and glazing). Literature shows that a window with a higher U-value may give a net energy gain to a building that is higher than a window with a smaller U-value. The net energy gain is calculated by subtracting the transmission losses through the window from the solar energy passing through the windows. The net energy gain depends on frame versus glazing area, solar factor, solar irradiance, calculation period and U-value. The frame research review also discusses heat transfer modeling issues related to window frames. Thermal performance increasing measures, surface modeling, and frame cavity modeling are among the topics discussed. The

  13. Topological insulator nanostructures and devices

    NASA Astrophysics Data System (ADS)

    Alegria, Loren D.

    We present a series of experiments developing the compounds Bi2 Te3, Bi2Se3, and Bi2Te 2Se for nanoscale device applications. New metal organic chemical vapor deposition techniques are employed to make high quality mesoscopic samples, focusing on the growth of nanowires and nanotubes with controlled structure and composition. Fundamental properties of nanowires are studied via transmission electron microscopy and magnetotransport experiments at low temperatures. We describe a method for promoting the self-assembly of pristine nanotubes of Bi2Te3, which have not been observed before. Finally, we demonstrate a method of introducing ferromagnetism precisely at the Bi 2Te3 surface by developing the epitaxy of Bi2Te 3 on the ferromagnetic insulator Cr2Ge2Te 6 and we study the hall effect in these new heterostructures at low temperature. Our results are promising for the development of advanced thermoelectric, optoelectronic, or magnetoresistive devices based on the unique properties of these materials, as well as for the realization of new states of matter, such as the quantum anomalous hall state and Majorana fermion states in heavy element nanowires.

  14. Nature of the Insulating Ground State of the Two-Dimensional Sn Atom Lattice on SiC(0001).

    PubMed

    Yi, Seho; Lee, Hunpyo; Choi, Jin-Ho; Cho, Jun-Hyung

    2016-01-01

    Semiconductor surfaces with narrow surface bands provide unique playgrounds to search for Mott-insulating state. Recently, a combined experimental and theoretical study of the two-dimensional (2D) Sn atom lattice on a wide-gap SiC(0001) substrate proposed a Mott-type insulator driven by strong on-site Coulomb repulsion U within a single-band Hubbard model. However, our systematic density-functional theory (DFT) study with local, semilocal, and hybrid exchange-correlation functionals shows that the Sn dangling-bond state largely hybridizes with the substrate Si 3p and C 2p states to split into three surface bands due to the crystal field. Such a hybridization gives rise to the stabilization of the antiferromagnetic order via superexchange interactions. The band gap and the density of states predicted by the hybrid DFT calculation agree well with photoemission data. Our findings not only suggest that the Sn/SiC(0001) system can be represented as a Slater-type insulator driven by long-range magnetism, but also have an implication that taking into account long-range interactions beyond the on-site interaction would be of importance for properly describing the insulating nature of Sn/SiC(0001). PMID:27465057

  15. Experimental Realizations of Magnetic Topological Insulator and Topological Crystalline Insulator

    NASA Astrophysics Data System (ADS)

    Xu, Suyang

    2013-03-01

    Over the past few years the experimental research on three-dimensional topological insulators have emerged as one of the most rapidly developing fields in condensed matter physics. In this talk, we report on two new developments in the field: The first part is on the dynamic interplay between ferromagnetism and the Z2 topological insulator state (leading to a magnetic topological insulator). We present our spin-resolved photoemission and magnetic dichroic experiments on MBE grown films where a hedgehog-like spin texture is revealed on the magnetically ordered surface of Mn-Bi2Se3 revealing a Berry's phase gradient in energy-momentum space of the crystal. A chemically/electrically tunable Berry's phase switch is further demonstrated via the tuning of the spin groundstate in Mn-Bi2Se3 revealed in our data (Nature Physics 8, 616 (2012)). The second part of this talk describes our experimental observation of a new topological phase of matter, namely a topological crystalline insulator where space group symmetries replace the role of time-reversal symmetry in an otherwise Z2 topological insulator predicted in theory. We experimentally investigate the possibility of a mirror symmetry protected topological phase transition in the Pb1-xSnxTe alloy system, which has long been known to contain an even number of band inversions based on band theory. Our experimental results show that at a composition below the theoretically predicted band inversion, the system is fully gapped, whereas in the band-inverted regime, the surface exhibits even number of spin-polarized Dirac cone states revealing mirror-protected topological order (Nature Communications 3, 1192 (2012)) distinct from that observed in Z2 topological insulators. We discuss future experimental possibilities opened up by these new developments in topological insulators research. This work is in collaboration with M. Neupane, C. Liu, N. Alidoust, I. Belopolski, D. Qian, D.M. Zhang, A. Richardella, A. Marcinkova, Q

  16. Superconductivity in the surface states of a Bi2X3 topological insulator: effects of a realistic model

    NASA Astrophysics Data System (ADS)

    Hao, Lei; Wang, Jun

    2015-06-01

    Superconductivity in the topological surface states is essential to both the surface spectrum of bulk superconducting state and the proximity-induced superconductivity of \\text{B}{{\\text{i}}2}{{\\text{X}}3} (X is Se or Te) topological insulators. While previous theories were mostly based on simplified models for the bulk topological insulator and the surface states, the accumulating experiments stimulate us to make an analysis using realistic model for the normal state electronic structures, incorporating terms responsible for particle-hole asymmetry and hexagonal warping. An effective low-energy model for the topological surface states is derived first. Then we identify all the bulk time-reversal-invariant superconducting pairings in the topological insulator that can open a gap in the topological surface states. Many more pairings are found to be able to gap the topological surface states as compared to conclusions based on simplified models. The number of proximity-induced pairing channels in the topological surface states increases by one as a result of the hexagonal warping term, but is not changed by the particle-hole asymmetry term.

  17. Sign reversal of the MN-O bond compressibility in La{sub 1.2}Sr{sub 1.8}Mn{sub 2}O{sub 7} below T{sub C}: Exchange striction in the ferromagnetic state

    SciTech Connect

    Argyriou, D.N.; Mitchell, J.F.; Chmaissem, O.; Short, S.; Jorgensen, J.D.; Goodenough, J.B.

    1997-03-01

    The crystal structure of the layered perovskite La{sub 1.2}Sr{sub 1.8}Mn{sub 2}O{sub 7} has been studied under hydrostatic pressure up to {approximately} 6 kbar, in the paramagnetic and ferromagnetic states, with neutron powder diffraction. The compressibility of the Mn-O apical bonds in the double layer of MnO{sub 6} octahedra changes sign from the paramagnetic insulator (PI) to the ferromagnetic metal (FM) state; in the Fm state the Mn-O-Mn linkage between MnO{sub 2} planes expands under applied pressure, whereas they contract in the PI state. This counterintuative behavior is interpreted in terms of exchange striction, which reflect the competition between super- and double-exchange. An increase of the Mn-moment with applied pressure in the FM state is consistent with a positive dT{sub C}/dP, as well as a cant angle {theta}{sub 0} between the magnetizations of neighboring MnO{sub 2} sheets that decreases with pressure.

  18. Sign Reversal of the MN-O Bond Compressibility in La{sub 1.2}Sr{sub 1.8}Mn{sub 2}O{sub 7} Below T{sub C}: Exchange Striction in the Ferromagnetic State

    DOE R&D Accomplishments Database

    Argyriou, D. N.; Mitchell, J. F.; Chmaissem, O.; Short, S.; Jorgensen, J. D.; Goodenough, J. B.

    1997-03-01

    The crystal structure of the layered perovskite La{sub 1.2}Sr{sub 1.8}Mn{sub 2}O{sub 7} has been studied under hydrostatic pressure up to {approximately} 6 kbar, in the paramagnetic and ferromagnetic states, with neutron powder diffraction. The compressibility of the Mn-O apical bonds in the double layer of MnO{sub 6} octahedra changes sign from the paramagnetic insulator (PI) to the ferromagnetic metal (FM) state; in the Fm state the Mn-O-Mn linkage between MnO{sub 2} planes expands under applied pressure, whereas they contract in the PI state. This counterintuative behavior is interpreted in terms of exchange striction, which reflect the competition between super- and double-exchange. An increase of the Mn-moment with applied pressure in the FM state is consistent with a positive dT{sub C}/dP, as well as a cant angle {theta}{sub 0} between the magnetizations of neighboring MnO{sub 2} sheets that decreases with pressure.

  19. Thermodynamic-state and kinetic-process dependent dual ferromagnetic states in high-Si content FeMn(PSi) alloys

    SciTech Connect

    Li, Guijiang; Eriksson, Olle; Johansson, Börje; Vitos, Levente

    2015-12-07

    We have found that thermodynamic state and kinetic process co-determine the dual ferromagnetic (FM) orders in high-Si content FeMnP{sub 1−x}Si{sub x} (0.25 < x < 0.5). Alloys undergoing high temperature annealing and quenching process prefer a high magnetic moment FM state in a chemically partial disordered structure with low c/a ratio. This mechanism is suggested to be responsible for the often discussed virgin effect as well. A chemically ordered structure obtained by a slow cooling process from a relatively low annealing temperature and the increase in Si content stabilize a metastable lattice with high c/a ratio and FM order with low magnetic moment. The non-simultaneity of the magnetic and structural transitions can be responsible for the occurrence of FM state in the high c/a range. Thus, a c/a ratio that changes from high to low is physically plausible to stabilize the metastable FM order at low temperature. Our theoretical observations indicate that suitable thermodynamic state and kinetic diffusion process is crucial for optimizing magnetocaloric properties and exploring feasible magnetocaloric materials.

  20. Ground States in Thin Ferromagnetic Nanorings with Four-Fold In-Plane Anisotropy

    NASA Astrophysics Data System (ADS)

    Chaves-O'Flynn, Gabriel; Muratov, Cyrill

    2014-03-01

    We present results of micromagnetic simulations based on the optimal grid algorithm for the study of metastable states in thin nano-rings with four-fold anisotropy. Previous work have demonstrated a rich energy landscape for these structures resulting from competition between shape and crystalline anisotropies. We present a quasi-1D framework for the analysis of nanorings. First, we calculate the energy of domain walls of different windings for magnetic strips oriented at different angles with respect to the easy anisotropy axes. We consider the dependence of wall energy on material parameters. With these numbers we build a reduced-model for the micromagnetic energy on the rings which allows to treat the micromagnetic energy minimization as a combinatorial problem: the walls in the ring are treated as separate entities each with an intrinsic energy calculated from the strip case and interacting with each other via dipole-dipole interactions. A comparison with the phase diagram for ground states provides information on the limits of validity of this simplified model.

  1. Ferromagnetic Microswimmers

    NASA Astrophysics Data System (ADS)

    Ogrin, Feodor Y.; Petrov, Peter G.; Winlove, C. Peter

    2008-05-01

    We propose a model for a novel artificial low Reynolds number swimmer, based on the magnetic interactions of a pair of ferromagnetic particles: one with hard and the other with soft magnetic properties, connected by a linear spring. Using a computational model, we analyze the behavior of the system and demonstrate that for realistic values of the parameters involved, the swimmer is capable of self-propelling with average speeds of the order of hundreds of micrometers per second.

  2. Ferromagnetic microswimmers.

    PubMed

    Ogrin, Feodor Y; Petrov, Peter G; Winlove, C Peter

    2008-05-30

    We propose a model for a novel artificial low Reynolds number swimmer, based on the magnetic interactions of a pair of ferromagnetic particles: one with hard and the other with soft magnetic properties, connected by a linear spring. Using a computational model, we analyze the behavior of the system and demonstrate that for realistic values of the parameters involved, the swimmer is capable of self-propelling with average speeds of the order of hundreds of micrometers per second. PMID:18518640

  3. Evolution of ferromagnetic and non-Fermi-liquid states with doping: The case of Ru-doped UCoGe

    NASA Astrophysics Data System (ADS)

    Vališka, Michal; Pospíšil, Jiří; Diviš, Martin; Prokleška, Jan; Sechovský, Vladimír; Abd-Elmeguid, Mohsen M.

    2015-07-01

    We have investigated the impact of Ru substitution for Co on the behavior of the ferromagnetic superconductor UCoGe by performing x-ray diffraction, magnetization, specific heat, and electrical resistivity measurements on polycrystalline samples of the UCo1 -xRuxGe series (0 ≥x ≤0.9 ) . The initial Ru substitution up to x ≈0.1 leads to a simultaneous sharp increase of the Curie temperature and spontaneous magnetization up to maximum values of TC=8.6 K and MS=0.1 μB per formula unit, respectively, whereas superconductivity vanishes already for x ≈0.03 . Further increase of the Ru content beyond x ≈0.1 leads to a precipitous decrease of both TC and MS towards a ferromagnetic quantum critical point (QCP) at xcr=0.31 . Consequently, the T -x magnetic phase diagram consists of a well-developed ferromagnetic dome. We discuss the evolution of ferromagnetism with x on the basis of band structure changes due to varying 5 f -ligand hybridization. This scenario is supported by the results of electronic structure calculations and consideration of the simplified periodic Anderson model. The analysis of the temperature dependencies of the electrical resistivity and heat capacity at low temperatures of the samples in the vicinity of the QCP reveals a non-Fermi-liquid behavior and assigns the ferromagnetic quantum phase transition to be most likely of a continuous Hertz-Millis type.

  4. RKKY interaction in P-N junction based on surface states of 3D topological insulator

    NASA Astrophysics Data System (ADS)

    Zhang, Shuhui; Yang, Wen; Chang, Kai

    The RKKY interaction mediated by conduction electrons supplies a mechanism to realize the long-range coupling of localized spins which is desired for the spin devices. Here, we examine the controllability of RKKY interaction in P-N junction (PNJ) based on surface states of 3D topological insulator (3DTI). In this study, through quantum way but not usual classical analogy to light propagation, the intuitive picture for electron waves across the interface of PNJ is obtained, e.g., Klein tunneling, negative refraction and focusing. Moreover, we perform the numerical calculations for all kinds of RKKY interaction including the Heisenberg, Ising, and Dzyaloshinskii-Moriya terms. We find the focusing of surface states leads to the local augmentation of RKKY interaction. Most importantly, a dimension transition occurs, i.e., the decay rate of RKKY interaction from the deserved 1/R 2 to 1/ R . In addition, the quadratic gate-dependence of RKKY interaction is also beneficial to the application of 3DTI PNJ in the fields of spintronics and quantum computation. This work was supported by the MOST (Grant No. 2015CB921503, and No. 2014CB848700) and NSFC (Grant No. 11434010, No. 11274036, No. 11322542, and No. 11504018).

  5. Manifestation of a Second Dirac Surface State and Bulk Bands in THz Radiation from Topological Insulators

    PubMed Central

    Tu, Chien-Ming; Yeh, Tien-Tien; Tzeng, Wen-Yen; Chen, Yi-Ru; Chen, Hsueh-Ju; Ku, Shin-An; Luo, Chih-Wei; Lin, Jiunn-Yuan; Wu, Kaung-Hsiung; Juang, Jenh-Yih; Kobayashi, Takayoshi; Cheng, Cheng-Maw; Tsuei, Ku-Ding; Berger, Helmuth; Sankar, Raman; Chou, Fang-Cheng

    2015-01-01

    Topological insulators (TIs) are interesting quantum matters that have a narrow bandgap for bulk and a Dirac-cone-like conducting surface state (SS). The recent discovered second Dirac surface state (SS) and bulk bands (BBs) located ~1.5 eV above the first SS are important for optical coupling in TIs. Here, we report on the time-domain measurements of THz radiation generated from TIs n-type Cu0.02Bi2Se3 and p-type Bi2Te3 single crystals by ultrafast optical pulse excitation. The observed polarity-reversal of the THz pulse originated from transient current is unusual, and cannot be reconciled with the photo-Dember effect. The second SS and BBs are found to be indispensable for the explanation of the unusual phenomenon. Thanks to the existence of the second SS and BBs, TIs manifest an effective wide band gap in THz generation. The present study demonstrates that time-domain THz spectroscopy provide rich information of the optical coupling and the electronic structure of TIs. PMID:26370337

  6. Manifestation of a Second Dirac Surface State and Bulk Bands in THz Radiation from Topological Insulators

    NASA Astrophysics Data System (ADS)

    Tu, Chien-Ming; Yeh, Tien-Tien; Tzeng, Wen-Yen; Chen, Yi-Ru; Chen, Hsueh-Ju; Ku, Shin-An; Luo, Chih-Wei; Lin, Jiunn-Yuan; Wu, Kaung-Hsiung; Juang, Jenh-Yih; Kobayashi, Takayoshi; Cheng, Cheng-Maw; Tsuei, Ku-Ding; Berger, Helmuth; Sankar, Raman; Chou, Fang-Cheng

    2015-09-01

    Topological insulators (TIs) are interesting quantum matters that have a narrow bandgap for bulk and a Dirac-cone-like conducting surface state (SS). The recent discovered second Dirac surface state (SS) and bulk bands (BBs) located ~1.5 eV above the first SS are important for optical coupling in TIs. Here, we report on the time-domain measurements of THz radiation generated from TIs n-type Cu0.02Bi2Se3 and p-type Bi2Te3 single crystals by ultrafast optical pulse excitation. The observed polarity-reversal of the THz pulse originated from transient current is unusual, and cannot be reconciled with the photo-Dember effect. The second SS and BBs are found to be indispensable for the explanation of the unusual phenomenon. Thanks to the existence of the second SS and BBs, TIs manifest an effective wide band gap in THz generation. The present study demonstrates that time-domain THz spectroscopy provide rich information of the optical coupling and the electronic structure of TIs.

  7. Manifestation of a Second Dirac Surface State and Bulk Bands in THz Radiation from Topological Insulators.

    PubMed

    Tu, Chien-Ming; Yeh, Tien-Tien; Tzeng, Wen-Yen; Chen, Yi-Ru; Chen, Hsueh-Ju; Ku, Shin-An; Luo, Chih-Wei; Lin, Jiunn-Yuan; Wu, Kaung-Hsiung; Juang, Jenh-Yih; Kobayashi, Takayoshi; Cheng, Cheng-Maw; Tsuei, Ku-Ding; Berger, Helmuth; Sankar, Raman; Chou, Fang-Cheng

    2015-01-01

    Topological insulators (TIs) are interesting quantum matters that have a narrow bandgap for bulk and a Dirac-cone-like conducting surface state (SS). The recent discovered second Dirac surface state (SS) and bulk bands (BBs) located ~1.5 eV above the first SS are important for optical coupling in TIs. Here, we report on the time-domain measurements of THz radiation generated from TIs n-type Cu(0.02)Bi2Se3 and p-type Bi2Te3 single crystals by ultrafast optical pulse excitation. The observed polarity-reversal of the THz pulse originated from transient current is unusual, and cannot be reconciled with the photo-Dember effect. The second SS and BBs are found to be indispensable for the explanation of the unusual phenomenon. Thanks to the existence of the second SS and BBs, TIs manifest an effective wide band gap in THz generation. The present study demonstrates that time-domain THz spectroscopy provide rich information of the optical coupling and the electronic structure of TIs. PMID:26370337

  8. A Low-Noise Solid-State Nanopore Platform Based on a Highly Insulating Substrate

    PubMed Central

    Lee, Min-Hyun; Kumar, Ashvani; Park, Kyeong-Beom; Cho, Seong-Yong; Kim, Hyun-Mi; Lim, Min-Cheol; Kim, Young-Rok; Kim, Ki-Bum

    2014-01-01

    A solid-state nanopore platform with a low noise level and sufficient sensitivity to discriminate single-strand DNA (ssDNA) homopolymers of poly-A40 and poly-T40 using ionic current blockade sensing is proposed and demonstrated. The key features of this platform are (a) highly insulating dielectric substrates that are used to mitigate the effect of parasitic capacitance elements, which decrease the ionic current RMS noise level to sub-10 pA and (b) ultra-thin silicon nitride membranes with a physical thickness of 5 nm (an effective thickness of 2.4 nm estimated from the ionic current) are used to maximize the signal-to-noise ratio and the spatial depth resolution. The utilization of an ultra-thin membrane and a nanopore diameter as small as 1.5 nm allow the successful discrimination of 40 nucleotide ssDNA poly-A40 and poly-T40. Overall, we demonstrate that this platform overcomes several critical limitations of solid-state nanopores and opens the door to a wide range of applications in single-molecule-based detection and analysis. PMID:25502421

  9. Werner states and the two-spinors Heisenberg anti-ferromagnet

    NASA Astrophysics Data System (ADS)

    Batle, J.; Casas, M.; Plastino, A.; Plastino, A. R.

    2005-08-01

    We ascertain, following ideas of Arnesen, Bose, and Vedral concerning thermal entanglement [Phys. Rev. Lett. 87 (2001) 017901] and using the statistical tool called entropic non-triviality [P.W. Lamberti, M.T. Martin, A. Plastino, O.A. Rosso, Physica A 334 (2004) 119], that there is a one-to-one correspondence between (i) the mixing coefficient x of a Werner state, on the one hand, and (ii) the temperature T of the one-dimensional Heisenberg two-spin chain with a magnetic field B along the z-axis, on the other one. This is true for each value of B below a certain critical value B. The pertinent mapping depends on the particular B-value one selects within such a range.

  10. Substitution-induced spin-splitted surface states in topological insulator (Bi 1-x Sbx)2Te3.

    PubMed

    He, Xiaoyue; Li, Hui; Chen, Lan; Wu, Kehui

    2015-01-01

    We present a study on surface states of topological insulator (Bi 1-x Sbx)2Te3 by imaging quasiparticle interference patterns (QPI) using low temperature scanning tunneling microscope. Besides the topological Dirac state, we observed another surface state with chiral spin texture within the conduction band range. The quasiparticle scattering in this state is selectively suppressed. Combined with first-principles calculations, we attribute this state to a spin-splitted band induced by the substitution of Bi with Sb atoms. Our results demonstrate that the coexistence of topological order and alloying may open wider tunability in quantum materials. PMID:25743262

  11. Spin-orbit driven magnetic insulating state with Jeff=1/2 character in a 4d oxide

    DOE PAGESBeta

    Calder, S.; Li, Ling; Okamoto, Satoshi; Choi, Yongseong; Mukherjee, Rupam; Haskel, Daniel; Mandrus, D.

    2015-11-30

    The unusual magnetic and electronic ground states of 5d iridates has been shown to be driven by intrinsically enhanced spin-orbit coupling (SOC). The influence of appreciable but reduced SOC in creating the manifested magnetic insulating states in 4d oxides is less clear, with one hurdle being the existence of such compounds. Here we present experimental and theoretical results on Sr4RhO6 that reveal SOC dominated behavior. Neutron measurements show the octahedra are both spatially separated and locally ideal, making the electronic ground state susceptible to alterations by SOC. Magnetic ordering is observed with a similar structure to an analogous Jeff=1/2 Mottmore » iridate. We consider the underlying role of SOC in this rhodate with density functional theory and x-ray absorption spectroscopy and find a magnetic insulating ground state with Jeff =1/2 character.The unusual magnetic and electronic ground states of 5d iridates have been shown to be driven by intrinsically enhanced spin-orbit coupling (SOC). The influence of appreciable but reduced SOC in creating the manifested magnetic insulating states in 4d oxides is less clear, with one hurdle being the existence of such compounds. Here, we present experimental and theoretical results on Sr4RhO6 that reveal SOC dominated behavior. Neutron measurements show the octahedra are both spatially separated and locally ideal, making the electronic ground state susceptible to alterations by SOC. Magnetic ordering is observed with a similar structure to an analogous Jeff=1/2 Mott iridate. We consider the underlying role of SOC in this rhodate with density functional theory and x-ray absorption spectroscopy, and find a magnetic insulating ground state with Jeff=12 character.« less

  12. La Displacement Driven Double-Exchange Like Mediation in Titanium dxy Ferromagnetism at the LaAlO3/SrTiO3

    NASA Astrophysics Data System (ADS)

    Odkhuu, Dorj; Rhim, Sonny H.; Shin, Dongbin; Park, Noejung

    2016-04-01

    The epitaxial atomistic interfaces of two insulating oxides, LaAlO3 (LAO)/SrTiO3 (STO), have attracted great interest owing to rich emergent phenomena such as interface metallicity, thickness dependent insulator-metal transition, superconductivity, ferromagnetism, and even their coexistence. However, the physics origin of ferromagnetic ordering in the n-type LAO/STO interface is in debate. Here, we propose that the polar distortion of La atom can ignite the ferromagnetism at the interface even without oxygen vacancy. The induced hybridization between La dz2 and O px,y states can mediate double-exchange like interaction between Ti dxy electrons. We further suggest that the structural and electrical modification of the outermost surface of LAO or switching the polarization direction of ferroelectric overlayers on LAO/STO can promote such La displacement.

  13. Manipulating the ferromagnetism in narrow-bandwidth Pr{sub 1-x}Ca{sub x}MnO{sub 3} (0 ≤ x ≤ 0.6) by means of the Mn-Ru t{sub 2g} ferromagnetic super-exchanges

    SciTech Connect

    Wang, Y. L.; Liu, M. F.; Xie, Y. L.; Yan, Z. B.; Dong, S.; Liu, J.-M.

    2015-09-28

    The concurrent ferromagnetic and metal-insulator transitions via the double-exchange route and electronic phase separation scenario represent the core ingredients of the physics of manganites. In this work, a Ca{sup 2+} and Ru{sup 4+} co-substitution of Pr{sup 3+} and Mn{sup 3+} in narrow-bandwidth and insulating PrMnO{sub 3}, namely, Pr{sub 1-x}Ca{sub x}Mn{sub 1-x}Ru{sub x}O{sub 3} (PCMRO, x ≤ 0.6), is carried out in order to investigate an alternative approach to effectively manipulate the ferromagnetism of PrMnO{sub 3}-based manganites. It is revealed that PCMRO over the whole substitution range is homogeneous solid solution with increased lattice distortion. The preference of Ru{sup 4+} valence state and the absence of Mn{sup 4+} valence state disable the Mn{sup 3+}-Mn{sup 4+} e{sub g}-orbital double-exchange, and the random occupation of Ru{sup 4+} in the lattice excludes the charge ordering and electronic phase separation. While all these consequences should favor antiferromagnetic insulating states, nevertheless, a high-temperature ferromagnetic transition is triggered by the co-substitution and the magnetization can reach up to ∼1.0 μ{sub B}/f.u. at x ∼ 0.2–0.3, much bigger than the moment (<0.1 μ{sub B}/f.u.) of Pr{sub 1−x}Ca{sub x}MnO{sub 3} in the weak ferromagnetic insulator state. It is suggested that this strong ferromagnetism is substantially ascribed to the Mn{sup 3+}-Ru{sup 4+} t{sub 2g}-orbital ferromagnetic super-exchange, and a simple geometric network illustration of the magnetism and electrical transport is presented.

  14. The public health benefits of insulation retrofits in existing housing in the United States

    PubMed Central

    Levy, Jonathan I; Nishioka, Yurika; Spengler, John D

    2003-01-01

    Background Methodological limitations make it difficult to quantify the public health benefits of energy efficiency programs. To address this issue, we developed a risk-based model to estimate the health benefits associated with marginal energy usage reductions and applied the model to a hypothetical case study of insulation retrofits in single-family homes in the United States. Methods We modeled energy savings with a regression model that extrapolated findings from an energy simulation program. Reductions of fine particulate matter (PM2.5) emissions and particle precursors (SO2 and NOx) were quantified using fuel-specific emission factors and marginal electricity analyses. Estimates of population exposure per unit emissions, varying by location and source type, were extrapolated from past dispersion model runs. Concentration-response functions for morbidity and mortality from PM2.5 were derived from the epidemiological literature, and economic values were assigned to health outcomes based on willingness to pay studies. Results In total, the insulation retrofits would save 800 TBTU (8 × 1014 British Thermal Units) per year across 46 million homes, resulting in 3,100 fewer tons of PM2.5, 100,000 fewer tons of NOx, and 190,000 fewer tons of SO2 per year. These emission reductions are associated with outcomes including 240 fewer deaths, 6,500 fewer asthma attacks, and 110,000 fewer restricted activity days per year. At a state level, the health benefits per unit energy savings vary by an order of magnitude, illustrating that multiple factors (including population patterns and energy sources) influence health benefit estimates. The health benefits correspond to $1.3 billion per year in externalities averted, compared with $5.9 billion per year in economic savings. Conclusion In spite of significant uncertainties related to the interpretation of PM2.5 health effects and other dimensions of the model, our analysis demonstrates that a risk-based methodology is viable

  15. Topological insulator states in a honeycomb lattice of s-triazines

    NASA Astrophysics Data System (ADS)

    Wang, Aizhu; Zhang, Xiaoming; Zhao, Mingwen

    2014-09-01

    Two-dimensional (2D) graphitic carbon nitride materials have been drawing increasing attentions in energy conversion, environment protection and spintronic devices. Here, based on first-principles calculations, we demonstrate that the already-synthesized honeycomb lattice of s-triazines with a chemical formula of C6N6 (g-C6N6) has topologically nontrivial electronic states characterized by px,y-orbital band structures with a topological invariant of Z2 = 1, and stronger spin-orbital coupling (SOC) than both graphene and silicene. The band gaps opened in the px,y-orbital bands due to SOC are 5.50 meV (K points) and 8.27 eV (Γ point), respectively, implying that the quantum spin Hall effect (QSHE) could be achieved in this 2D graphitic carbon nitride material at a temperature lower than 95 K. This offers a viable approach for searching for 2D Topological Insulators (TIs) in metal-free organic materials.Two-dimensional (2D) graphitic carbon nitride materials have been drawing increasing attentions in energy conversion, environment protection and spintronic devices. Here, based on first-principles calculations, we demonstrate that the already-synthesized honeycomb lattice of s-triazines with a chemical formula of C6N6 (g-C6N6) has topologically nontrivial electronic states characterized by px,y-orbital band structures with a topological invariant of Z2 = 1, and stronger spin-orbital coupling (SOC) than both graphene and silicene. The band gaps opened in the px,y-orbital bands due to SOC are 5.50 meV (K points) and 8.27 eV (Γ point), respectively, implying that the quantum spin Hall effect (QSHE) could be achieved in this 2D graphitic carbon nitride material at a temperature lower than 95 K. This offers a viable approach for searching for 2D Topological Insulators (TIs) in metal-free organic materials. Electronic supplementary information (ESI) available: A ruby model and the relevant tight-binding Hamiltonian, parity tables for the g-C6N6 lattice and the

  16. Tuning Dirac states at grain boundaries in the topological insulator Bi2Se3

    NASA Astrophysics Data System (ADS)

    Li, Lian

    2015-03-01

    Symmetry protected Dirac states have been experimentally observed in topological insulator (TI) bismuth chalcogenides. Recently, we have further demonstrated direct electrical generation and detection of spin accumulation induced by spin-momentum locking of Dirac surface states in Bi2Se3, a critical step forward towards future electronic and spintronic applications. In this talk, I will give an overview of the opportunities and challenges in the epitaxial growth of these layered TIs that exhibit a strong (covalent) intra-layer bonding and weak (van der Waals) inter-layer bonding. Using Bi2Se3 as an example, I will show that this characteristic anisotropic bonding facilitates a spiral growth mode on virtually any substrates by molecular beam epitaxy. The coalescence of these spirals results in a high density of grain boundaries (GBs). Using scanning tunneling and transmission electron microscopies, and density functional theory calculations, I will further show that near the zero-angle GBs (i.e., anti-phase domain boundaries), caused by vertical shifts of a fraction of a Bi2Se3 quintuple layer, the Dirac states are robust against scattering by these extended structural defects. However, electrostatic fields on the order of 108 V/m are found, which locally charge the Dirac state, shifting the Dirac point by up to 120 meV. On the other hand, low-angle (<15o) GBs are found to be of the tilt variant, consisting of alternating edge dislocation pairs, resulting in periodic in-plane stretching and compression. Scanning tunneling spectroscopy reveals that in-plane stretching reduces the van der Waals gap, enhancing the Dirac states; while in-plane compression expands the inter-quintuple separation, therefore destroying the Dirac states and opens a gap in the local density of states. These findings demonstrate the tunability of Dirac states by electric field and strain at the atomic scale, and also highlight the inherent formation of GBs during vapor phase epitaxy of layered

  17. Kondo insulators

    SciTech Connect

    Fisk, Z.; Sarrao, J.L.; Thompson, J.D.

    1994-10-01

    The Kondo insulating materials present a particularly simple limiting case of the strongly correlated electron lattice problem: one occupied f-state interacting with a single half-filled conduction band. Experiment shows that the solution to this problem has some remarkably simple aspects. Optical conductivity data display the strong coupling nature of this physics.

  18. Magnetic impurities on the surface of a topological insulator

    SciTech Connect

    Liu, Qin; Liu, Chao-Xing; Xu, Cenke; Qi, Xiao-Liang; Zhang, Shou-Cheng; /Stanford U., Phys. Dept.

    2010-03-25

    The surface states of a topological insulator are described by an emergent relativistic massless Dirac equation in 2+1 dimensions. In contrast to graphene, there is an odd number of Dirac points, and the electron spin is directly coupled to the momentum. We show that a magnetic impurity opens up a local gap and suppresses the local density of states. Furthermore, the Dirac electronic states mediate an RKKY interaction among the magnetic impurities which is always ferromagnetic, whenever the chemical potential lies near the Dirac point. These effects can be directly measured in STM experiments. We also study the case of quenched disorder through a renormalization group analysis.

  19. Edge States and Topological Insulating Phases Generated by Curving a Nanowire with Rashba Spin-Orbit Coupling

    NASA Astrophysics Data System (ADS)

    Gentile, Paola; Cuoco, Mario; Ortix, Carmine

    2015-12-01

    We prove that curvature effects in low-dimensional nanomaterials can promote the generation of topological states of matter by considering the paradigmatic example of quantum wires with Rashba spin-orbit coupling, which are bent in a nanoscale periodic serpentine structure. The effect of the periodic curvature generally results in the appearance of insulating phases with a corresponding novel butterfly spectrum characterized by the formation of finite measure complex regions of forbidden energies. When the Fermi energy lies in the gaps, the system displays localized end states protected by topology. We further show that for certain superstructure periods the system possesses topologically nontrivial insulating phases at half filling. Our results suggest that the local curvature and the topology of the electronic states are inextricably intertwined in geometrically deformed nanomaterials.

  20. Edge States and Topological Insulating Phases Generated by Curving a Nanowire with Rashba Spin-Orbit Coupling.

    PubMed

    Gentile, Paola; Cuoco, Mario; Ortix, Carmine

    2015-12-18

    We prove that curvature effects in low-dimensional nanomaterials can promote the generation of topological states of matter by considering the paradigmatic example of quantum wires with Rashba spin-orbit coupling, which are bent in a nanoscale periodic serpentine structure. The effect of the periodic curvature generally results in the appearance of insulating phases with a corresponding novel butterfly spectrum characterized by the formation of finite measure complex regions of forbidden energies. When the Fermi energy lies in the gaps, the system displays localized end states protected by topology. We further show that for certain superstructure periods the system possesses topologically nontrivial insulating phases at half filling. Our results suggest that the local curvature and the topology of the electronic states are inextricably intertwined in geometrically deformed nanomaterials. PMID:26722937

  1. Metallic 2D Surface State of Silicon by Ionic Liquid gating and observation of Reentrant Insulating behavior

    NASA Astrophysics Data System (ADS)

    Nelson, J. J.; Goldman, A. M.

    2015-03-01

    Metal insulator transitions are usually observed in high mobility and low carrier density 2D electron systems. There are several open questions regarding the metallic state including its existence in the limit of zero temperature. The current experimental focus is on the production of higher mobility samples to push the critical carrier density to even lower values, which will increase the effects of the Coulomb interaction. Here we report an unexpected result, the observation of the onset of a metallic state at high carrier densities in silicon gated with the ionic liquid DEME-TFSI. In addition we have observed a return to the insulating state as the carrier density was further increased. This reentrant insulting behavior is an effect that was recently predicted. Supported in part by NSF/DMR-1263316. Part of this work was carried out at the Minnesota Nanocenter.

  2. Metallic quantum ferromagnets

    NASA Astrophysics Data System (ADS)

    Brando, M.; Belitz, D.; Grosche, F. M.; Kirkpatrick, T. R.

    2016-04-01

    An overview of quantum phase transitions (QPTs) in metallic ferromagnets, discussing both experimental and theoretical aspects, is given. These QPTs can be classified with respect to the presence and strength of quenched disorder: Clean systems generically show a discontinuous, or first-order, QPT from a ferromagnetic to a paramagnetic state as a function of some control parameter, as predicted by theory. Disordered systems are much more complicated, depending on the disorder strength and the distance from the QPT. In many disordered materials the QPT is continuous, or second order, and Griffiths-phase effects coexist with QPT singularities near the transition. In other systems the transition from the ferromagnetic state at low temperatures is to a different type of long-range order, such as an antiferromagnetic or a spin-density-wave state. In still other materials a transition to a state with glasslike spin dynamics is suspected. The review provides a comprehensive discussion of the current understanding of these various transitions and of the relation between experiment and theory.

  3. Casimir Torque between Topological Insulators: a Physical Implication of the Surface State Hexagonal Warping Effect

    NASA Astrophysics Data System (ADS)

    Chen, Liang; Chang, Kai

    We use a variation of the Lifshitz formula to calculate the anisotropic Casimir energy density between two topological insulators in the vacuum. We find that the hexagonal warping effect can induce a Casimir torque between the two topological insulators, Tc ~ sin (6 θ) with twisted angle θ. The maximal Casimir torque at θ = π / 12 is estimated to be ~10-19 N . m / rad for Bi2Te3 on the [111] surface when the distance between the two topological insulators is about 20 nm and the surface areas are taken to be ~ 1 cm2 .

  4. Spin-torque generation in topological insulator based heterostructures

    NASA Astrophysics Data System (ADS)

    Fischer, Mark H.; Vaezi, Abolhassan; Manchon, Aurelien; Kim, Eun-Ah

    2016-03-01

    Heterostructures utilizing topological insulators exhibit a remarkable spin-torque efficiency. However, the exact origin of the strong torque, in particular whether it stems from the spin-momentum locking of the topological surface states or rather from spin-Hall physics of the topological-insulator bulk, remains unclear. Here, we explore a mechanism of spin-torque generation purely based on the topological surface states. We consider topological-insulator-based bilayers involving ferromagnetic metal (TI/FM) and magnetically doped topological insulators (TI/mdTI), respectively. By ascribing the key theoretical differences between the two setups to location and number of active surface states, we describe both setups within the same framework of spin diffusion of the nonequilibrium spin density of the topological surface states. For the TI/FM bilayer, we find large spin-torque efficiencies of roughly equal magnitude for both in-plane and out-of-plane spin torques. For the TI/mdTI bilayer, we elucidate the dominance of the spin-transfer-like torque. However, we cannot explain the orders of magnitude enhancement reported. Nevertheless, our model gives an intuitive picture of spin-torque generation in topological-insulator-based bilayers and provides theoretical constraints on spin-torque generation due to topological surface states.

  5. Topological surface states interacting with bulk excitations in the Kondo insulator SmB6 revealed via planar tunneling spectroscopy.

    PubMed

    Park, Wan Kyu; Sun, Lunan; Noddings, Alexander; Kim, Dae-Jeong; Fisk, Zachary; Greene, Laura H

    2016-06-14

    Samarium hexaboride (SmB6), a well-known Kondo insulator in which the insulating bulk arises from strong electron correlations, has recently attracted great attention owing to increasing evidence for its topological nature, thereby harboring protected surface states. However, corroborative spectroscopic evidence is still lacking, unlike in the weakly correlated counterparts, including Bi2Se3 Here, we report results from planar tunneling that unveil the detailed spectroscopic properties of SmB6 The tunneling conductance obtained on the (001) and (011) single crystal surfaces reveals linear density of states as expected for two and one Dirac cone(s), respectively. Quite remarkably, it is found that these topological states are not protected completely within the bulk hybridization gap. A phenomenological model of the tunneling process invoking interaction of the surface states with bulk excitations (spin excitons), as predicted by a recent theory, provides a consistent explanation for all of the observed features. Our spectroscopic study supports and explains the proposed picture of the incompletely protected surface states in this topological Kondo insulator SmB6. PMID:27233936

  6. High-mobility surface states and conductance fluctuations in Bismuth Telluro-Sulfide topological insulator devices

    NASA Astrophysics Data System (ADS)

    Trivedi, Tanuj; Sonde, Sushant; Movva, Hema C. P.; Banerjee, Sanjay K.

    Since the experimental discovery of three-dimensional topological insulators (TI), (Bi,Sb)2(Se,Te)3 binary compounds are the principal material systems to explore TI physics. However, transport experiments studying surface-states are complicated by parallel bulk conductivity contribution, which is expected to improve for ternary and quaternary chalcogenide compounds of Bi and Sb. A promising alternative is the Sulfur-based tetradymite with an ideal formula of Bi2Te2S, which has received little attention. We present van der Waals epitaxial growth and magnetotransport in Bismuth Telluro-Sulfide (BTS) crystalline nanosheets. Gating-enhanced Weak-antilocalization (WAL) and Universal Conductance Fluctuations (UCF) are observed in BTS devices. Empirical modeling of the data shows the existence of two-dimensional surface transport. A three-channel Hall conductivity model is proposed, which is utilized in conjunction with an extended-WAL analysis, showing the presence of a high-mobility surface component and indication for separation of transport channels in BTS devices. Our growth and comprehensive transport experiments demonstrate BTS as a promising candidate TI material.

  7. Control and characterization of the metallic surface state of bulk insulating Bi2Se3

    NASA Astrophysics Data System (ADS)

    Fuhrer, Michael

    2015-03-01

    Bi2Se3 is a three dimensional strong topological insulator with a conducting two-dimensional surface state whose existence is guaranteed by topology. The bulk Bi2Se3 has a 300 meV bandgap, but is often a degenerately n-doped metal in as-grown material. I will discuss our efforts to remove this doping in thin crystals and films to achieve surface-dominated conduction. Electrochemical gating (using PEO+LiClO4 electrolyte) or molecular doping (using F4-TCNQ) is shown to effectively bring the Fermi energy of thin (3-20 nm) exfoliated Bi2Se3 crystals to the conduction band edge, where it can be further modulated at low temperature using field-effect gating. These techniques allowed us to reveal the gapless ambipolar transport in the topological surface, and measure the minimum conductivity, electron-acoustic phonon scattering, thermopower, and inter-surface coupling of the topological surfaces. Recently we have developed techniques to measure the transport properties of Bi2Se3 in situ during growth in ultra-high vacuum, enabling better understanding of the doping mechanisms. We have also studied vacuum-deposited MoO3 as a highly effective acceptor dopant which remains stable on air exposure for time scales of days. This work is supported by NSF Grants DMR-1105224, DMR-0520471, and DMR-0952716, and an ARC Laureate Fellowship.

  8. Disorder-induced density of states on the surface of a spherical topological insulator

    NASA Astrophysics Data System (ADS)

    Durst, Adam C.

    2016-06-01

    We consider a topological insulator (TI) of spherical geometry and numerically investigate the influence of disorder on the density of surface states. The energy spectrum of the spherical TI surface is discrete, for a sphere of finite radius, and can be truncated by imposing a high-energy cutoff at the scale of the bulk band gap. To this clean system we add a surface disorder potential of the most general Hermitian form, V =V0(θ ,ϕ ) 1 +V (θ ,ϕ ) .σ , where V0 describes the spin-independent part of the disorder and the three components of V describe the spin-dependent part. We expand these four disorder functions in spherical harmonics and draw the expansion coefficients randomly from a four-dimensional, zero-mean Gaussian distribution. Different strengths and classes of disorder are realized by specifying the 4 ×4 covariance matrix. For each instantiation of the disorder, we solve for the energy spectrum via exact diagonalization. Then we compute the disorder-averaged density of states, ρ (E ) , by averaging over 200 000 different instantiations. Disorder broadens the Landau-level delta functions of the clean density of states into peaks that decay and merge together. If the spin-dependent term is dominant, these peaks split due to the breaking of the degeneracy between time-reversed partner states. Increasing disorder strength pushes states closer and closer to zero energy (the Dirac point), resulting in a low-energy density of states that becomes nonzero for sufficient disorder, typically approaching an energy-independent saturation value, for most classes of disorder. But for purely spin-dependent disorder with V either entirely out-of-surface or entirely in-surface, we identify intriguing disorder-induced features in the vicinity of the Dirac point. In the out-of-surface case, a new peak emerges at zero energy. In the in-surface case, we see a symmetry-protected zero at zero energy, with ρ (E ) increasing linearly toward nonzero-energy peaks. These

  9. Ramp-edge structured tunneling devices using ferromagnet electrodes

    DOEpatents

    Kwon, Chuhee; Jia, Quanxi

    2002-09-03

    The fabrication of ferromagnet-insulator-ferromagnet magnetic tunneling junction devices using a ramp-edge geometry based on, e.g., (La.sub.0.7 Sr.sub.0.3) MnO.sub.3, ferromagnetic electrodes and a SrTiO.sub.3 insulator is disclosed. The maximum junction magnetoresistance (JMR) as large as 23% was observed below 300 Oe at low temperatures (T<100 K). These ramp-edge junctions exhibited JMR of 6% at 200 K with a field less than 100 Oe.

  10. Hierarchy of bound states in the one-dimensional ferromagnetic Ising chain CoNb2O6 investigated by high-resolution time-domain terahertz spectroscopy.

    PubMed

    Morris, C M; Valdés Aguilar, R; Ghosh, A; Koohpayeh, S M; Krizan, J; Cava, R J; Tchernyshyov, O; McQueen, T M; Armitage, N P

    2014-04-01

    Kink bound states in the one-dimensional ferromagnetic Ising chain compound CoNb2O6 have been studied using high-resolution time-domain terahertz spectroscopy in zero applied magnetic field. When magnetic order develops at low temperature, nine bound states of kinks become visible. Their energies can be modeled exceedingly well by the Airy function solutions to a 1D Schrödinger equation with a linear confining potential. This sequence of bound states terminates at a threshold energy near 2 times the energy of the lowest bound state. Above this energy scale we observe a broad feature consistent with the onset of the two particle continuum. At energies just below this threshold we observe a prominent excitation that we interpret as a novel bound state of bound states--two pairs of kinks on neighboring chains. PMID:24745454

  11. Collapse of the low temperature insulating state in Cr-doped V2O3 thin films

    NASA Astrophysics Data System (ADS)

    Homm, P.; Dillemans, L.; Menghini, M.; Van Bilzen, B.; Bakalov, P.; Su, C.-Y.; Lieten, R.; Houssa, M.; Nasr Esfahani, D.; Covaci, L.; Peeters, F. M.; Seo, J. W.; Locquet, J.-P.

    2015-09-01

    We have grown epitaxial Cr-doped V2O3 thin films with Cr concentrations between 0% and 20% on (0001)-Al2O3 by oxygen-assisted molecular beam epitaxy. For the highly doped samples (>3%), a regular and monotonous increase of the resistance with decreasing temperature is measured. Strikingly, in the low doping samples (between 1% and 3%), a collapse of the insulating state is observed with a reduction of the low temperature resistivity by up to 5 orders of magnitude. A vacuum annealing at high temperature of the films recovers the low temperature insulating state for doping levels below 3% and increases the room temperature resistivity towards the values of Cr-doped V2O3 single crystals. It is well-know that oxygen excess stabilizes a metallic state in V2O3 single crystals. Hence, we propose that Cr doping promotes oxygen excess in our films during deposition, leading to the collapse of the low temperature insulating state at low Cr concentrations. These results suggest that slightly Cr-doped V2O3 films can be interesting candidates for field effect devices.

  12. Collapse of the low temperature insulating state in Cr-doped V2O3 thin films

    NASA Astrophysics Data System (ADS)

    Homm, Pia; Dillemans, Leander; Menghini, Mariela; van Bilzen, Bart; Bakalov, Petar; Su, Chen-Yi; Lieten, Ruben; Houssa, Michel; Seo, Jin Won; Locquet, Jean-Pierre; Nasr Esfahani, Davoud; Covani, Lucian; Peeters, Francois

    We have grown epitaxial Cr-doped V2O3 thin films with Cr concentrations between 0 and 20% on (0001)-Al2O3 by oxygen-assisted molecular beam epitaxy. For the highly doped samples (> 3%), a regular and monotonous increase of the resistance with decreasing temperature is measured. Strikingly, in the low doping samples (between 1% and 3%), a collapse of the insulating state is observed with a reduction of the low temperature resistivity by up to 5 orders of magnitude. A vacuum annealing at high temperature of the films recovers the low temperature insulating state for doping levels below 3% and increases the room temperature resistivity towards the values of Cr-doped V2O3 single crystals. It is well known that oxygen excess stabilizes a metallic state in V2O3 single crystals. Hence, we propose that Cr doping promotes oxygen excess in our films during deposition, leading to the collapse of the low temperature insulating state at low Cr concentrations. These results suggest that slightly Cr-doped V2O3 films can be interesting candidates for field effect devices.

  13. Surface Kondo effect and non-trivial metallic state of the Kondo insulator YbB12.

    PubMed

    Hagiwara, Kenta; Ohtsubo, Yoshiyuki; Matsunami, Masaharu; Ideta, Shin-Ichiro; Tanaka, Kiyohisa; Miyazaki, Hidetoshi; Rault, Julien E; Fèvre, Patrick Le; Bertran, François; Taleb-Ibrahimi, Amina; Yukawa, Ryu; Kobayashi, Masaki; Horiba, Koji; Kumigashira, Hiroshi; Sumida, Kazuki; Okuda, Taichi; Iga, Fumitoshi; Kimura, Shin-Ichi

    2016-01-01

    A synergistic effect between strong electron correlation and spin-orbit interaction has been theoretically predicted to realize new topological states of quantum matter on Kondo insulators (KIs), so-called topological Kondo insulators (TKIs). One TKI candidate has been experimentally observed on the KI SmB6(001), and the origin of the surface states (SS) and the topological order of SmB6 has been actively discussed. Here, we show a metallic SS on the clean surface of another TKI candidate YbB12(001) using angle-resolved photoelectron spectroscopy. The SS shows temperature-dependent reconstruction corresponding to the Kondo effect observed for bulk states. Despite the low-temperature insulating bulk, the reconstructed SS with c-f hybridization is metallic, forming a closed Fermi contour surrounding on the surface Brillouin zone and agreeing with the theoretically expected behaviour for SS on TKIs. These results demonstrate the temperature-dependent holistic reconstruction of two-dimensional states localized on KIs surface driven by the Kondo effect. PMID:27576449

  14. Surface-state-dominated transport in crystals of the topological crystalline insulator In-doped Pb1-xSnxTe

    DOE PAGESBeta

    Zhong, Ruidan; He, Xugang; Schneeloch, J. A.; Zhang, Cheng; Liu, Tiansheng; Pletikosić, I.; Yilmaz, T.; Sinkovic, B.; Li, Qiang; Ku, Wei; et al

    2015-05-29

    Three-dimensional topological insulators and topological crystalline insulators represent new quantum states of matter, which are predicted to have insulating bulk states and spin-momentum-locked gapless surface states. Experimentally, it has proven difficult to achieve the high bulk resistivity that would allow surface states to dominate the transport properties over a substantial temperature range. Here we report a series of indium-doped Pb1-xSnxTe compounds that manifest huge bulk resistivities together with evidence consistent with the topological character of the surface states for x ≳ 0.35, based on thickness-dependent transport studies and magnetoresistance measurements. For these bulk-insulating materials, the surface states determine the resistivitymore » for temperatures beyond 20 K.« less

  15. Surface-state-dominated transport in crystals of the topological crystalline insulator In-doped Pb1-xSnxTe

    SciTech Connect

    Zhong, Ruidan; He, Xugang; Schneeloch, J. A.; Zhang, Cheng; Liu, Tiansheng; Pletikosić, I.; Yilmaz, T.; Sinkovic, B.; Li, Qiang; Ku, Wei; Valla, T.; Tranquada, J. M.; Gu, Genda

    2015-05-29

    Three-dimensional topological insulators and topological crystalline insulators represent new quantum states of matter, which are predicted to have insulating bulk states and spin-momentum-locked gapless surface states. Experimentally, it has proven difficult to achieve the high bulk resistivity that would allow surface states to dominate the transport properties over a substantial temperature range. Here we report a series of indium-doped Pb1-xSnxTe compounds that manifest huge bulk resistivities together with evidence consistent with the topological character of the surface states for x ≳ 0.35, based on thickness-dependent transport studies and magnetoresistance measurements. For these bulk-insulating materials, the surface states determine the resistivity for temperatures beyond 20 K.

  16. Electrically-induced ferromagnetism at room temperature in (Ti,Co)O2: carrier-mediated ferromagnetism

    NASA Astrophysics Data System (ADS)

    Fukumura, Tomoteru

    2013-03-01

    Oxide-diluted magnetic semiconductors (DMS) is expected to have high Curie temperature via carrier-mediated ferromagnetism through heavy electron mass and large electron carrier density. We have studied various oxide-DMS such as (Zn,Mn)O, and discovered room temperature ferromagnetism in (Ti,Co)O2. The origin of ferromagnetism has been discussed for a decade. Previously, the control of ferromagnetism was demonstrated through carrier control by chemical doping. But it was difficult to exclude the defect-mediated ferromagnetism, since the electron donor was the oxygen vacancy. In order to evidence the carrier-mediated ferromagnetism, the electric field control of ferromagnetism is useful. The control of ferromagnetism at room temperature is also important for implementation of spintronic devices. By gating with electric double layer transistor, the ferromagnetism was induced at room temperature, representing electron carrier-mediated ferromagnetism. Chemical doping study in (Ti,Co)O2 for wider range of carrier density exhibited clearer paramagnetic insulator to ferromagnetic metal transition with increasing carrier density. At a medium carrier density, a ferromagnetic insulator phase appeared possibly related with a phase separation between ferromagnetic and paramagnetic phases. Also, a superparamagnetic phase appeared for excessively reduced sample. Taking all these results into account, previously proposed extrinsic mechanisms such as oxygen vacancy-mediated mechanism, metal segregation, and superparamagnetism are not correct picture of the ferromagnetism. This study was in collaboration with Y. Yamada, K. Ueno, M. Kawasaki, H. T. Yuan, H. Shimotani, Y. Iwasa, L. Gu, S. Tsukimoto, Y. Ikuhara, A. Fujimori, and T. Mizokawa. This research was in part supported by JSPS through NEXT Program initiated by CSTP.

  17. Spin Transport in Ferromagnetic and Antiferromagnetic Insulators

    NASA Astrophysics Data System (ADS)

    Su, Shanshan; Yin, Gen; Liu, Yizhou; Zang, Jiadong; Barlas, Yafis; Lake, Roger

    Recently, experiments of spin pumping have been done for system with antiferromagnetic oxides (AFMOs) as a spacer between YIG and Pt. Observation of spin transport through the AFMO and the enhancement of spin pumping signal in the system due to the insertion of AFMO have been reported. In this research, we model the spin transport in Pt/YIG/Pt and Pt/YIG/AFMO/Pt heterostructures using the Landau-Lifshitz-Gilbert equations coupled with the non-equilibrium Green's function equations. We show that a pure spin current generated at the first Rashba SOC electrode is carried by magnon through YIG, which can be converted back to spin pumping signal at the second electrode. The spin dynamical details at the heterostructure can determine the transport efficiency. The effect of different magnetization orientations and finite temperatures will be addressed. This work was supported by the SHINES under Award # SC0012670.

  18. Topological properties of ferromagnetic superconductors

    DOE PAGESBeta

    Cheung, Alfred K. C.; Raghu, S.

    2016-04-27

    Here, a variety of heavy fermion superconductors, such as UCoGe, UGe2, and URhGe exhibit a striking coexistence of bulk ferromagnetism and superconductivity. In the first two materials, the magnetic moment decreases with pressure, and vanishes at a ferromagnetic quantum critical point (qcp). Remarkably, the superconductivity in UCoGe varies smoothly with pressure across the qcp and exists in both the ferromagnetic and paramagnetic regimes. We argue that in UCoGe, spin-orbit interactions stabilize a time-reversal invariant odd-parity superconductor in the high pressure paramagnetic regime. Based on a simple phenomenological model, we predict that the transition from the paramagnetic normal state to themore » phase where superconductivity and ferromagnetism coexist is a first-order transition.« less

  19. Topological properties of ferromagnetic superconductors

    NASA Astrophysics Data System (ADS)

    Cheung, Alfred K. C.; Raghu, S.

    2016-04-01

    A variety of heavy fermion superconductors, such as UCoGe, UGe2, and URhGe exhibit a striking coexistence of bulk ferromagnetism and superconductivity. In the first two materials, the magnetic moment decreases with pressure, and vanishes at a ferromagnetic quantum critical point (qcp). Remarkably, the superconductivity in UCoGe varies smoothly with pressure across the qcp and exists in both the ferromagnetic and paramagnetic regimes. We argue that in UCoGe, spin-orbit interactions stabilize a time-reversal invariant odd-parity superconductor in the high pressure paramagnetic regime. Based on a simple phenomenological model, we predict that the transition from the paramagnetic normal state to the phase where superconductivity and ferromagnetism coexist is a first-order transition.

  20. Experimental Observation of Topological Edge States at the Surface Step Edge of the Topological Insulator ZrTe5

    NASA Astrophysics Data System (ADS)

    Li, Xiang-Bing; Huang, Wen-Kai; Lv, Yang-Yang; Zhang, Kai-Wen; Yang, Chao-Long; Zhang, Bin-Bin; Chen, Y. B.; Yao, Shu-Hua; Zhou, Jian; Lu, Ming-Hui; Sheng, Li; Li, Shao-Chun; Jia, Jin-Feng; Xue, Qi-Kun; Chen, Yan-Feng; Xing, Ding-Yu

    2016-04-01

    We report an atomic-scale characterization of ZrTe5 by using scanning tunneling microscopy. We observe a bulk band gap of ˜80 meV with topological edge states at the step edge and, thus, demonstrate that ZrTe5 is a two-dimensional topological insulator. We also find that an applied magnetic field induces an energetic splitting of the topological edge states, which can be attributed to a strong link between the topological edge states and bulk topology. The relatively large band gap makes ZrTe5 a potential candidate for future fundamental studies and device applications.

  1. Ultrafast Optical Excitation of a Persistent Surface-State Population in the Topological Insulator Bi2Se3

    SciTech Connect

    Sobota, Jonathan

    2012-03-14

    Using femtosecond time- and angle-resolved photoemission spectroscopy, we investigated the nonequilibrium dynamics of the topological insulator Bi{sub 2}Se{sub 3}. We studied p-type Bi{sub 2}Se{sub 3}, in which the metallic Dirac surface state and bulk conduction bands are unoccupied. Optical excitation leads to a meta-stable population at the bulk conduction band edge, which feeds a nonequilibrium population of the surface state persisting for >10 ps. This unusually long-lived population of a metallic Dirac surface state with spin texture may present a channel in which to drive transient spin-polarized currents.

  2. Hierarchy of Bound States in the One-Dimensional Ferromagnetic Ising Chain CoNb2O6 Investigated by High-Resolution Time-Domain Terahertz Spectroscopy

    NASA Astrophysics Data System (ADS)

    Morris, C. M.; Valdés Aguilar, R.; Ghosh, A.; Koohpayeh, S. M.; Krizan, J.; Cava, R. J.; Tchernyshyov, O.; McQueen, T. M.; Armitage, N. P.

    2014-04-01

    Kink bound states in the one-dimensional ferromagnetic Ising chain compound CoNb2O6 have been studied using high-resolution time-domain terahertz spectroscopy in zero applied magnetic field. When magnetic order develops at low temperature, nine bound states of kinks become visible. Their energies can be modeled exceedingly well by the Airy function solutions to a 1D Schrödinger equation with a linear confining potential. This sequence of bound states terminates at a threshold energy near 2 times the energy of the lowest bound state. Above this energy scale we observe a broad feature consistent with the onset of the two particle continuum. At energies just below this threshold we observe a prominent excitation that we interpret as a novel bound state of bound states—two pairs of kinks on neighboring chains.

  3. A study of changes in foundation insulation levels in the United States

    SciTech Connect

    Christian, J.E. ); Voss, M.K. )

    1992-10-01

    For almost 10 years the US Department of Energy (DOE) has sponsored a small research effort with. the objective of working with the building industry to see that cost-effective foundation insulation levels are installed in all US buildings. One of the first discoveries in 1983--1984 was that less than 5% of the existing buildings had foundation insulation and less than 30% of new construction included foundation insulation. After producing foundation handbooks, actively working with energy code and standards groups (ASHRAE and Council of American Building Officials: Model Energy Code Committee) conducting told experiments, and developing computer models for predicting energy savings has any measurable progress been made toward the fulfillment of this DOE objective Also, in order to derive maximum energy savings impact for this ongoing research activity, market feedback of progress toward attainment of the objective is needed. Using the network of building experts available to the Building Thermal Envelope Systems and Materials program, a short mail survey was developed, administered, and results analyzed. This study concludes that foundation insulation usage on new residential buildings has increased from 1982 levels of around 25% to 1992 levels of around 50%. Government handbooks and recent changes in local enforced codes and standards are identified as major contributors to this improvement. Progress has been made but more is needed to capture the remaining 40% of the foundations that should be insulated. Several issues are reported as obstacles that need to be overcome to capture the remainder of the foundation market.

  4. A study of changes in foundation insulation levels in the United States

    SciTech Connect

    Christian, J.E.; Voss, M.K.

    1992-10-01

    For almost 10 years the US Department of Energy (DOE) has sponsored a small research effort with. the objective of working with the building industry to see that cost-effective foundation insulation levels are installed in all US buildings. One of the first discoveries in 1983--1984 was that less than 5% of the existing buildings had foundation insulation and less than 30% of new construction included foundation insulation. After producing foundation handbooks, actively working with energy code and standards groups (ASHRAE and Council of American Building Officials: Model Energy Code Committee) conducting told experiments, and developing computer models for predicting energy savings has any measurable progress been made toward the fulfillment of this DOE objective? Also, in order to derive maximum energy savings impact for this ongoing research activity, market feedback of progress toward attainment of the objective is needed. Using the network of building experts available to the Building Thermal Envelope Systems and Materials program, a short mail survey was developed, administered, and results analyzed. This study concludes that foundation insulation usage on new residential buildings has increased from 1982 levels of around 25% to 1992 levels of around 50%. Government handbooks and recent changes in local enforced codes and standards are identified as major contributors to this improvement. Progress has been made but more is needed to capture the remaining 40% of the foundations that should be insulated. Several issues are reported as obstacles that need to be overcome to capture the remainder of the foundation market.

  5. Strong nonlinear terahertz response induced by Dirac surface states in Bi2Se3 topological insulator

    NASA Astrophysics Data System (ADS)

    Giorgianni, Flavio; Chiadroni, Enrica; Rovere, Andrea; Cestelli-Guidi, Mariangela; Perucchi, Andrea; Bellaveglia, Marco; Castellano, Michele; di Giovenale, Domenico; di Pirro, Giampiero; Ferrario, Massimo; Pompili, Riccardo; Vaccarezza, Cristina; Villa, Fabio; Cianchi, Alessandro; Mostacci, Andrea; Petrarca, Massimo; Brahlek, Matthew; Koirala, Nikesh; Oh, Seongshik; Lupi, Stefano

    2016-04-01

    Electrons with a linear energy/momentum dispersion are called massless Dirac electrons and represent the low-energy excitations in exotic materials such as graphene and topological insulators. Dirac electrons are characterized by notable properties such as a high mobility, a tunable density and, in topological insulators, a protection against backscattering through the spin-momentum locking mechanism. All those properties make graphene and topological insulators appealing for plasmonics applications. However, Dirac electrons are expected to present also a strong nonlinear optical behaviour. This should mirror in phenomena such as electromagnetic-induced transparency and harmonic generation. Here we demonstrate that in Bi2Se3 topological insulator, an electromagnetic-induced transparency is achieved under the application of a strong terahertz electric field. This effect, concomitantly determined by harmonic generation and charge-mobility reduction, is exclusively related to the presence of Dirac electron at the surface of Bi2Se3, and opens the road towards tunable terahertz nonlinear optical devices based on topological insulator materials.

  6. Strong nonlinear terahertz response induced by Dirac surface states in Bi2Se3 topological insulator.

    PubMed

    Giorgianni, Flavio; Chiadroni, Enrica; Rovere, Andrea; Cestelli-Guidi, Mariangela; Perucchi, Andrea; Bellaveglia, Marco; Castellano, Michele; Di Giovenale, Domenico; Di Pirro, Giampiero; Ferrario, Massimo; Pompili, Riccardo; Vaccarezza, Cristina; Villa, Fabio; Cianchi, Alessandro; Mostacci, Andrea; Petrarca, Massimo; Brahlek, Matthew; Koirala, Nikesh; Oh, Seongshik; Lupi, Stefano

    2016-01-01

    Electrons with a linear energy/momentum dispersion are called massless Dirac electrons and represent the low-energy excitations in exotic materials such as graphene and topological insulators. Dirac electrons are characterized by notable properties such as a high mobility, a tunable density and, in topological insulators, a protection against backscattering through the spin-momentum locking mechanism. All those properties make graphene and topological insulators appealing for plasmonics applications. However, Dirac electrons are expected to present also a strong nonlinear optical behaviour. This should mirror in phenomena such as electromagnetic-induced transparency and harmonic generation. Here we demonstrate that in Bi2Se3 topological insulator, an electromagnetic-induced transparency is achieved under the application of a strong terahertz electric field. This effect, concomitantly determined by harmonic generation and charge-mobility reduction, is exclusively related to the presence of Dirac electron at the surface of Bi2Se3, and opens the road towards tunable terahertz nonlinear optical devices based on topological insulator materials. PMID:27113395

  7. Strong nonlinear terahertz response induced by Dirac surface states in Bi2Se3 topological insulator

    PubMed Central

    Giorgianni, Flavio; Chiadroni, Enrica; Rovere, Andrea; Cestelli-Guidi, Mariangela; Perucchi, Andrea; Bellaveglia, Marco; Castellano, Michele; Di Giovenale, Domenico; Di Pirro, Giampiero; Ferrario, Massimo; Pompili, Riccardo; Vaccarezza, Cristina; Villa, Fabio; Cianchi, Alessandro; Mostacci, Andrea; Petrarca, Massimo; Brahlek, Matthew; Koirala, Nikesh; Oh, Seongshik; Lupi, Stefano

    2016-01-01

    Electrons with a linear energy/momentum dispersion are called massless Dirac electrons and represent the low-energy excitations in exotic materials such as graphene and topological insulators. Dirac electrons are characterized by notable properties such as a high mobility, a tunable density and, in topological insulators, a protection against backscattering through the spin–momentum locking mechanism. All those properties make graphene and topological insulators appealing for plasmonics applications. However, Dirac electrons are expected to present also a strong nonlinear optical behaviour. This should mirror in phenomena such as electromagnetic-induced transparency and harmonic generation. Here we demonstrate that in Bi2Se3 topological insulator, an electromagnetic-induced transparency is achieved under the application of a strong terahertz electric field. This effect, concomitantly determined by harmonic generation and charge-mobility reduction, is exclusively related to the presence of Dirac electron at the surface of Bi2Se3, and opens the road towards tunable terahertz nonlinear optical devices based on topological insulator materials. PMID:27113395

  8. A Study of Topological Insulator States of Half-Heusler Materials and the Momentum Density of Overdoped Cuprates

    NASA Astrophysics Data System (ADS)

    Al-Sawai, Wael M.

    We have applied first-principles calculations to investigate the topological insulator state of half-Heusler materials and the momentum density for overdoped cuprates. The specific topics addressed in this thesis are as follows. 1. Topological insulators are materials exhibiting a novel quantum state of matter; these insulators are characterized by a bulk excitation generated by the spin orbit interaction, and protected conducting states on their edge or surface. In this work we investigate in detail the electronic structure of a series of ternary half-Heusler compounds MM'X of MgAgAS-type with M = (Lu, La, Sc, Y) and M'X=(PtBi,AuPb,PdBi,PtSb,AuSn,NiBi,PdSb). The characteristic features of all half-Heusler compounds considered here are topological nontrivial semimetal, or nontrivial metal, or trivial insulator. The analysis of the relation among the band inversion strength (distance from the critical line), atomic number of constituents, and lattice constant could provide a methodology (a rule of thumb) to predict the topological order of hypothetical nonmagnetic half-Heusler compound more generally. 2. The bulk Fermi surface (FS) is observed in an overdoped ( x=0.3) single crystal of La2-xSrxCuO4 by using Compton scattering. A 2-D momentum density reconstruction from measured Compton profiles yields a clear FS signature in the third Brillouin zone along [100]. The quantitative agreement between density functional theory calculations and momentum density experiment suggests that Fermi-liquid physics is restored in the overdoped regime. The FS shows signs of a change topological of the FS found at lower doping. We find similar quantitative agreement between the measured 2D angular correlation of positron annihilation radiation (2D-ACAR) spectra and DFT simulations. However, 2D-ACAR does not give such a clear signature of the FS in the extended momentum space in either theory or experiment.

  9. Chiral magnetic conductivity and surface states of Weyl semimetals in topological insulator ultra-thin film multilayer

    NASA Astrophysics Data System (ADS)

    Owerre, S. A.

    2016-06-01

    We investigate an ultra-thin film of topological insulator (TI) multilayer as a model for a three-dimensional (3D) Weyl semimetal. We introduce tunneling parameters t S, {{t}\\bot} , and t D, where the former two parameters couple layers of the same thin film at small and large momenta, and the latter parameter couples neighbouring thin film layers along the z-direction. The Chern number is computed in each topological phase of the system and we find that for {{t}\\text{S}},{{t}\\text{D}}>0 , the tunneling parameter {{t}\\bot} changes from positive to negative as the system transits from Weyl semi-metallic phase to insulating phases. We further study the chiral magnetic effect (CME) of the system in the presence of a time dependent magnetic field. We compute the low-temperature dependence of the chiral magnetic conductivity and show that it captures three distinct phases of the system separated by plateaus. Furthermore, we propose and study a 3D lattice model of Porphyrin thin film, an organic material known to support topological Frenkel exciton edge states. We show that this model exhibits a 3D Weyl semi-metallic phase and also supports a 2D Weyl semi-metallic phase. We further show that this model recovers that of 3D Weyl semimetal in topological insulator thin film multilayer. Thus, paving the way for simulating a 3D Weyl semimetal in topological insulator thin film multilayer. We obtain the surface states (Fermi arcs) in the 3D model and the chiral edge states in the 2D model and analyze their topological properties.

  10. Chiral magnetic conductivity and surface states of Weyl semimetals in topological insulator ultra-thin film multilayer.

    PubMed

    Owerre, S A

    2016-06-15

    We investigate an ultra-thin film of topological insulator (TI) multilayer as a model for a three-dimensional (3D) Weyl semimetal. We introduce tunneling parameters t S, [Formula: see text], and t D, where the former two parameters couple layers of the same thin film at small and large momenta, and the latter parameter couples neighbouring thin film layers along the z-direction. The Chern number is computed in each topological phase of the system and we find that for [Formula: see text], the tunneling parameter [Formula: see text] changes from positive to negative as the system transits from Weyl semi-metallic phase to insulating phases. We further study the chiral magnetic effect (CME) of the system in the presence of a time dependent magnetic field. We compute the low-temperature dependence of the chiral magnetic conductivity and show that it captures three distinct phases of the system separated by plateaus. Furthermore, we propose and study a 3D lattice model of Porphyrin thin film, an organic material known to support topological Frenkel exciton edge states. We show that this model exhibits a 3D Weyl semi-metallic phase and also supports a 2D Weyl semi-metallic phase. We further show that this model recovers that of 3D Weyl semimetal in topological insulator thin film multilayer. Thus, paving the way for simulating a 3D Weyl semimetal in topological insulator thin film multilayer. We obtain the surface states (Fermi arcs) in the 3D model and the chiral edge states in the 2D model and analyze their topological properties. PMID:27157544

  11. Combined gate-tunable Josephson junctions and normal state transport in Bi2Te3 topological insulator thin films

    NASA Astrophysics Data System (ADS)

    Ngabonziza, Prosper; Stehno, Martin, P.; Myoren, Hiroaki; Brinkman, Alexander

    In recent years, extensive efforts have been made to improve the coupling between topological insulators and s-wave superconductors in topological insulator Josephson devices (TIJDs). Despite significant progress, essential questions remain open such as the bulk contribution to the Josephson critical current or the existence (and number) of 4 π -periodic bound states (Majoranas) in TIJDs. To address these issues, we fabricated Nb/Bi2Te3/Nb Josephson junctions alongside Hall bar devices on MBE-grown Bi2Te3 topological insulator thin films. Using the SrTiO3 [111] substrate as a gate dielectric, we tuned the carrier density electrostatically and measured the Josephson supercurrent and the normal state transport properties of our thin film devices. We identify three gate voltage ranges with distinct behavior: A region of intermediate gate bias where the measured quantities change rapidly with the applied electric field, and two saturation regions for large bias of either polarity. We discuss carrier distribution and band alignment in the material as well as implications for the effective Josephson coupling in TIJDs. This work is financially supported by the Dutch Foundation for Fundamental Research on Matter (FOM), the Netherlands Organization for Scientific Research (NWO), and by the European Research Council (ERC).

  12. Weak localization effect in topological insulator micro flakes grown on insulating ferrimagnet BaFe12O19.

    PubMed

    Zheng, Guolin; Wang, Ning; Yang, Jiyong; Wang, Weike; Du, Haifeng; Ning, Wei; Yang, Zhaorong; Lu, Hai-Zhou; Zhang, Yuheng; Tian, Mingliang

    2016-01-01

    Many exotic physics anticipated in topological insulators require a gap to be opened for their topological surface states by breaking time reversal symmetry. The gap opening has been achieved by doping magnetic impurities, which however inevitably create extra carriers and disorder that undermine the electronic transport. In contrast, the proximity to a ferromagnetic/ferrimagnetic insulator may improve the device quality, thus promises a better way to open the gap while minimizing the side-effects. Here, we grow thin single-crystal Sb1.9Bi0.1Te3 micro flakes on insulating ferrimagnet BaFe12O19 by using the van der Waals epitaxy technique. The micro flakes show a negative magnetoresistance in weak perpendicular fields below 50 K, which can be quenched by increasing temperature. The signature implies the weak localization effect as its origin, which is absent in intrinsic topological insulators, unless a surface state gap is opened. The surface state gap is estimated to be 10 meV by using the theory of the gap-induced weak localization effect. These results indicate that the magnetic proximity effect may open the gap for the topological surface attached to BaM insulating ferrimagnet. This heterostructure may pave the way for the realization of new physical effects as well as the potential applications of spintronics devices. PMID:26891682

  13. Weak localization effect in topological insulator micro flakes grown on insulating ferrimagnet BaFe12O19

    PubMed Central

    Zheng, Guolin; Wang, Ning; Yang, Jiyong; Wang, Weike; Du, Haifeng; Ning, Wei; Yang, Zhaorong; Lu, Hai-Zhou; Zhang, Yuheng; Tian, Mingliang

    2016-01-01

    Many exotic physics anticipated in topological insulators require a gap to be opened for their topological surface states by breaking time reversal symmetry. The gap opening has been achieved by doping magnetic impurities, which however inevitably create extra carriers and disorder that undermine the electronic transport. In contrast, the proximity to a ferromagnetic/ferrimagnetic insulator may improve the device quality, thus promises a better way to open the gap while minimizing the side-effects. Here, we grow thin single-crystal Sb1.9Bi0.1Te3 micro flakes on insulating ferrimagnet BaFe12O19 by using the van der Waals epitaxy technique. The micro flakes show a negative magnetoresistance in weak perpendicular fields below 50 K, which can be quenched by increasing temperature. The signature implies the weak localization effect as its origin, which is absent in intrinsic topological insulators, unless a surface state gap is opened. The surface state gap is estimated to be 10 meV by using the theory of the gap-induced weak localization effect. These results indicate that the magnetic proximity effect may open the gap for the topological surface attached to BaM insulating ferrimagnet. This heterostructure may pave the way for the realization of new physical effects as well as the potential applications of spintronics devices. PMID:26891682

  14. Mirror-symmetry protected non-TRIM surface state in the weak topological insulator Bi2TeI

    PubMed Central

    Rusinov, I. P.; Menshchikova, T. V.; Isaeva, A.; Eremeev, S. V.; Koroteev, Yu. M.; Vergniory, M. G.; Echenique, P. M.; Chulkov, E. V.

    2016-01-01

    Strong topological insulators (TIs) support topological surfaces states on any crystal surface. In contrast, a weak, time-reversal-symmetry-driven TI with at least one non-zero v1, v2, v3 ℤ2 index should host spin-locked topological surface states on the surfaces that are not parallel to the crystal plane with Miller indices (v1 v2 v3). On the other hand, mirror symmetry can protect an even number of topological states on the surfaces that are perpendicular to a mirror plane. Various symmetries in a bulk material with a band inversion can independently preordain distinct crystal planes for realization of topological states. Here we demonstrate the first instance of coexistence of both phenomena in the weak 3D TI Bi2TeI which (v1 v2 v3) surface hosts a gapless spin-split surface state protected by the crystal mirror-symmetry. The observed topological state has an even number of crossing points in the directions of the 2D Brillouin zone due to a non-TRIM bulk-band inversion. Our findings shed light on hitherto uncharted features of the electronic structure of weak topological insulators and open up new vistas for applications of these materials in spintronics. PMID:26864814

  15. Mirror-symmetry protected non-TRIM surface state in the weak topological insulator Bi2TeI

    NASA Astrophysics Data System (ADS)

    Rusinov, I. P.; Menshchikova, T. V.; Isaeva, A.; Eremeev, S. V.; Koroteev, Yu. M.; Vergniory, M. G.; Echenique, P. M.; Chulkov, E. V.

    2016-02-01

    Strong topological insulators (TIs) support topological surfaces states on any crystal surface. In contrast, a weak, time-reversal-symmetry-driven TI with at least one non-zero v1, v2, v3 ℤ2 index should host spin-locked topological surface states on the surfaces that are not parallel to the crystal plane with Miller indices (v1 v2 v3). On the other hand, mirror symmetry can protect an even number of topological states on the surfaces that are perpendicular to a mirror plane. Various symmetries in a bulk material with a band inversion can independently preordain distinct crystal planes for realization of topological states. Here we demonstrate the first instance of coexistence of both phenomena in the weak 3D TI Bi2TeI which (v1 v2 v3) surface hosts a gapless spin-split surface state protected by the crystal mirror-symmetry. The observed topological state has an even number of crossing points in the directions of the 2D Brillouin zone due to a non-TRIM bulk-band inversion. Our findings shed light on hitherto uncharted features of the electronic structure of weak topological insulators and open up new vistas for applications of these materials in spintronics.

  16. Quantum anomalous Hall effect in magnetic topological insulators

    SciTech Connect

    Wang, Jing; Lian, Biao; Zhang, Shou -Cheng

    2015-08-25

    The search for topologically non-trivial states of matter has become an important goal for condensed matter physics. Here, we give a theoretical introduction to the quantum anomalous Hall (QAH) effect based on magnetic topological insulators in two-dimensions (2D) and three-dimensions (3D). In 2D topological insulators, magnetic order breaks the symmetry between the counter-propagating helical edge states, and as a result, the quantum spin Hall effect can evolve into the QAH effect. In 3D, magnetic order opens up a gap for the topological surface states, and chiral edge state has been predicted to exist on the magnetic domain walls. We present the phase diagram in thin films of a magnetic topological insulator and review the basic mechanism of ferromagnetic order in magnetically doped topological insulators. We also review the recent experimental observation of the QAH effect. Furthermore, we discuss more recent theoretical work on the coexistence of the helical and chiral edge states, multi-channel chiral edge states, the theory of the plateau transition, and the thickness dependence in the QAH effect.

  17. Quantum anomalous Hall effect in magnetic topological insulators

    NASA Astrophysics Data System (ADS)

    Wang, Jing; Lian, Biao; Zhang, Shou-Cheng

    2015-12-01

    The search for topologically non-trivial states of matter has become an important goal for condensed matter physics. Here, we give a theoretical introduction to the quantum anomalous Hall (QAH) effect based on magnetic topological insulators in two-dimensions (2D) and three-dimensions (3D). In 2D topological insulators, magnetic order breaks the symmetry between the counter-propagating helical edge states, and as a result, the quantum spin Hall effect can evolve into the QAH effect. In 3D, magnetic order opens up a gap for the topological surface states, and chiral edge state has been predicted to exist on the magnetic domain walls. We present the phase diagram in thin films of a magnetic topological insulator and review the basic mechanism of ferromagnetic order in magnetically doped topological insulators. We also review the recent experimental observation of the QAH effect. We discuss more recent theoretical work on the coexistence of the helical and chiral edge states, multi-channel chiral edge states, the theory of the plateau transition, and the thickness dependence in the QAH effect.

  18. Quantum anomalous Hall effect in magnetic topological insulators

    DOE PAGESBeta

    Wang, Jing; Lian, Biao; Zhang, Shou -Cheng

    2015-08-25

    The search for topologically non-trivial states of matter has become an important goal for condensed matter physics. Here, we give a theoretical introduction to the quantum anomalous Hall (QAH) effect based on magnetic topological insulators in two-dimensions (2D) and three-dimensions (3D). In 2D topological insulators, magnetic order breaks the symmetry between the counter-propagating helical edge states, and as a result, the quantum spin Hall effect can evolve into the QAH effect. In 3D, magnetic order opens up a gap for the topological surface states, and chiral edge state has been predicted to exist on the magnetic domain walls. We presentmore » the phase diagram in thin films of a magnetic topological insulator and review the basic mechanism of ferromagnetic order in magnetically doped topological insulators. We also review the recent experimental observation of the QAH effect. Furthermore, we discuss more recent theoretical work on the coexistence of the helical and chiral edge states, multi-channel chiral edge states, the theory of the plateau transition, and the thickness dependence in the QAH effect.« less

  19. Josephson current in a graphene SG/ferromagnetic barrier/SG junction

    NASA Astrophysics Data System (ADS)

    Soodchomshom, Bumned; Tang, I.-Ming; Hoonsawat, Rassmidara

    2008-12-01

    The Josephson current passing through a SG1/FB/SG2 graphene junction, where SG and FB are those parts of a graphene layer which are induced into the superconducting state and into the ferromagnetic state, respectively, and where the small thickness of the FB layer L is studied. The ferromagnetic barrier strength is taken to be given by χH ∼ HL/ℏvF, where H is the strength of the exchange energy and vF ∼ 106m/s is the Fermi velocity of quasiparticles. The eigenstates of the relativistic quasiparticles in the graphene are taken to be the solutions of the Dirac Bogoliubov-de Gennes equations. It is found that the energy levels of the Andreev bound states for the Weyl-Dirac particles in the SG1/FB/SG2 junction are independent of the direction of the spins and that they depend on the strength of ferromagnetic barrier potential. The critical supercurrent is seen to vary in an oscillatory (periodic) manner as χH is varied. The oscillatory behavior of the critical supercurrent carried by the Cooper pairs formed by massless the Weyl-Dirac particles is different from the behavior of the supercurrent carried by the Cooper pairs formed by non-relativistic particles in a conventional SC/FI/SC (FI being a ferromagnetic insulator) junction. In those types of junctions, the supercurrent does not exhibit a similar oscillatory dependence.

  20. Imaging Spatially Varying Magnetic Order in Proximity Induced Magnetic Topological Insulator

    NASA Astrophysics Data System (ADS)

    Rosenberg, Aaron J.; Katmis, Ferhat; Wang, Yihua H.; Kirtley, John R.; Moodera, Jagadeesh S.; Moler, Kathryn A.

    Broken time-reversal symmetry on the surface states of a three dimensional topological insulator, such as Bi2Se3, results in quantized anomalous Hall conductance and is predicted to exhibit topological magneto-electric effects. We plan investigate how Dirac fermions interact with magnetism by imaging the magnetization of a topological insulator (Bi2Se3) sandwiched between two ferromagnetic insulator layers (EuS) with a scanning SQUID microscope. Cooling in an in-plane field leads to a magnetization that varies spatially on a micron scale, with 6-fold rotational symmetry. Understanding the origin of this magnetism may shed light on the exchange interaction and electronic properties of topological insulators.

  1. Towards spin injection from silicon into topological insulators

    NASA Astrophysics Data System (ADS)

    Ojeda-Aristizabal, Claudia; Fuhrer, Michael S.; Appelbaum, Ian

    2012-02-01

    Attempts to uncover evidence of spin-momentum coupling in a topological insulator (TI) using transport measurements are hampered by many challenges. Most importantly, injection of a spin polarized current from a ferromagnet that is in contact or close proximity to a topological insulator can easily give rise to anisotropic magnetoresistance signals or planar Hall effect from stray fields, which have the same symmetry and hence are indistinguishable from any signal coming from the spin-momentum-locked surface states. Here we propose a scheme to remove this difficulty by injecting spin-polarized electrons from undoped silicon into the TI surface states. In addition to providing a long-distance transport region to separate the ferromagnetic spin source from the TI by several hundred microns or even millimeters, this approach will also allow spin precession measurements (necessary for unambiguous identification of spin signals) whereas direct injection does not. Detection is provided by differential measurement from two ballistic current contacts on the topological insulator. We will describe our progress in fabrication and measurement of devices with exfoliated crystals of TI Bi2Se3, including the determination of the silicon-Bi2Se3 Schottky barrier height of 0.34 eV

  2. Ferromagnetic-organic interfacial states and their role on low voltage current injection in tris-8-hydroxyquinloline (Alq{sub 3}) organic spin valves

    SciTech Connect

    Zhang, H. T.; Drew, A. J.; Zhang, S. J. E-mail: t.kreouzis@qmul.ac.uk; Kreouzis, T. E-mail: t.kreouzis@qmul.ac.uk; Han, S.; Desai, P.; Scott, K.; Zhan, Y. Q.; Li, W.; Si, W.; Gillin, W. P.

    2014-11-17

    Organic Spin Valves (OSVs) operate at small bias (<100 mV) when carrier injection should not occur due to injection barriers and in built potentials. We explore the consequences of hybrid-interface states between a ferromagnetic electrode and an organic semiconductor in OSV carrier injection. By temperature-dependent Dark Injection measurements, we observe hole trapping due to these filled states and measure a low thermal activation energy (∼100 meV) of the carrier density within OSVs. The small injection barrier is consistent with a significant interfacial potential, due to hybrid-interface state filling, overcoming the injection barrier due to the electrode work function—transport level mismatch.

  3. Lower limits of spin detection efficiency for two-parameter two-qubit (TPTQ) states with non-ideal ferromagnetic detectors

    NASA Astrophysics Data System (ADS)

    Majd, Nayereh; Ghasemi, Zahra

    2016-07-01

    We have investigated a TPTQ state as an input state of a non-ideal ferromagnetic detectors. Minimal spin polarization required to demonstrate spin entanglement according to entanglement witness and CHSH inequality with respect to (w.r.t.) their two free parameters have been found, and we have numerically shown that the entanglement witness is less stringent than the direct tests of Bell's inequality in the form of CHSH in the entangled limits of its free parameters. In addition, the lower limits of spin detection efficiency fulfilling secure cryptographic key against eavesdropping have been derived. Finally, we have considered TPTQ state as an output of spin decoherence channel and the region of ballistic transmission time w.r.t. spin relaxation time and spin dephasing time has been found.

  4. Ground state properties and thermoelectric behavior of Ru2VZ (Z=Si, ge, sn) half-metallic ferromagnetic full-Heusler compounds

    NASA Astrophysics Data System (ADS)

    Yalcin, Battal Gazi

    2016-06-01

    The ground state properties namely structural, mechanical, electronic and magnetic properties and thermoelectric behavior of Ru2VZ (Z=Si, Ge and Sn) half-metallic ferromagnetic full-Heusler compounds are systematically investigated. These compounds are ferromagnetic and crystallize in the Heusler type L21 structure (prototype: Cu2MnAl, Fm-3m 225). This result is confirmed for Ru2VSi and Ru2VSn by experimental work reported by Yin and Nash using high temperature direct reaction calorimetry. The studied materials are half-metallic ferromagnets with a narrow direct band gap in the minority spin channel that amounts to 31 meV, 66 meV and 14 meV for Ru2VSi, Ru2VGe, and Ru2VSn, respectively. The total spin magnetic moment (Mtot) of the considered compounds satisfies a Slater-Pauling type rule for localized magnetic moment systems (Mtot=(NV-24)μB), where NV=25 is the number of valence electrons in the primitive cell. The Curie temperature within the random phase approximation (RPA) is found to be 23 K, 126 K and 447 K for Ru2VSi, Ru2VGe and Ru2VSn, respectively. Semi-classical Boltzmann transport theories have been used to obtain thermoelectric constants, such as Seebeck coefficient (S), electrical (σ/τ) and thermal conductivity (κ/τ), power factor (PF) and the Pauli magnetic susceptibility (χ). ZTMAX values of 0.016 (350 K), 0.033 (380 K) and 0.063 (315 K) are achieved for Ru2VSi, Ru2VGe and Ru2VSn, respectively. It is expected that the obtained results might be a trigger in future experimentally interest in this type of full-Heusler compounds.

  5. Field-induced transition of the magnetic ground state from A-type antiferromagnetic to ferromagnetic order in CsCo2Se2

    NASA Astrophysics Data System (ADS)

    von Rohr, Fabian; Krzton-Maziopa, Anna; Pomjakushin, Vladimir; Grundmann, Henrik; Guguchia, Zurab; Schnick, Wolfgang; Schilling, Andreas

    2016-07-01

    We report on the magnetic properties of CsCo2Se2 with ThCr2Si2 structure, which we have characterized through a series of magnetization and neutron diffraction measurements. We find that CsCo2Se2 undergoes a phase transition to an antiferromagnetically ordered state with a Néel temperature of {{T}\\text{N}}≈ 66 K. The nearest neighbour interactions are ferromagnetic as observed by the positive Curie–Weiss temperature of \\Theta≈ 51.0 K. We find that the magnetic structure of CsCo2Se2 consists of ferromagnetic sheets, which are stacked antiferromagnetically along the tetragonal c-axis, generally referred to as A-type antiferromagnetic order. The observed magnitude of the ordered magnetic moment at T  =  1.5 K is found to be only 0.20(1){μ\\text{Bohr}}  / Co. Already in comparably small magnetic fields of {μ0}H{{}\\text{MM}}(5~K)≈ 0.3 T, we observe a metamagnetic transition that can be attributed to spin-rearrangements of CsCo2Se2, with the moments fully ferromagnetically saturated in a magnetic field of {μ0}{{H}\\text{FM}}(5~K)≈ 6.4 T. We discuss the entire experimentally deduced magnetic phase diagram for CsCo2Se2 with respect to its unconventionally weak magnetic coupling. Our study characterizes CsCo2Se2, which is chemically and electronically posed closely to the A x Fe2‑y Se2 superconductors, as a host of versatile magnetic interactions.

  6. Field-induced transition of the magnetic ground state from A-type antiferromagnetic to ferromagnetic order in CsCo2Se2.

    PubMed

    von Rohr, Fabian; Krzton-Maziopa, Anna; Pomjakushin, Vladimir; Grundmann, Henrik; Guguchia, Zurab; Schnick, Wolfgang; Schilling, Andreas

    2016-07-13

    We report on the magnetic properties of CsCo2Se2 with ThCr2Si2 structure, which we have characterized through a series of magnetization and neutron diffraction measurements. We find that CsCo2Se2 undergoes a phase transition to an antiferromagnetically ordered state with a Néel temperature of [Formula: see text] K. The nearest neighbour interactions are ferromagnetic as observed by the positive Curie-Weiss temperature of [Formula: see text] K. We find that the magnetic structure of CsCo2Se2 consists of ferromagnetic sheets, which are stacked antiferromagnetically along the tetragonal c-axis, generally referred to as A-type antiferromagnetic order. The observed magnitude of the ordered magnetic moment at T  =  1.5 K is found to be only 0.20(1)[Formula: see text] / Co. Already in comparably small magnetic fields of [Formula: see text] T, we observe a metamagnetic transition that can be attributed to spin-rearrangements of CsCo2Se2, with the moments fully ferromagnetically saturated in a magnetic field of [Formula: see text] T. We discuss the entire experimentally deduced magnetic phase diagram for CsCo2Se2 with respect to its unconventionally weak magnetic coupling. Our study characterizes CsCo2Se2, which is chemically and electronically posed closely to the A x Fe2-y Se2 superconductors, as a host of versatile magnetic interactions. PMID:27195766

  7. Dirac states with knobs on: Interplay of external parameters and the surface electronic properties of three-dimensional topological insulators

    NASA Astrophysics Data System (ADS)

    Frantzeskakis, E.; de Jong, N.; Zwartsenberg, B.; Bay, T. V.; Huang, Y. K.; Ramankutty, S. V.; Tytarenko, A.; Wu, D.; Pan, Y.; Hollanders, S.; Radovic, M.; Plumb, N. C.; Xu, N.; Shi, M.; Lupulescu, C.; Arion, T.; Ovsyannikov, R.; Varykhalov, A.; Eberhardt, W.; de Visser, A.; van Heumen, E.; Golden, M. S.

    2015-05-01

    Topological insulators are a novel materials platform with high applications potential in fields ranging from spintronics to quantum computation. In the ongoing scientific effort to demonstrate controlled manipulation of their electronic structure by external means, i.e., the provision of knobs with which to tune properties, stoichiometric variation and surface decoration are two effective approaches that have been followed. In angle-resolved photoelectron spectroscopy (ARPES) experiments, both approaches are seen to lead to electronic band-structure changes. Most importantly, such approaches result in variations of the energy position of bulk and surface-related features and the creation of two-dimensional electron gases. The data presented here demonstrate that a third manipulation handle is accessible by utilizing the amount of super-band-gap light a topological insulator surface has been exposed to under typical ARPES experimental conditions. Our results show that this third knob acts on an equal footing with stoichiometry and surface decoration as a modifier of the electronic band structure, and that it is in continuous and direct competition with the latter. The data clearly point towards surface photovoltage and photoinduced desorption as the physical phenomena behind modifications of the electronic band structure under exposure to high-flux photons. We show that the interplay of these phenomena can minimize and even eliminate the adsorbate-related surface band bending on typical binary, ternary, and quaternary Bi-based topological insulators. Including the influence of the sample temperature, these data set up a detailed framework for the external control of the electronic band structure in topological insulator compounds in an ARPES setting. Four external knobs are available: bulk stoichiometry, surface decoration, temperature, and photon exposure. These knobs can be used in conjunction to fine tune the band energies near the surface and consequently

  8. Anomalous charge and negative-charge-transfer insulating state in cuprate chain compound KCuO2

    NASA Astrophysics Data System (ADS)

    Choudhury, D.; Rivero, P.; Meyers, D.; Liu, X.; Cao, Y.; Middey, S.; Whitaker, M. J.; Barraza-Lopez, S.; Freeland, J. W.; Greenblatt, M.; Chakhalian, J.

    2015-11-01

    Using a combination of x-ray absorption spectroscopy (XAS) experiments and first-principles calculations, we demonstrate that insulating KCuO2 contains Cu in an unusually high formal 3+ valence state, and the ligand-to-metal (O-to-Cu) charge-transfer energy is intriguingly negative (Δ ˜-1.5 eV) and has a dominant (˜60 % ) ligand-hole character in the ground state akin to the high Tc cuprate Zhang-Rice state. Unlike most other formal Cu3 + compounds, the Cu 2 p XAS spectra of KCuO2 exhibit pronounced 3 d8 (Cu3 +) multiplet structures, which account for ˜40 % of its ground state wave function. Ab initio calculations elucidate the origin of the band gap in KCuO2 as arising primarily from strong intracluster Cu 3 d -O 2 p hybridizations (tpd); the value of the band gap decreases with a reduced value of tpd. Further, unlike conventional negative-charge-transfer insulators, the band gap in KCuO2 persists even for vanishing values of Coulomb repulsion U , underscoring the importance of single-particle band-structure effects connected to the one-dimensional nature of the compound.

  9. Ferromagnetism in armchair graphene nanoribbons

    NASA Astrophysics Data System (ADS)

    Lin, Hsiu-Hau; Hikihara, Toshiya; Jeng, Horng-Tay; Huang, Bor-Luen; Mou, Chung-Yu; Hu, Xiao

    2009-01-01

    Due to the weak spin-orbit interaction and the peculiar relativistic dispersion in graphene, there are exciting proposals to build spin qubits in graphene nanoribbons with armchair boundaries. However, the mutual interactions between electrons are neglected in most studies so far and thus motivate us to investigate the role of electronic correlations in armchair graphene nanoribbon by both analytical and numerical methods. Here we show that the inclusion of mutual repulsions leads to drastic changes and the ground state turns ferromagnetic in a range of carrier concentrations. Our findings highlight the crucial importance of the electron-electron interaction and its subtle interplay with boundary topology in graphene nanoribbons. Furthermore, since the ferromagnetic properties sensitively depend on the carrier concentration, it can be manipulated at ease by electric gates. The resultant ferromagnetic state with metallic conductivity is not only surprising from an academic viewpoint, but also has potential applications in spintronics at nanoscale.

  10. Edge-state-induced Andreev oscillation in quantum anomalous Hall insulator-superconductor junctions

    NASA Astrophysics Data System (ADS)

    Lian, Biao; Wang, Jing; Zhang, Shou-Cheng

    2016-04-01

    We study the quantum Andreev oscillation induced by interference of the edge chiral Majorana fermions in junctions made of quantum anomalous Hall (QAH) insulators and superconductors (SCs). We show two chiral Majorana fermions on a QAH edge with SC proximity generically have a momentum difference Δ k , which depends on the chemical potentials of both the QAH insulator and the SC. Due to the spatial interference induced by Δ k , the longitudinal conductance of QAH-SC junctions oscillates with respect to the edge lengths and the chemical potentials, which can be probed via charge transport. Furthermore, we show the dynamical SC phase fluctuation will give rise to a geometrical correction to the longitudinal conductance of the junctions.

  11. Nonmagnetic Insulator State in Na1CoO2 and Phase Separation of Na Vacancies

    NASA Astrophysics Data System (ADS)

    de Vaulx, C.; Julien, M.-H.; Berthier, C.; Horvatić, M.; Bordet, P.; Simonet, V.; Chen, D. P.; Lin, C. T.

    2005-10-01

    Crystallographic, magnetic, and NMR properties of a NaxCoO2 single crystal with x≃1 are presented. We identify the stoichiometric Na1CoO2 phase, which is shown to be a nonmagnetic insulator, as expected for homogeneous planes of Co3+ ions with S=0. In addition, we present evidence that, because of slight average Na deficiency, chemical and electronic phase separation leads to a segregation of Na vacancies into the well-defined, magnetic, Na0.8CoO2 phase. The importance of phase separation is discussed in the context of magnetic order for x≃0.8 and the occurrence of a metal-insulator transition for x→1.

  12. Optically detecting the edge-state of a three-dimensional topological insulator under ambient conditions by ultrafast infrared photoluminescence spectroscopy

    PubMed Central

    Maezawa, Shun-ya; Watanabe, Hiroshi; Takeda, Masahiro; Kuroda, Kenta; Someya, Takashi; Matsuda, Iwao; Suemoto, Tohru

    2015-01-01

    Ultrafast infrared photoluminescence spectroscopy was applied to a three-dimensional topological insulator TlBiSe2 under ambient conditions. The dynamics of the luminescence exhibited bulk-insulating and gapless characteristics bounded by the bulk band gap energy. The existence of the topologically protected surface state and the picosecond-order relaxation time of the surface carriers, which was distinguishable from the bulk response, were observed. Our results provide a practical method applicable to topological insulators under ambient conditions for device applications. PMID:26552784

  13. Long-range supercurrents induced by the interference effect of opposite-spin triplet state in clean superconductor-ferromagnet structures

    NASA Astrophysics Data System (ADS)

    Meng, Hao; Wu, Jiansheng; Wu, Xiuqiang; Ren, Mengyuan; Ren, Yajie

    2016-07-01

    By now it is known that in an s-wave superconductor-ferromagnet-superconductor (SF S) structure the supercurrent induced by spin singlet pairs can only transmit a short distance of the order of magnetic coherence length. The long-range supercurrent, taking place on the length scale of the normal metal coherence length, will be maintained by equal-spin triplet pairs, which can be generated by magnetic inhomogeneities in the system. In this paper, we have shown an unusual long-range supercurrent, which can take place in clean {{SF}}1{F}2S junction with non-parallel orientation of magnetic moments. The mechanism behind the enhancement of Josephson current is provided by the interference of the opposite-spin triplet states deriving from S/{F}1 and {F}2/S interfaces when both ferromagnetic layers have the same values of the length and exchange field. This discovery can offer a natural explanation for recent experiments (Robinson et al 2010 Phys. Rev. Lett. 104 207001; Baek et al 2014 Nat. Commun. 5 3888).

  14. Metal-insulator transition and tunable Dirac-cone surface state in the topological insulator TlBi1 -xSbxTe2 studied by angle-resolved photoemission

    NASA Astrophysics Data System (ADS)

    Trang, Chi Xuan; Wang, Zhiwei; Yamada, Keiko; Souma, Seigo; Sato, Takafumi; Takahashi, Takashi; Segawa, Kouji; Ando, Yoichi

    2016-04-01

    We report a systematic angle-resolved photoemission spectroscopy on topological insulator (TI) TlBi1 -xSbxTe2 which is bulk insulating at 0.5 ≲x ≲0.9 and undergoes a metal-insulator-metal transition with the Sb content x . We found that this transition is characterized by a systematic hole doping with increasing x , which results in the Fermi-level crossings of the bulk conduction and valence bands at x ˜0 and x ˜1 , respectively. The Dirac point of the topological surface state is gradually isolated from the valence-band edge, accompanied by a sign reversal of Dirac carriers. We also found that the Dirac velocity is the largest among known solid-solution TI systems. The TlBi1 -xSbxTe2 system thus provides an excellent platform for Dirac-cone engineering and device applications of TIs.

  15. Strain controlled orbital state and magnetization in insulating LaMnO3+δ films

    NASA Astrophysics Data System (ADS)

    Zhang, A. M.; Cheng, S. L.; Lin, J. G.; Wu, X. S.

    2015-05-01

    LaMnO3+δ films with various thicknesses were grown on LaAlO3 (001) single crystal substrate to investigate the effect of in-plane compressive strain (˜-0.57%) on magnetic properties. All films exhibit a blocking temperature Tb at which the zero field cooled magnetization reaches a maximum, indicating the ferromagnetic (FM) nanoclusters are embedded in the background of antiferromagnetic (AFM) matrix. The onset temperature of FM transition Tc and Tb is increased by 24% and 89%, respectively, with the thickness decreasing from 82.4 nm to 9.2 nm. Simultaneously, the saturation magnetization greatly increases by 309%, which is ascribed to the strain-induced transition of AFM to FM phase due to the orbital order structure switching from x2- 1/y2-1 [A-type] to (x2- y2) + (z2- 1) [F-type].

  16. Strong reflection and periodic resonant transmission of helical edge states in topological-insulator stub-like resonators

    SciTech Connect

    Takagaki, Y.

    2015-08-07

    The helical edge states of two-dimensional topological insulators (TIs) experience appreciable quantum mechanical scattering in narrow channels when the width changes abruptly. The interference of the geometry scattering in narrow-wide-narrow waveguide structures is shown to give rise to the strong suppression of transmission when the incident energy is barely above the propagation threshold. Periodic resonant transmission takes place in this high reflection regime while the length of the wide section is varied. The resonance condition is governed by the transverse confinement in the wide section, where the form of quantization is manifested to differ for the two orthogonal directions. The confined energy levels in TI quantum dots are derived based on this observation. In addition, the off-diagonal spin-orbit term is found to produce an anomalous resonance state, which merges with the bottom ordinary resonance state to annihilate.

  17. Theory of disordered Heisenberg ferromagnets

    NASA Technical Reports Server (NTRS)

    Stubbs, R. M.

    1973-01-01

    A Green's function technique is used to calculate the magnetic properties of Heisenberg ferromagnets in which the exchange interactions deviate randomly in strength from the mean interaction. Systems of sc, bcc, and fcc topologies and of general spin values are treated. Disorder produces marked effects in the density of spin wave states, in the form of enhancement of the low-energy density and extension of the energy band to higher values. The spontaneous magnetization and the Curie temperature decrease with increasing disorder. The effects of disorder are shown to be more pronounced in the ferromagnetic than in the paramagnetic phase.

  18. Jeff=1 /2 Mott-Insulating State in Rh and Ir Fluorides

    NASA Astrophysics Data System (ADS)

    Birol, Turan; Haule, Kristjan

    2015-03-01

    Discovery of new transition metal compounds with large spin orbit coupling coexisting with strong electron-electron correlation among the d electrons is essential for understanding the physics that emerges from the interplay of these two effects. In this study, we predict a novel class of Jeff=1 /2 Mott insulators in a family of fluoride compounds that are previously synthesized, but not characterized extensively. First principles calculations in the level of all electron density functional theory +dynamical mean field theory indicate that these compounds have large Mott gaps and some of them exhibit unprecedented proximity to the ideal, S U (2 ) symmetric Jeff=1 /2 limit.

  19. Chiral tunneling modulated by a time-periodic potential on the surface states of a topological insulator

    PubMed Central

    Li, Yuan; Jalil, Mansoor B. A.; Tan, S. G.; Zhao, W.; Bai, R.; Zhou, G. H.

    2014-01-01

    Time-periodic perturbation can be used to modify the transport properties of the surface states of topological insulators, specifically their chiral tunneling property. Using the scattering matrix method, we study the tunneling transmission of the surface states of a topological insulator under the influence of a time-dependent potential and finite gate bias voltage. It is found that perfect transmission is obtained for electrons which are injected normally into the time-periodic potential region in the absence of any bias voltage. However, this signature of Klein tunneling is destroyed when a bias voltage is applied, with the transmission probability of normally incident electrons decreasing with increasing gate bias voltage. Likewise, the overall conductance of the system decreases significantly when a gate bias voltage is applied. The characteristic left-handed helicity of the transmitted spin polarization is also broken by the finite gate bias voltage. In addition, the time-dependent potential modifies the large-angle transmission profile, which exhibits an oscillatory or resonance-like behavior. Finally, time-dependent transport modes (with oscillating potential in the THz frequency) can result in enhanced overall conductance, irrespective of the presence or absence of the gate bias voltage. PMID:24713634

  20. Control of magnetic direction in multi-layer ferromagnetic devices by bias voltage

    DOEpatents

    You, Chun-Yeol; Bader, Samuel D.

    2001-01-01

    A system for controlling the direction of magnetization of materials comprising a ferromagnetic device with first and second ferromagnetic layers. The ferromagnetic layers are disposed such that they combine to form an interlayer with exchange coupling. An insulating layer and a spacer layer are located between the first and second ferromagnetic layers. A direct bias voltage is applied to the interlayer exchange coupling, causing the direction of magnetization of the second ferromagnetic layer to change. This change of magnetization direction occurs in the absence of any applied external magnetic field.

  1. X-ray spectroscopic study of the charge state and local orderingof room-temperature ferromagnetic Mn oped ZnO

    SciTech Connect

    Guo, J.-H.; Gupta, Amita; Sharma, Parmanand; Rao, K.V.; Marcus,M.A.; Dong, C.L.; Guillen, J.M.O.; Butorin, S.M.; Mattesini, M.; Glans,P.A.; Smith, K.E.; Chang, C.L.; Ahuja, R.

    2007-08-07

    The charge state and local ordering of Mn doped into a pulsed laser deposited single-phase thin film of ZnO are investigated by using X-ray absorption spectroscopy at the O K-, Mn K- and L-edges, and X-ray emission spectroscopy at the O K- and Mn L-edge. This film is found to be ferromagnetic at room temperature. EXAFS measurement shows that Mn{sup 2+} replaces Zn site in tetrahedral symmetry, and there is no evidence for either metallic Mn or MnO in the film. Upon Mn doping, the top of O 2p valence band extends into the bandgap indicating additional charge carries being created.

  2. Direct search for a ferromagnetic phase in a heavily overdoped nonsuperconducting copper oxide.

    PubMed

    Sonier, J E; Kaiser, C V; Pacradouni, V; Sabok-Sayr, S A; Cochrane, C; MacLaughlin, D E; Komiya, S; Hussey, N E

    2010-10-01

    The doping of charge carriers into the CuO(2) planes of copper oxide Mott insulators causes a gradual destruction of antiferromagnetism and the emergence of high-temperature superconductivity. Optimal superconductivity is achieved at a doping concentration p beyond which further increases in doping cause a weakening and eventual disappearance of superconductivity. A potential explanation for this demise is that ferromagnetic fluctuations compete with superconductivity in the overdoped regime. In this case, a ferromagnetic phase at very low temperatures is predicted to exist beyond the doping concentration at which superconductivity disappears. Here we report on a direct examination of this scenario in overdoped La(2-x)Sr(x)CuO(4) using the technique of muon spin relaxation. We detect the onset of static magnetic moments of electronic origin at low temperature in the heavily overdoped nonsuperconducting region. However, the magnetism does not exist in a commensurate long-range ordered state. Instead it appears as a dilute concentration of static magnetic moments. This finding places severe restrictions on the form of ferromagnetism that may exist in the overdoped regime. Although an extrinsic impurity cannot be absolutely ruled out as the source of the magnetism that does occur, the results presented here lend support to electronic band calculations that predict the occurrence of weak localized ferromagnetism at high doping. PMID:20855579

  3. Direct search for a ferromagnetic phase in a heavily overdoped nonsuperconducting copper oxide

    PubMed Central

    Sonier, J. E.; Kaiser, C. V.; Pacradouni, V.; Sabok-Sayr, S. A.; Cochrane, C.; MacLaughlin, D. E.; Komiya, S.; Hussey, N. E.

    2010-01-01

    The doping of charge carriers into the CuO2 planes of copper oxide Mott insulators causes a gradual destruction of antiferromagnetism and the emergence of high-temperature superconductivity. Optimal superconductivity is achieved at a doping concentration p beyond which further increases in doping cause a weakening and eventual disappearance of superconductivity. A potential explanation for this demise is that ferromagnetic fluctuations compete with superconductivity in the overdoped regime. In this case, a ferromagnetic phase at very low temperatures is predicted to exist beyond the doping concentration at which superconductivity disappears. Here we report on a direct examination of this scenario in overdoped La2-xSrxCuO4 using the technique of muon spin relaxation. We detect the onset of static magnetic moments of electronic origin at low temperature in the heavily overdoped nonsuperconducting region. However, the magnetism does not exist in a commensurate long-range ordered state. Instead it appears as a dilute concentration of static magnetic moments. This finding places severe restrictions on the form of ferromagnetism that may exist in the overdoped regime. Although an extrinsic impurity cannot be absolutely ruled out as the source of the magnetism that does occur, the results presented here lend support to electronic band calculations that predict the occurrence of weak localized ferromagnetism at high doping. PMID:20855579

  4. Ferromagnetic interactions between transition-metal impurities in topological and 3D Dirac semimetals

    NASA Astrophysics Data System (ADS)

    Dietl, Tomasz

    The magnitude of ferromagnetic coupling driven by inter-band (Bloembergen-Rowland - BR) and intra-band (Ruderman-Kittel-Kasuya-Yoshida - RKKY) spin polarization is evaluated within kp theory for topological semimetals Hg1-xMnxTe and Hg1-xMnxSe as well as for 3D Dirac semimetal (Cd1-xMnx)3As2. In these systems Mn2+ ions do not introduce any carriers. Since, however, both conduction and valence bands are built from anion p-type wave functions, hybridization of Mn d levels with neighboring anion p states leads to spin-dependent p - d coupling of both electrons and holes to localized Mn spins, resulting in sizable inter-band spin polarization and, thus in large BR interactions. We demonstrate that this ferromagnetic coupling, together with antiferromagnetic superexchange, elucidate a specific dependence of spin-glass freezing temperature on x, determined experimentally for these systems. Furthermore, by employing a multi-orbital tight-binding method, we find that superexchange becomes ferromagnetic when Mn is replaced by Cr or V. Since Cr should act as an isoelectronic impurity in HgTe, this opens a road for realization of ferromagnetic topological insulators based on (Hg,Cr)Te.

  5. An unsteady-state method for determining overall coefficient of heat transfer (k-value) of insulated bodies at variable external temperatures

    NASA Astrophysics Data System (ADS)

    Gvozdenac, Dušan

    2015-02-01

    A method for determining k-value of insulated bodies at variable external temperatures is proposed, theoretically described and results of experimental verification are presented in this paper. Theoretical analyses include descriptions of both physical and mathematical models and definition of the extrapolation formula. The method is tested in laboratory conditions on a simple model of insulated chamber and compliance with all testing conditions prescribed by Agreement on Transport of Perishables. The advantage of this method in comparison to any other unsteady- or steady-state method is that it enables k-value determination out of the specialized test stations. This further makes it possible to carry out cheaper and more frequent k-value measurements/control in insulated bodies. Also, the proposed method can be used for testing the k-value of stationary insulated chambers which cannot be objectively tested by means of stationary methods.

  6. Controlling the metal-to-insulator relaxation of the metastable hidden quantum state in 1T-TaS2

    PubMed Central

    Vaskivskyi, Igor; Gospodaric, Jan; Brazovskii, Serguei; Svetin, Damjan; Sutar, Petra; Goreshnik, Evgeny; Mihailovic, Ian A.; Mertelj, Tomaz; Mihailovic, Dragan

    2015-01-01

    Controllable switching between metastable macroscopic quantum states under nonequilibrium conditions induced either by light or with an external electric field is rapidly becoming of great fundamental interest. We investigate the relaxation properties of a “hidden” (H) charge density wave (CDW) state in thin single crystals of the layered dichalcogenide 1T-TaS2, which can be reached by either a single 35-fs optical laser pulse or an ~30-ps electrical pulse. From measurements of the temperature dependence of the resistivity under different excitation conditions, we find that the metallic H state relaxes to the insulating Mott ground state through a sequence of intermediate metastable states via discrete jumps over a “Devil’s staircase.” In between the discrete steps, an underlying glassy relaxation process is observed, which arises because of reciprocal-space commensurability frustration between the CDW and the underlying lattice. We show that the metastable state relaxation rate may be externally stabilized by substrate strain, thus opening the way to the design of nonvolatile ultrafast high-temperature memory devices based on switching between CDW states with large intrinsic differences in electrical resistance. PMID:26601218

  7. Accumulation capacitance frequency dispersion of III-V metal-insulator-semiconductor devices due to disorder induced gap states

    SciTech Connect

    Galatage, R. V.; Zhernokletov, D. M.; Dong, H.; Brennan, B.; Hinkle, C. L.; Wallace, R. M.; Vogel, E. M.

    2014-07-07

    The origin of the anomalous frequency dispersion in accumulation capacitance of metal-insulator-semiconductor devices on InGaAs and InP substrates is investigated using modeling, electrical characterization, and chemical characterization. A comparison of the border trap model and the disorder induced gap state model for frequency dispersion is performed. The fitting of both models to experimental data indicate that the defects responsible for the measured dispersion are within approximately 0.8 nm of the surface of the crystalline semiconductor. The correlation between the spectroscopically detected bonding states at the dielectric/III-V interface, the interfacial defect density determined using capacitance-voltage, and modeled capacitance-voltage response strongly suggests that these defects are associated with the disruption of the III-V atomic bonding and not border traps associated with bonding defects within the high-k dielectric.

  8. Accumulation capacitance frequency dispersion of III-V metal-insulator-semiconductor devices due to disorder induced gap states

    NASA Astrophysics Data System (ADS)

    Galatage, R. V.; Zhernokletov, D. M.; Dong, H.; Brennan, B.; Hinkle, C. L.; Wallace, R. M.; Vogel, E. M.

    2014-07-01

    The origin of the anomalous frequency dispersion in accumulation capacitance of metal-insulator-semiconductor devices on InGaAs and InP substrates is investigated using modeling, electrical characterization, and chemical characterization. A comparison of the border trap model and the disorder induced gap state model for frequency dispersion is performed. The fitting of both models to experimental data indicate that the defects responsible for the measured dispersion are within approximately 0.8 nm of the surface of the crystalline semiconductor. The correlation between the spectroscopically detected bonding states at the dielectric/III-V interface, the interfacial defect density determined using capacitance-voltage, and modeled capacitance-voltage response strongly suggests that these defects are associated with the disruption of the III-V atomic bonding and not border traps associated with bonding defects within the high-k dielectric.

  9. Microscopic theory of the insulating electronic ground states of the actinide dioxides AnO2 (An = U, Np, Pu, Am, and Cm)

    NASA Astrophysics Data System (ADS)

    Suzuki, M.-T.; Magnani, N.; Oppeneer, P. M.

    2013-11-01

    The electronic states of the actinide dioxides AnO2 (with An = U, Np, Pu, Am, and Cm) are investigated employing first-principles calculations within the framework of the local density approximation +U (LDA+U) approach, implemented in a full-potential linearized augmented plane-wave scheme. A systematic analysis of the An-5f states is performed which provides intuitive connections between the electronic structures and the local crystalline fields of the f states in the AnO2 series. Particularly the mechanisms leading to the experimentally observed insulating ground states are investigated. These are found to be caused by the strong spin-orbit and Coulomb interactions of the 5f orbitals; however, as a result of the different configurations, this mechanism works in distinctly different ways for each of the AnO2 compounds. In agreement with experimental observations, the nonmagnetic states of plutonium and curium dioxide are computed to be insulating, whereas those of uranium, neptunium, and americium dioxides require additional symmetry breaking to reproduce the insulator ground states, a condition which is met with magnetic phase transitions. We show that the occupancy of the An-f orbitals is closely connected to each of the appearing insulating mechanisms. We furthermore investigate the detailed constitution of the noncollinear multipolar moments for transverse 3q magnetic ordered states in UO2 and longitudinal 3q high-rank multipolar ordered states in NpO2 and AmO2.

  10. Insulating state to quantum Hall-like state transition in a spin-orbit-coupled two-dimensional electron system

    SciTech Connect

    Lo, Shun-Tsung; Hsu, Chang-Shun; Lin, Y. M.; Lin, S.-D.; Lee, C. P.; Ho, Sheng-Han; Chuang, Chiashain; Wang, Yi-Ting; Liang, C.-T.

    2014-07-07

    We study interference and interactions in an InAs/InAsSb two-dimensional electron system. In such a system, spin-orbit interactions are shown to be strong, which result in weak antilocalization (WAL) and thereby positive magnetoresistance around zero magnetic field. After suppressing WAL by the magnetic field, we demonstrate that classical positive magnetoresistance due to spin-orbit coupling plays a role. With further increasing the magnetic field, the system undergoes a direct insulator-quantum Hall transition. By analyzing the magnetotransport behavior in different field regions, we show that both electron-electron interactions and spin-related effects are essential in understanding the observed direct transition.

  11. Transport studies of mesoscopic and magnetic topological insulators

    NASA Astrophysics Data System (ADS)

    Kandala, Abhinav

    Topological Insulators (TI) are a novel class of materials that are ideally insulating in the bulk, but have gapless, metallic states at the surface. These surface states have very exciting properties such as suppressed backscattering and spin-momentum locking, which are of great interest for research efforts towards dissipation-less electronics and spintronics. The popular thermo-electrics from the Bi chalcogenide family -- Bi2Se3 and Bi 2Te3 -- have been experimentally demonstrated to be promising candidate TI materials, and form the chosen material system for this dissertation research. The first part of this dissertation research focuses on low temperature magneto-transport measurements of mesoscopic topological insulator devices (Chapter 3). The top-down patterning of epitaxial thin films of Bi2Se 3 and Bi2Te3 (that are plagued with bulk conduction) is motivated, in part, by an effort to enhance the surface-to-volume ratio in mesoscopic channels. At cryogenic temperatures, transport measurements of these devices reveal periodic conductance fluctuations in straight channel devices, despite the lack of any explicit patterning of the TI film into a ring or a loop. A careful analysis of the surface morphology and comparison with the transport data then demonstrate that scattering off the edges of triangular plateaus at the surface leads to the creation of Aharonov-Bohm electronic orbits responsible for the periodicity. Another major focus of this dissertation work is on combining topological insulators with magnetism. This has been shown to open a gap in the surface states leading to possibilities of magnetic "gating" and the realization of dissipation-less transport at zero-field, amongst several other exotic quantum phenomena. In this dissertation, I present two different schemes for probing these effects in electrical transport devices -- interfacing with insulating ferromagnets (Chapter 4) and bulk magnetic doping (Chapter 5). In Chapter 4, I shall present the

  12. The electronic structure of the Mott insulator VO2: the strongly correlated metal state is screened by impurity band

    NASA Astrophysics Data System (ADS)

    Kim, Hyun-Tak

    ="0pt"> 2 3 3) EF case, it sustains the insulator state. However, when both Uc > (2 2 3 3) EF and Δact = 0 by excitation are satisfied, the IMT occurs in Vg >= 0. This indicates that the excitation (Δact = 0) breaks the Coulomb equilibrium (Vg<0 and insulator sustaining Uc) in Eq. (1) ; the Coulomb energy changes from Uc to a UNtotntot ntot to the conduction band, by applying external parameters such as heat, pressure, doping etc. The direct band gap (semiconductor gap) in the Mott insulator smaller than that of dielectric insulators can be explained by Uc.[1] NewJ.Phys.6(2004)52.

  13. High-Resolution Identification of Chemical States in Individual Metal Clusters in an Insulating Amorphous Polymer.

    PubMed

    Kubo, Yugo; Mizoguchi, Akira; Fujita, Jun-Ichi

    2016-05-17

    The effectivity of cryo-scanning transmission electron microscopy-electron energy loss spectroscopy was demonstrated for nanoscale analysis of the cross-section of the Cu/polyimide interface. The nanoscale Cu/Cu2O/CuO layer structure at the interface was clearly observed for the first time. In addition, a Cu atom was identified, embedded in the polyimide matrix, and the average valence of diffusing Cu atoms or nanoclusters was determined using (cryo-)scanning transmission electron microscopy-electron energy loss spectroscopy. On the basis of these results, we have proposed a mechanism for the diffusion of Cu atoms in polyimide. To the best of our knowledge, this is the first report of the observation of a metal atom embedded in an insulating amorphous polymer. PMID:27104743

  14. Growth of high quality single crystals of Bi2Se3 topological insulator via solid state reaction method

    NASA Astrophysics Data System (ADS)

    Yadav, Anil K.; Majhi, Kunjalata; Banerjee, Abhishek; Devi, Poonam; Ganesan, R.; Mishra, P.; Lohani, H.; Sekhar, B. R.; Kumar, P. S. Anil

    2016-05-01

    Recently discovered, Topological Insulators (TIs) have garnered enormous amount of attention owing to its unique surface properties which has potential applications in the field of spintronics and other modern technologies. For all this, it should require a very good quality samples. There are a number of techniques suggested by people for the growth of good quality TIs. Here, we are reporting the growth of high quality single crystals of Bi2Se3 (a TI) by slow cooling solid-state reaction method. X-ray diffraction measurements performed on a cleaved flake of single crystal Bi2Se3 showed up with proper orientations of the crystal planes. High energy X-ray diffraction has been performed to confirm the stoichiometry of the compound and also recorded Laue patterns prove the single crystalline nature of Bi2Se3. Moreover, angle resolved photo-emission spectroscopy (ARPES) carried out on a flat crystal flake shows distinct Dirac dispersion of surface bands at the gamma point clarifying it as a 3D topological insulator.

  15. Laser-driven parametric instability and generation of entangled photon-plasmon states in graphene and topological insulators

    NASA Astrophysics Data System (ADS)

    Belyanin, Alexey; Wang, Yongrui; Oladyshkin, Ivan; Tokman, Mikhail

    Massless Dirac electrons in graphene and on the surface of topological insulators such as Bi2Se3 demonstrate strong nonlinear optical response and support tightly confined surface plasmon modes. Although both systems constitute an isotropic medium for low-energy in-plane electron excitations, their second-order nonlinear susceptibility becomes non-zero when its spatial dispersion is taken into account. In this case the anisotropy is induced by in-plane wave vectors of obliquely incident or in-plane propagating electromagnetic waves. In this work we show that a strong (0.1-1 MW/cm2) near-infrared or mid-infrared laser beam obliquely incident on graphene can experience a parametric instability with respect to decay into lower-frequency (idler) photons and THz surface plasmons. The parametric gain leads to efficient generation of THz plasmons. Furthermore, the parametric decay process gives rise to quantum entanglement of idler photon and surface plasmon states. This enables diagnostics and control of surface plasmons by detecting idler photons. A similar parametric process can be implemented in topological insulator thin films. This work has been supported in part by the Air Force Office for Scientific Research through Grant FA9550-15-1-0153.

  16. Ferromagnetism in the two-dimensional periodic Anderson model

    SciTech Connect

    Batista, C. D.; Bonca, J.; Gubernatis, J. E.

    2001-05-01

    Using the constrained-path Monte Carlo method, we studied the magnetic properties of the two-dimensional periodic Anderson model for electron fillings between 1/4 and 1/2. We also derived two effective low-energy theories to assist in interpreting the numerical results. For 1/4 filling, we found that the system can be a Mott or a charge-transfer insulator, depending on the relative values of the Coulomb interaction and the charge-transfer gap between the two noninteracting bands. The insulator may be a paramagnet or antiferromagnet. We concentrated on the effect of electron doping on these insulating phases. Upon doping we obtained a partially saturated ferromagnetic phase for low concentrations of conduction electrons. If the system were a charge-transfer insulator, we would find that the ferromagnetism is induced by the well-known Ruderman-Kittel-Kasuya-Yosida interaction. However, we found a novel correlated hopping mechanism inducing the ferromagnetism in the region where the nondoped system is a Mott insulator. Our regions of ferromagnetism spanned a much smaller doping range than suggested by recent slave boson and dynamical mean-field theory calculations, but they were consistent with that obtained by density-matrix renormalization group calculations of the one-dimensional periodic Anderson model.

  17. Exact many-electron ground states on the diamond Hubbard chain

    NASA Astrophysics Data System (ADS)

    Gulacsi, Zsolt; Kampf, Arno; Vollhardt, Dieter

    2008-03-01

    Exact ground states of interacting electrons on the diamond Hubbard chain in a magnetic field are constructed which exhibit a wide range of properties such as flat-band ferromagnetism, correlation induced metallic, half-metallic, or insulating behavior [1]. The properties of these ground states can be tuned by changing the magnetic flux, local potentials, or electron density.The results show that the studied simple one-dimensional structure displays remarkably complex physical properties. The virtue of tuning different ground states through external parameters points to new possibilities for the design of electronic devices which can switch between insulating or conducting and nonmagnetic or (fully or partially spin polarized) ferromagnetic states, open new routes for the design of spin-valve devices and gate induced ferromagnetism. [1] Z. Gulacsi, A. Kampf, D. Vollhardt, Phys. Rev. Lett. 99, 026404(2007).

  18. Novel room temperature ferromagnetic semiconductors

    SciTech Connect

    Gupta, Amita

    2004-11-01

    for Zn a 2+ state in the ZnO lattice. Ferromagnetic Resonance (FMR) technique is used to confirm the existence of ferromagnetic ordering at temperatures as high as 425K. The ab initio calculations were found to be consistent with the observation of ferromagnetism arising from fully polarized Mn 2+ state. The key to observed room temperature ferromagnetism in this system is the low temperature processing, which prevents formation of clusters, secondary phases and the host ZnO from becoming n-type. The electronic structure of the same Mn doped ZnO thin films studied using XAS, XES and RIXS, revealed a strong hybridization between Mn 3d and O 2p states, which is an important characteristic of a Dilute magnetic Semiconductor (DMS). It is shown that the various processing conditions like sintering temperature, dopant concentration and the properties of precursors used for making of DMS have a great influence on the final properties. Use of various experimental techniques to verify the physical properties, and to understand the mechanism involved to give rise to ferromagnetism is presented. Methods to improve the magnetic moment in Mn doped ZnO are also described. New promising DMS materials (such as Cu doped ZnO are explored). The demonstrated new capability to fabricate powder, pellets, and thin films of room temperature ferromagnetic semiconductors thus makes possible the realization of a wide range of complex elements for a variety of new multifunctional phenomena related to Spintronic devices as well as magneto-optic components.

  19. The zero-field glassy ground state and field-induced ferromagnetic transition in (La0.4Pr0.6)1.2Sr1.8Mn2O7

    NASA Astrophysics Data System (ADS)

    Tackett, R.; Lawes, G.; Suryanarayanan, R.; Apostu, M.; Revcolevschi, A.

    2011-04-01

    We have investigated glassy magnetic freezing in (La0.4Pr0.6)1.2Sr1.8Mn2O7 single crystals together with the field-induced transition to a metastable ferromagnetic phase using ac magnetic susceptibility and heat capacity measurements. The magnetization measurements show evidence for the development of a zero-field glassy ground state below 45 K along with a hysteretic, field-induced change in susceptibility associated with the transition to the ferromagnetic phase above 5 T. The heat capacity develops a clear peak at higher temperatures with the application of large magnetic fields, consistent with the development of a ferromagnetic order, while at low temperatures the Sommerfeld coefficient is monotonically reduced by an applied field, suggesting suppression of spin fluctuations. The heat capacity shows hysteretic behaviour, accompanied by a sharp decrease at a critical field, when held at fixed temperature, which does not recover on reducing the field back to zero. These measurements suggest that the zero-field ground state for (La0.4Pr0.6)1.2Sr1.8Mn2O7 consists of frozen disordered spin clusters, which develop into a metastable ferromagnetic state in modest magnetic fields.

  20. Very large thermophase in ferromagnetic Josephson junctions.

    PubMed

    Giazotto, F; Heikkilä, T T; Bergeret, F S

    2015-02-13

    The concept of thermophase refers to the appearance of a phase gradient inside a superconductor originating from the presence of an applied temperature bias across it. The resulting supercurrent flow may, in suitable conditions, fully counterbalance the temperature-bias-induced quasiparticle current therefore preventing the formation of any voltage drop, i.e., a thermovoltage, across the superconductor. Yet, the appearance of a thermophase is expected to occur in Josephson-coupled superconductors as well. Here, we theoretically investigate the thermoelectric response of a thermally biased Josephson junction based on a ferromagnetic insulator. In particular, we predict the occurrence of a very large thermophase that can reach π/2 across the contact for suitable temperatures and structure parameters; i.e., the quasiparticle thermal current can reach the critical current. Such a thermophase can be several orders of magnitude larger than that predicted to occur in conventional Josephson tunnel junctions. In order to assess experimentally the predicted very large thermophase, we propose a realistic setup realizable with state-of-the-art nanofabrication techniques and well-established materials, based on a superconducting quantum interference device. This effect could be of strong relevance in several low-temperature applications, for example, for revealing tiny temperature differences generated by coupling the electromagnetic radiation to one of the superconductors forming the junction. PMID:25723238

  1. Effective J=1/2 Insulating State in Ruddlesden-Popper Iridates: An LDA+DMFT Study

    NASA Astrophysics Data System (ADS)

    Zhang, Hongbin; Haule, Kristjan; Vanderbilt, David

    2013-12-01

    Using ab initio methods for correlated electrons in solids, we investigate the metal-insulator transition across the Ruddlesden-Popper (RP) series of iridates and explore the robustness of the Jeff=1/2 state against band effects due to itineracy, tetragonal distortion, octahedral rotation, and Coulomb interaction. We predict the effects of epitaxial strain on the optical conductivity, magnetic moments, and Jeff=1/2 ground-state wave functions in the RP series. To describe the solution of the many-body problem in an intuitive picture, we introduce a concept of energy-dependent atomic states, which strongly resemble the atomic Jeff=1/2 states but with coefficients that are energy or time dependent. We demonstrate that the deviation from the ideal Jeff=1/2 state is negligible at short time scales for both single- and double-layer iridates, while it becomes quite significant for Sr3Ir2O7 at long times and low energy. Interestingly, Sr2IrO4 is positioned very close to the SU(2) limit, with only ˜3% deviation from the ideal Jeff=1/2 situation.

  2. Momentum-Resolved Spin Dynamics of Bulk and Surface Excited States in the Topological Insulator Bi2Se3

    NASA Astrophysics Data System (ADS)

    Cacho, C.; Crepaldi, A.; Battiato, M.; Braun, J.; Cilento, F.; Zacchigna, M.; Richter, M. C.; Heckmann, O.; Springate, E.; Liu, Y.; Dhesi, S. S.; Berger, H.; Bugnon, Ph.; Held, K.; Grioni, M.; Ebert, H.; Hricovini, K.; Minár, J.; Parmigiani, F.

    2015-03-01

    The prospect of optically inducing and controlling a spin-polarized current in spintronic devices has generated wide interest in the out-of-equilibrium electronic and spin structure of topological insulators. In this Letter we show that only measuring the spin intensity signal over several orders of magnitude by spin-, time-, and angle-resolved photoemission spectroscopy can provide a comprehensive description of the optically excited electronic states in Bi2Se3. Our experiments reveal the existence of a surface resonance state in the second bulk band gap that is benchmarked by fully relativistic ab initio spin-resolved photoemission calculations. We propose that the newly reported state plays a major role in the ultrafast dynamics of the system, acting as a bottleneck for the interaction between the topologically protected surface state and the bulk conduction band. In fact, the spin-polarization dynamics in momentum space show that these states display macroscopically different temperatures and, more importantly, different cooling rates over several picoseconds.

  3. Insulated Honeycomb

    NASA Technical Reports Server (NTRS)

    Bhat, Balakrishna T.

    1989-01-01

    Proposed insulated honeycomb structure similar to reinforced honeycomb structure described in NPO-17538. Panels of insulated honeycomb used to make supports for solar-energy collectors and radar antennas.

  4. Random-anisotropy ferromagnetic state in the C u5G d0.54C a0.42 intermetallic compound

    NASA Astrophysics Data System (ADS)

    Krnel, M.; Vrtnik, S.; Koželj, P.; Kocjan, A.; Jagličić, Z.; Boulet, P.; de Weerd, M. C.; Dubois, J. M.; Dolinšek, J.

    2016-03-01

    By applying the alloy design concept that stable intermetallic phases between two immiscible elements can be formed by adding a third element that forms stable compounds with both elements, we have synthesized the first known stable intermetallic compound of Cu, Gd, and Ca, where copper acts as the mediating element between the immiscible Gd and Ca. A compound with the composition C u84G d9C a7 (equivalent to C u5G d0.54C a0.42 ) was synthesized by the Czochralski technique in the form of a large single crystal of high structural perfection, and the structural model was determined by x-ray diffraction (XRD). The compound crystallizes in the hexagonal system, space group P 6 /mmm, and the crystal structure is isotypic to the C u5.44T b0.78 . The unit cell contains inherent disorder due to partial occupation of the Cu3 site and the substitutional disorder at the Gd/Ca mixed site located at the vertices of the hexagonal unit cell, where Gd and Ca randomly substitute each other. The random substitution of magnetic Gd by nonmagnetic Ca atoms makes the magnetic Gd lattice disordered, which leads to interesting magnetic ordering at low temperatures that occurs below TC=24 K in zero and low external magnetic fields. By performing a large set of complementary experiments along two perpendicular crystallographic directions (the [001] hexagonal-axis direction and the [100] hexagonal-plane direction), we show that the zero-field collective magnetic state can be described as a random-anisotropy ferromagnetic state, where random magnetic anisotropies originate from the magnetic dipole interactions between the Gd moments in the magnetically disordered lattice. The random-anisotropy ferromagnetic state in the C u84G d9C a7 is characterized by randomness and frustration of magnetic interactions, which are the two ingredients that allow classifying this state into the generic class of spin glasses. Our paper opens the possibility to search for new ternary intermetallic phases in the

  5. Quantum Hall effect on top and bottom surface states of topological insulator (Bi1-xSbx)2Te3 films.

    PubMed

    Yoshimi, R; Tsukazaki, A; Kozuka, Y; Falson, J; Takahashi, K S; Checkelsky, J G; Nagaosa, N; Kawasaki, M; Tokura, Y

    2015-01-01

    The three-dimensional topological insulator is a novel state of matter characterized by two-dimensional metallic Dirac states on its surface. To verify the topological nature of the surface states, Bi-based chalcogenides such as Bi2Se3, Bi2Te3, Sb2Te3 and their combined/mixed compounds have been intensively studied. Here, we report the realization of the quantum Hall effect on the surface Dirac states in (Bi1-xSbx)2Te3 films. With electrostatic gate-tuning of the Fermi level in the bulk band gap under magnetic fields, the quantum Hall states with filling factor ±1 are resolved. Furthermore, the appearance of a quantum Hall plateau at filling factor zero reflects a pseudo-spin Hall insulator state when the Fermi level is tuned in between the energy levels of the non-degenerate top and bottom surface Dirac points. The observation of the quantum Hall effect in three-dimensional topological insulator films may pave a way toward topological insulator-based electronics. PMID:25868494

  6. Heavy surface state in a possible topological Kondo insulator: Magnetothermoelectric transport on the (011) plane of SmB6

    DOE PAGESBeta

    Luo, Yongkang; Chen, Hua; Dai, Jianhui; Xu, Zhu -an; Thompson, J. D.

    2015-02-25

    Motivated by the high sensitivity to Fermi surface topology and scattering mechanisms in magnetothermoelectric transport, we have measured the thermopower and Nernst effect on the (011) plane of the proposed topological Kondo insulator SmB6. These experiments, together with electrical resistivity and Hall effect measurements, suggest that the (011) plane also harbors a metallic surface with an effective mass on the order of 10–102 m0. The surface and bulk conductances are well distinguished in these measurements and are categorized into metallic and nondegenerate semiconducting regimes, respectively. As a result, electronic correlations play an important role in enhancing scattering and also contributemore » to the heavy surface state.« less

  7. Dynamic conductivity of the bulk states of n-type HgTe/CdTe quantum well topological insulator

    SciTech Connect

    Chen, Qinjun; Sanderson, Matthew; Cao, J. C.; Zhang, Chao

    2014-11-17

    We theoretically studied the frequency-dependent current response of the bulk state of topological insulator HgTe/CdTe quantum well. The optical conductivity is mainly due to the inter-band process at high frequencies. At low frequencies, intra-band process dominates with a dramatic drop to near zero before the inter-band contribution takes over. The conductivity decreases with temperature at low temperature and increases with temperature at high temperature. The transport scattering rate has an opposite frequency dependence in the low and high temperature regime. The different frequency dependence is due to the interplay of the carrier-impurity scattering and carrier population near the Fermi surface.

  8. Dynamic conductivity of the bulk states of n-type HgTe/CdTe quantum well topological insulator

    NASA Astrophysics Data System (ADS)

    Chen, Qinjun; Sanderson, Matthew; Cao, J. C.; Zhang, Chao

    2014-11-01

    We theoretically studied the frequency-dependent current response of the bulk state of topological insulator HgTe/CdTe quantum well. The optical conductivity is mainly due to the inter-band process at high frequencies. At low frequencies, intra-band process dominates with a dramatic drop to near zero before the inter-band contribution takes over. The conductivity decreases with temperature at low temperature and increases with temperature at high temperature. The transport scattering rate has an opposite frequency dependence in the low and high temperature regime. The different frequency dependence is due to the interplay of the carrier-impurity scattering and carrier population near the Fermi surface.

  9. PREFACE: Half Metallic Ferromagnets

    NASA Astrophysics Data System (ADS)

    Dowben, Peter

    2007-08-01

    Since its introduction by de Groot and colleagues in the early 1980s [1], the concept of half metallic ferromagnetism has attracted great interest. Idealized, half-metals have only one spin channel for conduction: the spin-polarized band structure exhibits metallic behavior for one spin channel, while the other spin band structure exhibits a gap at the Fermi level. Due to the gap for one spin direction, the density of states at the Fermi level has, theoretically, 100 & spin polarization. This gap in the density of states in one spin at the Fermi level, for example ↓ so N↓ (EF) = 0, also causes the resistance of that channel to go to infinity. At zero or low temperatures, the nonquasiparticle density of states (electron correlation effects), magnons and spin disorder reduce the polarization from the idealized 100 & polarization. At higher temperatures magnon-phonon coupling and irreversible compositional changes affect polarization further. Strategies for assessing and reducing the effects of finite temperatures on the polarization are now gaining attention. The controversies surrounding the polarization stability of half metallic ferromagnets are not, however, limited to the consideration of finite temperature effects alone. While many novel half metallic materials have been predicted, materials fabrication can be challenging. Defects, surface and interface segregation, and structural stability can lead to profound decreases in polarization, but can also suppress long period magnons. There is a 'delicate balance of energies required to obtain half metallic behaviour: to avoid spin flip scattering, tiny adjustments in atomic positions might occur so that a gap opens up in the other spin channel' [2]. When considering 'spintronics' devices, a common alibi for the study of half metallic systems, surfaces and interfaces become important. Free enthalpy differences between the surface and the bulk will lead to spin minority surface and interface states, as well as

  10. Decoupling of the antiferromagnetic and insulating states in Tb-doped Sr2IrO4

    SciTech Connect

    Wang, J. C.; Aswartham, S.; Ye, Feng; Terzic, J.; Zheng, H.; Haskel, Daniel; Chikara, Shalinee; Choi, Yong; Schlottmann, P.; Custelcean, Radu; Yuan, S. J.; Cao, G.

    2015-12-08

    Sr2IrO4 is a spin-orbit coupled insulator with an antiferromagnetic (AFM) transition at TN = 240 K. We report results of a comprehensive study of single-crystal Sr2Ir1-xTbxO4 (0≤x≤0.03). This study found that mere 3% (x=0.03) tetravalent Tb4+(4f7) substituting for Ir4+ (rather than Sr2+) completely suppresses the long-range collinear AFM transition but retains the insulating state, leading to a phase diagram featuring a decoupling of magnetic interactions and charge gap. The insulating state at x = 0.03 is characterized by an unusually large specific heat at low temperatures and an incommensurate magnetic state having magnetic peaks at (0.95, 0, 0) and (0, 0.95, 0) in the neutron diffraction, suggesting a spiral or spin density wave order. It is apparent that Tb doping effectively changes the relative strength of the SOI and the tetragonal CEF and enhances the Hund’s rule coupling that competes with the SOI, and destabilizes the AFM state. However, the disappearance of the AFM accompanies no metallic state chiefly because an energy level mismatch for the Ir and Tb sites weakens charge carrier hopping and renders a persistent insulating state. Furthermore, this work highlights an unconventional correlation between the AFM and insulating states in which the magnetic transition plays no critical role in the formation of the charge gap in the iridate.

  11. Topological Kondo Insulators

    NASA Astrophysics Data System (ADS)

    Dzero, Maxim; Xia, Jing; Galitski, Victor; Coleman, Piers

    2016-03-01

    This article reviews recent theoretical and experimental work on a new class of topological material -- topological Kondo insulators, which develop through the interplay of strong correlations and spin-orbit interactions. The history of Kondo insulators is reviewed along with the theoretical models used to describe these heavy fermion compounds. The Fu-Kane method of topological classification of insulators is used to show that hybridization between the conduction electrons and localized f electrons in these systems gives rise to interaction-induced topological insulating behavior. Finally, some recent experimental results are discussed, which appear to confirm the theoretical prediction of the topological insulating behavior in samarium hexaboride, where the long-standing puzzle of the residual low-temperature conductivity has been shown to originate from robust surface states.

  12. Development of a ferromagnetic component in the superconducting state of Fe-excess Fe1.12Te1-xSex by electronic charge redistribution

    NASA Astrophysics Data System (ADS)

    Li, Wen-Hsien; Karna, Sunil K.; Hsu, Han; Li, Chi-Yen; Lee, Chi-Hung; Sankar, Raman; Cheng Chou, Fang

    2015-06-01

    The general picture established so far for the links between superconductivity and magnetic ordering in iron chalcogenide Fe1+y(Te1-xSex) is that the substitution of Se for Te directly drives the system from the antiferromagnetic end into the superconducting regime. Here, we report on the observation of a ferromagnetic component that developed together with the superconducting transition in Fe-excess Fe1.12Te1-xSex crystals using neutron and x-ray diffractions, resistivity, magnetic susceptibility and magnetization measurements. The superconducting transition is accompanied by a negative thermal expansion of the crystalline unit cell and an electronic charge redistribution, where a small portion of the electronic charge flows from around the Fe sites toward the Te/Se sites. First-principles calculations show consistent results, revealing that the excess Fe ions play a more significant role in affecting the magnetic property in the superconducting state than in the normal state and the occurrence of an electronic charge redistribution through the superconducting transition.

  13. Ising-Type Ferromagnetic Ground State Driven by Anisotropic c-f Hybridization in CeRu2Al2B

    NASA Astrophysics Data System (ADS)

    Matsuno, Haruki; Kotegawa, Hisashi; Matsuoka, Eiichi; Tomiyama, Yo; Sugawara, Hitoshi; Tou, Hideki

    2014-10-01

    The magnetic correlations between Ce 4f electrons and conduction electrons in the new tetragonal compound CeRu2Al2B have been investigated by 27Al nuclear magnetic resonance (NMR). The 27Al NMR spin-lattice relaxation rate 1/T1 exhibits a large magnetic anisotropy for field directions. Within a localized moment picture, the Ce 4f spin-fluctuation rates Γ|| for the c-axis and Γ⊥ for the c-plane are evaluated by taking account of the magnetic anisotropy. The relation of Γ allel gg Γ bot , which holds in the entire temperature range, indicates that the c-f hybridization between the ligand conduction electrons and the Ce 4f electrons with the Γ 7(1) crystal electric field ground state is anisotropic. From the temperature dependence of Γ, it is found that the Kondo effect dominates the Ruderman-Kittel-Kasuya-Yosida (RKKY) interaction at high temperatures, whereas below 60 K the RKKY interaction overcomes the Kondo effect and causes the magnetic order. These results indicate that the anisotropic c-f hybridization plays a vital role in realization of the Ising-type ferromagnetic magnetic ground state in CeRu2Al2B.

  14. Ferromagnetism in defect-ridden oxides and related materials

    NASA Astrophysics Data System (ADS)

    Coey, J. M. D.; Stamenov, P.; Gunning, R. D.; Venkatesan, M.; Paul, K.

    2010-05-01

    The existence of high-temperature ferromagnetism in thin films and nanoparticles of oxides containing small quantities of magnetic dopants remains controversial. Some regard these materials as dilute magnetic semiconductors, while others think they are ferromagnetic only because the magnetic dopants form secondary ferromagnetic impurity phases such as cobalt metal or magnetite. There are also reports in d0 systems and other defective oxides that contain no magnetic ions. Here, we investigate TiO2 (rutile) containing 1-5% of iron cations and find that the room temperature ferromagnetism of films prepared by pulsed-laser deposition is not due to magnetic ordering of the iron. The films are neither dilute magnetic semiconductors nor hosts to an iron-based ferromagnetic impurity phase. A new model is developed for defect-related ferromagnetism, which involves a spin-split defect band populated by charge transfer from a proximate charge reservoir—in the present case a mixture of Fe2+ and Fe3+ ions in the oxide lattice. The phase diagram for the model shows how inhomogeneous Stoner ferromagnetism depends on the total number of electrons Ntot, the Stoner exchange integral I and the defect bandwidth W; the band occupancy is governed by the d-d Coulomb interaction U. There are regions of ferromagnetic metal, half-metal and insulator as well as non-magnetic metal and insulator. A characteristic feature of the high-temperature Stoner magnetism is an anhysteretic magnetization curve, which is practically temperature independent below room temperature. This is related to a wandering ferromagnetic axis, which is determined by local dipole fields. The magnetization is limited by the defect concentration, not by the 3d doping. Only 1-2% of the volume of the films is magnetically ordered.

  15. Anisotropic Fabry-Pérot resonant states confined within nano-steps on the topological insulator surface

    PubMed Central

    Fu, Zhen-Guo; Zhang, Ping; Chen, Mu; Wang, Zhigang; Zheng, Fa-Wei; Lin, Hai-Qing

    2014-01-01

    The peculiar nature of topological surface states, such as absence of backscattering, weak anti-localization, and quantum anomalous Hall effect, has been demonstrated mainly in bulk and film of topological insulator (TI), using surface sensitive probes and bulk transport probes. However, it is equally important and experimentally challenging to confine massless Dirac fermions with nano-steps on TI surfaces. This potential structure has similar ground with linearly-dispersed photons in Fabry-Pérot resonators, while reserving fundamental differences from well-studied Fabry-Pérot resonators and quantum corrals on noble metal surfaces. In this paper, we study the massless Dirac fermions confined within steps along the x (Γ–K) or y (Γ–M) direction on the TI surface, and the Fabry-Pérot-like resonances in the electronic local density of states (LDOS) between the steps are found. Due to the remarkable warping effect in the topological surface states, the LDOS confined in the step-well running along Γ-M direction exhibit anisotropic resonance patterns as compared to those in the step-well along Γ-K direction. The transmittance properties and spin orientation of Dirac fermion in both cases are also anisotropic in the presence of warping effect. PMID:24986567

  16. Spin-filtered Edge States with an Electrically Tunable Gap in a Two-Dimensional Topological Crystallin Insulator

    NASA Astrophysics Data System (ADS)

    Liu, Junwei; Hsieh, Timothy H.; Wei, Peng; Duan, Wenhui; Moodera, Jagadeesh; Fu, Liang

    2014-03-01

    Three-dimensional topological crystalline insulators (TCIs) were recently predicted and observed in the SnTe class of IV-VI semiconductors, which host metallic surface states protected by crystal symmetries. In this work, we study thin films of these materials and expose their potential device applications. We demonstrate that thin films of SnTe and Pb1-xSnxSe(Te) grown along the (001) direction are topologically nontrivial in a wide range of film thickness and carry conducting spin-filtered edge states that are protected by the (001) mirror symmetry via a topological invariant. Application of an electric field perpendicular to the film will break the mirror symmetry and generate a band gap in these edge states. This functionality motivates us to propose a novel topological transistor device, in which charge and spin transport are maximally entangled and simultaneously controlled by an electric field. The high on/off operation speed and coupling of spin and charge in such a device may lead to electronic and spintronic applications for TCIs. This work is supported by the U.S. Department of Energy, Office of Basic Energy Sciences, Division of Materials Sciences and Engineering under Award DE-SC0010526.

  17. Intrinsic ferromagnetism in hexagonal boron nitride nanosheets

    SciTech Connect

    Si, M. S.; Gao, Daqiang E-mail: xueds@lzu.edu.cn; Yang, Dezheng; Peng, Yong; Zhang, Z. Y.; Xue, Desheng E-mail: xueds@lzu.edu.cn; Liu, Yushen; Deng, Xiaohui; Zhang, G. P.

    2014-05-28

    Understanding the mechanism of ferromagnetism in hexagonal boron nitride nanosheets, which possess only s and p electrons in comparison with normal ferromagnets based on localized d or f electrons, is a current challenge. In this work, we report an experimental finding that the ferromagnetic coupling is an intrinsic property of hexagonal boron nitride nanosheets, which has never been reported before. Moreover, we further confirm it from ab initio calculations. We show that the measured ferromagnetism should be attributed to the localized π states at edges, where the electron-electron interaction plays the role in this ferromagnetic ordering. More importantly, we demonstrate such edge-induced ferromagnetism causes a high Curie temperature well above room temperature. Our systematical work, including experimental measurements and theoretical confirmation, proves that such unusual room temperature ferromagnetism in hexagonal boron nitride nanosheets is edge-dependent, similar to widely reported graphene-based materials. It is believed that this work will open new perspectives for hexagonal boron nitride spintronic devices.

  18. Direct Spectroscopic Observation of a Shallow Hydrogenlike Donor State in Insulating SrTiO3

    NASA Astrophysics Data System (ADS)

    Salman, Z.; Prokscha, T.; Amato, A.; Morenzoni, E.; Scheuermann, R.; Sedlak, K.; Suter, A.

    2014-10-01

    We present a direct spectroscopic observation of a shallow hydrogenlike muonium state in SrTiO3 which confirms the theoretical prediction that interstitial hydrogen may act as a shallow donor in this material. The formation of this muonium state is temperature dependent and appears below ˜70 K. From the temperature dependence we estimate an activation energy of ˜50 meV in the bulk and ˜23 meV near the free surface. The field and directional dependence of the muonium precession frequencies further supports the shallow impurity state with a rare example of a fully anisotropic hyperfine tensor. From these measurements we determine the strength of the hyperfine interaction and propose that the muon occupies an interstitial site near the face of the oxygen octahedron in SrTiO3. The observed shallow donor state provides new insight for tailoring the electronic and optical properties of SrTiO3-based oxide interface systems.

  19. Excitation of the S-1(0) State of Trivalent Praseodymium in Insulating Crystals

    NASA Astrophysics Data System (ADS)

    Levey, Christopher Gerrit

    In this dissertation we present an investigation of the fluorescence and two-photon absorption properties of the ('1)S(,0) state of Pr('3+) in LaF(,3) and several other crystal hosts. The high-lying (4f('2)) ('1)S(,0) state of Pr('3+) is of particular interest because it is the closest of the meta-stable rare earth levels to the next configuration of opposite parity (the 4f5d band), and this proximity varies from host to host. However, the ('1)S(,0) state has received relatively little attention because its energy is beyond the range of dye lasers and frequency doubling crystals. We excite the ('1)S(,0) state directly, through a two-photon excited state absorption process, and indirectly, through pumping the 4f5d band. In most host materials the ('1)S(,0) level of Pr('3+) lies above the lowest 4f5d band component, and consequently only 4f5d band emission is observed. In LaF(,3), however, the ('1)S(,0) state lies below the band, and only the ('1)S(,0) emission is observed. In Pr('3+):CaF(,2) there are sites with each of these properties, and both ('1)S(,0) and 4f5d emission are observed, with a relative strength that is concentration dependent. Two-photon absorption from the ('3)H(,4) ground state into the ('1)S(,0) state is doubly forbidden by the two-photon spin and J selection rules ((DELTA)S = 0 and (DELTA)J <= 2), and we have not observed this transition. However, we do observe a two-photon absorption into the ('1)S(,0) state from each of the lowest two Stark components of the meta-stable ('1)D(,2) state. These transitions are allowed by all two-photon selection rules, though not for all polarizations, and we observe a strong polarization dependence. We also observe line narrowing of this transition as a smaller subset of the ('1)D(,2) population is excited, and we see homogeneous broadening of the transition as the temperature is raised to 77K. The ('1)S(,0) lifetime is much shorter than that predicted by the Judd-Ofelt theory, and is only weakly concentration

  20. High Spin-Chern Insulators with Magnetic Order

    PubMed Central

    Ezawa, Motohiko

    2013-01-01

    As a topological insulator, the quantum Hall (QH) effect is indexed by the Chern and spin-Chern numbers and . We have only in conventional QH systems. We investigate QH effects in generic monolayer honeycomb systems. We search for spin-resolved characteristic patterns by exploring Hofstadter's butterfly diagrams in the lattice theory and fan diagrams in the low-energy Dirac theory. It is shown that the spin-Chern number can takes an arbitrary high value for certain QH systems. This is a new type of topological insulators, which we may call high spin-Chern insulators. Samples may be provided by graphene on the SiC substrate with ferromagnetic order, transition-metal dichalcogenides with ferromagnetic order, transition-metal oxide with antiferromagnetic order and silicene with ferromagnetic order. Actually high spin-Chern insulators are ubiquitous in any systems with magnetic order. Nevertheless, the honeycomb system would provide us with unique materials for practical materialization. PMID:24310394

  1. High Spin-Chern Insulators with Magnetic Order

    NASA Astrophysics Data System (ADS)

    Ezawa, Motohiko

    2013-12-01

    As a topological insulator, the quantum Hall (QH) effect is indexed by the Chern and spin-Chern numbers and . We have only in conventional QH systems. We investigate QH effects in generic monolayer honeycomb systems. We search for spin-resolved characteristic patterns by exploring Hofstadter's butterfly diagrams in the lattice theory and fan diagrams in the low-energy Dirac theory. It is shown that the spin-Chern number can takes an arbitrary high value for certain QH systems. This is a new type of topological insulators, which we may call high spin-Chern insulators. Samples may be provided by graphene on the SiC substrate with ferromagnetic order, transition-metal dichalcogenides with ferromagnetic order, transition-metal oxide with antiferromagnetic order and silicene with ferromagnetic order. Actually high spin-Chern insulators are ubiquitous in any systems with magnetic order. Nevertheless, the honeycomb system would provide us with unique materials for practical materialization.

  2. A scheme for a topological insulator field effect transistor

    NASA Astrophysics Data System (ADS)

    Vali, Mehran; Dideban, Daryoosh; Moezi, Negin

    2015-05-01

    We propose a scheme for a topological insulator field effect transistor. The idea is based on the gate voltage control of the Dirac fermions in a ferromagnetic topological insulator channel with perpendicular magnetization connecting to two metallic topological insulator leads. Our theoretical analysis shows that the proposed device displays a switching effect with high on/off current ratio and a negative differential conductance with a good peak to valley ratio.

  3. A metallic mosaic phase and the origin of Mott-insulating state in 1T-TaS2.

    PubMed

    Ma, Liguo; Ye, Cun; Yu, Yijun; Lu, Xiu Fang; Niu, Xiaohai; Kim, Sejoong; Feng, Donglai; Tománek, David; Son, Young-Woo; Chen, Xian Hui; Zhang, Yuanbo

    2016-01-01

    Electron-electron and electron-phonon interactions are two major driving forces that stabilize various charge-ordered phases of matter. In layered compound 1T-TaS2, the intricate interplay between the two generates a Mott-insulating ground state with a peculiar charge-density-wave (CDW) order. The delicate balance also makes it possible to use external perturbations to create and manipulate novel phases in this material. Here, we study a mosaic CDW phase induced by voltage pulses, and find that the new phase exhibits electronic structures entirely different from that of the original Mott ground state. The mosaic phase consists of nanometre-sized domains characterized by well-defined phase shifts of the CDW order parameter in the topmost layer, and by altered stacking relative to the layers underneath. We discover that the nature of the new phase is dictated by the stacking order, and our results shed fresh light on the origin of the Mott phase in 1T-TaS2. PMID:26961788

  4. A metallic mosaic phase and the origin of Mott-insulating state in 1T-TaS2

    NASA Astrophysics Data System (ADS)

    Ma, Liguo; Ye, Cun; Yu, Yijun; Lu, Xiu Fang; Niu, Xiaohai; Kim, Sejoong; Feng, Donglai; Tománek, David; Son, Young-Woo; Chen, Xian Hui; Zhang, Yuanbo

    2016-03-01

    Electron-electron and electron-phonon interactions are two major driving forces that stabilize various charge-ordered phases of matter. In layered compound 1T-TaS2, the intricate interplay between the two generates a Mott-insulating ground state with a peculiar charge-density-wave (CDW) order. The delicate balance also makes it possible to use external perturbations to create and manipulate novel phases in this material. Here, we study a mosaic CDW phase induced by voltage pulses, and find that the new phase exhibits electronic structures entirely different from that of the original Mott ground state. The mosaic phase consists of nanometre-sized domains characterized by well-defined phase shifts of the CDW order parameter in the topmost layer, and by altered stacking relative to the layers underneath. We discover that the nature of the new phase is dictated by the stacking order, and our results shed fresh light on the origin of the Mott phase in 1T-TaS2.

  5. Development of 1200-kV compressed-gas-insulated transmission and substation equipment in the United States

    SciTech Connect

    Walldorf, S.P.; Gnadt, P.A.

    1986-01-01

    Major research and development (R and D) programs for the development of sulfur hexafluoride (SF/sub 6/) compressed-gas-insulated (CGI) equipment for ultrahighvoltage (uhv) application at 100-1200 kV have been under way independently in both the United States and Japan. The CGI transmission and substation equipment under study and development in these countries has included the surge arrester, circuit breaker, CGI transmission bus, disconnect and ground switches, and an air-SF/sub 6/ termination. The US CGI program has also included development of a power transformer, shunt reactor, voltage- and current-sensing instrumentation, and a line trap, all for 1200-kV application. This paper presents the overall status of development of CGI substation equipment for uhv application in the United States. It describes the major CGI components developed, highlights their key design aspects, indicates the present status of development and testing, and addresses the work remaining to establish final design consepts. The paper also includes 23 references, a number of which have not been widely distributed or included in any of the recently published bibliographies of uhv substation equipment.

  6. Ultrafast terahertz spectroscopy study of Kondo insulating thin film SmB6: evidence for an emergent surface state

    NASA Astrophysics Data System (ADS)

    Zhang, Jingdi; Yong, Jie; Takeuchi, Ichiro; Greene, Richard; Averitt, Richard

    We utilize terahertz time domain spectroscopy to investigate thin films of the heavy fermion compound SmB6, a prototype Kondo insulator. Temperature dependent terahertz (THz) conductivity measurements reveal a rapid decrease in the Drude weight and carrier scattering rate at ~T* =20 K, well below the hybridization gap onset temperature (100 K). Moreover, a low-temperature conductivity plateau (below 20K) indicates the emergence of a surface state with an effective electron mass of 0.1me. Conductivity dynamics following optical excitation are also measured and interpreted using Rothwarf-Taylor (R-T) phenomenology, yielding a hybridization gap energy of 17 meV. However, R-T modeling of the conductivity dynamics reveals a deviation from the expected thermally excited quasiparticle density at temperatures below 20K, indicative of another channel opening up in the low energy electrodynamics. Taken together, these results suggest the onset of a surface state well below the crossover temperature (100K) after long-range coherence of the f-electron Kondo lattice is established. JZ and RDA acknowledge support from DOE - Basic Energy Sciences under Grant No. DE-FG02-09ER46643, under which the THz measurements and data analysis were performed. JY, IT and RLG acknowledge support from ONR N00014-13-1-0635 and NSF DMR 1410665.

  7. A metallic mosaic phase and the origin of Mott-insulating state in 1T-TaS2

    PubMed Central

    Ma, Liguo; Ye, Cun; Yu, Yijun; Lu, Xiu Fang; Niu, Xiaohai; Kim, Sejoong; Feng, Donglai; Tománek, David; Son, Young-Woo; Chen, Xian Hui; Zhang, Yuanbo

    2016-01-01

    Electron–electron and electron–phonon interactions are two major driving forces that stabilize various charge-ordered phases of matter. In layered compound 1T-TaS2, the intricate interplay between the two generates a Mott-insulating ground state with a peculiar charge-density-wave (CDW) order. The delicate balance also makes it possible to use external perturbations to create and manipulate novel phases in this material. Here, we study a mosaic CDW phase induced by voltage pulses, and find that the new phase exhibits electronic structures entirely different from that of the original Mott ground state. The mosaic phase consists of nanometre-sized domains characterized by well-defined phase shifts of the CDW order parameter in the topmost layer, and by altered stacking relative to the layers underneath. We discover that the nature of the new phase is dictated by the stacking order, and our results shed fresh light on the origin of the Mott phase in 1T-TaS2. PMID:26961788

  8. Scattering states of the Majorana-bound-state-supporting vortices at the interface of a topological insulator and an s-wave superconductor

    NASA Astrophysics Data System (ADS)

    Durst, Adam

    We consider an isolated vortex in the 2D proximity-induced superconducting state formed at the interface of a 3D topological insulator (TI) and an s-wave superconductor (sSC). Prior calculations of the bound states of this system famously revealed a zero-energy state that is its own conjugate, a Majorana fermion bound to the vortex core. We calculate, not the bound states, but the scattering states of this system, and ask how the spin-momentum-locked massless Dirac form of the single-particle Hamiltonian, inherited from the TI surface, affects the cross section for scattering Bogoliubov quasiparticles from the vortex. As in the case of an ordinary superconductor, this is a two-channel problem with the vortex mixing particle-like and hole-like excitations. And as in the ordinary case, the same-channel differential cross section diverges in the forward direction due to the Aharonov-Bohm effect, resulting in an infinite total cross section but finite transport and skew cross sections. We calculate the transport and skew cross sections numerically, via a partial wave analysis, as a function of both quasiparticle excitation energy and chemical potential. Novel effects emerge as particle-like or hole-like excitations are tuned through the Dirac point.

  9. Scattering states of a vortex in the proximity-induced superconducting state at the interface of a topological insulator and an s -wave superconductor

    NASA Astrophysics Data System (ADS)

    Durst, Adam C.

    2016-02-01

    We consider an isolated vortex in the two-dimensional proximity-induced superconducting state formed at the interface of a three-dimensional strong topological insulator (TI) and an s -wave superconductor. Prior calculations of the bound states of this system famously revealed a zero-energy state that is its own conjugate, a Majorana fermion bound to the vortex core. We calculate, not the bound states, but the scattering states of this system, and ask how the spin-momentum-locked massless Dirac form of the single-particle Hamiltonian, inherited from the TI surface, affects the cross section for scattering Bogoliubov quasiparticles from the vortex. As in the case of an ordinary superconductor, this is a two-channel problem with the vortex mixing particlelike and holelike excitations. As in the ordinary case, the same-channel differential cross section diverges in the forward direction due to the Aharonov-Bohm effect, resulting in an infinite total cross section but finite transport and skew cross sections. We calculate the transport and skew cross sections numerically, via a partial wave analysis, as a function of both quasiparticle excitation energy and chemical potential. Novel effects emerge as particlelike or holelike excitations are tuned through the Dirac point.

  10. Influence of the FFLO-like state on the upper critical field of a superconductor/ferromagnet bilayer: Angular and temperature dependence

    NASA Astrophysics Data System (ADS)

    Lenk, D.; Hemmida, M.; Morari, R.; Zdravkov, V. I.; Ullrich, A.; Müller, C.; Sidorenko, A. S.; Horn, S.; Tagirov, L. R.; Loidl, A.; von Nidda, H.-A. Krug; Tidecks, R.

    2016-05-01

    We investigated the upper critical magnetic field Hc of a superconductor-ferromagnet (S/F) bilayer of Nb/Cu41Ni59 and a Nb film (as reference). We obtained the dependence of Hc ⊥ and Hc ∥ (perpendicular and parallel to the film plane, respectively) on the temperature T by measurements of the resistive transitions and the dependence on the inclination angle θ of the applied field to the film plane, by nonresonant microwave absorption. Over a wide range, Hc ⊥ and Hc ∥ show the temperature dependence predicted by the Ginzburg-Landau theory. At low temperatures and close to the critical temperature, deviations are observed. While Hc(θ ) of the Nb film follows the Tinkham prediction for thin superconducting films, the Nb/Cu41Ni59 -bilayer data exhibit deviations when θ approaches zero. We attribute this finding to the additional anisotropy induced by the quasi-one-dimensional Fulde-Ferrell-Larkin-Ovchinnikov (FFLO)-like state and propose a new vortex structure in S/F bilayers, adopting the segmentation approach from high-temperature superconductors.

  11. Occupation probabilities and current densities of bulk and edge states of a Floquet topological insulator

    NASA Astrophysics Data System (ADS)

    Dehghani, Hossein; Mitra, Aditi

    2016-05-01

    Results are presented for the occupation probabilities and current densities of bulk and edge states of half-filled graphene in a cylindrical geometry and irradiated by a circularly polarized laser. It is assumed that the system is closed and that the laser has been switched on as a quench. Laser parameters corresponding to some representative topological phases are studied: one where the Chern number of the Floquet bands equals the number of chiral edge modes, a second where anomalous edge states appear in the Floquet Brillouin zone boundaries, and a third where the Chern number is zero, yet topological edge states appear at the center and boundaries of the Floquet Brillouin zone. Qualitative differences are found for the high-frequency off-resonant and low-frequency on-resonant laser with edge states arising due to resonant processes occupied with a high effective temperature on the one hand, while edge states arising due to off-resonant processes occupied with a low effective temperature on the other. For an ideal half-filled system where only one of the bands in the Floquet Brillouin zone is occupied and the other empty, particle-hole and inversion symmetry of the Floquet Hamiltonian implies zero current density. However the laser switch-on protocol breaks the inversion symmetry, resulting in a net cylindrical sheet of current density at steady state. Due to the underlying chirality of the system, this current density profile is associated with a net charge imbalance between the top and bottom of the cylinders.

  12. Spin-orbit driven magnetic insulating state with Jeff=1/2 character in a 4d oxide

    SciTech Connect

    Calder, S.; Li, Ling; Okamoto, Satoshi; Choi, Yongseong; Mukherjee, Rupam; Haskel, Daniel; Mandrus, D.

    2015-11-30

    The unusual magnetic and electronic ground states of 5d iridates has been shown to be driven by intrinsically enhanced spin-orbit coupling (SOC). The influence of appreciable but reduced SOC in creating the manifested magnetic insulating states in 4d oxides is less clear, with one hurdle being the existence of such compounds. Here we present experimental and theoretical results on Sr4RhO6 that reveal SOC dominated behavior. Neutron measurements show the octahedra are both spatially separated and locally ideal, making the electronic ground state susceptible to alterations by SOC. Magnetic ordering is observed with a similar structure to an analogous Jeff=1/2 Mott iridate. We consider the underlying role of SOC in this rhodate with density functional theory and x-ray absorption spectroscopy and find a magnetic insulating ground state with Jeff =1/2 character.The unusual magnetic and electronic ground states of 5d iridates have been shown to be driven by intrinsically enhanced spin-orbit coupling (SOC). The influence of appreciable but reduced SOC in creating the manifested magnetic insulating states in 4d oxides is less clear, with one hurdle being the existence of such compounds. Here, we present experimental and theoretical results on Sr4RhO6 that reveal SOC dominated behavior. Neutron measurements show the octahedra are both spatially separated and locally ideal, making the electronic ground state susceptible to alterations by SOC. Magnetic ordering is observed with a similar structure to an analogous Jeff=1/2 Mott iridate. We consider the underlying role of SOC in this rhodate with density functional theory and x-ray absorption spectroscopy, and find a magnetic insulating ground state with Jeff=12 character.

  13. Cooper Pairs in Insulators?!

    ScienceCinema

    James Valles

    2010-01-08

    Nearly 50 years elapsed between the discovery of superconductivity and the emergence of the microscopic theory describing this zero resistance state. The explanation required a novel phase of matter in which conduction electrons joined in weakly bound pairs and condensed with other pairs into a single quantum state. Surprisingly, this Cooper pair formation has also been invoked to account for recently uncovered high-resistance or insulating phases of matter. To address this possibility, we have used nanotechnology to create an insulating system that we can probe directly for Cooper pairs. I will present the evidence that Cooper pairs exist and dominate the electrical transport in these insulators and I will discuss how these findings provide new insight into superconductor to insulator quantum phase transitions. 

  14. Imaging of room-temperature ferromagnetic nano-domains at the surface of a non-magnetic oxide

    NASA Astrophysics Data System (ADS)

    Taniuchi, T.; Motoyui, Y.; Morozumi, K.; Rödel, T. C.; Fortuna, F.; Santander-Syro, A. F.; Shin, S.

    2016-06-01

    Two-dimensional electron gases at oxide surfaces or interfaces show exotic ordered states of matter, like superconductivity, magnetism or spin-polarized states, and are a promising platform for alternative oxide-based electronics. Here we directly image a dense population of randomly distributed ferromagnetic domains of ~40 nm typical sizes at room temperature at the oxygen-deficient surface of SrTiO3, a non-magnetic transparent insulator in the bulk. We use laser-based photoemission electron microscopy, an experimental technique that gives selective spin detection of the surface carriers, even in bulk insulators, with a high spatial resolution of 2.6 nm. We furthermore find that the Curie temperature in this system is as high as 900 K. These findings open perspectives for applications in nano-domain magnetism and spintronics using oxide-based devices, for instance through the nano-engineering of oxygen vacancies at surfaces or interfaces of transition-metal oxides.

  15. Direct spectroscopic observation of a shallow hydrogenlike donor state in insulating SrTiO3.

    PubMed

    Salman, Z; Prokscha, T; Amato, A; Morenzoni, E; Scheuermann, R; Sedlak, K; Suter, A

    2014-10-10

    We present a direct spectroscopic observation of a shallow hydrogenlike muonium state in SrTiO(3) which confirms the theoretical prediction that interstitial hydrogen may act as a shallow donor in this material. The formation of this muonium state is temperature dependent and appears below ∼ 70K. From the temperature dependence we estimate an activation energy of ∼ 50 meV in the bulk and ∼ 23 meV near the free surface. The field and directional dependence of the muonium precession frequencies further supports the shallow impurity state with a rare example of a fully anisotropic hyperfine tensor. From these measurements we determine the strength of the hyperfine interaction and propose that the muon occupies an interstitial site near the face of the oxygen octahedron in SrTiO(3). The observed shallow donor state provides new insight for tailoring the electronic and optical properties of SrTiO(3)-based oxide interface systems. PMID:25375730

  16. Exploring interfacial ferromagnetism in manganite-based superlattices

    NASA Astrophysics Data System (ADS)

    Yi, Di; Flint, Charles; Suzuki, Yuri

    Heterointerface of complex oxides provides a rich playground to explore the emergent phenomena that are not found in bulk. In particular, emergent interfacial ferromagnetism has been successfully demonstrated in heterostructures composed of materials which are paramagnetic and antiferromagnetic in bulk. In our previous work, leakage of itinerant electrons from a paramagnetic metal to an antiferromagnetic insulator has been shown to give rise to interfacial ferromagnetism in CaMnO3 based superlattices. However interfacial ferromagnetism in insulating superlattices suggests a more complicated scenario. Therefore a thorough investigation of coupling between charge, lattice and spin degrees of freedom is necessary. In this talk, we focus on the NdNiO3 /CaMnO3 system. By choosing a paramagnetic layer that undergoes a metal-insulator transition, we can explore the role of electron itinerancy in interfacial ferromagnetism in the same sample to eliminate the inconsistencies that may originate from the deposition of multiple samples. We demonstrate that NdNiO3 exhibits a metal-insulator transition as a function of temperature, which can be tuned as a function of film thickness. We have also grown NdNiO3 /CaMnO3 heterostructures with excellent crystallinity. Preliminary transport measurements indicate that the presence of an adjacent CaMnO3 layer also affects the transport in NdNiO3 so that charge transfer from the itinerant layer into the adjacent antiferromagnetic insulating CaMnO3 is likely not the only contribution to interfacial ferromagnetism.

  17. Magnetic Exchange Coupling in Ferromagnetic/Superconducting/Ferromagnetic Multilayers

    NASA Astrophysics Data System (ADS)

    de Melo, C. A. R. Sa

    2001-03-01

    The possibility of magnetic exchange coupling between ferromagnets (F) separated by superconductor (S) spacers in F/S/F multilayers is analysed theoretically [1,2]. Ideal systems for the observation of magnetic coupling through superconductors are complex oxide multilayers consisting of Colossal Magneto-Resistance (CMR) Ferromagnets and High Critical Temperature Cuprate Superconductors. For this coupling to occur, three "prima facie" conditions need to be satisfied. First, an indirect exchange coupling between the ferromagnets must exist when the superconductor is in its normal state. Second, superconductivity must not be destroyed due to the proximity of ferromagnetic boundaries. Third, roughness of the F/S interfaces must be small. Under these conditions, when the superconductor is cooled below its critical temperature T_c, the magnetic coupling changes. The appearance of the superconducting gap introduces a new length scale (the coherence length of the superconductor) and modifies the temperature dependence of the indirect exchange coupling existent in the normal state. The magnetic coupling is oscillatory both above and below T_c, as well as strongly temperature-dependent. However at low temperatures the indirect exchange coupling decay length is controlled by the coherence length of the superconductor, while at temperatures close to and above Tc the magnetic coupling decay length is controlled by the thermal length. [I would like to thank the Georgia Institute of Technology, NSF (Grant No. DMR-9803111) and NATO (Grant No. CRG-972261) for financial support.] [1] C. A. R. Sa de Melo, Phys. Rev. Lett. 79, 1933 (1997). [2] C. A. R. Sa de Melo, Phys. Rev. B 62, 12303 (2000).

  18. Thermal insulator

    SciTech Connect

    Yamamoto, R.; Asada, Y.; Matsuo, Y.; Mikoda, M.

    1985-07-16

    A thermal insulator comprises an expanded resin body having embedded therein an evacuated powder insulation portion which consists of fine powder and a container of film-like plastics or a film-like composite of plastics and metal for enclosing the powder. The resin body has been expanded by a Freon gas as a blowing agent. Since a Freon gas has a larger molecular diameter than the constituent gases of air, it is less likely to permeate through the container than air. Thus present invention provides a novel composite insulator which fully utilizes the benefits of vacuum insulation without necessitating a strong and costly material for a vacuum container.

  19. Systematic study of transport via surface and bulk states in Bi2Te3 topological insulator

    NASA Astrophysics Data System (ADS)

    de Castro, S.; Peres, M. L.; Chitta, V. A.; Gratens, X.; Soares, D. A. W.; Fornari, C. I.; Rappl, P. H. O.; Abramof, E.; Oliveira, N. F., Jr.

    2016-07-01

    We performed magnetoresistance measurements on Bi2Te3 thin film in the temperature range of T = 1.2–4.0 K and for magnetic fields up to 2 T. The curves exhibited anomalous behavior for temperatures below 4.0 K. Different temperature intervals revealed electrical transport through different conductive channels with clear signatures of weak antilocalization. The magnetoresistance curves were explained using the Hikami–Larkin–Nagaoka model and the 2D Dirac modified model. The comparison between the parameters obtained from the two models revealed the transport via topological surface states and bulk states. In addition, a superconductive like transition is observed for the lowest temperatures and we suggest that this effect can be originated from the misfit dislocations caused by strain, giving rise to a superconductive channel between the interface of the film and the substrate.

  20. Polyanion Driven Antiferromagnetic and Insulating Ground State of Olivine Phosphates: LiMPO4

    NASA Astrophysics Data System (ADS)

    Jena, Ajit Kumar; Nanda, B. R. K.; Condensed Matter Theory; Computation Team

    Through density functional calculations we have investigated the electronic and magnetic properties of LiMPO4, where M is a 3d transition metal element. We find that contrary to many transition metal oxides, in these Olivine phosphates the band gap is originated due to crystal field anisotropy as well as weak O-p - M-d covalent interaction. Both of them are attributed to the presence of PO43- polyanion. The anisotropic crystal field, in the absence of covalent interactions, creates atomically localized non-degenerate M-d states and therefore the gap is a natural outcome. Onsite repulsion, due to strong correlation effect, further enhances the gap. These localized d states favor high-spin configuration which leads to antiferromagnetic ordering due to Hund's coupling. Experimentally observed low Neel temperature of this family of compounds is explained from the DFT obtained spin exchange interaction parameters. Work supported by Nissan Research Program.

  1. Mn-based ferromagnetic semiconductors

    NASA Astrophysics Data System (ADS)

    Dietl, Tomasz; Sawicki, Maciej

    2003-07-01

    The present status of research and prospects for device applications of ferromagnetic (diluted magnetic) semiconductors (DMS) is presented. We review the nature of the electronic states and the mechanisms of the carrier-mediated exchange interactions (mean-field Zener model) in p-type Mn-based III-V and II-VI compounds, highlighting a good correspondence of experimental findings and theoretical predictions. An account of the latest progress on the road of increasing the Currie point to above the room temperature is given for both families of compounds. We comment on a possibility of obtaining ferromagnetism in n-type materials, taking (Zn,Mn)O:Al as the example. Concerning technologically important issue of easy axis and domain engineering, we present theoretical predictions and experimental results on the temperature and carrier concentration driven change of magnetic anisotropy in (Ga,Mn)As.

  2. Photoinduced switching to metallic states in the two-dimensional organic Mott insulator dimethylphenazine-tetrafluorotetracyanoquinodimethane with anisotropic molecular stacks

    NASA Astrophysics Data System (ADS)

    Matsuzaki, Hiroyuki; Ohkura, Masa-aki; Ishige, Yu; Nogami, Yoshio; Okamoto, Hiroshi

    2015-06-01

    A photoinduced phase transition was investigated in an organic charge-transfer (CT) complex M2P -TCNQ F4 , [M2P : 5,10-dihydro-5,10-dimethylphenazine, donor (D) molecule; TCNQ F4 : 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane, acceptor (A) molecule] by means of femtosecond pump-probe reflection spectroscopy. This is an ionic compound and has a peculiar two-dimensional (2D) molecular arrangement; the same A (or D) molecules arrange along the [100] direction, and A and D molecules alternately arrange along the [111] direction. It results in a strongly anisotropic two-dimensional electronic structure. This compound shows a structural and magnetic phase transition at 122 K below which the two neighboring molecules are dimerized along both the [100] and [111] directions. We demonstrate that two kinds of photoinduced phase transitions occur by irradiation of a femtosecond laser pulse; in the high-temperature lattice-uniform phase, a quasi-one-dimensional (1D) metallic state along the AA(DD) stack is generated, and in the low-temperature lattice-dimerized phase, a quasi-2D metallic state is initially produced and molecular dimerizations are subsequently released. Mixed-stack CT compounds consisting of DA stacks are generally insulators or semiconductors in the ground state. Here, such a dynamical metallization in the DA stack is demonstrated. The release of the dimerizations drives several kinds of coherent oscillations which play an important role in the stabilization of the lattice-dimerized phase. The mechanisms of those photoinduced phase transitions are discussed in terms of the magnitudes of the anisotropic bandwidths and molecular dimerizations along two different directions of the molecular stacks.

  3. Instability of the Jeff = 1/2 Insulating State in Srn+1IrnO3n+1 (n = 1 and 2)

    NASA Astrophysics Data System (ADS)

    Cao, G.; Delong, L. E.

    2013-06-01

    The most profound consequence of the spin-orbit interaction in layered iridates is the realization of a recently proposed Jeff = 1/2 insulating state, which explains novel insulating behavior observed in the Srn+1IrnO3n+1 materials. We review the basic physical properties of layered iridates, including our recent transport and thermodynamic data for Srn+1IrnO3n+1 (n = 1, 2). These results indicate the spin-orbit interaction vigorously competes with Coulomb interactions, non-cubic crystal electric field interactions, and the Hund's rule coupling, which leads to states with exotic properties that are highly susceptible to small perturbations. The effects of chemical doping, application of pressure and magnetic field are emphasized; these perturbations are relatively weak, but capable of influencing the spin-orbit interaction and generating a rich phase diagram of strongly competing ground states.

  4. Excitation-Photon-Energy Selectivity of Photoconversions in Halogen-Bridged Pd-Chain Compounds: Mott Insulator to Metal or Charge-Density-Wave State

    NASA Astrophysics Data System (ADS)

    Matsuzaki, H.; Iwata, M.; Miyamoto, T.; Terashige, T.; Iwano, K.; Takaishi, S.; Takamura, M.; Kumagai, S.; Yamashita, M.; Takahashi, R.; Wakabayashi, Y.; Okamoto, H.

    2014-08-01

    Ultrafast photoinduced transitions of a one-dimensional Mott insulator into two distinct electronic phases, metal and charge-density-wave (CDW) state, were achieved in a bromine-bridged Pd-chain compound [Pd(en)2Br](C5-Y)2H2O (en =ethylenediamine and C5-Y=dialkylsulfosuccinate), by selecting the photon energy of a femtosecond excitation pulse. For the resonant excitation of the Mott-gap transition, excitonic states are generated and converted to one-dimensional CDW domains. For the higher-energy excitation, free electron and hole carriers are produced, giving rise to a transition of the Mott insulator to a metal. Such selectivity in photoconversions by the choice of initial photoexcited states opens a new possibility for the developments of advanced optical switching and memory functions.

  5. Weak anisotropic x-ray magnetic linear dichroism at the Eu M4,5 edges of ferromagnetic EuO(001): evidence for 4f-state contributions.

    PubMed

    van der Laan, Gerrit; Arenholz, Elke; Schmehl, Andreas; Schlom, Darrell G

    2008-02-15

    We have observed a weak anisotropic x-ray magnetic linear dichroism (AXMLD) at the Eu M4,5 edges of ferromagnetic EuO(001), which indicates that the 4f states are not rotationally invariant. A quantitative agreement of the AXMLD is obtained with multiplet calculations where the 4f state is split by an effective cubic crystalline electrostatic field. The results indicate that the standard model of rare earths, where 4f electrons are treated as core states, is not correct and that the 4f orbitals contribute weakly to the magnetic anisotropy. PMID:18352515

  6. Magneto-elastic coupling in a potential ferromagnetic 2D atomic crystal

    NASA Astrophysics Data System (ADS)

    Tian, Yao; Gray, Mason J.; Ji, Huiwen; Cava, R. J.; Burch, Kenneth S.

    2016-06-01

    Cr2Ge2Te6 has been of interest for decades, as it is one of only a few naturally forming ferromagnetic semiconductors. Recently, this material has been revisited due to its potential as a two-dimensional semiconducting ferromagnet and a substrate to induce anomalous quantum Hall states in topological insulators. However, many relevant properties of Cr2Ge2Te6 still remain poorly understood, especially the spin-phonon coupling crucial to spintronic, multiferrioc, thermal conductivity, magnetic proximity and the establishment of long range order on the nanoscale. We explore the interplay between the lattice and magnetism through high resolution micro-Raman scattering measurements over the temperature range from 10 to 325 K. Strong spin-phonon coupling effects are confirmed from multiple aspects: two low energy modes splits in the ferromagnetic phase, magnetic quasielastic scattering in the paramagnetic phase, the phonon energies of three modes show clear upturn below T C, and the phonon linewidths change dramatically below T C as well. Our results provide the first demonstration of spin-phonon coupling in a potential two-dimensional atomic crystal.

  7. Observation of a ferromagnetic instability in repulsively interacting Fermi gases of 6 Li

    NASA Astrophysics Data System (ADS)

    Scazza, Francesco; Valtolina, Giacomo; Amico, Andrea; Burchianti, Alessia; Fort, Chiara; Zaccanti, Matteo; Inguscio, Massimo; Roati, Giacomo

    2016-05-01

    The fine control of interaction strengths and optical trapping potentials in ultracold atomic ensembles provide unique opportunities to explore strongly correlated fermion phenomena, such as superfluidity and magnetism. In our setup, we produce 6 Li quantum gases in the vicinity of a broad Feshbach resonance and we subsequently superimpose to the samples a thin optical barrier to engineer either a Josephson weak link or a ferromagnetic domain wall. This technique recently enabled the experimental study of the Josephson dynamics of superfluid Fermi gases flowing through an insulating barrier, spanning a wide range of interaction strengths across the BEC-BCS crossover. On the other hand, by preparing adjacent and fully spin-polarized domains, we are able to experimentally address the upper branch of a repulsively interacting Fermi gas and its magnetic properties. Here, we report on the investigation of the onset and the stability of the ferromagnetic state. Measurements of spin diffusion dynamics in the system reveal a total suppression of spin conductance above a critical interaction strength, accompanied by a softening of the collective spin-dipole mode, indicating the existence of a ferromagnetic instability.

  8. Orographic insulation in the South Asian summer monsoon: mean state and model bias

    NASA Astrophysics Data System (ADS)

    Boos, W.; Kuang, Z.; Hurley, J. V.

    2012-12-01

    Monsoons are thermally direct, continental-scale circulations that supply water to billions of people, so controls on the intensity and location of monsoons are widely studied. Here we discuss several aspects of the interaction of orography with the South Asian summer monsoon. We begin by reviewing the observed thermodynamic mean state of this monsoon, and explain how it motivates the hypothesis that orography creates a strong monsoon by preventing the intrusion of cold and dry extratropical air into the monsoon thermal maximum. This hypothesis contrasts sharply with the long-standing view that the South Asian summer monsoon is driven by elevated heating from the Tibetan Plateau, but numerical model results confirm that the monsoon remains largely unchanged when the plateau heat source is eliminated. Next we show that almost all models participating in the Coupled Model Intercomparison Project (CMIP) exhibit a common bias in the thermodynamic structure of the South Asian summer monsoon that is caused by a poor representation of orography. In most CMIP models, the simulated Asian upper-tropospheric temperature maximum is too weak during summer and is displaced southeast of its observed location over northwest India; maxima of surface air moist static energy are also too weak and displaced to the southeast. The spatial structure of this bias and its relation to low-level wind suggest that it is caused by an overly smoothed representation of topography west of the Tibetan Plateau, which allows dry air from the deserts of western Asia to pass over topography and penetrate the thermal maximum. A model with a decent representation of the thermodynamic state of the Asian monsoon is integrated with standard topography and then with truncated topography just west of the Tibetan Plateau. This relatively minor modification recreates the thermodynamic bias seen in the CMIP models; it also reduces precipitation over India in simulations of modern climate and reduces the next

  9. Collapse of the low temperature insulating state in Cr-doped V{sub 2}O{sub 3} thin films

    SciTech Connect

    Homm, P. Dillemans, L.; Menghini, M.; Van Bilzen, B.; Bakalov, P.; Su, C.-Y.; Lieten, R.; Houssa, M.; Locquet, J.-P.; Nasr Esfahani, D.; Covaci, L.; Peeters, F. M.; Seo, J. W.

    2015-09-14

    We have grown epitaxial Cr-doped V{sub 2}O{sub 3} thin films with Cr concentrations between 0% and 20% on (0001)-Al{sub 2}O{sub 3} by oxygen-assisted molecular beam epitaxy. For the highly doped samples (>3%), a regular and monotonous increase of the resistance with decreasing temperature is measured. Strikingly, in the low doping samples (between 1% and 3%), a collapse of the insulating state is observed with a reduction of the low temperature resistivity by up to 5 orders of magnitude. A vacuum annealing at high temperature of the films recovers the low temperature insulating state for doping levels below 3% and increases the room temperature resistivity towards the values of Cr-doped V{sub 2}O{sub 3} single crystals. It is well-know that oxygen excess stabilizes a metallic state in V{sub 2}O{sub 3} single crystals. Hence, we propose that Cr doping promotes oxygen excess in our films during deposition, leading to the collapse of the low temperature insulating state at low Cr concentrations. These results suggest that slightly Cr-doped V{sub 2}O{sub 3} films can be interesting candidates for field effect devices.

  10. RKKY interaction on surfaces of topological insulators with superconducting proximity effect

    NASA Astrophysics Data System (ADS)

    Zyuzin, Alexander A.; Loss, Daniel

    2014-09-01

    We consider the Ruderman-Kittel-Kasuya-Yosida (RKKY) interaction between magnetic impurities on the surface of a three-dimensional topological insulator with proximity induced superconductivity. A superconductor placed on the top of the topological insulator induces a gap in the surface electron states and gives rise to a long-range in-plane antiferromagnetic RKKY interaction. This interaction is frustrated due to strong spin-orbit coupling, inversely proportional to the distance between magnetic impurities if the distance between magnetic impurities is smaller than the superconducting coherence length, and dominates over the ferromagnetic and Dzialoshinskii-Moriya type interactions if the distance between magnetic impurities is larger than the superconducting coherence length. The condition for the subgap states that are bound to the magnetic impurities is found.

  11. Superconducting dome and crossover to an insulating state in [Tl{sub 4}]Tl{sub 1−x}Sn{sub x}Te{sub 3}

    SciTech Connect

    Arpino, K. E.; Wasser, B. D.; McQueen, T. M.

    2015-04-01

    The structural, superconducting, and electronic phase diagram of [Tl{sub 4}]Tl{sub 1−x}Sn{sub x}Te{sub 3} is reported. Magnetization and specific heat measurements show bulk superconductivity exists for 0 ≤ x ≤ 0.4. Resistivity measurements indicate a crossover from a metallic state at x = 0 to a doped insulator at x = 1. Universally, there is a large non-Debye specific heat contribution, characterized by an Einstein temperature of θ{sub E} ≈ 35 K. Density functional theory calculations predict x = 0 to be a topological metal, while x = 1 is a topological crystalline insulator. The disappearance of superconductivity correlates with the transition between these distinct topological states.

  12. Electrical Probing of Inherent Spin Polarization in a Topological Insulator with Electrical Gating

    NASA Astrophysics Data System (ADS)

    Lee, Joon Sue; Richardella, Anthony; Samarth, Nitin

    2015-03-01

    The hallmark of a time-reversal symmetry protected three-dimensional topological insulator is the helically spin-textured surface state. Although electrical detection of spin polarization in topological insulators has been demonstrated very recently, there have not been any electrical measurements to demonstrate the entire mapping of the spin polarization throughout the surface state. We report the electrical probing of the spin-polarized surface state using a magnetic tunnel junction as a spin detector while the chemical potential of a topological insulator (Bi,Sb)2Te3 is tuned by back gating. Hysteretic spin signals were observed as the magnetization of the detector ferromagnet (permalloy) switches with in-plane magnetic field. Changing the direction of bias current through the topological insulator channel flips the direction of the spin polarization, resulting in the reverse of sign of the detected spin signals. We demonstrate the control of the Fermi energy, which has importance not only in further understanding of the spin-momentum locking in the surface state but also in possible electrical tuning of the spin polarization for potential spin-based devices. Supported by C-SPIN & DARPA/SRC.

  13. Automotive Insulation

    NASA Technical Reports Server (NTRS)

    1997-01-01

    Under a Space Act Agreement between Boeing North America and BSR Products, Space Shuttle Thermal Protection System (TPS) materials are now used to insulate race cars. BSR has created special TPS blanket insulation kits for use on autos that take part in NASCAR events, and other race cars through its nationwide catalog distribution system. Temperatures inside a race car's cockpit can soar to a sweltering 140 to 160 degrees, with the extreme heat coming through the engine firewall, transmission tunnel, and floor. It is common for NASCAR drivers to endure blisters and burns due to the excessive heat. Tests on a car insulated with the TPS material showed a temperature drop of some 50 degrees in the driver's cockpit. BSR-TPS Products, Inc. now manufactures insulation kits for distribution to race car teams around the world.

  14. Ferromagnetic Ground States with High Transition Temperatures in New Tetragonal Rare-Earth Compounds CeRu2Al2B and PrRu2Al2B

    NASA Astrophysics Data System (ADS)

    Matsuoka, Eiichi; Tomiyama, Yo; Sugawara, Hitoshi; Sakurai, Takahiro; Ohta, Hitoshi

    2012-04-01

    We report on the discovery of ferromagnetic ground states in the newly synthesized tetragonal compounds CeRu2Al2B and PrRu2Al2B. Polycrystalline samples of these compounds were synthesized and their magnetization M(B) and electrical resistivity ρ(T) were measured as functions of the magnetic field B and the temperature T. CeRu2Al2B is a metallic compound with trivalent Ce ions and is classified into a Kondo-lattice system: a Kondo-like -\\ln T dependence of ρ(T) is observed below 32 K. The cusp at TN{}Ce = 14.3 K and the steep increase at approximately TC{}Ce = 13 K observed in the magnetic susceptibility M/B(T) correspond to antiferromagnetic (AFM) and ferromagnetic (FM) transitions, respectively. TC{}Ce is the second highest transition temperature of the Ce-based ferromagnet with no other magnetic elements. PrRu2Al2B is also a metallic compound with trivalent Pr ions. The M/B(T) of PrRu2Al2B shows a cusp at TN{}Pr = 26 K and a steep increase at approximately TC{}Pr = 11 K, indicating that AFM and FM transitions occur at TN{}Pr and TC{}Pr, respectively.

  15. Topological Mott Insulators

    SciTech Connect

    Raghu, S.

    2010-03-02

    We consider extended Hubbard models with repulsive interactions on a honeycomb lattice, and the transitions from the semimetal to Mott insulating phases at half-filling. Because of the frustrated nature of the second-neighbor interactions, topological Mott phases displaying the quantum Hall and the quantum spin Hall effects are found for spinless and spin fermion models, respectively. The mean-field phase diagram is presented and the fluctuations are treated within the random phase approximation. Renormalization group analysis shows that these states can be favored over the topologically trivial Mott insulating states.

  16. Doping an antiferromagnetic insulator : A route to an antiferromagnetic metallic phase

    NASA Astrophysics Data System (ADS)

    Mahadevan, Priya; Pandey, Shishir; Sarma, D. D.

    Usually antiferromagnetism is accompanied by an insulating character of the ground state, while ferromagnetism is accompanied by metallicity. In the limit of half-filling, the Hubbard model yields an antiferromagnetic insulator as the ground state. From the Nagaoka theorem we expect ferromagnetism at any finite electron doping of this half filled state. Numerical studies on the other hand, have however shown, that at low doping concentrations one has a narrow region of an antiferromagnetic metallic phase. The question is whether this is realizable in real materials. Among the 3d transition metal oxides, this antiferromagnetic metallic phase has remained elusive as strong electron-phonon coupling results in a different phase diagram. The 5d transition metal oxides are therefore more suitable. In this work we solve a multiband Hubbard model relevant for a 5d transition metal oxide within a mean-field approach and show that the large bandwidth and the small intra-atomic Hund's exchange associated with this limit gives us a robust AFM-M ground state for 25% electron doping. The conclusions are supported by ab-initio electronic structure calculations for NaOsO3.

  17. Thermal Insulation

    NASA Technical Reports Server (NTRS)

    1984-01-01

    Commercially known as Solimide, Temptronics, Inc.'s thermal insulation has application in such vehicles as aircraft, spacecraft and surface transportation systems (i.e. rapid transit cars, trains, buses, and ships) as acoustical treatment for door, wall, and ceiling panels, as a means of reducing vibrations, and as thermal insulation (also useful in industrial equipment). Product originated from research conducted by Johnson Space Center on advanced flame-resistant materials for minimizing fire hazard in the Shuttle and other flight vehicles.

  18. Gradual Localization of 5f States in Orthorhombic UTX Ferromagnets:Polarized Neutron Diffraction Study of Ru Substituted UCoGe

    NASA Astrophysics Data System (ADS)

    Vališka, Michal; Pospíšil, Jiří; Stunault, Anne; Takeda, Yukiharu; Gillon, Béatrice; Haga, Yoshinori; Prokeš, Karel; Abd-Elmeguid, Mohsen M.; Nénert, Gwilherm; Okane, Tetsuo; Yamagami, Hiroshi; Chapon, Laurent; Gukasov, Arsene; Cousson, Alain; Yamamoto, Etsuji; Sechovský, Vladimír

    2015-08-01

    We report on a microscopic study of the evolution of ferromagnetism in the Ru substituted ferromagnetic superconductor (FM SC) UCoGe crystallizing in the orthorhombic TiNiSi-type structure. For that purpose, two single crystals with composition UCo0.97Ru0.03Ge and UCo0.88Ru0.12Ge have been prepared and characterized by magnetization, AC susceptibility, specific heat and electrical resistivity measurements. Both compounds have been found to order ferromagnetically below TC = 6.5 and 7.5 K, respectively, which is considerably higher than the TC = 3 K of the parent compound UCoGe. The higher values of TC are accompanied by enhanced values of the spontaneous moment μspont = 0.11 μB/f.u. and μspont = 0.21 μB/f.u., respectively in comparison to the tiny spontaneous moment of UCoGe (about 0.07 μB/f.u.). No sign of superconductivity was detected in either compound. The magnetic moments of the samples were investigated on the microscopic scale using polarized neutron diffraction (PND) and for UCo0.88Ru0.12Ge also by soft X-ray magnetic circular dichroism (XMCD). The analysis of the PND results indicates that the observed enhancement of ferromagnetism is mainly due to the growth of the orbital part of the uranium 5f moment μ LU, reflecting a gradual localization of the 5f electrons with Ru substitution. In addition, the parallel orientation of the U and Co moments has been established in both substituted compounds. The results are discussed and compared with related isostructural ferromagnetic UTX compounds (T: transition metals, X: Si, Ge) in the context of a varying degree of the 5f-ligand hybridization.

  19. Magnetic Topological Insulators and Quantum Anomalous Hall Effect

    NASA Astrophysics Data System (ADS)

    Kou, Xufeng

    The engineering of topological surface states is a key to realize applicable devices based on topological insulators (TIs). Among various proposals, introducing magnetic impurities into TIs has been proven to be an effective way to open a surface gap and integrate additional ferromagnetism with the original topological order. In this Dissertation, we study both the intrinsic electrical and magnetic properties of the magnetic TI thin films grown by molecular beam epitaxy. By doping transition element Cr into the host tetradymite-type V-VI semiconductors, we achieve robust ferromagnetic order with a strong perpendicular magnetic anisotropy. With additional top-gating capability, we realize the electric-field-controlled ferromagnetism in the magnetic TI systems, and demonstrate such magneto-electric effects can be effectively manipulated, depending on the interplays between the band topology, magnetic exchange coupling, and structural engineering. Most significantly, we report the observation of quantum anomalous Hall effect (QAHE) in the Cr-doped (BiSb)2Te3 samples where dissipationless chiral edge conduction is realized in the macroscopic millimeter-size devices without the presence of any external magnetic field, and the stability of the quantized Hall conductance of e2/h is well-maintained as the film thickness varies across the 2D hybridization limit. With additional quantum confinement, we discover the metal-to-insulator switching between two opposite QAHE states, and reveal the universal QAHE phase diagram in the thin magnetic TI samples. In addition to the uniform magnetic TIs, we further investigate the TI/Cr-doped TI bilayer structures prepared by the modulation-doped growth method. By controlling the magnetic interaction profile, we observe the Dirac hole-mediated ferromagnetism and develop an effective way to manipulate its strength. Besides, the giant spin-orbit torque in such magnetic TI-based heterostructures enables us to demonstrate the current

  20. Interface-Driven Ferromagnetism within the Quantum Wells of a Rare Earth Titanate Superlattice.

    PubMed

    Need, R F; Isaac, B J; Kirby, B J; Borchers, J A; Stemmer, S; Wilson, Stephen D

    2016-07-15

    Here we present polarized neutron reflectometry measurements exploring thin film heterostructures composed of a strongly correlated Mott state, GdTiO_{3}, embedded with SrTiO_{3} quantum wells. Our results reveal that the net ferromagnetism inherent to the Mott GdTiO_{3} matrix propagates into the nominally nonmagnetic SrTiO_{3} quantum wells and tracks the magnetic order parameter of the host Mott insulating matrix. Beyond a well thickness of 5 SrO layers, the magnetic moment within the wells is dramatically suppressed, suggesting that quenched well magnetism comprises the likely origin of quantum critical magnetotransport in this thin film architecture. Our data demonstrate that the interplay between proximate exchange fields and polarity-induced carrier densities can stabilize extended magnetic states within SrTiO_{3} quantum wells. PMID:27472135