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.
NASA Astrophysics Data System (ADS)
Zhang, Duming; Richardella, Anthony; Rench, David W.; Xu, Su-Yang; Kandala, Abhinav; Flanagan, Thomas C.; Beidenkopf, Haim; Yeats, Andrew L.; Buckley, Bob B.; Klimov, Paul V.; Awschalom, David D.; Yazdani, Ali; Schiffer, Peter; Hasan, M. Zahid; Samarth, Nitin
2012-11-01
The breaking of time-reversal symmetry by ferromagnetism is predicted to yield profound changes to the electronic surface states of a topological insulator. Here, we report on a concerted set of structural, magnetic, electrical, and spectroscopic measurements of Mn-Bi2Se3 thin films wherein photoemission and x-ray magnetic circular dichroism studies have recently shown surface ferromagnetism in the temperature range 15K≤T≤100 K, accompanied by a suppressed density of surface states at the Dirac point. Secondary-ion mass spectroscopy and scanning tunneling microscopy reveal an inhomogeneous distribution of Mn atoms, with a tendency to segregate towards the sample surface. Magnetometry and anisotropic magnetoresistance measurements are insensitive to the high-temperature ferromagnetism seen in surface studies, revealing instead a low-temperature ferromagnetic phase at T≲5 K. The absence of both a magneto-optical Kerr effect and an anomalous Hall effect suggests that this low-temperature ferromagnetism is unlikely to be a homogeneous bulk phase but likely originates in nanoscale near-surface regions of the bulk where magnetic atoms segregate during sample growth. Although the samples are not ideal, with both bulk and surface contributions to electron transport, we measure a magnetoconductance whose behavior is qualitatively consistent with predictions that the opening of a gap in the Dirac spectrum drives quantum corrections to the conductance in topological insulators from the symplectic to the orthogonal class.
Noise signatures of metastable resistivity states in ferromagnetic insulating manganite
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.
Chang, Cui-Zu; Zhao, Weiwei; Kim, Duk Y; Zhang, Haijun; Assaf, Badih A; Heiman, Don; Zhang, Shou-Cheng; Liu, Chaoxing; Chan, Moses H W; Moodera, Jagadeesh S
2015-05-01
The discovery of the quantum Hall (QH) effect led to the realization of a topological electronic state with dissipationless currents circulating in one direction along the edge of a two-dimensional electron layer under a strong magnetic field. The quantum anomalous Hall (QAH) effect shares a similar physical phenomenon to that of the QH effect, whereas its physical origin relies on the intrinsic spin-orbit coupling and ferromagnetism. Here, we report the experimental observation of the QAH state in V-doped (Bi,Sb)2Te3 films with the zero-field longitudinal resistance down to 0.00013 ± 0.00007h/e(2) (~3.35 ± 1.76 Ω), Hall conductance reaching 0.9998 ± 0.0006e(2)/h and the Hall angle becoming as high as 89.993° ± 0.004° at T = 25 mK. A further advantage of this system comes from the fact that it is a hard ferromagnet with a large coercive field (Hc > 1.0 T) and a relative high Curie temperature. This realization of a robust QAH state in hard ferromagnetic topological insulators (FMTIs) is a major step towards dissipationless electronic applications in the absence of external fields.
Orbital domain state and finite size scaling in ferromagnetic insulating manganites.
Papavassiliou, G; Pissas, M; Belesi, M; Fardis, M; Dolinsek, J; Dimitropoulos, C; Ansermet, J P
2003-10-03
55Mn and 139La NMR measurements on a high quality single crystal of ferromagnetic (FM) La0.80Ca0.20MnO3 demonstrate the formation of localized Mn(3+,4+) states below 70 K, accompanied by a strong cooling-rate dependent increase of certain FM neutron Bragg peaks. (55,139)(1/T(1)) spin-lattice and (139)(1/T(2)) spin-spin relaxation rates are strongly enhanced on approaching this temperature from below, signaling a genuine phase transition at T(tr) approximately 70 K. The disappearance of the FM metallic signal by applying a weak external magnetic field, the different NMR radio-frequency enhancement of the FM metallic and insulating states, and the observed finite size scaling of T(tr) with Ca (hole) doping, as observed in powder La(1-x)CaxMnO3 samples, are suggestive of freezing into an inhomogeneous FM insulating and orbitally ordered state embodying "metallic" hole-rich walls.
Nyquist noise as probe of hot-electron effects in the ferromagnetic insulating state of manganites
NASA Astrophysics Data System (ADS)
Samanta, Sudeshna; Raychaudhuri, Arup K.
2009-03-01
Hole-doped rare-earth manganites (like La1-xCaxMnO3) in the ferromagnetic insulating (FMI) state show large non-linear conductance. Such non-linear conductance can arise due to hot-electron effect which originates from decoupling of the electron and lattice temperatures at high power level. The non-linear conductance manifests as electro-resistance or current induced resistance change. We report here low frequency temperature dependent noise measurement which allows us to estimate the electronic temperature by measuring Nyquist noise (``white noise'' in contrast to 1/f noise) in La0.8Ca0.2MnO3 single crystals which has a distinct FMI state below 100K. The measurement was performed with low ac biasing current which was mixed with a high current density d.c that leads to electron heating. We observed that in the insulating state, above a certain input d.c power, the Nyquist noise increases by a large extent and this is coupled to the onset of non-linear conduction as signalled by the power dependence of the differential conductance. The experiment establishes a direct link between hot-electron effect and non-linear conductance.
Highly insulating ferromagnetic cobaltite heterostructures
Choi, Woo Seok; Kang, Kyeong Tae; Jeen, Hyoungjeen; ...
2017-04-02
Ferromagnetic insulators are rather rare but possess great technological potential in, for example, spintronics. Individual control of ferromagnetic properties and electronic transport provides a useful design concept of multifunctional oxide heterostructures. We studied the close correlation among the magnetism, atomic structure, and electronic structure of oxide heterostructures composed of the ferromagnetic perovskite LaCoO3 and the antiferromagnetic brownmillerite SrCoO2.5 epitaxial thin film layers. By reversing the stacking sequence of the two layers, we could individually modify the electric resistance and saturation magnetic moment. Lastly, the ferromagnetic insulating behavior in the heterostructures was understood in terms of the electronic reconstruction at themore » oxide surface/interfaces and crystalline quality of the constituent layers.« less
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.
Visualizing ferromagnetic domains in magnetic topological insulators
Wang, Wenbo; Gu, G. D.; Yang, Fang; ...
2015-05-13
We report a systematic study of ferromagnetic domains in both single-crystal and thin-film specimens of magnetic topological insulators Cr doped (Bi0.1Sb0.9)2Te3 using magnetic force microscopy (MFM). The temperature and field dependences of MFM and in situ resistance data are consistent with previous bulk transport and magnetic characterization. Bubble-like ferromagnetic domains were observed in both single crystals and thin films. Significantly, smaller domain size (~500 nm) with narrower domain wall (~150 – 300 nm) was observed in thin films of magnetic topological insulators, likely due to vertical confinement effect. As a result, these results suggest that thin films are more promisingmore » for visualization of chiral edge states.« less
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.
Spin pumping in Ferromagnet-Topological Insulator-Ferromagnet Heterostructures
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
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.
NASA Astrophysics Data System (ADS)
Xiao, Xianbo; Liu, Zhengfang; Du, Yan; Ai, Guoping
2017-10-01
The spin-resolved edge states transport in a normal/ferromagnetic/normal topological insulator (TI) junction is investigated numerically. It is shown that the transport properties of the hybrid junction strongly depend on the interface shape. For the junction with two sharp interfaces, a nonzero spin conductance can be generated besides the spin-split energy windows. Moreover, the axial symmetries of the in-plane spin conductance amplitude are broken. The underlying physics is attributed to the sharp-interface-induced quantum interference effect. However, for the hybrid junction with two smooth interfaces, a non-zero spin conductance can only be achieved in the spin-split energy windows. Further, the axial symmetries of the in-plane spin conductance amplitude recover. These findings may not only benefit to further apprehend the spin-dependent edge states transport in the hybrid TI junctions but also provide some theoretical bases to the application of the topological spintronics devices.
Yoshida, Nobukatsu; Yamashiro, Masashi
2012-09-12
We present a general formula for tunneling conductance in ballistic ferromagnet/ferromagnetic insulator/superconductor junctions where the superconducting state has the opposite spin pairing symmetry. The formula shows, correctly, that ferromagnetism has been induced by the effective mass difference between up- and down-spin electrons. This effectively mass mismatched ferromagnet and a standard Stoner ferromagnet have been employed in this paper. As an application of the formulation, we have studied the tunneling effect for junctions including a spin-triplet p-wave superconductor, where we choose a normal insulator for the insulating region, although our formula can be used for a ferromagnetic insulator. Then, we have been able to devote our attention to features of a ferromagnetic metal. The conductance spectra show a clear difference between the two ferromagnets depending upon the method of normalization of the conductance. In particular, an essential difference is seen in the zero-bias conductance peaks, reflecting the characteristics of each ferromagnet. From the obtained results, we suggest that the measurements of the tunneling conductance in the junction provide us with useful information about the mechanism of itinerant ferromagnetism in metals.
Pure spin current devices based on ferromagnetic topological insulators
Götte, Matthias; Joppe, Michael; Dahm, Thomas
2016-01-01
Two-dimensional topological insulators possess two counter propagating edge channels with opposite spin direction. Recent experimental progress allowed to create ferromagnetic topological insulators realizing a quantum anomalous Hall (QAH) state. In the QAH state one of the two edge channels disappears due to the strong ferromagnetic exchange field. We investigate heterostructures of topological insulators and ferromagnetic topological insulators by means of numerical transport calculations. We show that spin current flow in such heterostructures can be controlled with high fidelity. Specifically, we propose spintronic devices that are capable of creating, switching and detecting pure spin currents using the same technology. In these devices electrical currents are directly converted into spin currents, allowing a high conversion efficiency. Energy independent transport properties in combination with large bulk gaps in some topological insulator materials may allow operation even at room temperature. PMID:27782187
Pure spin current devices based on ferromagnetic topological insulators.
Götte, Matthias; Joppe, Michael; Dahm, Thomas
2016-10-26
Two-dimensional topological insulators possess two counter propagating edge channels with opposite spin direction. Recent experimental progress allowed to create ferromagnetic topological insulators realizing a quantum anomalous Hall (QAH) state. In the QAH state one of the two edge channels disappears due to the strong ferromagnetic exchange field. We investigate heterostructures of topological insulators and ferromagnetic topological insulators by means of numerical transport calculations. We show that spin current flow in such heterostructures can be controlled with high fidelity. Specifically, we propose spintronic devices that are capable of creating, switching and detecting pure spin currents using the same technology. In these devices electrical currents are directly converted into spin currents, allowing a high conversion efficiency. Energy independent transport properties in combination with large bulk gaps in some topological insulator materials may allow operation even at room temperature.
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.
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
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.
Controllable ferromagnetism of iron doped topological insulator
NASA Astrophysics Data System (ADS)
Qiao, Shan; Liu, Zhen; Ji, Fuhao; Li, Bin; Xi, Fuchun; Kuroda, K.; Ye, Mao; Miyamoto, K.; Kimura, A.
2012-02-01
The higher than room temperature ferromagnetism was found in iron doped Bi2Se3. Samples generated by different processes have different magnetic characters. The Curie temperature is independent on iron concentration which against all discovered dilute magnetic systems. EXAFS observations show that the local structure of iron in samples with paramagnetic character is complex. On the contrary, that with ferromagnetic character is very simple that the iron atoms make up small single atom, dimer or trimer structures and these structures randomly distributed in Bi2Se3 crystal. The ferromagnetism can be enhanced or suppressed by the shift of Fermi edge by co-doping of Mg and Fe to Bi2Se3 crystal. The less than 3 atoms small structure cannot have room temperature ferromagnetism, so we believe that the higher than room temperature controllable ferromagnetism is intrinsic character of iron doped topological insulator.
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.
Ferromagnetism and glassiness on the surface of topological insulators
NASA Astrophysics Data System (ADS)
Liu, Chun-Xiao; Roy, Bitan; Sau, Jay D.
2016-12-01
We investigate the nature of the ordering among magnetic adatoms, randomly deposited on the surface of topological insulators. Restricting ourselves to dilute impurity and weak coupling (between itinerant fermion and magnetic impurities) limit, we show that for arbitrary amount of chemical doping away from the apex of the surface Dirac cone the magnetic impurities tend to arrange themselves in a spin-density-wave pattern, with the periodicity approximately π /kF , where kF is the Fermi wave vector, when magnetic moment for impurity adatoms is isotropic. However, when magnetic moment possesses strong Ising or easy-axis anisotropy, pursuing both analytical and numerical approaches we show that the ground state is ferromagnetic for low to moderate chemical doping, despite the fragmentation of the system into multiple ferromagnetic islands. For high doping away from the Dirac point as well, the system appears to fragment into many ferromagnetic islands, but the magnetization in these islands is randomly distributed. Such magnetic ordering with net zero magnetization is referred to here as ferromagnetic spin glass, which is separated from the pure ferromagnet state by a first order phase transition. We generalize our analysis for cubic topological insulators (supporting three Dirac cones on a surface) and demonstrate that the nature of magnetic orderings and the transition between them remains qualitatively the same. We also discuss the possible relevance of our analysis to recent experiments.
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.
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.
Spin and Lattice excitations in Ferromagnetic Insulating Manganites
NASA Astrophysics Data System (ADS)
Mesa, Dalgis; Zhang, Jiandi; Fernandez-Baca, Jaime; Ye, Feng; Hagen, Mark; Tomioka, T.; Tokura, Yoshinori
2011-03-01
Though double-exchange interaction has been recognized as a major driving force for the couple magnetic and electronic phase transition, the nature of insulating ground state with ferromagnetic ordering in low-doping manganites is still not fully understood. Here we report on an inelastic neutron scattering study of spin and lattice excitations in the ferromagnetic insulating (FMI) phase of La 1-x Ca x Mn O3 with x(Ca) = 0.2. Dispersion relations for both phonons and spin waves along high-symmetry directions were obtained for temperatures of 5 and 225 K, respectively. At low temperatures, our results indicate an anomalous softening and broadening of the magnons near the zone boundary, especially when the magnon energy E ~ 20 meV, where a longitudinal optical phonon is present. Additional phonon and magnon branches observed will also be discussed. Acknowledgement: NSF DMR1005562.
Possible Chern insulators based on novel ferromagnetic substrates
NASA Astrophysics Data System (ADS)
Liu, Jianpeng; Vanderbilt, David
2015-03-01
Previous work has opened the possibility that one can obtain non-zero Chern numbers from the surface bands of atomic layers with strong spin-orbit coupling deposited on insulating ferromagnetic thin films. Following this idea, we carry out a theoretical search for Chern (i.e., quantum anomalous Hall) insulators formed by depositing heavy adatoms on top of stable insulating ferromagnetic substrates such as CrSiTe3 and CrGeTe3. These materials have a layered structure with weak van der Waals interlayer coupling, so it seems likely that thin films with clean surfaces will be experimentally stable. Moreover, the insulating character of these materials, with magnetization normal to the surface, are promising for realizing Chern-insulator states when adatoms are added. By searching over a series of heavy elements, we found non-zero Chern numbers in Bi and Tl-deposited CrSiTe3 thin films, although with slightly negative indirect energy gaps. We expect that a global energy gap could be opened up by some further engineering, such as by applying epitaxial strain or additional atomic substitution. Supported by NSF Grant DMR-14-08838.
A high-temperature ferromagnetic topological insulating phase by proximity coupling.
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.
A high-temperature ferromagnetic topological insulating phase by proximity coupling
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.
2016-05-09
Topological insulators are insulating materials that display conducting surface states protected by time-reversal symmetry(1,)2, 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(3-5). 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(6,7). 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 similar to 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(2,8) could allow efficient manipulation of the magnetization dynamics by an electric field, providing an energy-efficient topological control mechanism for future spin-based technologies.
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.
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.
Novel Majorana mode and magnetoresistance in ferromagnetic superconducting topological insulator
NASA Astrophysics Data System (ADS)
Goudarzi, H.; Khezerlou, M.; Asgarifar, S.
2017-03-01
Among the potential applications of topological insulators, we investigate theoretically the effect of coexistence of proximity-induced ferromagnetism and superconductivity on the surface states of 3-dimensional topological insulator, where the superconducting electron-hole excitations can be significantly affected by the magnetization of ferromagnetic order. We find that, Majorana mode energy, as a verified feature of TI F/S structure, along the interface sensitively depends on the magnitude of magnetization mzfs in FS region, while its slope in perpendicular incidence presents steep and no change. Since the superconducting gap is renormalized by a factor η (mzfs) , hence Andreev reflection is more or less suppressed, and, in particular, resulting subgap tunneling conductance is more sensitive to the magnitude of magnetizations in FS and F regions. Furthermore, an interesting scenario happens at the antiparallel configuration of magnetizations mzf and mzfs resulting in magnetoresistance in N/F/FS junction, which can be controlled and decreased by tuning the magnetization magnitude in FS region.
NASA Astrophysics Data System (ADS)
Khezerlou, Maryam; Goudarzi, Hadi; Asgarifar, Samin
2017-03-01
Among the potential applications of topological insulators, we theoretically study the coexistence of proximity-induced ferromagnetic and superconducting orders in the surface states of a 3-dimensional topological insulator. The superconducting electron-hole excitations can be significantly affected by the magnetic order induced by a ferromagnet. In one hand, the surface state of the topological insulator, protected by the time-reversal symmetry, creates a spin-triplet and, on the other hand, magnetic order causes to renormalize the effective superconducting gap. We find Majorana mode energy along the ferromagnet/superconductor interface to sensitively depend on the magnitude of magnetization m zfs from superconductor region, and its slope around perpendicular incidence is steep with very low dependency on m zfs . The superconducting effective gap is renormalized by a factor η( m zfs ), and Andreev bound state in ferromagnet-superconductor/ferromagnet/ferromagnet-superconductor (FS/F/FS) Josephson junction is more sensitive to the magnitude of magnetizations of FS and F regions. In particular, we show that the presence of m zfs has a noticeable impact on the gap opening in Andreev bound state, which occurs in finite angle of incidence. This directly results in zero-energy Andreev state being dominant. By introducing the proper form of corresponding Dirac spinors for FS electron-hole states, we find that via the inclusion of m zfs , the Josephson supercurrent is enhanced and exhibits almost abrupt crossover curve, featuring the dominant zero-energy Majorana bound states.
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.
Quantum Corrections Crossover and Ferromagnetism in Magnetic Topological Insulators
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
Ferromagnetic-Insulator-Based Superconducting Junctions as Sensitive Electron Thermometers
NASA Astrophysics Data System (ADS)
Giazotto, F.; Solinas, P.; Braggio, A.; Bergeret, F. S.
2015-10-01
We present an exhaustive theoretical analysis of charge and thermoelectric transport in a normal-metal-ferromagnetic-insulator-superconductor junction and explore the possibility of its use as a sensitive thermometer. We investigate the transfer functions and the intrinsic noise performance for different measurement configurations. A common feature of all configurations is that the best temperature-noise performance is obtained in the nonlinear temperature regime for a structure based on an Europium chalcogenide ferromagnetic insulator in contact with a superconducting Al film structure. For an open-circuit configuration, although the maximal intrinsic temperature sensitivity can achieve 10 nK Hz-1 /2 , a realistic amplifying chain will reduce the sensitivity up to 10 μ K Hz-1 /2 . To overcome this limitation, we propose a measurement scheme in a closed-circuit configuration based on state-of-the-art superconducting-quantum-interference-device detection technology in an inductive setup. In such a case, we show that temperature-noise can be as low as 35 nK Hz-1 /2 . We also discuss a temperature-to-frequency converter where the obtained thermovoltage developed over a Josephson junction operated in the dissipative regime is converted into a high-frequency signal. We predict that the structure can generate frequencies up to approximately 120 GHz and transfer functions up to 200 GHz /K at around 1 K. If operated as an electron thermometer, the device may provide temperature-noise lower than 35 nK Hz-1 /2 thereby being potentially attractive for radiation-sensing applications.
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.
Superfluid Spin Transport Through Easy-Plane Ferromagnetic Insulators
NASA Astrophysics Data System (ADS)
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.
Theory of the spin-Seebeck effect at a topological-insulator/ferromagnetic-insulator interface
NASA Astrophysics Data System (ADS)
Okuma, Nobuyuki; Masir, Massoud Ramezani; MacDonald, Allan H.
2017-04-01
The spin-Seebeck effect refers to voltage signals induced in metals by thermally driven spin currents in adjacent magnetic systems. We present a theory of the spin-Seebeck signal in the case where the conductor that supports the voltage signal is the topologically protected two-dimensional surface-state system at the interface between a ferromagnetic insulator (FI) and a topological insulator (TI). Our theory uses a Dirac model for the TI surface states and assumes Heisenberg exchange coupling between the TI quasiparticles and the FI magnetization. The spin-Seebeck voltage is induced by the TI surface states scattering off the nonequilibrium magnon population at the surface of the semi-infinite thermally driven FI. Our theory is readily generalized to spin-Seebeck voltages in any two-dimensional conductor that is exchange-coupled to the surface of a FI. Surface-state carrier-density-dependent signal strengths calculated using Bi2Te3 and yttrium iron garnet material parameters are consistent with recent experiments.
Kondo dynamics in one-dimensional doped ferromagnetic insulators
NASA Astrophysics Data System (ADS)
Pimenta, Hudson; Oliveira, Luiz N.; Pereira, Rodrigo G.
2015-04-01
Some well-established examples of itinerant-electron ferromagnetism in one dimension occur in a Mott-insulating phase. We examine the consequences of doping a ferromagnetic insulator and coupling magnons to gapless charge fluctuations. Using a bosonization scheme for strongly interacting electrons, we derive an effective field theory for the magnon-holon interaction. When the magnon momentum matches the Fermi momentum of the holons, the backscattering of the magnon at low energies gives rise to a Kondo effect of a pseudospin defined from the chirality degree of freedom (right- or left-moving particles). The crossover between weak-coupling and strong-coupling fixed points of the effective mobile-impurity model is then investigated using a numerical renormalization group approach.
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.
Interfacial Coupling-Induced Ferromagnetic Insulator Phase in Manganite Film.
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.
Tuning Ferromagnetism at Interfaces between Insulating Perovskite Oxides
NASA Astrophysics Data System (ADS)
Ganguli, Nirmal; Kelly, Paul J.
2014-09-01
We use density functional theory calculations to show that the LaAlO3|SrTiO3 interface between insulating perovskite oxides is borderline in satisfying the Stoner criterion for itinerant ferromagnetism and explore other oxide combinations with a view to satisfying it more amply. The larger lattice parameter of a LaScO3|BaTiO3 interface is found to be less favorable than the greater interface distortion of LaAlO3|CaTiO3. Compared to LaAlO3|SrTiO3, the latter is predicted to exhibit robust magnetism with a larger saturation moment and a higher Curie temperature. Our results provide support for a "two phase" picture of coexistent superconductivity and ferromagnetism.
Tuning ferromagnetism at interfaces between insulating perovskite oxides.
Ganguli, Nirmal; Kelly, Paul J
2014-09-19
We use density functional theory calculations to show that the LaAlO3|SrTiO3 interface between insulating perovskite oxides is borderline in satisfying the Stoner criterion for itinerant ferromagnetism and explore other oxide combinations with a view to satisfying it more amply. The larger lattice parameter of a LaScO3|BaTiO3 interface is found to be less favorable than the greater interface distortion of LaAlO3|CaTiO3. Compared to LaAlO3|SrTiO3, the latter is predicted to exhibit robust magnetism with a larger saturation moment and a higher Curie temperature. Our results provide support for a "two phase" picture of coexistent superconductivity and ferromagnetism.
Superfluid Spin Transport through Easy-Plane Ferromagnetic Insulators
NASA Astrophysics Data System (ADS)
Takei, So; Tserkovnyak, Yaroslav
2014-03-01
Superfluid spin transport | dissipationless transport of spin | is theoretically studied in a ferromagnetic insulator with easy-plane anisotropy. We consider an open geometry where 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 through the device 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 using a magnetically-mediated electron-drag experiment. This work was supported in part by FAME (an SRC STARnet center sponsored by MARCO and DARPA), the NSF under Grant No. DMR-0840965, and Grant No. 228481 from the Simons Foundation.
Domain wall of a ferromagnet on a three-dimensional topological insulator
Wakatsuki, Ryohei; Ezawa, Motohiko; Nagaosa, Naoto
2015-01-01
Topological insulators (TIs) show rich phenomena and functions which can never be realized in ordinary insulators. Most of them come from the peculiar surface or edge states. Especially, the quantized anomalous Hall effect (QAHE) without an external magnetic field is realized in the two-dimensional ferromagnet on a three-dimensional TI which supports the dissipationless edge current. Here we demonstrate theoretically that the domain wall of this ferromagnet, which carries edge current, is charged and can be controlled by the external electric field. The chirality and relative stability of the Neel wall and Bloch wall depend on the position of the Fermi energy as well as the form of the coupling between the magnetic moments and orbital of the host TI. These findings will pave a path to utilize the magnets on TI for the spintronics applications. PMID:26323943
Domain wall of a ferromagnet on a three-dimensional topological insulator.
Wakatsuki, Ryohei; Ezawa, Motohiko; Nagaosa, Naoto
2015-09-01
Topological insulators (TIs) show rich phenomena and functions which can never be realized in ordinary insulators. Most of them come from the peculiar surface or edge states. Especially, the quantized anomalous Hall effect (QAHE) without an external magnetic field is realized in the two-dimensional ferromagnet on a three-dimensional TI which supports the dissipationless edge current. Here we demonstrate theoretically that the domain wall of this ferromagnet, which carries edge current, is charged and can be controlled by the external electric field. The chirality and relative stability of the Neel wall and Bloch wall depend on the position of the Fermi energy as well as the form of the coupling between the magnetic moments and orbital of the host TI. These findings will pave a path to utilize the magnets on TI for the spintronics applications.
Interfacial Coupling-Induced Ferromagnetic Insulator Phase in Manganite Film
Zhang, Bangmin; Wu, Lijun; Yin, Wei-Guo; ...
2016-06-08
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 themore » 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. In conclusion, 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.« less
Ferromagnetism in vanadium doped thin films of a topological insulator Bi2Te3
NASA Astrophysics Data System (ADS)
Zhao, Lukas; Chen, Zhiyi; Korzhovska, Inna; Deng, Haiming; Raoux, Simone; Jordan-Sweet, Jean; Sarachik, Myriam; Krusin-Elbaum, Lia
2012-02-01
Recent first-principle calculations predict a new class of ferromagnetic systems that are distinctly different from the conventional dilute magnetic semiconductors. A novel ferromagnetic topological insulator (ferro-TI) state can be obtained when topological insulator are doped with certain transition metal elements. In the the quasi-2D limit these ferro-TIs are expected to support a quantized anomalous Hall effect. Here we report on electrical and magnetic characterization of vanadium doped thin (˜50 nm) films of a topological insulator Bi2Te3. Films were grown by rf sputtering on S3N4/Si substrates with lithographically pre-patterned contact pads. Low-temperature in-plane and Hall resistivity measurements were performed in magnetic fields up to 5 T fields. We find that below 100 K, V-doped films display negative linear magnetoresistance, which at lower temperatures becomes hysteretic. Hall resistivity is also hysteretic, suggesting an unusual ferromagnetic ordering below 10 K. Moreover, V-doping turns the p-type conduction in as-grown films into n-type. The doping and thickness dependence of these effects will be discussed.
Interfacial Coupling-Induced Ferromagnetic Insulator Phase in Manganite Film
Zhang, Bangmin; Wu, Lijun; Yin, Wei-Guo; Sun, Cheng-Jun; Yang, Ping; Venkatesan, T.; Chen, Jingsheng; Zhu, Yimei; Chow, Gan Moog
2016-06-08
Interfaces with subtle differences in atomic and electronic structures in perovskite ABO_{3} 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 Pr_{0.67}Sr_{0.33}MnO_{3} film on SrTiO_{3} substrate. The variations in the out-of-plane lattice constant and BO_{6} octahedral rotation across the Pr_{0.67}Sr_{0.33}MnO_{3}/SrTiO_{3} interface strongly depend on the thickness of the Pr_{0.67}Sr_{0.33}MnO_{3} 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 SrTiO_{3}, 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. In conclusion, 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.
Interfacial Coupling-Induced Ferromagnetic Insulator Phase in Manganite Film
Zhang, Bangmin; Wu, Lijun; Yin, Wei-Guo; Sun, Cheng-Jun; Yang, Ping; Venkatesan, T.; Chen, Jingsheng; Zhu, Yimei; Chow, Gan Moog
2016-06-08
Interfaces with subtle differences in atomic and electronic structures in perovskite ABO_{3} 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 Pr_{0.67}Sr_{0.33}MnO_{3} film on SrTiO_{3} substrate. The variations in the out-of-plane lattice constant and BO_{6} octahedral rotation across the Pr_{0.67}Sr_{0.33}MnO_{3}/SrTiO_{3} interface strongly depend on the thickness of the Pr_{0.67}Sr_{0.33}MnO_{3} 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 SrTiO_{3}, 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. In conclusion, 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.
Magnon excitation and transport in Ferromagnetic Insulator/metal multilayers
NASA Astrophysics Data System (ADS)
Liu, Tao; Ren, Jie; Zhang, Jianwei
We studied magnon excitation and transport in a Ferromagnetic Insulator(FI) layer(such as YIG), which connected with Ferromagnetic/normal metal multilayers in two sides. In our modeling, we adopted self-consistent spin dependent Boltzmann equations in metal layers and magnon Boltzmann equation in FI layer. When applying an in-plane current in FM layer, a transverse spin current was generated due to Anomalous Hall effect, after crossing normal metal layer, it will produce magnon excitation at N/FI interface. With carrying spin information, magnon excitation in FI can eventually excite a new spin current at second F/N interface. This is so call magnon-drag effect. In our work, we focused on magnon propagation in FI, with all two-magnon, three magnon, and four magnon scattering. Associated with spin dependent Boltzmann equation, we can investigate magnon excitation and transport properties in FI layer from the interface to bulk scale. The magnon excitation in FI layer is dominated not only by the interface interaction at Normal/FI boundary, but also by the bulk scattering in FI. Our results show the magnon in FI layer has decay behaviors to low energy model. We also showed a new way to manipulate magnon transport in FI This work was supported by NSFC Grant No. 11274240 and 51471119.
Magnon excitation and decay in Ferromagnetic Insulator/metal multilayers
NASA Astrophysics Data System (ADS)
Liu, Tao; Li, Jiaxi; Zhang, Jianwei
2014-03-01
We studied magnon excitation in a Ferromagnetic Insulator(FI) layer(such as YIG), which connected two Ferromagnetic/normal metal multilayers in two sides. In our modeling, we adopted self-consistent spin dependent Boltzmann equations in metal layers and magnon Boltzmann equation in FI layer. When applying an in-plane current in first FM layer, a transverse spin current was generated due to Anomalous Hall effect(AHE), after crossing normal metal layer, this transverse spin current will produce magnon excitation at N/FI interface. With carrying spin information, magnon excitations in FI can eventually excite a new spin current at second F/N interface. Although the FI cannot support any spin current propagation across it, but spin polarization information was passed through FI with propagation of magnon. Finally, the transverse spin current in second FM layer can also generate another in-plane spin current by AHE. The magnon excitation in FI layer is dominated by the interfacial interaction at Normal/FI boundary. Our results show the magnon in FI layer have decay behaviors to low energy model. We also showed that when applying a magnetic field on FI layer, spin current in final FM layer can be manipulated by varying magnon excitation.
Integration of the ferromagnetic insulator EuO onto graphene.
Swartz, Adrian G; Odenthal, Patrick M; Hao, Yufeng; Ruoff, Rodney S; Kawakami, Roland K
2012-11-27
We have demonstrated the deposition of EuO films on graphene by reactive molecular beam epitaxy in a special adsorption-controlled and oxygen-limited regime, which is a critical advance toward the realization of the exchange proximity interaction (EPI). It has been predicted that when the ferromagnetic insulator (FMI) EuO is brought into contact with graphene, an overlap of electronic wave functions at the FMI/graphene interface can induce a large spin splitting inside the graphene. Experimental realization of this effect could lead to new routes for spin manipulation, which is a necessary requirement for a functional spin transistor. Furthermore, EPI could lead to novel spintronic behavior such as controllable magnetoresistance, gate tunable exchange bias, and quantized anomalous Hall effect. However, experimentally, EuO has not yet been integrated onto graphene. Here we report the successful growth of high-quality crystalline EuO on highly oriented pyrolytic graphite and single-layer graphene. The epitaxial EuO layers have (001) orientation and do not induce an observable D peak (defect) in the Raman spectra. Magneto-optic measurements indicate ferromagnetism with a Curie temperature of 69 K, which is the value for bulk EuO. Transport measurements on exfoliated graphene before and after EuO deposition indicate only a slight decrease in mobility.
NASA Astrophysics Data System (ADS)
Kawamura, Minoru; Yoshimi, Ryutaro; Tsukazaki, Atsushi; Takahashi, Kei S.; Kawasaki, Masashi; Tokura, Yoshinori
2017-07-01
The instability of the quantum anomalous Hall (QAH) effect has been studied as a function of the electric current and temperature in ferromagnetic topological insulator thin films. We find that a characteristic current for the breakdown of the QAH effect is roughly proportional to the Hall-bar width, indicating that the Hall electric field is relevant to the breakdown. We also find that electron transport is dominated by variable range hopping (VRH) at low temperatures. Combining the current and temperature dependences of the conductivity in the VRH regime, the localization length of the QAH state is evaluated to be about 5 μ m . The long localization length suggests a marginally insulating nature of the QAH state due to a large number of in-gap states.
Josephson current in d-wave superconductor junctions with ferromagnetic insulator
NASA Astrophysics Data System (ADS)
Liao, Yan-Hua; Yang, Meng; Ma, Chang; Cao, Yu-Bin
2012-05-01
We investigate the temperature dependence of the critical current and current-phase relation by taking into account the ferromagnetic scattering effect at interface in a d-wave superconductor (S)/ferromagnetic insulator layer (FI)/d-wave superconductor (S) junction. It is shown that both the barrier scattering and the roughness scattering at the interface always suppress the Andreev reflection. The Josephson critical currents depend to a great extent on the effective exchange field of the interface and the crystal orientation of the d-wave superconductor. The exchange field can lead to the change of the junction from 0 to π states and the alteration of the oscillation periods. It can also enhance the Josephson critical current in the junction under certain conditions.
Ferromagnetism in chromium doped topological insulator thin films and nanoplate crystals
NASA Astrophysics Data System (ADS)
Chen, Zhiyi; Zhao, Lukas; Korzhovska, Inna; Deng, Haiming; Huang, Limin; Raoux, Simone; Jordan-Sweet, Jean; O'Brien, Stephen; Krusin-Elbaum, Lia
2012-02-01
The surface states of topological insulators are protected by time-reversal symmetry. Introducing magnetic impurities should break this symmetry and open a gap in the otherwise gapless surface states. Recent first-principle calculations predict that when topological insulators are doped with transition metal elements, such as Cr or Fe, a magnetically ordered insulating state will form -- a state that in thin (quasi-2D) samples may support a quantized Hall conductance. Here we report on electrical and magnetic characterization of thin Cr doped topological insulators: Sb2Te3 nanoplate crystals and ˜50 nm thin films of Bi2Te3. Electrical contacts to samples were lithographically defined, with rf sputtered films grown on pre-patterned substrates. Low-temperature in-plane resistivity, Hall, and magnetization measurements were performed in up to 5 T magnetic fields. For 5 at% Cr content, a distinct ferromagnetic hysteretic response is observed at temperatures below 10 K. Hysteretic loops, also observed in Hall resistivity, indicate low-T coercive fields of the order of 0.5 T. Correlation of transport and magnetic measurements indicating anomalous Hall effect, and strong dependence on dopant concentration and sample thickness will be presented.
Spin-dependent delay time in ferromagnet/insulator/ferromagnet heterostructures
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.
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.
Above 400-K robust perpendicular ferromagnetic phase in a topological insulator
Tang, Chi; Chang, Cui-Zu; Zhao, Gejian; Liu, Yawen; Jiang, Zilong; Liu, Chao-Xing; McCartney, Martha R.; Smith, David J.; Chen, Tingyong; Moodera, Jagadeesh S.; Shi, Jing
2017-01-01
The quantum anomalous Hall effect (QAHE) that emerges under broken time-reversal symmetry in topological insulators (TIs) exhibits many fascinating physical properties for potential applications in nanoelectronics and spintronics. However, in transition metal–doped TIs, the only experimentally demonstrated QAHE system to date, the QAHE is lost at practically relevant temperatures. This constraint is imposed by the relatively low Curie temperature (Tc) and inherent spin disorder associated with the random magnetic dopants. We demonstrate drastically enhanced Tc by exchange coupling TIs to Tm3Fe5O12, a high-Tc magnetic insulator with perpendicular magnetic anisotropy. Signatures showing that the TI surface states acquire robust ferromagnetism are revealed by distinct squared anomalous Hall hysteresis loops at 400 K. Point-contact Andreev reflection spectroscopy confirms that the TI surface is spin-polarized. The greatly enhanced Tc, absence of spin disorder, and perpendicular anisotropy are all essential to the occurrence of the QAHE at high temperatures. PMID:28691097
Above 400-K robust perpendicular ferromagnetic phase in a topological insulator.
Tang, Chi; Chang, Cui-Zu; Zhao, Gejian; Liu, Yawen; Jiang, Zilong; Liu, Chao-Xing; McCartney, Martha R; Smith, David J; Chen, Tingyong; Moodera, Jagadeesh S; Shi, Jing
2017-06-01
The quantum anomalous Hall effect (QAHE) that emerges under broken time-reversal symmetry in topological insulators (TIs) exhibits many fascinating physical properties for potential applications in nanoelectronics and spintronics. However, in transition metal-doped TIs, the only experimentally demonstrated QAHE system to date, the QAHE is lost at practically relevant temperatures. This constraint is imposed by the relatively low Curie temperature (Tc) and inherent spin disorder associated with the random magnetic dopants. We demonstrate drastically enhanced Tc by exchange coupling TIs to Tm3Fe5O12, a high-Tc magnetic insulator with perpendicular magnetic anisotropy. Signatures showing that the TI surface states acquire robust ferromagnetism are revealed by distinct squared anomalous Hall hysteresis loops at 400 K. Point-contact Andreev reflection spectroscopy confirms that the TI surface is spin-polarized. The greatly enhanced Tc, absence of spin disorder, and perpendicular anisotropy are all essential to the occurrence of the QAHE at high temperatures.
Zero-bias photocurrent in ferromagnetic topological insulator
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
Zero-bias photocurrent in ferromagnetic topological insulator.
Ogawa, N; Yoshimi, R; Yasuda, K; Tsukazaki, A; Kawasaki, M; Tokura, Y
2016-07-20
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.
Qin, Wei; Zhang, Zhenyu
2014-12-31
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.
Linear response theory for magnon transport in ferromagnetic insulators
NASA Astrophysics Data System (ADS)
Murakami, Shuichi; Matsumoto, Ryo
2012-02-01
We study transverse response of magnons in ferromagnetic insulators within linear response theory. In analogy with the corresponding theory for electrons [1], magnon transverse response is described, including the Hall effect, Nernst effect, and thermal Hall effect. As is also the case for electrons [1], the response functions for magnons consist of the Kubo-formula term, and the term corresponding to the orbital angular momentum. We can rewrite the response functions in terms of the Berry curvature in momentum space [2]. We apply this theory to the (quantum-mechanical) magnons and to the classical magnetostatic waves. For the magnetostatic waves, the eigenmodes are given by a generalized eigenvalue problem, giving rise to the special form of the Berry curvature [2]. We explain various properties of this Berry curvature for the generalized eigenvalue problem, and discuss its implications for the physical properties of magnetostatic modes. [1] L. Smrcka and P. Streda, J. Phys. C, 10, 2153 (1977); H. Oji, P. Streda, Phys. Rev. B 31, 7291 (1985); [2] R. Matsumoto and S. Murakami, Phys. Rev. Lett. 106, 197202 (2011); Phys. Rev. B 84, 184406 (2011).
2015-01-01
principle calculations predicted that the insulating magnetic ground state can indeed be obtained by a proper choice of TM dopants, through van Vleck... magnetic TI Cr- and V-doped (Bi,Sb)2Te3, where the insulating FM order [21] excludes the RKKY-type interaction and indicates the FM mechanism to be of...Experimental Verification of Van Vleck Nature of Long-Range Ferromagnetic Order in Vanadium-Doped Three-Dimensional Topological Insulator Sb2Te3
Ba2NiOsO6 : A Dirac-Mott insulator with ferromagnetism near 100 K
Feng, Hai L.; Calder, Stuart; Ghimire, Madhav Prasad; ...
2016-12-28
In this study, the ferromagnetic semiconductor Ba2NiOsO6 ( Tmag ~ 100 K ) was synthesized at 6 GPa and 1500 °C. It crystallizes into a double perovskite structure [Fm - 3m ; a = 8.0428 ( 1 ) Å], where the Ni2+ and Os6+ ions are perfectly ordered at the perovskite B site. We show that the spin-orbit coupling of Os6+ plays an essential role in opening the charge gap. The magnetic state was investigated by density functional theory calculations and powder neutron diffraction. The latter revealed a collinear ferromagnetic order in a > 21 kOe magnetic field at 5more » K. The ferromagnetic gapped state is fundamentally different from that of known dilute magnetic semiconductors such as (Ga,Mn)As and (Cd,Mn)Te ( Tmag < 180 K ), the spin-gapless semiconductor Mn2 CoAl ( Tmag ~ 720 K ), and the ferromagnetic insulators EuO ( Tmag ~ 70 K ) and Bi3Cr3O11 ( Tmag ~ 220 K ). It is also qualitatively different from known ferrimagnetic insulators and semiconductors, which are characterized by an antiparallel spin arrangement. Our finding of the ferromagnetic semiconductivity of Ba2NiOsO6 should increase interest in the platinum group oxides, because this alternative class of materials should be useful in the development of spintronic, quantum magnetic, and related devices.« less
B a2NiOs O6 : A Dirac-Mott insulator with ferromagnetism near 100 K
NASA Astrophysics Data System (ADS)
Feng, Hai L.; Calder, Stuart; Ghimire, Madhav Prasad; Yuan, Ya-Hua; Shirako, Yuichi; Tsujimoto, Yoshihiro; Matsushita, Yoshitaka; Hu, Zhiwei; Kuo, Chang-Yang; Tjeng, Liu Hao; Pi, Tun-Wen; Soo, Yun-Liang; He, Jianfeng; Tanaka, Masahiko; Katsuya, Yoshio; Richter, Manuel; Yamaura, Kazunari
2016-12-01
The ferromagnetic semiconductor B a2NiOs O6 (Tmag˜100 K ) was synthesized at 6 GPa and 1500 °C. It crystallizes into a double perovskite structure [F m -3 m ; a =8.0428 (1 )Å ], where the N i2 + and O s6 + ions are perfectly ordered at the perovskite B site. We show that the spin-orbit coupling of O s6 + plays an essential role in opening the charge gap. The magnetic state was investigated by density functional theory calculations and powder neutron diffraction. The latter revealed a collinear ferromagnetic order in a >21 kOe magnetic field at 5 K. The ferromagnetic gapped state is fundamentally different from that of known dilute magnetic semiconductors such as (Ga,Mn)As and (Cd,Mn)Te (Tmag<180 K ), the spin-gapless semiconductor M n2CoAl (Tmag˜720 K ), and the ferromagnetic insulators EuO (Tmag˜70 K ) and B i3C r3O11 (Tmag˜220 K ). It is also qualitatively different from known ferrimagnetic insulators and semiconductors, which are characterized by an antiparallel spin arrangement. Our finding of the ferromagnetic semiconductivity of B a2NiOs O6 should increase interest in the platinum group oxides, because this alternative class of materials should be useful in the development of spintronic, quantum magnetic, and related devices.
Insulating Ferromagnetic LaCoO3-δ Films: A Phase Induced by Ordering of Oxygen Vacancies
NASA Astrophysics Data System (ADS)
Biškup, Neven; Salafranca, Juan; Mehta, Virat; Oxley, Mark P.; Suzuki, Yuri; Pennycook, Stephen J.; Pantelides, Sokrates T.; Varela, Maria
2014-02-01
The origin of ferromagnetism in strained epitaxial LaCoO3 films has been a long-standing mystery. Here, we combine atomically resolved Z-contrast imaging, electron-energy-loss spectroscopy, and density-functional calculations to demonstrate that, in epitaxial LaCoO3 films, oxygen-vacancy superstructures release strain, control the film's electronic properties, and produce the observed ferromagnetism via the excess electrons in the Co d states. Although oxygen vacancies typically dope a material n-type, we find that ordered vacancies induce Peierls-like minigaps which, combined with strain relaxation, trigger a nonlinear rupture of the energy bands, resulting in insulating behavior.
Gangopadhyay, Shruba; Pickett, Warren E.
2015-01-15
The double perovskite Ba2NaOsO6 (BNOO), an exotic example of a very high oxidation state (heptavalent) osmium d1 compound and also uncommon by being a ferromagnetic open d-shell (Mott) insulator without Jahn-Teller (JT) distortion, is modeled using a density functional theory based hybrid functional incorporating exact exchange for correlated electronic orbitals and including the large spin-orbit coupling (SOC). The experimentally observed narrow-gap ferromagnetic insulating ground state is obtained, but only when including spin-orbit coupling, making this a Dirac-Mott insulator. The calculated easy axis along [110] is in accord with experiment, providing additional support that this approach provides a realistic method formore » studying this system. The predicted spin density for [110] spin orientation is nearly cubic (unlike for other directions), providing an explanation for the absence of JT distortion. An orbital moment of –0.4μB strongly compensates the +0.5μB spin moment on Os, leaving a strongly compensated moment more in line with experiment. Remarkably, the net moment lies primarily on the oxygen ions. An insulator-metal transition, by rotating the magnetization direction with an external field under moderate pressure, is predicted as one consequence of strong SOC, and metallization under moderate pressure is predicted. In conclusion, a comparison is made with the isostructural, isovalent insulator Ba2LiOsO6, which, however, orders antiferromagnetically.« less
Critical thickness for ferromagnetism in insulating LaMnO3 films
NASA Astrophysics Data System (ADS)
Renshaw Wang, X.; Poccia, N.; Leusink, D. P.; Paudel, Tura R.; Tsymbal, E. Y.; Li, C. J.; Lv, W. M.; Venkatesan, T.; Ariando, Ariando; Hilgenkamp, H.
2014-03-01
The interplay between exchange interactions, interfacial charges, and confinement effects controls the electronic, magnetic, and transport properties of complex oxide thin films. Here we report the emergence of ferromagnetism in insulating LaMnO3 thin films grown on SrTiO3 substrates beyond a critical thickness. LaMnO3 (001) films are deposited by a pulsed laser deposition technique with thicknesses varying from 1 unit cell to 24 unit cells. The position dependent local magnetization is then mapped with micrometer resolution using scanning superconducting quantum interference device microscopy. We find that the magnetic ground state switches from non-ferromagnetic to ferromagnetic within a change of one unit cell above the critical thickness of 5 unit cells with characteristic domain size of about 20 μm. Further increase of film thickness up to 24 unit cells leads to reduction of the domain size to about 10 μm. The critical thickness is qualitatively explained in terms of the charge transfer in polar LaMnO3 (001) thin films based on results of additional experimental data, density-functional calculations, and the electrostatic modeling.
Microscopic derivation of magnon spin current in a topological insulator/ferromagnet heterostructure
NASA Astrophysics Data System (ADS)
Okuma, Nobuyuki; Nomura, Kentaro
2017-03-01
We investigate a spin-electricity conversion effect in a topological insulator/ferromagnet heterostructure. In the spin-momentum-locked surface state, an electric current generates nonequilibrium spin accumulation, which causes a spin-orbit torque that acts on the ferromagnet. When spins in the ferromagnet are completely parallel to the accumulated spin, this spin-orbit torque is zero. In the presence of spin excitations, however, a coupling between magnons and electrons enables us to obtain a nonvanishing torque. In this paper, we consider a model of the heterostructure in which a three-dimensional magnon gas is coupled with a two-dimensional massless Dirac electron system at the interface. We calculate the torque induced by an electric field, which can be interpreted as a magnon spin current, up to the lowest order of the electron-magnon interaction. We derive the expressions for high and low temperatures and estimate the order of magnitude of the induced spin current for realistic materials at room temperature.
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.
Large anomalous Hall effect in ferromagnetic insulator-topological insulator heterostructures
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.
Jiang, Zilong; Chang, Cui-Zu; Tang, Chi; Wei, Peng; Moodera, Jagadeesh S; Shi, Jing
2015-09-09
The quantum anomalous Hall effect (QAHE) has been recently demonstrated in Cr- and V-doped three-dimensional topological insulators (TIs) at temperatures below 100 mK. In those materials, the spins of unfilled d-electrons in the transition metal dopants are exchange coupled to develop a long-range ferromagnetic order, which is essential for realizing QAHE. However, the addition of random dopants does not only introduce excess charge carriers that require readjusting the Bi/Sb ratio, but also unavoidably introduces paramagnetic spins that can adversely affect the chiral edge transport in QAHE. In this work, we show a heterostructure approach to independently tune the electronic and magnetic properties of the topological surface states in (BixSb1-x)2Te3 without resorting to random doping of transition metal elements. In heterostructures consisting of a thin (BixSb1-x)2Te3 TI film and yttrium iron garnet (YIG), a high Curie temperature (∼550 K) magnetic insulator, we find that the TI surface in contact with YIG becomes ferromagnetic via proximity coupling which is revealed by the anomalous Hall effect (AHE). The Curie temperature of the magnetized TI surface ranges from 20 to 150 K but is uncorrelated with the Bi fraction x in (BixSb1-x)2Te3. In contrast, as x is varied, the AHE resistivity scales with the longitudinal resistivity. In this approach, we decouple the electronic properties from the induced ferromagnetism in TI. The independent optimization provides a pathway for realizing QAHE at higher temperatures, which is important for novel spintronic device applications.
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.
Pulsed laser deposition of high-quality thin films of the insulating ferromagnet EuS
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.
Liu, Wenqing; He, Liang; Zhang, Rong E-mail: rzhang@nju.edu.cn; Zhou, Yan; Murata, Koichi; Wang, Kang L. E-mail: rzhang@nju.edu.cn; Onbasli, Mehmet C.; Ross, Caroline A.; Jiang, Ying; Wang, Yong; Xu, Yongbing E-mail: rzhang@nju.edu.cn
2016-05-15
One of the major obstacles of the magnetic topological insulators (TIs) impeding their practical use is the low Curie temperature (T{sub c}). Very recently, we have demonstrated the enhancement of the magnetic ordering in Cr-doped Bi{sub 2}Se{sub 3} by means of proximity to the high-T{sub c} ferrimagnetic insulator (FMI) Y{sub 3}Fe{sub 5}O{sub 12} 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 Bi{sub 2−x}Cr{sub x}Se{sub 3}/Y{sub 3}Fe{sub 5}O{sub 12} interface. We found that the Bi{sub 2−x}Cr{sub x}Se{sub 3} grown on Y{sub 3}Fe{sub 5}O{sub 12}(111) predominately contains Cr{sup 3+} cations, and the spin direction of the Cr{sup 3+} is aligned parallel to that of tetrahedral Fe{sup 3+} of the YIG, revealing a ferromagnetic exchange coupling between the Bi{sub 2−x}Cr{sub x}Se{sub 3} and the Y{sub 3}Fe{sub 5}O{sub 12}.
Visualization of a ferromagnetic metallic edge state in manganite strips.
Du, Kai; Zhang, Kai; Dong, Shuai; Wei, Wengang; Shao, Jian; Niu, Jiebin; Chen, Jinjie; Zhu, Yinyan; Lin, Hanxuan; Yin, Xiaolu; Liou, Sy-Hwang; Yin, Lifeng; Shen, Jian
2015-02-04
Recently, broken symmetry effect induced edge states in two-dimensional electronic systems have attracted great attention. However, whether edge states may exist in strongly correlated oxides is not yet known. In this work, using perovskite manganites as prototype systems, we demonstrate that edge states do exist in strongly correlated oxides. Distinct appearance of ferromagnetic metallic phase is observed along the edge of manganite strips by magnetic force microscopy. The edge states have strong influence on the transport properties of the strips, leading to higher metal-insulator transition temperatures and lower resistivity in narrower strips. Model calculations show that the edge states are associated with the broken symmetry effect of the antiferromagnetic charge-ordered states in manganites. Besides providing a new understanding of the broken symmetry effect in complex oxides, our discoveries indicate that novel edge state physics may exist in strongly correlated oxides beyond the current two-dimensional electronic systems.
NASA Astrophysics Data System (ADS)
Nolting, W.; Borgiel, W.; Borstel, G.
1988-05-01
We present a method for calculating the temperature dependence of the electronic quasiparticle density of states (QDOS) of a ferromagnetic rare-earth insulator like EuO. Special attention is devoted to how the ``localized'' ferromagnetism manifests itself in x-ray photoemission and bremsstrahlung isochromat spectra. Our study includes the first six conduction bands of EuO (the first five are Eu 5d like, the sixth is mainly of Eu 6s character) as well as the rather flat 4f levels. The starting point is an extended d-f exchange model, the main parts of which are an exchange interaction between 4f moments and conduction electrons, a Coulomb repulsion between highly correlated 4f electrons, and a hybridization of 4f with conduction-band states. We use an exact T=0 relationship between spin-up quasiparticle energies and one-electron Bloch energies ɛm(k) for an optimal determination of the latter by performing a self-consistent, spin-polarized band-structure calculation based on density-functional theory. For finite temperatures the model is approximately solved by a many-body procedure. The QDOS exhibits a striking temperature dependence mainly due to the d-f exchange. Two 4f-like peaks appear in the spin-polarized QDOS, the low-energy one being occupied, the high-energy one being empty. The temperature dependence of the localized ferromagnetism appears in the QDOS as a temperature-dependent shift of spectral weight between the low- and the high-energy peak.
Zero field conductance singularity in two terminal ferromagnet-topological insulator device
NASA Astrophysics Data System (ADS)
Duan, Xiaopeng; Semenov, Yuriy G.; Kim, Ki Wook
2014-03-01
Spin-momentum interlocking of surface electronic states on 3D topological insulator (TI) grants the unique opportunity to generate electric current directed according to the spin polarization of injected electrons instead of the applied electric field. Such asymmetry in momentum distribution of injected electrons takes place in the vicinity of ferromagnetic contact but vanishes on the length of few mean free passes. We propose to use this property in two terminal devices consisting of two parallel ferromagnetic contacts deposited on the surface of 3D TI. When the injected spin polarization leads to electron momentum pointing towards the other electrode, it facilitate the direct transmission, resulting in a lower resistance; in contrast with a reversed bias, the spin-determined momentum points away from the other electrode, because of which the electrons could gain the right momentum only after multiple scatterings to approach the second electrode, thus resulting in a higher resistance. We stress that this asymmetry in the resistance keeps up to arbitrarily small applied voltage since it does not need the formation of space charge region that is essential in conventional diodes. The rectification ratio near zero voltage are estimated and potential application are discussed. This work was supported, in part, by the US Army Research Office and FAME (one of six centers of STARnet, a SRC program sponsored by MARCO and DARPA).
Ferromagnetism in the Mott insulator Ba2NaOsO6
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.
Orbital ordering in the ferromagnetic insulator Cs2AgF4 from first principles
NASA Astrophysics Data System (ADS)
Wu, Hua; Khomskii, D. I.
2007-10-01
We found, using density-functional theory calculations within the generalized gradient approximation, that Cs2AgF4 is stabilized in the insulating orthorhombic phase rather than in the metallic tetragonal phase. The lattice distortion present in the orthorhombic phase corresponds to the x2-z2/y2-z2 hole-orbital ordering of the Ag2+4d9 ions, and this orbital ordering leads to the observed ferromagnetism, as confirmed by the present total-energy calculations. This picture holds in the presence of moderate 4d -electron correlation. The results are compared with the picture of ferromagnetism based on the metallic tetragonal phase.
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.
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.
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.
Adaptive microwave impedance memory effect in a ferromagnetic insulator
NASA Astrophysics Data System (ADS)
Lee, Hanju; Friedman, Barry; Lee, Kiejin
2016-12-01
Adaptive electronics, which are often referred to as memristive systems as they often rely on a memristor (memory resistor), are an emerging technology inspired by adaptive biological systems. Dissipative systems may provide a proper platform to implement an adaptive system due to its inherent adaptive property that parameters describing the system are optimized to maximize the entropy production for a given environment. Here, we report that a non-volatile and reversible adaptive microwave impedance memory device can be realized through the adaptive property of the dissipative structure of the driven ferromagnetic system. Like the memristive device, the microwave impedance of the device is modulated as a function of excitation microwave passing through the device. This kind of new device may not only helpful to implement adaptive information processing technologies, but also may be useful to investigate and understand the underlying mechanism of spontaneous formation of complex and ordered structures.
Adaptive microwave impedance memory effect in a ferromagnetic insulator
Lee, Hanju; Friedman, Barry; Lee, Kiejin
2016-01-01
Adaptive electronics, which are often referred to as memristive systems as they often rely on a memristor (memory resistor), are an emerging technology inspired by adaptive biological systems. Dissipative systems may provide a proper platform to implement an adaptive system due to its inherent adaptive property that parameters describing the system are optimized to maximize the entropy production for a given environment. Here, we report that a non-volatile and reversible adaptive microwave impedance memory device can be realized through the adaptive property of the dissipative structure of the driven ferromagnetic system. Like the memristive device, the microwave impedance of the device is modulated as a function of excitation microwave passing through the device. This kind of new device may not only helpful to implement adaptive information processing technologies, but also may be useful to investigate and understand the underlying mechanism of spontaneous formation of complex and ordered structures. PMID:27966536
Adaptive microwave impedance memory effect in a ferromagnetic insulator.
Lee, Hanju; Friedman, Barry; Lee, Kiejin
2016-12-14
Adaptive electronics, which are often referred to as memristive systems as they often rely on a memristor (memory resistor), are an emerging technology inspired by adaptive biological systems. Dissipative systems may provide a proper platform to implement an adaptive system due to its inherent adaptive property that parameters describing the system are optimized to maximize the entropy production for a given environment. Here, we report that a non-volatile and reversible adaptive microwave impedance memory device can be realized through the adaptive property of the dissipative structure of the driven ferromagnetic system. Like the memristive device, the microwave impedance of the device is modulated as a function of excitation microwave passing through the device. This kind of new device may not only helpful to implement adaptive information processing technologies, but also may be useful to investigate and understand the underlying mechanism of spontaneous formation of complex and ordered structures.
Excitonic instabilities and insulating states in bilayer graphene
NASA Astrophysics Data System (ADS)
Song, Kok Wee; Liang, Yung-Ching; Haas, Stephan
2012-11-01
The competing ground states of bilayer graphene are studied by applying renormalization group techniques to a bilayer honeycomb lattice with nearest neighbor hopping. In the absence of interactions, the Fermi surface of this model at half-filling consists of two nodal points with momenta K, K', where the conduction band and valence band touch each other, yielding a semimetal. Since near these two points the energy dispersion is quadratic with perfect particle-hole symmetry, excitonic instabilities are inevitable if interband interactions are present. Using a perturbative renormalization group analysis up to the one-loop level, we find different competing ordered ground states, including ferromagnetism, superconductivity, spin and charge density wave states with ordering vector Q=K-K', and excitonic insulator states. In addition, two states with valley symmetry breaking are found in the excitonic insulating and ferromagnetic phases. This analysis strongly suggests that the ground state of bilayer graphene should be gapped, and with the exception of superconductivity, all other possible ground states are insulating.
NASA Astrophysics Data System (ADS)
Pugach, N. G.; Kupriyanov, M. Yu.; Goldobin, E.; Kleiner, R.; Koelle, D.
2011-10-01
The proximity effect and the Josephson current in a superconductor-insulator-ferromagnet-superconductor junction are investigated within the framework of the quasiclassical Eilenberger equations. This investigation allows us to compare the dirty and the clean limits, to investigate an arbitrary impurity scattering, and to determine the applicability limits of the Usadel equations for such structures. The role of different types of the FS interface is analyzed. It is shown that the decay length ξ1 and the spatial oscillation period 2πξ2 of the Eilenberger function may exhibit a nonmonotonic dependence on the properties of the ferromagnetic layer such as exchange field or electron mean-free path. The results of our calculations are applied to the interpretation of experimentally observed dependencies of the critical current density on the ferromagnet thickness in Josephson junctions containing a Ni layer with an arbitrary scattering.
Microwave-induced spin currents in ferromagnetic-insulator|normal-metal bilayer system
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.
Ferromagnetism in vanadium-doped Bi2Se3 topological insulator films
NASA Astrophysics Data System (ADS)
Zhang, Liguo; Zhao, Dapeng; Zang, Yunyi; Yuan, Yonghao; Jiang, Gaoyuan; Liao, Menghan; Zhang, Ding; He, Ke; Ma, Xucun; Xue, Qikun
2017-07-01
With molecular beam epitaxy, we grew uniformly vanadium-doped Bi2Se3 films which exhibit ferromagnetism with perpendicular magnetic anisotropy. A systematic study on the magneto-transport properties of the films revealed the crucial role of topological surface states in ferromagnetic coupling. The enhanced ferromagnetism with reduced carrier density can support quantum anomalous Hall phase in the films, though the anomalous Hall resistance is far from quantization due to high carrier density. The topological surface states of films exhibit a gap of ˜180 meV which is unlikely to be magnetically induced but may significantly influence the quantum anomalous Hall effect in the system.
Thermodynamic transport theory of spin waves in ferromagnetic insulators
NASA Astrophysics Data System (ADS)
Basso, Vittorio; Ferraro, Elena; Piazzi, Marco
2016-10-01
We use the Boltzmann transport theory in the relaxation time approximation to describe the thermal transport of spin waves in a ferromagnet. By treating spin waves as magnon excitations we are able to compute analytically and numerically the coefficients of the constitutive thermomagnetic transport equations. As a main result, we find that the absolute thermomagnetic power coefficient ɛM, relating the gradient of the potential of the magnetization current and the gradient of the temperature, in the limit of low temperature and low field, is a constant ɛM=-0.6419 kB/μB . The theory correctly describes the low-temperature and magnetic-field dependencies of spin Seebeck experiments. Furthermore, the theory predicts that in the limit of very low temperatures the spin Peltier coefficient ΠM, relating the heat and the magnetization currents, tends to a finite value which depends on the amplitude of the magnetic field. This indicates the possibility to exploit the spin Peltier effect as an efficient cooling mechanism in cryogenics.
Quantum Capacitance of a Topological Insulator-Ferromagnet Interface
NASA Astrophysics Data System (ADS)
Siu, Zhuo Bin; Chowdhury, Debashree; Jalil, Mansoor B. A.; Basu, Banasri
2017-03-01
We study the quantum capacitance in a topological insulator thin film system magnetized in the in-plane direction in the presence of an out-of-plane magnetic field and hexagonal warping. To first order, the modification in quantum capacitance due to hexagonal warping compared to the clean case, where both the in-plane magnetization and hexagonal warping are absent, is always negative, and increases in magnitude monotonically with the energy difference from the charge neutrality point. In contrast, the change in the quantum capacitance due to in-plane magnetization oscillates with the energy in general, except when a certain relation between the inter-surface coupling, out of plane Zeeman energy splitting and magnetic field strength is satisfied. In this special case, the quantum capacitance remains unchanged by the in-plane magnetization for all energies.
Quantum Capacitance of a Topological Insulator-Ferromagnet Interface.
Siu, Zhuo Bin; Chowdhury, Debashree; Jalil, Mansoor B A; Basu, Banasri
2017-03-24
We study the quantum capacitance in a topological insulator thin film system magnetized in the in-plane direction in the presence of an out-of-plane magnetic field and hexagonal warping. To first order, the modification in quantum capacitance due to hexagonal warping compared to the clean case, where both the in-plane magnetization and hexagonal warping are absent, is always negative, and increases in magnitude monotonically with the energy difference from the charge neutrality point. In contrast, the change in the quantum capacitance due to in-plane magnetization oscillates with the energy in general, except when a certain relation between the inter-surface coupling, out of plane Zeeman energy splitting and magnetic field strength is satisfied. In this special case, the quantum capacitance remains unchanged by the in-plane magnetization for all energies.
Quantum Capacitance of a Topological Insulator-Ferromagnet Interface
Siu, Zhuo Bin; Chowdhury, Debashree; Jalil, Mansoor B. A.; Basu, Banasri
2017-01-01
We study the quantum capacitance in a topological insulator thin film system magnetized in the in-plane direction in the presence of an out-of-plane magnetic field and hexagonal warping. To first order, the modification in quantum capacitance due to hexagonal warping compared to the clean case, where both the in-plane magnetization and hexagonal warping are absent, is always negative, and increases in magnitude monotonically with the energy difference from the charge neutrality point. In contrast, the change in the quantum capacitance due to in-plane magnetization oscillates with the energy in general, except when a certain relation between the inter-surface coupling, out of plane Zeeman energy splitting and magnetic field strength is satisfied. In this special case, the quantum capacitance remains unchanged by the in-plane magnetization for all energies. PMID:28337992
NASA Astrophysics Data System (ADS)
Kioussis, Nicholas
2016-10-01
The realization of the MeRAM is based on the voltage control of the interfacial magnetocrystalline anisotropy (MCA) of heavy-metal/ferromagnet/insulator (HM/FM/I) nanojunctions, where the non-magnetic HM contact electrode (Ta, Pd, Pt, Au) has strong spin-orbit coupling (SOC). Employing ab initio electronic structure calculations we have investigated the effect of electric-field (E-field) and epitaxial strain on the MCA of Ta/FeCo/MgO heterostructure. We predict that uniaxial strain leads to a wide range of interesting voltage behavior of the MCA ranging from linear behavior with positive or negative magnetoelectronic coefficient, to non-monotonic ⋁-shape or inverse-⋀-shape E-field dependence with asymmetric magnetoelectronic coefficients. The calculations reveal that under a 4% compressive strain on MgO reaches the giant value of 1126 fJ/(V.m). The underlying mechanism is the synergistic effects of strain and E-field on the orbital characters, the energy level shifts of the SOC d-states, and the dielectric constant of MgO. These results demonstrate for the first time the feasibility of highly sensitive E-field-controlled MCA through strain engineering, which in turn open a viable pathway towards tailoring magnetoelectric properties for spintronic applications. * nick.kioussis@csun.edu This research was supported by NSF Grant No. ERC-TANMS-1160504
NASA Astrophysics Data System (ADS)
Chikara, Shalinee; Schlottmann, Pedro
2005-03-01
We report results of a magnetic and transport study of SrRu1-xMnxO3 (0<= x<0.60), i.e., Mn doped SrRuO3. The Mn doping drives the system from the itinerant ferromagnetic state (TC=165 K for x=0) through a quantum critical point at xc=0.39 to an insulating antiferromagnetic state. The onset of antiferromagnetism is abrupt with a Néel temperature increasing from 205 K for x=0.44 to 250 K for x=0.59. Accompanying this quantum phase transition is a drastic change in resistivity by as much as 8 orders of magnitude as a function of x at low temperatures. The critical composition xc=0.39 sharply separates the two distinct ground states, namely the ferromagnetic metal from the antiferromagnetic insulator.
NASA Astrophysics Data System (ADS)
Mukherjee, Anamitra; Cole, William S.; Woodward, Patrick; Randeria, Mohit; Trivedi, Nandini
2013-04-01
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.
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
Thin-film topological insulator-ferromagnet heterostructures for terahertz detection
Li, Xiaodong; Semenov, Yuriy G.; Kim, Ki Wook
2014-02-10
An atomically thin topological insulator is investigated theoretically for long-wavelength photodetection when it interacts with a magnetic material. Through the coupling between top and bottom surfaces as well as the exchange interaction with the proximate ferromagnet, the distribution of optically excited carriers exhibits unique patterns that depend sensitively on the frequency of the incoming light. This effect results in the generation of strong nonzero photocurrent, leading potentially to room-temperature detection of far-infrared/THz radiation with the advantage of low noise and fast response. The ease of frequency tuning by an external electrical bias offers an added versatility in the realistic implementation.
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.
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.
Growth and characterization of insulating ferromagnetic semiconductor (Al,Fe)Sb
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.
Unconventional Fermi surface in an insulating state
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 SmB_{6}, which reveal quantum oscillation frequencies characteristic of a large three-dimensional conduction electron Fermi surface similar to the metallic rare earth hexaborides such as PrB_{6} and LaB_{6}. As a result, the quantum oscillation amplitude strongly increases at low temperatures, appearing strikingly at variance with conventional metallic behavior.
Unconventional Fermi surface in an insulating state
NASA Astrophysics Data System (ADS)
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.
2015-07-01
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.
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/Sb_{2–x}V_{x}Te_{3} hybrid heterostructure, where V doping is used to drive the TI (Sb_{2}Te_{3}) 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.
Li, Mingda; Zhu, Yimei; Chang, Cui -Zu; ...
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.
NASA Astrophysics Data System (ADS)
Schmitz, R.; Entin-Wohlman, O.; Aharony, A.; Müller-Hartmann, E.
2005-09-01
[Dedicated to Bernhard Mühlschlegel on the occasion ofhis 80th birthday]Using a point-charge calculation of the electrostatic crystal field, we determine the non-degenerate orbital ground state of the ferromagnetic Mott insulator YTiO3, which is found to agree perfectly with experiment. Based on the orbital order, we obtain by perturbation theory an effective spin Hamiltonian that describes the magnetic superexchange between nearest-neighbor Ti ions. The superexchange Hamiltonian includes, in addition to the isotropic Heisenberg coupling, antisymmetric (Dzyaloshinskii-Moriya) and symmetric anisotropy terms, caused by the spin-orbit interaction on the Ti ions. We find ferromagnetic Heisenberg couplings for Ti-Ti bonds in the crystallographic ab planes, but antiferromagnetic ones for Ti-Ti bonds between planes, in contradiction with experiment (which gives ferromagnetic couplings for both). Difficulties in calculating realistic values for the isotropic couplings of YTiO3 have been already reported in the literature. We discuss possible origins for these discrepancies. However, the much smaller values we obtain for the symmetric and antisymmetric anisotropies may be expected to be reliable. We therefore combine the experimentally-deduced isotropic coupling with the calculated anisotropic ones to determine the magnetic order of the Ti ions, which is found to be in satisfactory agreement with experiment. Based on this magnetic order, we derive the spin-wave spectrum. We find an acoustic branch with a very small zone-center gap and three optical spin-wave modes with sizeable zone-center gaps. The acoustic branch reproduces the one reported in experiment, and the optical ones are in a satisfactory agreement with experiment, upon a proper folding of the magnetic Brillouin zone.
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.
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.
Magnetic-field-modulated resonant tunneling in ferromagnetic-insulator-nonmagnetic junctions.
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.
Driving and detecting ferromagnetic resonance in insulators with the spin Hall effect.
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.
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.
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.
Carrier-mediated ferromagnetism in the magnetic topological insulator Cr-doped (Sb,Bi)2Te3
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
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.
Coupling of Crystal Structure and Magnetism in the Layered, Ferromagnetic Insulator CrI 3
McGuire, Michael A.; Dixit, Hemant; Cooper, Valentino R.; ...
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
NASA Astrophysics Data System (ADS)
Nomura, Kentaro
2012-02-01
Topologically nontrivial gapped phases can be characterized by the bulk topological indices and the surface gapless modes. The topological magneto-electric (ME) effect is a novel manifestation of the bulk-surface correspondence in which the bulk magnetization is generated by a circulating quantized Hall current flowing at the surface of topological insulators. To realize the topological ME effect, there are two difficulties: (a) one needs to attach an insulating ferromagnetic layer with the magnetization normal to the surface all pointing out or in. (b) The Fermi energy must be tuned accurately within the small gap of the surface Dirac spectrum opened by the exchange interaction. In this talk we discuss the anomalous quantized Hall current on the surface of a magnetically doped topological insulator, basing on the two-dimensional surface Dirac Hamiltonian with magnetic disorder. The scaling analysis indicates that, in sharp contrast to the time-reversal-invariant cases, the all surface states tend to be localized while the Hall conductivity is quantized no matter whether the Fermi level resides within or out of the surface gap. This resolves problem(b). Furthermore it is shown that this also resolves problem (a) with the simultaneous application of magnetic and electric fields parallel or antiparallel to each other. By this method, doped local spins can be controlled by the bulk energy which can overcome the magnetic anisotropy and Zeeman splitting at the surface. We also comment on the generalization of the topological responses to the case of topological superconductors and superfluids. This work was done in collaboration with Naoto Nagaosa, Shinsei Ryu, and Akira Furusaki. K. Nomura and N. Nagaosa, Phys. Rev. Lett. 106, 166802 (2011); K. Nomura, S. Ryu, A. Furusaki, N. Nagaosa, arXiv:1108.5054.
Transport properties of silicene-based ferromagnetic-insulator-superconductor junction
NASA Astrophysics Data System (ADS)
Vosoughi-nia, Sakineh; Hajati, Yaser; Rashedi, Gholamreza
2017-07-01
We study the tunneling conductance of a silicene-based ferromagnet/insulator/superconductor (FIS) junction by the use of the spin-dependent Dirac-Bogoliubov de-Gennes equation. We demonstrate that the conductance spectra are strongly affected by exchange energy h , Fermi energy EF, and external perpendicular electric field Ez. In the thin barrier limit of insulator silicene IS, the zero-bias charge conductance of the FIS silicene junction oscillates as a function of barrier strength χG. It is shown that the period of oscillations changes from π/2 to π corresponding to undoped and doped silicene. Remarkably, in contrast to that of the graphene FIS junction where the conductance only vanishes at the exchange energy h =EF, here due to the buckled structure of silicene, there is a transport gap region for the range of h values and the magnitude of such a gap region can be controlled by Ez. Moreover, it is found that by appropriate choice of h and Ez, it is possible to achieve a fully spin and valley-polarized charge conductance through the FIS silicene junction. This property suggests experimentally measuring the Fermi energy of silicene.
Impurity states in the magnetic topological insulator V :(Bi,Sb ) 2Te3
NASA Astrophysics Data System (ADS)
Peixoto, Thiago R. F.; Bentmann, Hendrik; Schreyeck, Steffen; Winnerlein, Martin; Seibel, Christoph; Maaß, Henriette; Al-Baidhani, Mohammed; Treiber, Katharina; Schatz, Sonja; Grauer, Stefan; Gould, Charles; Brunner, Karl; Ernst, Arthur; Molenkamp, Laurens W.; Reinert, Friedrich
2016-11-01
The ferromagnetic topological insulator V :(Bi,Sb ) 2Te3 has been recently reported as a quantum anomalous Hall (QAH) system. Yet the microscopic origins of the QAH effect and the ferromagnetism remain unclear. One key aspect is the contribution of the V atoms to the electronic structure. Here the valence band of V :(Bi,Sb ) 2Te3 thin films was probed in an element-specific way by resonant photoemission spectroscopy. The signature of the V 3 d impurity band was extracted and exhibits a high density of states near the Fermi level, in agreement with spin-polarized first-principles calculations. Our results indicate the occurrence of a ferromagnetic superexchange interaction mediated by the observed impurity band, contributing to the ferromagnetism in this system.
An Emergent Spin-Filter at the interface between Ferromagnetic and Insulating Layered Oxides
NASA Astrophysics Data System (ADS)
Liu, Yaohua
2014-03-01
Complex oxide heterostructures are of keen interest because modified bonding at the interfaces can give rise to fundamentally new phenomena and valuable functionalities. Particularly, an induced magnetization is widely observed at epitaxial interfaces between layered transition-metal oxides; however, much less effort has been spent on investigating how it affects the charge transport properties. To this end, we have studied magnetic tunneling junctions consisting of ferromagnetic manganite La0.7Ca0.3MnO3 (LCMO) and insulating cuprate PrBa2Cu3O7 (PBCO). Contrary to the typically observed steady increase of the tunnel magnetoresistance with decreasing temperature, this system exhibits an anomalous decrease at low temperatures. Polarized neutron reflectometry (PNR) and x-ray magnetic circular dichroism (XMCD) studies on LCMO/PBCO/LCMO trilayers show that the saturation magnetization of the LCMO contacts increase as the temperature decreases. In other words, degradation of the ferromagnetic contacts is ruled out as a cause. Interestingly, there exists induced net Cu moments, which indicates that the spin degeneracy of the conduction band of the PBCO barrier is lifted and thus the barrier becomes spin selective. Our calculations, within the Wentzel-Kramers-Brillouin approximation, show that the complex temperature dependence can arise from a competition between the high positive spin polarization of the manganite electrodes and a negative spin-filter effect from the interfacial Cu magnetization. This work illustrates that the interface-induced magnetization in layered oxide heterostructures can have non-trivial effects on the macroscopic transport properties. Work performed in collaboration with FA Cuellar, Z Sefrioui, C Leon, J Santamaria (Universidad Complutense de Madrid), JW Freeland, SGE te Velthuis (ANL) and MR Fitzsimmons (LANL). Work at Argonne National Laboratory was supported by the U.S. Department of Energy, Office of Basic Energy Sciences under contract no
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 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.
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
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.
Stone, P.R.; Alberi, K.; Tardif, S.K.Z.; Beeman, J.W.; Yu, K.M.; Walukiewicz, W.; Dubon, O.D.
2008-02-07
We have investigated the effect of partial isovalent anion substitution in Ga1-xMnxAs on electrical transport and ferromagnetism. Substitution of only 2.4percent of As by P induces a metal-insulator transition at a constant Mn doping of x=0.046 while the replacement of 0.4 percent As with N results in the crossover from metal to insulator for x=0.037. This remarkable behavior is consistent with a scenario in which holes located within an impurity band are scattered by alloy disorder in the anion sublattice. The shorter mean free path of holes, which mediate ferromagnetism, reduces the Curie temperature TC from 113 K to 60 K (100 K to 65 K) upon the introduction of 3.1 percent P (1percent N) into the As sublattice.
Niu, Wei; Du, Kai; Wang, Shuangbao; Zhang, Minhao; Gao, Ming; Chen, Yongda; Liu, Hao; Zhou, Wei; Song, Fengqi; Wang, Peng; Xu, Yongbing; Wang, Xuefeng; Shen, Jian; Zhang, Rong
2017-08-31
Time-reversal symmetry is broken by magnetic doping in topological insulators (TIs). An energy gap at the Dirac point opens and thus, generates numerous surface carriers. TI nanostructures are an ideal platform to investigate exotic surface transport behavior due to their large surface-to-volume ratio, which enhances the contribution of the TI surface states. However, magnetic doping into TI nanostructures has been challenging, and induced magnetic behavior has remained elusive. Herein, we have synthesized Fe-doped Bi2Se3 nanowires using a facile chemical vapor deposition with a doping concentration of ∼1 at%. The combined structural characterizations illustrate the homogeneous distribution of the Fe dopants. Cryogenic magnetic force microscopy gives direct evidence of the spontaneous magnetization with a Curie temperature of ∼40 K in a single nanowire. The transport measurements show a quantum transition from weak anti-localization to weak localization behavior. All the evidence indicates the existence of intrinsic ferromagnetism and gapped topological surface states in the TI nanowires, paving a way for future memory and magnetoelectric nanodevice applications.
Magnetic Nanoparticles in "Amorphous Ferromagnetic Metal-Insulator" Nanogranular thin Films
NASA Astrophysics Data System (ADS)
Granovsky, A.; Kalinin, Yu.; Sitnikov, A.; Stognei, O.
A lot of factors limit possible applications of magnetic nanoparticles in medicine for drug delivery and magnetic hyperthermia therefore it is of primary importance to understand influence of classical and quantum-size effects, surface layers, interparticle distance, shape of nanoparticles on their magnetic properties. Magnetic nanogranular thin films, known also as nanocomposites, can be considered as a convenient model for such investigations as it is possible to tune easily many of mentioned above parameters by varying of the fabrication conditions. The ion-beam sputtering technique has been developed to prepare "amorphous ferromagnetic metal-insulator" nanocomposites with different concentration and parameters simultaneously in one technological cycle. This feature is achieved by using of a composite target (consisting of a metal and dielectric parts) with an asymmetric arrangement of the dielectric parts on the metal base. Influence of sputtering conditions and post-fabrication treatment on structural, magnetic, electrical and magnetotransport properties of magnetic nanocomposites in a wide range of metal volume fraction and distance between magnetic nanoparticles is being discussed.
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.
Driving and detecting ferromagnetic resonance in insulators with the spin Hall effect
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/Y_{3}Fe_{5}O_{12} (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. In conclusion, 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.
Driving and detecting ferromagnetic resonance in insulators with the spin Hall effect
Sklenar, Joseph; Zhang, Wei; Jungfleisch, Matthias B.; ...
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 inversemore » spin Hall effect. The voltage is measured with applied magnetic field directions that range in-plane to nearly perpendicular. In conclusion, 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.« less
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.
Topological insulator state in gated bilayer silicene
NASA Astrophysics Data System (ADS)
Zhang, Ming-Ming; Xu, Lei; Zhang, Jun
2015-11-01
We investigate the topological insulator state of gated bilayer silicene in the presence of extrinsic Rashba spin-orbit (SO) coupling. The system exhibits a band insulator (BI) phase for small Rashba SO coupling, and then translate to a strong topological insulator (TI) phase with both spin and valley filtered at large Rashba SO coupling. The strong TI phase is robust in the presence of intrinsic SO and intrinsic Rashba SO couplings. When a titled electric field is introduced, the in-plane component of the electric field gives rise to an interlayer Rashba SO coupling, and the system turns to a BI phase no matter how large the Rashab SO coupling and bias voltage are. This will provide potential application in nanoelectronics based on silicene.
New type of ferromagnetic insulator: Double perovskite La2NiMO6 (M=Mn, Tc, Re, Ti, Zr, and Hf)
NASA Astrophysics Data System (ADS)
Fuh, H. R.; Liu, Y. P.; Xiao, Z. R.; Wang, Y. K.
2014-05-01
Electronic structures of transition metal pnictides double perovskite La2NiMO6 (M=Mn, Tc, Re, Ti, Zr, and Hf) were shown as ferromagnetic (FM) insulators based on density functional calculation results. The FM state observed in La2NiMO6 (M=Mn, Tc, and Re) was most likely a mixture of high spin (HS) and low spin (LS) states; the electrons were transferred from the filled LS Ni eg states to the half-filled HS Mn (Tc). On the other hand, the FM state in La2NiMO6 (M=Ti, Zr, and Hf) was caused by the electron transfer from the half-filled LS Ti (Zr and Hf) eg orbital of HS Ni to the empty eg orbital of LS Ti (Zr and Hf). The FM insulating state of La2NiMO6 (M=Mn. Tc, Ti, Zr, and Hf) remained the same, whereas it changed from metal to insulator for La2NiReO6 based on the generalized gradient approximation+U calculation.
NASA Astrophysics Data System (ADS)
Zhang, Kunhua; Zeng, Junjie; Ren, Yafei; Qiao, Zhenhua
2017-08-01
We demonstrate that a zero-energy Majorana bound state in a ferromagnetic insulator (FI)-superconductor (SC) junction formed on the edge of a two-dimensional topological insulator exhibits three types of spin-triplet pairing correlations, its spin-polarization direction is position independent in a ferromagnetic insulator, and demonstrates a spin-helix structure in a superconductor. These spin properties of Majorana bound states lead to anomalous selective equal-spin Andreev reflection. Similar behavior is found when the coupling between two Majorana bound states in a FI-SC-FI junction is invoked, though an additional weak spin-singlet pairing correlation is generated. These signatures can readily facilitate the experimental detection of spin-triplet correlations and spin polarization of Majorana bound states.
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.
NASA Astrophysics Data System (ADS)
Cao, G.; Chikara, S.; Lin, X. N.; Elhami, E.; Durairaj, V.; Schlottmann, P.
2005-01-01
We report results of a magnetic and transport study of SrRu1-xMnxO3(0⩽x<0.60) , i.e., Mn doped SrRuO3 . The Mn doping drives the system from the itinerant ferromagnetic state ( TC=165K for x=0 ) through a quantum critical point at xc=0.39 to an insulating antiferromagnetic state. The onset of antiferromagnetism is abrupt with a Néel temperature increasing from 205 K for x=0.44 to 250 K for x=0.59 . Accompanying this quantum phase transition is a drastic change in resistivity by as much as eight orders of magnitude as a function of x at low temperatures. The critical composition xc=0.39 sharply separates the two distinct ground states, namely the ferromagnetic metal from the antiferromagnetic insulator.
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.
Ferromagnetic Ground States in Face-Centered Cubic Hubbard Clusters
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
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.
NASA Astrophysics Data System (ADS)
Quan, Zhiyong; Wu, Biao; Zhang, Fei; Zhou, Guowei; Zang, Julu; Xu, Xiaohong
2017-02-01
The achievement of high temperature ferromagnetism in perovskite manganites has proved both fundamentally and technologically important for spintronics devices. However, high operating temperatures have not been achieved due to the depression of the Curie temperature and the rapid spin filtering efficiency loss, which are the main obstacles for practical applications. Here, we report unexpected room temperature insulating ferromagnetism in ultrathin (110) oriented La0.7Sr0.3MnO3 (LSMO) films. The relationships between room temperature ferromagnetism, charge transfer, and orbital occupancy are investigated, with X-ray absorption spectroscopy (XAS) and X-ray linear dichroism (XLD) measurements. Our results suggest that the room temperature insulating ferromagnetism is originated from super-exchange interaction between Mn2+ and Mn3+. The formation of Mn2+ ions is related to the charge transfer induced by oxygen vacancies. Moreover, a preferential orbital occupancy of eg(3z2-r2) in Mn3+ ions is crucial to the in-plane super-exchange coupling in ultrathin (110) LSMO films, resulting in insulating ferromagnetic behavior. This work may lead to the development of barrier materials in spin filter tunnel junctions and understanding of ferromagnetic coupling in insulating perovskite films.
Surface state transport suppression in topological insulators
NASA Astrophysics Data System (ADS)
Reijnders, Anjan A.; Tian, Y.; Pohl, G.; Kivlichan, I. D.; Zhao, S. Y. Frank; Kim, Y.-J.; Jia, S.; Cava, R. J.; Kwok, D. C.; Lee, N.; Cheong, S. W.; Burch, Kenneth S.
2013-03-01
An unresolved question in experimental research on topological insulators (TI) is the suppression mechanism of a TI's surface state transport. While room temperature ARPES studies reveal clear evidence of surface states, their observation in transport measurements is limited to low temperatures. A better understanding of this suppression is of fundamental interest, and crucial for pushing the boundary of device applications towards room-temperature operation. In this talk, we report the temperature dependent optical properties of the topological insulator Bi2Te2Se (BTS), obtained by infrared spectroscopy and ellipsometry, probing surface and bulk states simultaneously. We see clear evidence of coherent surface state transport at low temperature and find that electron-phonon coupling causes the gradual suppression of surface state transport as temperature rises to 43K. In the bulk, electron-phonon coupling enables the emergence of an indirect band gap transition, which peaks at 43K, and is limited by thermal ionization of the bulk valance band above 43K. For comparison with other resistive TIs, we also discuss the optical properties to BiSbSe2Te. Financially supported by NSERC CRSNG, Ontario Research Fund, Canadian Foundation for Innovation, Prins Bernhard Cultuurfonds, NSF
NASA Astrophysics Data System (ADS)
Zitzler, R.; Pruschke, Th.; Bulla, R.
2004-05-01
We discuss the magnetic phase diagram for the Hubbard model with magnetic frustration obtained within the dynamical mean-field theory. Most interesting is the appearance of a first-order paramagnetic metal to antiferromagnetic insulator transition for the magnetically frustrated lattice at half filling. For finite doping the antiferromagnetic phase is susceptible to phase separation and competes with an itinerant ferromagnetic phase (Nagaoka ferromagnetism), leading to an unexpectedly rich magnetic phase diagram.
Core contribution to magnetotransport of ferromagnetic dots in vortex state
NASA Astrophysics Data System (ADS)
Segal, A.; Gerber, A.
2012-04-01
We study the influence of the vortex core on magnetotransport of ferromagnetic dots in a vortex state. The extraordinary Hall effect generated in the core region has a different field symmetry compared to contributions of anisotropic magnetoresistance and the planar Hall effect, which can be used to detect chirality and polarity of the vortex. We propose a method for realization of two-bit per dot magnetic random access memory, in which two states are contributed by clockwise and counter-clockwise chirality and two by up and down core polarity. Dependence of the signal on vortex location, core diameter, and other parameters is discussed.
Ferromagnetism in neutron matter and its implication for the neutron star equation of state
NASA Astrophysics Data System (ADS)
Diener, J. P. W.; Scholtz, F. G.
2011-09-01
We investigate the possible contribution of the ferromagnetic phase of neutron matter in the neutron star interior to the star's magnetic field. We introduce a relativistic, self-consistent calculation of the ferromagnetic phase in neutron matter within the context of the relativistic mean-field approximation. The presence of the ferromagnetic phase stiffens the star's equation of state which implies a larger neutron star radius compared to the non-ferromagnetic case.
Ferromagnetism in neutron matter and its implication for the neutron star equation of state
Diener, J. P. W.; Scholtz, F. G.
2011-09-21
We investigate the possible contribution of the ferromagnetic phase of neutron matter in the neutron star interior to the star's magnetic field. We introduce a relativistic, self-consistent calculation of the ferromagnetic phase in neutron matter within the context of the relativistic mean-field approximation. The presence of the ferromagnetic phase stiffens the star's equation of state which implies a larger neutron star radius compared to the non-ferromagnetic case.
Lee, Changmin; Katmis, Ferhat; Jarillo-Herrero, Pablo; ...
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
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 the interface between the ferromagnetic insulator (FMI) EuS and the three-dimensional TI Bi_{2}Se_{3}. 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.
NASA Astrophysics Data System (ADS)
Mironov, V. L.; Skorohodov, E. V.; Blackman, J. A.
2014-05-01
We present a theoretical investigation of magnetostatic interaction effects in geometrically frustrated arrays of anisotropic one-layer and multilayer ferromagnetic nanoparticles arranged in different spatially configured systems with triangular symmetry. The peculiarities of the magnetization reversal and microwave excitation of such systems are discussed. We show that the use of multilayer stacks significantly expands the opportunities to create magnetically frustrated systems due to additional interlayer interaction. In particular, the interlayer coupling leads to the considerable splitting of the ferromagnetic resonance (FMR) spectrum. In addition, the magnetizing and remagnetizing of the two- and three-layer systems induce transitions between different states with ferromagnetic, antiferromagnetic, or mixed ferromagnetic-antiferromagnetic interlayer ordering that are accompanied by dramatic changes of FMR spectra. These effects can be potentially used in developing field controlled tunable microwave devices.
Reentrant Insulating State in Ultrathin Manganite Films
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.
Coherent transport of topological insulator surface states
NASA Astrophysics Data System (ADS)
Adroguer, Pierre; Carpentier, David; Orignac, Edmond; Cayssol, Jerome
2012-02-01
Topological insulators (TIs) are a new state of matter recently predicted theoreticallyootnotetextC. L. Kane and E. J. Mele, Phys. Rev. Lett. 95, 226801 (2005).^,ootnotetextX.-L. Qi, T. L. Hughes, and S.-C. Zhang, Phys. Rev. B 78,195424 (2008). and realized experimentally. In 3D they are characterized by the presence of gapless surface states which exhibit a linear dispersion, typical of Dirac fermions. Moreover, contrary to conventionnal materials, these Dirac cones occur in an odd number of Dirac fermions at the surface: ARPES experimentsootnotetextY. Xia, D. Qian, D. Hsieh, L. Wray, A. Pal, H. Lin, A. Bansil, D. Grauer, Y. S. Hor, R. J. Cava, and M. Z. Hasan, Nature Physics 5, 398 (2009).^,ootnotetextY. L. Chen, J. G. Analytis, J.-H. Chu, Z. K. Liu, S.-K. Mo, X.L.Qi,H.J.Zhang,D.H.Lu,X.Dai,Z.Fang,S.C. Zhang, I. R. Fisher, Z. Hussain, and Z.-X. Shen, Science 325, 178 (2009). have found a single Dirac cone at the surface of Bi2Se3, Bi2Te3. This work focuses on the electronic transport properties calculations in the diffusive limite of a single Dirac cone. Specificities of the TI surface states, like the hexagonal warping coupling are taken into account.
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.
NASA Astrophysics Data System (ADS)
Heim, D. M.; Pugach, N. G.; Kupriyanov, M. Yu; Goldobin, E.; Koelle, D.; Kleiner, R.; Ruppelt, N.; Weides, M.; Kohlstedt, H.
2015-11-01
Using the Usadel approach, we provide a formalism that allows us to calculate the critical current density of 21 different types of Josephson junctions (JJs) with a ferromagnetic (F) barrier and additional insulating (I) or/and normal (N) layers inserted between the F layer and superconducting (S) electrodes. In particular, we obtain that in SFS JJs, even a thin additional N layer between the S layer and F layer may noticeably change the thickness {d}{{F}} of the F layer at which the 0-π transitions occur. For certain values of {d}{{F}}, a 0-π transition can even be achieved by changing only the N layer thickness. We use our model to fit experimental data of SIFS and SINFS tunnel junctions.
High-Mobility Sm-Doped Bi2 Se3 Ferromagnetic Topological Insulators and Robust Exchange Coupling.
Chen, Taishi; Liu, Wenqing; Zheng, Fubao; Gao, Ming; Pan, Xingchen; van der Laan, Gerrit; Wang, Xuefeng; Zhang, Qinfang; Song, Fengqi; Wang, Baigeng; Wang, Baolin; Xu, Yongbing; Wang, Guanghou; Zhang, Rong
2015-09-02
High-mobility (Smx Bi1-x )2 Se3 topological insulators (with x = 0.05) show a Curie temperature of about 52 K, and the carrier concentration and Fermi wave vector can be manipulated by intentional Te introduction with no significant influence on the Curie temperature. The origin of the ferromagnetism is attributed to the trivalent Sm dopant, as confirmed by X-ray magnetic circular dichroism and first-principles calculations. The carrier concentration is on the order of 10(19) cm(-3) and the mobility can reach about 7200 cm(2) V(-1) s(-1) with pronounced Shubnikov-de Haas oscillations. © 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
Coupling of Crystal Structure and Magnetism in the Layered, Ferromagnetic Insulator CrI _{3}
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 CrI_{3}, 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$\\bar{3}$, BiI_{3}-type) and the high-temperature structure is monoclinic (C2/m, AlCl_{3}-type). Evidence for coupling between the crystallographic and magnetic degrees of freedom in CrI_{3} 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 CrI_{3} and other chromium trihalides may be promising materials for spintronic and magnetoelectronic research.
Magnetotransport properties of topological surface states in the presence of ferromagnetic order
NASA Astrophysics Data System (ADS)
Tiwari, Kunal; Coish, William; Pereg-Barnea, Tami
The surface of a 3D topological insulator hosts a two dimensional Dirac cone which is robust to weak, non-magnetic perturbation. Its presence will dominate low energy transport since the bulk is gapped. However, once magnetic impurities are introduced to the surface they may gap the Dirac dispersion, suppressing or modifying the system's surface transport properties. In particular, in the presence of uniform ferromagnetic order, the Dirac cone becomes massive and should not conduct for energies near the Dirac point. On the other hand, if the ferromagnetic order has domains with different magnetization directions, current may be carried on the domain walls where the Dirac mass vanishes. Our research aims to elucidate the transport properties of topological insulators in the presence of magnetic domain structures. Our work may be relevant to recent studies on the Kondo topological insulator SmB6.
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.
Mahfouzi, Farzad; Nagaosa, Naoto; Nikolić, Branislav K
2012-10-19
We predict an unconventional spin-transfer torque (STT) acting on the magnetization of a free ferromagnetic (F) layer within N/TI/F vertical heterostructures, which originates from strong spin-orbit coupling on the surface of a three-dimensional topological insulator (TI), as well as from charge current becoming spin polarized in the direction of transport as it flows perpendicularly from the normal metal (N) across the bulk of the TI layer. The STT vector has both in-plane and perpendicular components that are comparable in magnitude to conventional torque in F'/I/F (where I stands for insulator) magnetic tunnel junctions, while not requiring additional spin-polarizing F' layer with fixed magnetization, which makes it advantageous for spintronics applications. Our principal formal result is a derivation of the nonequilibrium Green function-based formula and the corresponding gauge-invariant nonequilibrium density matrix, which makes it possible to analyze the components of the STT vector in the presence of arbitrary strong spin-orbit coupling either in the bulk or at the interface of the free F layer.
Yang, Yi; Chen, Jin-Feng; Hu, Lei; Lin, Chen-Sheng; Cheng, Wen-Dan
2014-10-21
We studied the electronic and magnetic properties of hole doped KTaO{sub 3}/PbTiO{sub 3} interface using density functional theory methods. Ferromagnetic-nonmagnetic phase transition and metal-insulator phase transition occur simultaneously at the interface with ferroelectric polarization reversal. Furthermore, these two transitions are coupled with each other because hole doping with large concentration of holes gives rise to ferromagnetism. The interfacial magnetization, which is proportional to hole concentration at the interface, can be tuned by ferroelectric polarization, leading to strong intrinsic magnetoelectric effect at the interface of originally nonmagnetic KTaO{sub 3} and PbTiO{sub 3}.
NASA Astrophysics Data System (ADS)
Kasinathan, Deepa; Koepernik, Klaus; Nitzsche, Ulrike; Rosner, Helge
2007-12-01
Cs2AgF4 was proposed to be an orbitally ordered ferromagnet based on recent neutron scattering data. Here, we report a detailed electronic structure study within the local spin density approximation also including strong Coulomb repulsion U. We investigate the influence of an orthorhombic distortion of the Ag environment and the importance of the on-site Coulomb repulsion. We find good quantitative agreement with both the experimentally observed exchange coupling and structural changes. Thus, our results strongly support that Cs2AgF4 is a strongly correlated charge-transfer insulator where the ferromagnetism is driven by orbital order.
Ground state properties of perovskite and post-perovskite CaRuO 3: Ferromagnetism reduction
NASA Astrophysics Data System (ADS)
Zhong, Guohua; Li, Yanling; Liu, Zhuang; Zeng, Zhi
2010-12-01
Aiming at the disputed ground state properties of perovskite (Pv) CaRuO 3, we have investigated the variations of electronic structures and magnetism between Pv and post-perovskite (PPv) phases of CaRuO 3, based on the generalized gradient approximation (GGA) plus on-site Coulomb interaction U and spin-orbital coupling (SOC) effect correction, namely GGA + U + SOC method. Both Pv and PPv phases have Mott-Hubbard insulating characteristics. Under Pnma symmetry, the 4 d electrons have the stronger SOC effect and the weaker electronic correlation. Under Cmcm symmetry, however, the situation is just reversed. The G-type antiferromagnetic (AFM) superexchange interaction of Ru-Ru is perfected in Pv phase, antiferromagnetically mediated by O atoms. PPv phase exhibits the AFM ground state along c direction, but the FM-AFM fluctuation exists in a- b plane of PPv phase. The strong magnetic anisotropy and big exchange constants are the inexistent evidences of spin-glass behavior in Pv and PPv phases. Ru 4+ is in low-spin state, S ˜ 1. Pv-PPv phase transition changed the electronic and magnetic structures, but the magnetism is not sensitive to pressure in each phase. The suppression of ferromagnetism in Pv and PPv phases arises from the AFM interaction induced by the SOC effect and the FM-AFM fluctuation, respectively.
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)
Park, Jihwey; Soh, Yeong-Ah; Aeppli, Gabriel; Feng, Xiao; Ou, Yunbo; He, Ke; Xue, Qi-Kun
2015-06-30
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.
Park, Jihwey; Soh, Yeong-Ah; Aeppli, Gabriel; Feng, Xiao; Ou, Yunbo; He, Ke; Xue, Qi-Kun
2015-06-30
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)_{2}Te_{3} (CBST) films grown on SrTiO_{3} (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.
Zvonarev, M B; Cheianov, V V; Giamarchi, T
2009-09-11
We investigate the dynamics of the one-dimensional strongly repulsive spin-1/2 Bose-Hubbard model for filling nu
NASA Astrophysics Data System (ADS)
Kim, Chung Koo; Lee, Inhee; Lee, Jinho; Billinge, Simon; Zhong, Ruidan; Schneeloch, John; Liu, Tiansheng; Tranquada, John; Gu, Genda; Davis, J. C. Seamus
2015-03-01
Topological insulators (TI) have a gapless surface state of Dirac fermions protected by the time reversal symmetry (TRS). However, TRS can be broken in the ferromagnetic state induced by magnetic doping. This leads to the opening of ``mass gap'' at the Dirac point. Such a gap is predicted to involve many exotic phenomena for which understanding the microscopic role of magnetic dopants is critical. But it is unknown how the spatial arrangements of the magnetic dopant atoms influence the Dirac-mass gap at the atomic scale. Here we image the locations of the magnetic (Cr) dopant atoms in the ferromagnetic TI Cr0.08(Bi0.1Sb0.9)1.92 Te3. Simultaneous visualization of the Dirac-mass gap Δ(r) reveals its intense disorder, which we demonstrate directly is related to fluctuations in n(r), the areal Cr atom density at the surface. The relationship of the surface-state Fermi wavevectors to both the correlation length and anisotropic structure of Δ(r) are found consistent with predictions for ferromagnetism mediated by the surface states.
Electrical Control of Metallic Heavy-Metal-Ferromagnet Interfacial States
NASA Astrophysics Data System (ADS)
Bi, Chong; Sun, Congli; Xu, Meng; Newhouse-Illige, Ty; Voyles, Paul M.; Wang, Weigang
2017-09-01
Voltage-control effects provide an energy-efficient means of tailoring material properties, especially in highly integrated nanoscale devices. However, only insulating and semiconducting systems can be controlled so far. In metallic systems, there is no electric field due to electron screening effects and thus no such control effect exists. Here, we demonstrate that metallic systems can also be controlled electrically through ionic rather than electronic effects. In a Pt /Co structure, the control of the metallic Pt /Co interface can lead to unprecedented control effects on the magnetic properties of the entire structure. Consequently, the magnetization and perpendicular magnetic anisotropy of the Co layer can be independently manipulated to any desired state, the efficient spin toques can be enhanced about 3.5 times, and the switching current can be reduced about one order of magnitude. This ability to control a metallic system may be extended to control other physical phenomena.
Tuning the Weak Ferromagnetic States in Dysprosium Orthoferrite
Cao, Shixun; Chen, Lei; Zhao, Weiyao; Xu, Kai; Wang, Guohua; Yang, Yali; Kang, Baojuan; Zhao, Hongjian; Chen, Peng; Stroppa, Alessandro; Zheng, Ren-Kui; Zhang, Jincang; Ren, Wei; Íñiguez, Jorge; Bellaiche, L.
2016-01-01
RFeO3 orthoferrites, where R is a rare-earth ion of the lanthanide series, are attracting attention mostly because of their promising fast spin dynamics. The magnetic properties of these materials seem to crucially depend on whether the magnetizations of the R and Fe ions’ weak ferromagnetic (WFM) components are parallel or antiparallel to each other. Here, we report an extensive investigation of a high-quality DyFeO3 single crystal in which the induced Dy3+ magnetization (FDy) has a natural tendency to be antiparallel to Fe3+ sublattice magnetization (FFe) within a large temperature window. Moreover, we find that specific variations of temperature and applied magnetic fields allow us to make FDy parallel to FFe, or force a spin-flip transition in FFe, among other effects. We found three different magnetic states that respond to temperature and magnetic fields, i.e. linear versus constant or, alternatively, presenting either behavior depending on the history of the sample. An original magnetic field-versus-temperature phase diagram is constructed to indicate the region of stability of the different magnetic phases, and to reveal the precise conditions yielding sudden spin switching and reversals. Knowledge of such a phase diagram is of potential importance to applications in spintronics and magnetic devices. PMID:27886220
Tuning the Weak Ferromagnetic States in Dysprosium Orthoferrite
NASA Astrophysics Data System (ADS)
Cao, Shixun; Chen, Lei; Zhao, Weiyao; Xu, Kai; Wang, Guohua; Yang, Yali; Kang, Baojuan; Zhao, Hongjian; Chen, Peng; Stroppa, Alessandro; Zheng, Ren-Kui; Zhang, Jincang; Ren, Wei; Íñiguez, Jorge; Bellaiche, L.
2016-11-01
RFeO3 orthoferrites, where R is a rare-earth ion of the lanthanide series, are attracting attention mostly because of their promising fast spin dynamics. The magnetic properties of these materials seem to crucially depend on whether the magnetizations of the R and Fe ions’ weak ferromagnetic (WFM) components are parallel or antiparallel to each other. Here, we report an extensive investigation of a high-quality DyFeO3 single crystal in which the induced Dy3+ magnetization (FDy) has a natural tendency to be antiparallel to Fe3+ sublattice magnetization (FFe) within a large temperature window. Moreover, we find that specific variations of temperature and applied magnetic fields allow us to make FDy parallel to FFe, or force a spin-flip transition in FFe, among other effects. We found three different magnetic states that respond to temperature and magnetic fields, i.e. linear versus constant or, alternatively, presenting either behavior depending on the history of the sample. An original magnetic field-versus-temperature phase diagram is constructed to indicate the region of stability of the different magnetic phases, and to reveal the precise conditions yielding sudden spin switching and reversals. Knowledge of such a phase diagram is of potential importance to applications in spintronics and magnetic devices.
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.
Classical electromagnetic model of surface states in topological insulators
NASA Astrophysics Data System (ADS)
Lakhtakia, Akhlesh; Mackay, Tom G.
2016-07-01
A topological insulator is classically modeled as an isotropic material with a magnetoelectric pseudoscalar Ψ existing in its bulk while its surface is charge free and current free. An alternative model is obtained by setting Ψ≡0 and incorporating surface charge and current densities characterized by an admittance γ. Analysis of planewave reflection and refraction due to a topological-insulator half space reveals that the parameters Ψ and γ arise identically in the reflection and transmission coefficients, implying that the two classical models cannot be distinguished on the basis of any scattering scenario. However, as Ψ disappears from the Maxwell equations applicable to any region occupied by the topological insulator, and because surface states exist on topological insulators as protected conducting states, the alternative model must be chosen.
Self-localized states in photonic topological insulators.
Lumer, Yaakov; Plotnik, Yonatan; Rechtsman, Mikael C; Segev, Mordechai
2013-12-13
We propose solitons in a photonic topological insulator: self-localized wave packets forming topological edge states residing in the bulk of a nonlinear photonic topological insulator. These self-forming entities exhibit, despite being in the bulk, the property of unidirectional transport, similar to the transport their linear counterparts display on the edge of a topological insulator. In the concrete case of a Floquet topological insulator, such a soliton forms when a wave packet induces, through nonlinearity, a defect region in a honeycomb lattice of helical optical waveguides, and at the same time the wave packet populates a continuously rotating outer (or inner) edge state of that region. The concept is universal and applicable to topological systems with nonlinear response or mean-field interactions.
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
2015-01-01
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 Δ(r)∝n(r) is confirmed throughout and exhibits an electron–dopant interaction energy J* = 145 meV·nm2. 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
Lee, Inhee; Kim, Chung Koo; Lee, Jinho; ...
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
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
2015-02-03
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.
In-surface confinement of topological insulator nanowire surface states
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.
NASA Astrophysics Data System (ADS)
Li, Yun; Choi, Euiyoung; Kim, Shin-Ik; Baek, Seung-Hyub; Park, Seung-Young; Jo, Younghun; Seo, Jiwon
2017-08-01
We investigate the origin of insulating weak-ferromagnetic phase in ultra-thin epitaxial La0.67Sr0.33MnO3 (LSMO) films on SrTiO3 substrate using density functional theory calculation together with X-ray linear dichroism (XLD). The calculations show that symmetry breaking of the crystal field at the LSMO surface largely lowers the energy level of Mn d3z2 orbital at the surface and leads to full occupancy of the d3z2 orbital in majority spin channel, and XLD spectra clearly show the preferential occupation of Mn d3z2 orbital at the surface. Such an orbital reconstruction and charge redistribution in the ultra-thin films largely suppresses double-exchange interaction and favors super-exchange interaction, resulting in G-type antiferromagnetic spin ordering and insulating state. The anisotropic exchange interaction due to spin-orbital interaction leads to spin canting, and thus the films show weak ferromagnetism.
Interplay of imperfections and surface states in topological crystalline insulators
NASA Astrophysics Data System (ADS)
Plekhanov, Evgeny; Weber, Cedric
The conducting states, recently discovered at the surface of a special class of insulators - topological insulators - are distinguished for their insensitivity to local and non-magnetic surface defects. Their behavior in the presence of magnetic impurities and macroscopic imperfections of the surface is puzzling and hard to analyze quantitatively. Here, we present a systematic study of the imperfections (magnetic impurities and deviations from perfect surface cleavage) in topological crystalline insulators of the tin telluride family by using realistic first-principles-derived tight-binding models. The theoretical framework proposed is quite general and easily permits the extensions to other TI families and impurity types. The influence of the strong local correlations of the impurity atoms on the topological states stability is also discussed within the frame of the Dynamical Mean Field Theory.
Electrically Tunable Magnetism in Magnetic Topological Insulators
Wang, Jing; Lian, Biao; Zhang, Shou-Cheng
2015-07-14
The external controllability of the magnetic properties in topological insulators would be important both for fundamental and practical interests. Here we predict the electric-field control of ferromagnetism in a thin film of insulating magnetic topological insulators. The decrease of band inversion by the application of electric fields results in a reduction of magnetic susceptibility, and hence in the modification of magnetism. Remarkably, the electric field could even induce the magnetic quantum phase transition from ferromagnetism to paramagnetism. We further propose a transistor device in which the dissipationless charge transport of chiral edge states is controlled by an electric field. In particular, the field-controlled ferromagnetism in a magnetic topological insulator can be used for voltage based writing of magnetic random access memories in magnetic tunnel junctions. The simultaneous electrical control of magnetic order and chiral edge transport in such devices may lead to electronic and spintronic applications for topological insulators.
Electrically Tunable Magnetism in Magnetic Topological Insulators.
Wang, Jing; Lian, Biao; Zhang, Shou-Cheng
2015-07-17
The external controllability of the magnetic properties in topological insulators would be important both for fundamental and practical interests. Here we predict the electric-field control of ferromagnetism in a thin film of insulating magnetic topological insulators. The decrease of band inversion by the application of electric fields results in a reduction of magnetic susceptibility, and hence in the modification of magnetism. Remarkably, the electric field could even induce the magnetic quantum phase transition from ferromagnetism to paramagnetism. We further propose a transistor device in which the dissipationless charge transport of chiral edge states is controlled by an electric field. In particular, the field-controlled ferromagnetism in a magnetic topological insulator can be used for voltage based writing of magnetic random access memories in magnetic tunnel junctions. The simultaneous electrical control of magnetic order and chiral edge transport in such devices may lead to electronic and spintronic applications for topological insulators.
Edge-states ferromagnetism of WS{sub 2} nanosheets
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.
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…
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…
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.
Chern insulating state in laterally patterned semiconductor heterostructures
NASA Astrophysics Data System (ADS)
Li, Tommy; Sushkov, Oleg P.
2016-10-01
Hexagonally patterned two-dimensional p -type semiconductor systems are quantum simulators of graphene with strong and highly tunable spin-orbit interactions. We show that application of purely in-plane magnetic fields, in combination with the crystallographic anisotropy present in low-symmetry semiconductor interfaces, allows Chern insulating phases to emerge from an originally topologically insulating state after a quantum phase transition. These phases are characterized by pairs of co-propagating edge modes and Hall conductivities σx y=+2/e2 h ,-2/e2 h in the absence of Landau levels or cyclotron motion. With current lithographic technology, the Chern insulating transitions are predicted to occur in GaAs heterostructures at magnetic fields of ˜5 T .
Ferromagnetic superexchange in insulating Cr2MoO6 by controlling orbital hybridization
Zhu, M.; Do, D.; Dela Cruz, Clarina R.; ...
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
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.
Surface states, surface metal-insulator, and surface insulator-metal transitions
NASA Astrophysics Data System (ADS)
Tosatti, E.
1995-05-01
An informal discussion of various cases where two-dimensional surface metal-insulator structural and charge-density-wave instabilities driven by partly filled surface states have been advocated is presented. These include reconstructions of clean semiconductor surfaces and of W(100) and Mo(100), as well as anomalies on the hydrogen-covered surfaces H/W(110) and H/Mo(110), and possibly alkali-covered surfaces such as K/Cu(111). In addition, there is a discussion of the opposite type of phenomena, namely surface insulator-metal transitions, which can be argued to occur on (alpha)-Ga(001), high-temperature Ge(111), and probably Be(0001).
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.
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.
Spin-wave-induced corrections to the electronic density of states in metallic ferromagnets
NASA Astrophysics Data System (ADS)
Ricottone, A.; Danon, J.; Brouwer, P. W.
2013-12-01
We calculate the correction to the electronic density of states in a disordered ferromagnetic metal induced by spin-wave mediated interaction between the electrons. Our calculation is valid for the case that the exchange splitting Δ in the ferromagnet is much smaller than the Fermi energy, but we make no assumption on the relative magnitude of Δ and the elastic electronic scattering time τel. In the ‘clean limit’ Δτel/ℏ ≫ 1 we find a correction with a Td/2 temperature dependence, where d is the effective dimensionality of the ferromagnet. In the ‘dirty limit’ Δτel/ℏ ≪ 1, the density-of-states correction is a non-monotonous function of energy and temperature.
Di Bernardo, A; Diesch, S; Gu, Y; Linder, J; Divitini, G; Ducati, C; Scheer, E; Blamire, M G; Robinson, J W A
2015-09-02
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.
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
Surface state photoelectrons in topological insulators: Green's function approach.
Schmeltzer, D; Saxena, A
2015-12-09
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.
Magnon nodal-line semimetals and drumhead surface states in anisotropic pyrochlore ferromagnets
NASA Astrophysics Data System (ADS)
Mook, Alexander; Henk, Jürgen; Mertig, Ingrid
2017-01-01
We introduce a type of topological magnon matter: the magnonic pendant to electronic nodal-line semimetals. Magnon spectra of anisotropic pyrochlore ferromagnets feature twofold degeneracies of magnon bands along a closed loop in reciprocal space. These magnon nodal lines are topologically protected by the coexistence of inversion and time-reversal symmetry; they require the absence of spin-orbit interaction (no Dzyaloshinskii-Moriya interaction). We calculate the topological invariants of the nodal lines and show that details of the associated magnon drumhead surface states depend strongly on the termination of the surface. Magnon nodal-line semimetals complete the family of topological magnons in three-dimensional ferromagnetic materials.
Linear-optical access to topological insulator surface states
NASA Astrophysics Data System (ADS)
Panna, Dmitry; Marjieh, Raja; Sabag, Evyatar; Rybak, Leonid; Ribak, Amit; Kanigel, Amit; Hayat, Alex
2017-05-01
We demonstrate efficient linear-optical access to surface-state spin dynamics in Bi2Se3 by probing transitions between two surface-state Dirac cones, providing a practical technique for spin-current dynamics studies in topological-insulator devices. Using broadband transient-reflectivity pump-probe measurements, we distinguish bulk and surface state-responses, by controlling photon energy and circular polarization at oblique incidence. For pump-photon energies corresponding to bulk-state transitions, the probe polarized co-circularly with the pump shows stronger reflectivity change, compared to the anti-circularly polarized probe. However, pump photon energies corresponding to surface-state transitions result in an opposite effect, with the anti-circularly polarized probe exhibiting stronger reflectivity change. This surprising behavior stems from the surface-state in-plane spin orientation near the Dirac point, and the surface-state spin population remains at the injected energy for several ps. These results enable an efficient approach for studying spin current dynamics in topological-insulator based technologies.
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.
Metal-to-insulator switching in quantum anomalous Hall states
Kou, Xufeng; Pan, Lei; Wang, Jing; ...
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
Metal-to-insulator switching in quantum anomalous Hall states
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 (Cr_{0.12}Bi_{0.26}Sb_{0.62})_{2}Te_{3} 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.
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.
Interacting Surface States of Three-Dimensional Topological Insulators
NASA Astrophysics Data System (ADS)
Neupert, Titus; Rachel, Stephan; Thomale, Ronny; Greiter, Martin
2015-07-01
We numerically investigate the surface states of a strong topological insulator in the presence of strong electron-electron interactions. We choose a spherical topological insulator geometry to make the surface amenable to a finite size analysis. The single-particle problem maps to that of Landau orbitals on the sphere with a magnetic monopole at the center that has unit strength and opposite sign for electrons with opposite spin. Assuming density-density contact interactions, we find superconducting and anomalous (quantum) Hall phases for attractive and repulsive interactions, respectively, as well as chiral fermion and chiral Majorana fermion boundary modes between different phases. Our setup is preeminently adapted to the search for topologically ordered surface terminations that could be microscopically stabilized by tailored surface interaction profiles.
Topologically insulating states in ternary transition metal dichalcogenides
NASA Astrophysics Data System (ADS)
Lin, Xianqing; Ni, Jun
2017-01-01
The topological and electronic properties of monolayered monoclinic transition metal dichalcogenide (TMD) alloys (1T '-M1-xNxX2 with M, N = Cr, Mo, W and X = S, Se) have been studied through calculations based on the projected Wannier functions obtained from first-principles calculations. We predict that the ternary compounds 1T '-Mo1-xCrxS2 with x up to 7/12 and all 1T '-Mo1-xWxSe2 host topologically insulating states with band gaps comparable to the pure systems. For Cr contained alloys, the mechanism of sign changing of Berry curvature is proposed to explain the trivial band topology of some configurations. The predicted topologically insulating ternary TMDs may be promising candidates for future realization of topological devices.
Interaction-induced insulating states in multilayer graphenes
NASA Astrophysics Data System (ADS)
Koshino, Mikito; Sugisawa, Kyoka; McCann, Edward
2017-06-01
We explore the electronic ground states of Bernal-stacked multilayer graphenes using the Hartree-Fock mean-field approximation and the full-parameter band model. We find that the electron-electron interaction tends to open a band gap in multilayer graphenes from bilayer to eight-layer, while the nature of the insulating ground state sensitively depends on the band parameter γ2, which is responsible for the semimetallic nature of graphite. In four-layer graphene, particularly, the ground state assumes an odd-spatial-parity staggered phase at γ2=0 , while an increasing, finite value of γ2 stabilizes a different state with even parity, where the electrons are attracted to the top layer and the bottom layer. The two phases are topologically distinct insulating states with different Chern numbers, and they can be distinguished by spin or valley Hall conductivity measurements. Multilayers with more than five layers also exhibit similar ground states with potential minima at the outermost layers, although the opening of a gap in the spectrum as a whole is generally more difficult than in four-layer because of a larger number of energy bands overlapping at the Fermi energy.
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).
Tunable storage states' transition in slotted ferromagnetic nanorings
NASA Astrophysics Data System (ADS)
Zhou, Kan; Wang, Xiaokun; Li, Shichao; Liu, Bailin; Zhang, Baoshan; Tang, Dongming; Yang, Yi
2017-03-01
In magnetic random access memory, free layers are used to store data bits as "0" or "1." For free layers with slotted nanoring structures, the magnetization configuration of counterclockwise vortex state can be defined as "0" state, while clockwise vortex state as "1" state. It is important to have a controllable and stable state transition process to make sure that "0" state can switch to "1" state and vice versa. Up to now, it has seldom been reported that the transition process is heavily affected by the static anisotropy field Hk-stat of free layers. A sufficient Hk-stat will substantially reduce the probability of successful state transition. To increase the accuracy of writing data, the free layers must be prepared with a low anisotropy field. In this paper, we present a rotational sputtering method, which can finish the desired isotropic film preparation, and thus realize a stable state transition.
NASA Astrophysics Data System (ADS)
Agarwal, Vasudha; Siwach, P. K.; Varma, G. D.; Awana, V. P. S.; Srivastava, A. K.; Singh, H. K.
2014-12-01
Magnetic and magnetotransport properties of oriented polycrystalline Pr0.58Ca0.42MnO3 thin films prepared in flowing oxygen (O2) and air ambient have been investigated. In the air, annealed film charge order (CO) is quenched and ferromagnetic (FM) transition, which appears at TC ≈ 148 K is followed by antiferromagnetic (AFM) transition at TN ≈ 104 K. This film shows self-field insulator-metal transition (IMT) at {{T}IM}C ≈ 89 K and {{T}IM}W ≈ 148 K in the cooling and warming cycle, respectively. Magnetic field (H) enhances {{T}IM}C and {{T}IM}W, reduces the thermo-resistive hysteresis. The film annealed in O2 shows a CO transition at TCO ≈ 236 K, which is followed by FM and AFM transitions at TC ≈ 158 K and TN ≈ 140 K, respectively. No self-field IMT is observed in this film up to H = 20 kOe. At H ≥ 30 kOe, IMT having sharp resistivity jumps appears at {{T}IM}C ≈ 66 K and {{T}IM}W ≈ 144 K in the cooling and warming cycle, respectively. As H increases the resistivity jumps disappear and ΔTIM decreases. In the lower temperature regime (T = 5 K and 40 K) the H dependent resistivity (ρ-H) measurements show that the frozen cluster state is more robust in the O2 annealed film. At temperatures around TC, the ρ-H hysteresis and H induced drop in resistivity are more prominent in the O2 annealed film. At TC < T < TCO, higher H is required to induce IMT in the O2 annealed film. The magnetic and magnetotransport data clearly show that the film annealed in O2 has higher fraction of the AFM/COI phase, while the air annealed film has higher fraction of FMM phase. The microstructural analysis of the two set of films employing high resolution transmission electron microscopy reveals that the air annealed film has higher density of microstructural disorder and lattice defects, which could be responsible for CO quenching, FM transition and self-field IMT.
Phase states of a 2D easy-plane ferromagnet with strong inclined anisotropy
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.
Feng, Hai L.; Calder, Stuart; Ghimire, Madhav Prasad; Yuan, Ya-Hua; Shirako, Yuichi; Tsujimoto, Yoshihiro; Matsushita, Yoshitaka; Hu, Zhiwei; Kuo, Chang-Yang; Tjeng, Liu Hao; Pi, Tun-Wen; Soo, Yun-Liang; He, Jianfeng; Tanaka, Masahiko; Katsuya, Yoshio; Richter, Manuel; Yamaura, Kazunari
2016-12-28
In this study, the ferromagnetic semiconductor Ba_{2}NiOsO_{6} ( T_{mag} ~ 100 K ) was synthesized at 6 GPa and 1500 °C. It crystallizes into a double perovskite structure [Fm - 3m ; a = 8.0428 ( 1 ) Å], where the Ni^{2+} and Os^{6+} ions are perfectly ordered at the perovskite B site. We show that the spin-orbit coupling of Os^{6+} plays an essential role in opening the charge gap. The magnetic state was investigated by density functional theory calculations and powder neutron diffraction. The latter revealed a collinear ferromagnetic order in a > 21 kOe magnetic field at 5 K. The ferromagnetic gapped state is fundamentally different from that of known dilute magnetic semiconductors such as (Ga,Mn)As and (Cd,Mn)Te ( T_{mag} < 180 K ), the spin-gapless semiconductor Mn_{2} CoAl ( T_{mag} ~ 720 K ), and the ferromagnetic insulators EuO ( T_{mag} ~ 70 K ) and Bi_{3}Cr_{3}O_{11} ( T_{mag} ~ 220 K ). It is also qualitatively different from known ferrimagnetic insulators and semiconductors, which are characterized by an antiparallel spin arrangement. Our finding of the ferromagnetic semiconductivity of Ba_{2}NiOsO_{6} should increase interest in the platinum group oxides, because this alternative class of materials should be useful in the development of spintronic, quantum magnetic, and related devices.
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.
Gangopadhyay, Shruba; Pickett, Warren E.
2015-01-15
The double perovskite Ba_{2}NaOsO_{6} (BNOO), an exotic example of a very high oxidation state (heptavalent) osmium d1 compound and also uncommon by being a ferromagnetic open d-shell (Mott) insulator without Jahn-Teller (JT) distortion, is modeled using a density functional theory based hybrid functional incorporating exact exchange for correlated electronic orbitals and including the large spin-orbit coupling (SOC). The experimentally observed narrow-gap ferromagnetic insulating ground state is obtained, but only when including spin-orbit coupling, making this a Dirac-Mott insulator. The calculated easy axis along [110] is in accord with experiment, providing additional support that this approach provides a realistic method for studying this system. The predicted spin density for [110] spin orientation is nearly cubic (unlike for other directions), providing an explanation for the absence of JT distortion. An orbital moment of –0.4μ_{B} strongly compensates the +0.5μ_{B} spin moment on Os, leaving a strongly compensated moment more in line with experiment. Remarkably, the net moment lies primarily on the oxygen ions. An insulator-metal transition, by rotating the magnetization direction with an external field under moderate pressure, is predicted as one consequence of strong SOC, and metallization under moderate pressure is predicted. In conclusion, a comparison is made with the isostructural, isovalent insulator Ba_{2}LiOsO_{6}, which, however, orders antiferromagnetically.
Interference in the Mott Insulator State of Distinguishable Particles
NASA Astrophysics Data System (ADS)
Tian, Lin; Fujiwara, Fumitaka; Byrnes, Tim; Yamamoto, Yoshihisa
2008-03-01
Particle statistics plays a crucial role in strongly interacting quantum many-body systems. Here, we study the Hubbard model for distinguishable particles at unit filling. We show that when on-site repulsive interaction dominates over tunneling, the ground state is a Mott insulator state with higher order coherence between the particles. This result can be experimentally confirmed by the recovery of the interference pattern in the density correlation functions and is robust against non- uniformity of the interaction and tunneling parameters. We also show that this state is a maximally entangled state, in contrast to its bosonic counterpart. L. Tian, F. Fujiwara, T. Byrnes, and Y. Yamamoto, preprint, arXiv/0705.2023.
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.
Frequency mixer having ferromagnetic film
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.
Enhanced ferromagnetism and glassy state in phase separated La0.95Sr0.05MnO3 + δ
NASA Astrophysics Data System (ADS)
De, K.; Das, S.; Roy, A.; Dhak, P.; Willinger, M.; Amaral, J. S.; Amaral, V. S.; Giri, S.; Majumder, S.; Silva, C. J. R.; Gomes, M. J. M.; Mahapatra, P. K.
2012-11-01
Unusual high temperature ferromagentism is reported for La0.95Sr0.05MnO3+δ with Curie temperatures (TC1 and TC2) TC1 at 290 K and TC2 ˜ 150 K. Weak antiferromagnetism (AFM) is thought to give low moment, low temperature irreversibility, and non-saturation up to 10 T at 5 K. A short-range ferromagnetism is predicted from well-defined hysteresis at 5 K and field-cooled (FC) magnetization curves. Strong irreversibility between zero field-cooled (ZFC) and FC at 5 T is due to "hard" spins, not from ferromagnetic clusters. Field-dependent peak shift of spin freezing temperature in ZFC, memory effect, magnetic relaxation shows cluster glass like transition in the system. A metal-insulator transition at 115 K also implies a completion of percolation of short range ferromagnetic clusters, giving an onset of metallic state at 115 K. A complete magnetic phase diagram is presented showing its temperature dependent rich magnetic behavior.
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.
Andreev bound states in superconductor/ferromagnet point contact Andreev reflection spectra
NASA Astrophysics Data System (ADS)
Yates, K. A.; Olde Olthof, L. A. B.; Vickers, M. E.; Prabhakaran, D.; Egilmez, M.; Robinson, J. W. A.; Cohen, L. F.
2017-03-01
As charge carriers traverse a single superconductor ferromagnet interface, they experience an additional spin-dependent phase angle that results in spin mixing and the formation of a bound state called the Andreev bound state. Here we explore whether point contact Andreev reflection can be used to detect the Andreev bound state and, within the limits of our experiment, we extract the resulting spin mixing angle. By examining spectra taken from L a1.15S r1.85M n2O7-Pb junctions, together with a compilation of literature data on highly spin polarized systems, we suggest that the existence of the Andreev bound state would resolve a number of long standing controversies in the literature of Andreev reflection, as well as defining a route to quantify the strength of spin mixing at superconductor-ferromagnet interfaces. Intriguingly, we find that for high transparency junctions, the spin mixing angle appears to take a relatively narrow range of values across all the samples studied. The ferromagnets we have chosen to study share a common property in terms of their spin arrangement, and our observations may point to the importance of this property in determining the spin mixing angle under these circumstances.
Quantum Anomalous Hall State in Ferromagnetic SrRuO3 (111) Bilayers
NASA Astrophysics Data System (ADS)
Si, Liang; Janson, Oleg; Li, Gang; Zhong, Zhicheng; Liao, Zhaoliang; Koster, Gertjan; Held, Karsten
2017-07-01
SrRuO 3 heterostructures grown in the (111) direction are a rare example of thin film ferromagnets. By means of density functional theory plus dynamical mean field theory we show that the half-metallic ferromagnetic state with an ordered magnetic moment of 2 μB /Ru survives the ultimate dimensional confinement down to a bilayer, even at elevated temperatures of 500 K. In the minority channel, the spin-orbit coupling opens a gap at the linear band crossing corresponding to 3/4 filling of the t2 g shell. We predict that the emergent phase is Haldane's quantum anomalous Hall state with Chern number C =1 , without an external magnetic field or magnetic impurities.
Deformation effects of droplet fluctuations on dynamics in an Ising ferromagnetic state
NASA Technical Reports Server (NTRS)
Nakanishi, Hiizu
1990-01-01
Deformation effects of droplet fluctuations on the dynamics in an Ising ferromagnetic state in two dimensions are studied in the case of an order-parameter-nonconserving system by investigating an equation of motion for a domain boundary. Analytic and numerical studies show that the deformation effects on the survival probability of the droplets simply result in changing the time scale or renormalizing a kinetic coefficient.
Tang, Jianshi; Chang, Li-Te; Kou, Xufeng; Murata, Koichi; Choi, Eun Sang; Lang, Murong; Fan, Yabin; Jiang, Ying; Montazeri, Mohammad; Jiang, Wanjun; Wang, Yong; He, Liang; Wang, Kang L
2014-09-10
Strong spin-orbit interaction and time-reversal symmetry in topological insulators enable the spin-momentum locking for the helical surface states. To date, however, there has been little report of direct electrical spin injection/detection in topological insulator. In this Letter, we report the electrical detection of spin-polarized surface states conduction using a Co/Al2O3 ferromagnetic tunneling contact in which the compound topological insulator (Bi0.53Sb0.47)2Te3 was used to achieve low bulk carrier density. Resistance (voltage) hysteresis with the amplitude up to about 10 Ω was observed when sweeping the magnetic field to change the relative orientation between the Co electrode magnetization and the spin polarization of surface states. The two resistance states were reversible by changing the electric current direction, affirming the spin-momentum locking in the topological surface states. Angle-dependent measurement was also performed to further confirm that the abrupt change in the voltage (resistance) was associated with the magnetization switching of the Co electrode. The spin voltage amplitude was quantitatively analyzed to yield an effective spin polarization of 1.02% for the surface states conduction in (Bi0.53Sb0.47)2Te3. Our results show a direct evidence of spin polarization in the topological surface states conduction. It might open up great opportunities to explore energy-efficient spintronic devices based on topological insulators.
Ryzhov, V A; Lazuta, A V; Khavronin, V P; Molkanov, P L; Mukovskii, Ya M
2014-02-19
The transport and magnetic properties (ac linear and nonlinear (second and third orders) susceptibilities) are presented for La0.8Ca0.2MnO3 and La0.8Ca0.2CoO3 single crystals with insulator ground states. The ferromagnetic (FM) clusters with similar magnetic characteristics originate in the paramagnetic phases of both compounds below some temperature T(∗). At high temperatures the FM clusters arise at the preferable sites that can be attributed to the chemical inhomogeneities, their density being weakly T-dependent. On cooling a homogeneous nucleation of the FM clusters develops below a definite temperature T(#) that is characterized by a fast growth of their density. These two stages are observed in both compounds. At the third stage a coalescence of the FM clusters starts in the doped cobaltite, whereas in the manganite the development of matrix FM ordering occurs which changes a cluster's behavior. The indicated features support the common nature of the cluster state in the doped cobaltite and manganite. The difference in their evolution is a consequence of the different magnetic properties of the matrices in the manganite and cobaltite.
Coexistence of metallic and insulating-like states in graphene
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
Coexistence of metallic and insulating-like states in graphene
NASA Astrophysics Data System (ADS)
Wu, Fang; Huang, Jing; Li, Qunxiang; Deng, Kaiming; Kan, Erjun
2015-03-01
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.
Coexistence of metallic and insulating-like states in graphene.
Wu, Fang; Huang, Jing; Li, Qunxiang; Deng, Kaiming; Kan, Erjun
2015-03-10
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.
Type-II Dirac surface states in topological crystalline insulators
NASA Astrophysics Data System (ADS)
Chiu, Ching-Kai; Chan, Y.-H.; Li, Xiao; Nohara, Y.; Schnyder, A. P.
2017-01-01
Recently, it has been realized that topological Weyl semimetals come in two different varieties: (i) with standard Weyl cones with pointlike Fermi surfaces (type I) and (ii) with tilted Weyl cones that appear at the contact of electron and hole pockets (type II). These two types of Weyl semimetals have very different physical properties, in particular, in their thermodynamics and magnetotransport. Here, we show that Dirac cone surface states of topological crystalline insulators can be distinguished in a similar way. We demonstrate this in terms of a general surface theory and then apply this knowledge to a family of antiperovskites of the form A3E O , where A denotes an alkaline earth metal, while E stands for Pb or Sn. Using ab initio DFT calculations, we investigate the bulk and surface topology of these antiperovskites and show that they exhibit type-I as well as type-II Dirac surface states protected by reflection symmetry. We find that the type-II Dirac states, as opposed to the type-I Dirac states, exhibit characteristic van Hove singularities in their dispersion, which lead to different thermodynamic properties, and which can serve as an experimental fingerprint of type-II surface states. The different magnetotransport characteristics between type-I and type-II surface states are discussed. In addition, we show that both type-I and type-II surface states exhibit an unusual helical spin polarization, which could lead to topological surface superconductivity.
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.
Optical Properties of a Vibrationally Modulated Solid State Mott Insulator
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
Properties of an excitonic insulator in the superconducting state
Batyev, E. G.
2012-01-15
The properties of an excitonic insulator with embedded (nondissipative) current are studied using the self-consistent field approximation, in which the wavefunction of the system has the form of the known Bardeen-Cooper-Schrieffer trial function with time-dependent coefficients; the equations for these coefficients are derived. Such a formulation holds for the homogeneous case (in the absence of a coordinate dependence). We consider two problems: (i) time evolution of the system in the case when an embedded current exists at the initial instant; and (ii) the response of the system to an abrupt perturbation (the vector potential changes jumpwise from zero to a certain finite value). In both cases, the state of the system depends on time, but some characteristics (e.g., undamped current) tend to a constant value. For a weak perturbation, the system behaves as an insulator. If the perturbation is not small (on the order of the gap in the spectrum), nonlinear effects lead to substantial differences: a certain part of the embedded current is preserved in the former case, while the initial current in the latter case acquires a certain addition.
Accessing Rashba states in electrostatically gated topological insulator devices
NASA Astrophysics Data System (ADS)
Banerjee, Abhishek; Sundaresh, Ananthesh; Majhi, Kunjalata; Ganesan, R.; Anil Kumar, P. S.
2016-12-01
We study the low temperature electrical transport in gated BiSbTe1.25Se1.75/hexagonal-Boron Nitride van der Waals heterostructure devices. Our experiments indicate the presence of Rashba spin-split states confined to the sample surface. While such states have been observed previously in photo-emission spectroscopy and STM experiments, it has not been possible to unambiguously detect them by electrical means and their transport properties remain mostly unknown. We show that these states support high mobility conduction with Hall effect mobilities ˜2000 to 3000 cm2/V-s that are paradoxically much larger than the mobilities of the topological surface states ˜300 cm2/V-s at T = 2 K. The spin-split nature of these states is confirmed by magneto-resistance measurements that reveal multi-channel weak anti-localization. Our work shows that Rashba spin split states can be electrically accessed in Topological insulators paving the way for future spintronic applications.
Edge states of a three-dimensional topological insulator.
Deb, Oindrila; Soori, Abhiram; Sen, Diptiman
2014-08-06
We use the bulk Hamiltonian for a three-dimensional topological insulator such as Bi(2) Se(3) to study the states which appear on its various surfaces and along the edge between two surfaces. We use both analytical methods based on the surface Hamiltonians (which are derived from the bulk Hamiltonian) and numerical methods based on a lattice discretization of the bulk Hamiltonian. We find that the application of a potential barrier along an edge can give rise to states localized at that edge. These states have an unusual energy-momentum dispersion which can be controlled by applying a potential along the edge; in particular, the velocity of these states can be tuned to zero. The scattering and conductance across the edge is studied as a function of the edge potential. We show that a magnetic field in a particular direction can also give rise to zero energy states on certain edges. We point out possible experimental ways of looking for the various edge states.
Edge states of a three-dimensional topological insulator
NASA Astrophysics Data System (ADS)
Deb, Oindrila; Soori, Abhiram; Sen, Diptiman
2014-08-01
We use the bulk Hamiltonian for a three-dimensional topological insulator such as Bi2 Se3 to study the states which appear on its various surfaces and along the edge between two surfaces. We use both analytical methods based on the surface Hamiltonians (which are derived from the bulk Hamiltonian) and numerical methods based on a lattice discretization of the bulk Hamiltonian. We find that the application of a potential barrier along an edge can give rise to states localized at that edge. These states have an unusual energy-momentum dispersion which can be controlled by applying a potential along the edge; in particular, the velocity of these states can be tuned to zero. The scattering and conductance across the edge is studied as a function of the edge potential. We show that a magnetic field in a particular direction can also give rise to zero energy states on certain edges. We point out possible experimental ways of looking for the various edge states.
A Continuous Family of Equilibria in Ferromagnetic Media are Ground States
NASA Astrophysics Data System (ADS)
Su, Xifeng; de la Llave, Rafael
2017-09-01
We show that a foliation of equilibria (a continuous family of equilibria whose graph covers all the configuration space) in ferromagnetic transitive models are ground states. The result we prove is very general, and it applies to models with long range and many-body interactions. As an application, we consider several models of networks of interacting particles including models of Frenkel-Kontorova type on Z^d and one-dimensional quasi-periodic media. The result above is an analogue of several results in the calculus of variations (fields of extremals) and in PDE's. Since the models we consider are discrete and long range, new proofs need to be given. We also note that the main hypothesis of our result (the existence of foliations of equilibria) is the conclusion (using KAM theory) of several recent papers. Hence, we obtain that the KAM solutions recently established are minimizers when the interaction is ferromagnetic and transitive (these concepts are defined later).
Floquet Fractional Chern Insulators
NASA Astrophysics Data System (ADS)
Grushin, Adolfo G.; Gómez-León, Álvaro; Neupert, Titus
2014-04-01
We show theoretically that periodically driven systems with short range Hubbard interactions offer a feasible platform to experimentally realize fractional Chern insulator states. We exemplify the procedure for both the driven honeycomb and the square lattice, where we derive the effective steady state band structure of the driven system by using the Floquet theory and subsequently study the interacting system with exact numerical diagonalization. The fractional Chern insulator state equivalent to the 1/3 Laughlin state appears at 7/12 total filling (1/6 filling of the upper band). The state also features spontaneous ferromagnetism and is thus an example of the spontaneous breaking of a continuous symmetry along with a topological phase transition. We discuss light-driven graphene and shaken optical lattices as possible experimental systems that can realize such a state.
Particle-hole asymmetry in gapped topological insulator surface states
NASA Astrophysics Data System (ADS)
Tabert, C. J.; Carbotte, J. P.
2015-06-01
We consider the combined effect of a gap and the Zeeman interaction on the helical Dirac fermions that exist on the surface of a topological insulator. Magneto-optical properties, the magnetization, Hall effect, and the density of states are considered with emphasis on the particle-hole asymmetry, which arises when a subdominant Schrödinger piece is included along with the dominant Dirac part of the Hamiltonian. When appropriate, we compare our results with those of a single-valley gapped graphene system for which Zeeman splitting behaves differently. We provide a derivation of the phase offset in the magnetic oscillations brought about by the combined effect of the gap and Schrödinger term without requiring the semiclassical Onsager quantization condition. Our results agree with previous discussions based on semiclassical arguments.
NASA Astrophysics Data System (ADS)
Marmolejo-Tejada, Juan Manuel; Dolui, Kapildeb; Lazić, Predrag; Chang, Po-Hao; Smidstrup, Søren; Stradi, Daniele; Stokbro, Kurt; Nikolić, Branislav K.
2017-09-01
The control of recently observed spintronic effects in topological-insulator/ferromagnetic-metal (TI/FM) heterostructures is thwarted by the lack of understanding of band structure and spin texture around their interfaces. Here we combine density functional theory with Green's function techniques to obtain the spectral function at any plane passing through atoms of Bi$_2$Se$_3$ and Co or Cu layers comprising the interface. In contrast to widely assumed but thinly tested Dirac cone gapped by the proximity exchange field, we find that the Rashba ferromagnetic model describes the spectral function on the surface of Bi$_2$Se$_3$ in contact with Co near the Fermi level $E_F^0$, where circular and snowflake-like constant energy contours coexist around which spin locks to momentum. The remnant of the Dirac cone is hybridized with evanescent wave functions injected by metallic layers and pushed, due to charge transfer from Co or Cu layers, few tenths of eV below $E_F^0$ for both Bi$_2$Se$_3$/Co and Bi$_2$Se$_3$/Cu interfaces while hosting distorted helical spin texture wounding around a single circle. These features explain recent observation [K. Kondou {\\em et al.}, Nat. Phys. {\\bf 12}, 1027 (2016)] of sensitivity of spin-to-charge conversion signal at TI/Cu interface to tuning of $E_F^0$. Interestingly, three monolayers of Co adjacent to Bi$_2$Se$_3$ host spectral functions very different from the bulk metal, as well as in-plane spin textures signifying the spin-orbit proximity effect. We predict that out-of-plane tunneling anisotropic magnetoresistance in vertical heterostructure Cu/Bi$_2$Se$_3$/Co, where current flowing perpendicular to its interfaces is modulated by rotating magnetization from parallel to orthogonal to current flow, can serve as a sensitive probe of spin texture residing at $E_F^0$.
Marmolejo-Tejada, Juan Manuel; Dolui, Kapildeb; Lazić, Predrag; Chang, Po-Hao; Smidstrup, Søren; Stradi, Daniele; Stokbro, Kurt; Nikolić, Branislav K
2017-09-13
The control of recently observed spintronic effects in topological-insulator/ferromagnetic-metal (TI/FM) heterostructures is thwarted by the lack of understanding of band structure and spin textures around their interfaces. Here we combine density functional theory with Green's function techniques to obtain the spectral function at any plane passing through atoms of Bi2Se3 and Co or Cu layers comprising the interface. Instead of naively assumed Dirac cone gapped by the proximity exchange field spectral function, we find that the Rashba ferromagnetic model describes the spectral function on the surface of Bi2Se3 in contact with Co near the Fermi level EF(0), where circular and snowflake-like constant energy contours coexist around which spin locks to momentum. The remnant of the Dirac cone is hybridized with evanescent wave functions from metallic layers and pushed, due to charge transfer from Co or Cu layers, a few tenths of an electron-volt below EF(0) for both Bi2Se3/Co and Bi2Se3/Cu interfaces while hosting distorted helical spin texture wounding around a single circle. These features explain recent observation of sensitivity of spin-to-charge conversion signal at TI/Cu interface to tuning of EF(0). Crucially for spin-orbit torque in TI/FM heterostructures, few monolayers of Co adjacent to Bi2Se3 host spectral functions very different from the bulk metal, as well as in-plane spin textures (despite Co magnetization being out-of-plane) due to proximity spin-orbit coupling in Co induced by Bi2Se3. We predict that out-of-plane tunneling anisotropic magnetoresistance in Cu/Bi2Se3/Co vertical heterostructure can serve as a sensitive probe of the type of spin texture residing at EF(0).
Probing strongly hybridized nuclear-electronic states in a model quantum ferromagnet
NASA Astrophysics Data System (ADS)
Kovacevic, I.; Babkevich, P.; Jeong, M.; Piatek, J. O.; Boero, G.; Rønnow, H. M.
2016-12-01
We present direct local-probe evidence for strongly hybridized nuclear-electronic spin states of an Ising ferromagnet LiHoF4 in a transverse magnetic field. The nuclear-electronic states are addressed via a magnetic resonance in the GHz frequency range using coplanar resonators and a vector network analyzer. The magnetic resonance spectrum is successfully traced over the entire field-temperature phase diagram, which is remarkably well reproduced by mean-field calculations. Our method can be directly applied to a broad class of materials containing rare-earth ions for probing the substantially mixed nature of the nuclear and electronic moments.
Thickness Dependence of the Quantum Anomalous Hall Effect in Magnetic Topological Insulator Films.
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.
Takahashi, Masao
2010-01-01
The theoretical study of magnetic semiconductors using the dynamical coherent potential approximation (dynamical CPA) is briefly reviewed. First, we give the results for ferromagnetic semiconductors (FMSs) such as EuO and EuS by applying the dynamical CPA to the s-f model. Next, applying the dynamical CPA to a simple model for A1−xMnxB-type diluted magnetic semiconductors (DMSs), we show the results for three typical cases to clarify the nature and properties of the carrier states in DMSs. On the basis of this model, we discuss the difference in the optical band edges between II-V DMSs and III-V-based DMSs, and show that two types of ferromagnetism can occur in DMSs when carriers are introduced. The carrier-induced ferromagnetism of Ga1−xMnxAs is ascribed to a double-exchange (DE)-like mechanism realized in the magnetic impurity band/or in the band tail.
Exotic Ground State Phases of S=1/2 Heisenberg Δ-Chain with Ferromagnetic Main Chain
NASA Astrophysics Data System (ADS)
Hida, Kazuo
2008-04-01
The ground state phase diagram of the spin-1/2 Heisenberg frustrated Δ-chain with a ferromagnetic main chain is investigated. In addition to the ferromagnetic phase, various nonmagnetic ground states are found. If the ferromagnetic coupling between apical spins and the main chain is strong, this model is approximated by a spin-1 bilinear-biquadratic chain and the spin quadrupolar phase with spin-2 gapless excitation is realized in addition to the Haldane and ferromagnetic phases. In the regime where the coupling between the apical spins and the main chain is weak, the numerical results which suggest the possibility of a series of phase transitions among different nonmagnetic phases are obtained. Physical pictures of these phases are discussed based on the numerical results.
Electrically Tunable Magnetism in Magnetic Topological Insulators
NASA Astrophysics Data System (ADS)
Zhang, Shou-Cheng; Wang, Jing; Lian, Biao
2015-03-01
The external controllability of the magnetic properties in topological insulators would be important both for fundamental and practical interests. Here we predict the electric-field control of ferromagnetism in a thin film of insulating magnetic topological insulators. The decrease of band inversion by the application of electric fields results in a reduction of magnetic susceptibility, and hence in the modication of magnetism. Remarkably, the electric field could even induce the magnetic quantum phase transition from ferromagnetism to paramagnetism. We further propose a topological transistor device in which the dissipationless charge transport of chiral edge states is controlled by an electric field. The simultaneous electrical control of magnetic order and chiral edge transport in such a device may lead to electronic and spintronic applications for topological insulators. This work is supported by the U.S. Department of Energy, Office of Basic Energy Sciences, Division of Materials Sciences and Engineering, under Contract No. DE-AC02-76SF00515.
Interfacial Ferromagnetism in LaNiO3/CaMnO3 Superlattices
Grutter, Alexander J.; Yang, Hao; Kirby, B. J.; Fitzsimmons, M. R.; Aguiar, Jeffery A.; Browning, Nigel D.; Jenkins, C. A.; Arenholz, Elke; Mehta, V. V.; Alaan, U. S.; Suzuki, Y.
2013-08-01
We observe interfacial ferromagnetism in superlattices of the paramagnetic metal LaNiO3 and the antiferromagnetic insulator CaMnO3. LaNiO3 exhibits a thickness dependent metal-insulator transition and we find the emergence of ferromagnetism to be coincident with the conducting state of LaNiO3. That is, only superlattices in which the LaNiO3 layers are metallic exhibit ferromagnetism. Using several magnetic probes, we have determined that the ferromagnetism arises in a single unit cell of CaMnO3 at the interface. Together these results suggest that ferromagnetism can be attributed to a double exchange interaction among Mn ions mediated by the adjacent itinerant metal.
NASA Astrophysics Data System (ADS)
Awoga, Oladunjoye A.; Björnson, Kristofer; Black-Schaffer, Annica M.
2017-05-01
Majorana bound states (MBSs) are well established in the clean limit in chains of ferromagnetically aligned impurities deposited on conventional superconductors with finite spin-orbit coupling. Here we show that these MBSs are very robust against disorder. By performing self-consistent calculations we find that the MBSs are protected as long as the surrounding superconductor show no large signs of inhomogeneity. We also find that longer chains offer more stability against disorder for the MBSs, albeit the minigap decreases, as do increasing strengths of spin-orbit coupling and superconductivity.
Ferromagnets-induced splitting of molecular states of T-shaped double quantum dots
NASA Astrophysics Data System (ADS)
Wójcik, Krzysztof P.
2015-05-01
The exchange field for molecular states of double quantum dot, induced by two ferromagnets coupled to the device in T-shaped configuration, is defined and calculated. It is found, that in the regime of strong coupling between quantum dots, the dependence of the exchange field on this coupling becomes nontrivial. In particular, it changes the sign a few times to eventually vanish in the limit of infinite inter-dot coupling. The excitation energies of double quantum dot are calculated and the results used to predict the conditions for suppression of the two-stage Kondo effect in the considered nanostructure.
Effective Hamiltonian for surface states of topological insulator nanotubes.
Siu, Zhuo Bin; Tan, Seng Ghee; Jalil, Mansoor B A
2017-04-03
In this work we derive an effective Hamiltonian for the surface states of a hollow topological insulator (TI) nanotube with finite width walls. Unlike a solid TI cylinder, a TI nanotube possesses both an inner as well as outer surface on which the states localized at each surface are coupled together. The curvature along the circumference of the nanotube leads to a spatial variation of the spin orbit interaction field experienced by the charge carriers as well as an asymmetry between the inner and outer surfaces of the nanotube. Both of these features result in terms in the effective Hamiltonian for a TI nanotube absent in that of a flat TI thin film of the same thickness. We calculate the numerical values of the parameters for a Bi2Se3 nanotube as a function of the inner and outer radius, and show that the differing relative magnitudes between the parameters result in qualitatively differing behaviour for the eigenstates of tubes of different dimensions.
Effective Hamiltonian for surface states of topological insulator nanotubes
NASA Astrophysics Data System (ADS)
Siu, Zhuo Bin; Tan, Seng Ghee; Jalil, Mansoor B. A.
2017-04-01
In this work we derive an effective Hamiltonian for the surface states of a hollow topological insulator (TI) nanotube with finite width walls. Unlike a solid TI cylinder, a TI nanotube possesses both an inner as well as outer surface on which the states localized at each surface are coupled together. The curvature along the circumference of the nanotube leads to a spatial variation of the spin orbit interaction field experienced by the charge carriers as well as an asymmetry between the inner and outer surfaces of the nanotube. Both of these features result in terms in the effective Hamiltonian for a TI nanotube absent in that of a flat TI thin film of the same thickness. We calculate the numerical values of the parameters for a Bi2Se3 nanotube as a function of the inner and outer radius, and show that the differing relative magnitudes between the parameters result in qualitatively differing behaviour for the eigenstates of tubes of different dimensions.
Effective Hamiltonian for surface states of topological insulator nanotubes
Siu, Zhuo Bin; Tan, Seng Ghee; Jalil, Mansoor B. A.
2017-01-01
In this work we derive an effective Hamiltonian for the surface states of a hollow topological insulator (TI) nanotube with finite width walls. Unlike a solid TI cylinder, a TI nanotube possesses both an inner as well as outer surface on which the states localized at each surface are coupled together. The curvature along the circumference of the nanotube leads to a spatial variation of the spin orbit interaction field experienced by the charge carriers as well as an asymmetry between the inner and outer surfaces of the nanotube. Both of these features result in terms in the effective Hamiltonian for a TI nanotube absent in that of a flat TI thin film of the same thickness. We calculate the numerical values of the parameters for a Bi2Se3 nanotube as a function of the inner and outer radius, and show that the differing relative magnitudes between the parameters result in qualitatively differing behaviour for the eigenstates of tubes of different dimensions. PMID:28367970
Ferromagnetism in the Hubbard model: Spin waves and instability of the Nagaoka state
NASA Astrophysics Data System (ADS)
Wurth, P.; Müller-Hartmann, E.
We discuss two single spin flip variational wave functions describing spin wave excitations which were proposed earlier by Shastry, Krishnamurthy and Anderson (SKA) and by Basile and Elser (BE), respectively, in order to investigate the instability of the fully polarized ferromagnetic state (Nagaoka state) in the infinite U Hubbard model. We calculate the energy of these variational states for the square lattice and for multiple chains. At the zone boundary in the vicinity of the point (0, ) the spin wave energy is reduced substantially by the binding of the spin up hole to the flipped down spin. For the square lattice this leads to a critical hole density of cr = 0.407 for the SKA spin wave and of cr = 0.322 for the BE spin wave which implies remarkable improvements in comparison to the corresponding scattering states investigated previously.
Ando, Yuichiro; Hamasaki, Takahiro; Kurokawa, Takayuki; Ichiba, Kouki; Yang, Fan; Novak, Mario; Sasaki, Satoshi; Segawa, Kouji; Ando, Yoichi; Shiraishi, Masashi
2014-11-12
We detected the spin polarization due to charge flow in the spin nondegenerate surface state of a three-dimensional topological insulator by means of an all-electrical method. The charge current in the bulk-insulating topological insulator Bi1.5Sb0.5Te1.7Se1.3 (BSTS) was injected/extracted through a ferromagnetic electrode made of Ni80Fe20, and an unusual current-direction-dependent magnetoresistance gave evidence for the appearance of spin polarization, which leads to a spin-dependent resistance at the BSTS/Ni80Fe20 interface. In contrast, our control experiment on Bi2Se3 gave null result. These observations demonstrate the importance of the Fermi-level control for the electrical detection of the spin polarization in topological insulators.
NASA Astrophysics Data System (ADS)
Babu, S. Harinath; Kaleemulla, S.; Rao, N. Madhusudhana; Rao, G. Venugopal; Krishnamoorthi, C.
2016-11-01
Indium-tin-oxide (ITO) (In0.95Sn0.05)2O3 and Cr doped indium-tin-oxide (In0.90Sn0.05Cr0.05)2O3 nanoparticles were prepared using simple low cost solid state reaction method and characterized by different techniques to study their structural, optical and magnetic properties. Microstructures, surface morphology, crystallite size of the nanoparticles were studied using X-ray diffractometer (XRD), field emission scanning electron microscope (FE-SEM). From these methods it was found that the particles were about 45 nm. Chemical composition and valence states of the nanoparticles were studied using energy dispersive analysis of X-rays (EDAX) and X-ray photoelectron spectroscopy (XPS). From these techniques it was observed that the elements of indium, tin, chromium and oxygen were present in the system in appropriate ratios and they were in +3, +4, +3 and -2 oxidation states. Raman studies confirmed that the nanoparticle were free from unintentional impurities. Two broad emission peaks were observed at 330 nm and 460 nm when excited wavelength of 300 nm. Magnetic studies were carried out at 300 K and 100 K using vibrating sample magnetometer (VSM) and found that the ITO nanoparticles were ferromagnetic at 100 K and 300 K. Where-as the room temperature ferromagnetism completely disappeared in Cr doped ITO nanoparticles at 100 K and 300 K.
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.
Effects of Co doping on the metamagnetic states of the ferromagnetic fcc Fe-Co alloy.
Ortiz-Chi, Filiberto; Aguayo, Aarón; de Coss, Romeo
2013-01-16
The evolution of the metamagnetic states in the ferromagnetic face centered cubic (fcc) Fe(1-x)Co(x) alloy as a function of Co concentration has been studied by means of first-principles calculations. The ground state properties were obtained using the full-potential linear augmented plane wave method and the generalized gradient approximation for the exchange-correlation functional. The alloying was modeled using the virtual crystal approximation and the magnetic states were obtained from the calculations of the total energy as a function of the spin moment, using the fixed spin moment method. For ferromagnetic fcc Fe, the binding-energy curve shows metamagnetic behavior, with two minima corresponding to a small-volume, low-spin (LS) state and a large-volume, high-spin (HS) state, which are separated by a small energy (E(LS) ≲ E(HS)). The evolution of the magnetic moment, the exchange integral (J), and the binding-energy curve is analyzed in the whole range of Co concentrations (x). The magnetic moment corresponding to the HS state decreases monotonically from 2.6 μ(B)/atom in fcc Fe to 1.7 μ(B)/atom in fcc Co. In contrast, the exchange integral for the HS state shows a maximum at around x = 0.45. The thermal dependence of the lattice parameter is evaluated with a method based on statistical mechanics using the binding-energy curve as an effective potential. It is observed that the behavior of the lattice parameter with temperature is tuned by Co doping, from negative thermal expansion in fcc Fe to positive thermal expansion in fcc Co, through the modification of the energetics of the metamagnetic states.
Quantum filter of spin polarized states: Metal–dielectric–ferromagnetic/semiconductor device
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.
Steady-state and transient results on insulation materials
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.
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
Optical study of nonuniform quantum-Hall ferromagnetic states in bilayer and trilayer graphene
NASA Astrophysics Data System (ADS)
Barrette, Manuel; Côté, René
2015-03-01
The chiral two-dimensional electron gas in the N = 0 Landau level of a Bernal-stacked bilayer graphene is host to a variety of broken-symmetry ground states that can be described as layer, spin, or orbital quantum Hall ferromagnets (QHFs). At filling factors ν = 1 , 3 , an externally applied electric field between the two layers can induce a transition from uniform to nonuniform orbital QHF states with an helical or skyrmionic texture of electric dipoles. A similar skyrmionic texture can also arise in the N = 0 Landau level of an ABC-stacked trilayer graphene. In this talk, we discuss the optical properties of these textured ground states. We compute their electromagnetic absorption as well as the Kerr and Faraday rotations induced by their collective excitations and show that each textured phase has a distinct optical signature.
Interplay between chemical state, electric properties, and ferromagnetism in Fe-doped ZnO films
NASA Astrophysics Data System (ADS)
Chen, G.; Peng, J. J.; Song, C.; Zeng, F.; Pan, F.
2013-03-01
Valence state of Fe ions plays an important role in the physical properties of Fe doped ZnO films. Here, a series of Zn1-xFexO 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 Zn2+ site changes from Fe3+ to Fe2+ with the increase of Fe dopant concentration. We found enhanced piezoelectric and ferroelectric properties in Zn0.977Fe0.023O films with more Fe3+ due to the smaller Fe3+ ionic size in comparison with Zn2+ while the increase of Fe2+ concentration by a larger amount of Fe dopant results in the worse ferroelectric and piezoelectric performance. All Pt/Zn1-xFexO/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/Zn1-xFexO/Pt devices. The multifunctional properties of Fe-doped ZnO films are promising for communication systems and information storage devices.
NASA Astrophysics Data System (ADS)
Estes, William E.; Losee, D. Bruce; Hatfield, William E.
1980-01-01
Magnetic susceptibility measurements on powdered samples of bis(benzylammonium) tetrachlorocuprate (II), bis(phenethylammonium) tetrachlorocuprate(II), bis(3-phenyl-1-propylammonium) tetrachlorocuprate(II), and bis(benzylammonium) tetrabromocuprate(II) have shown that these four substances order ferromagnetically with Curie temperatures of 8.0±0.5, 9.0±0.2, 7.0±0.5, and 12.3±1.0 K, respectively. Fits of a series expansion for a two-dimensional lattice to the magnetic susceptibility data obtained from powdered samples in the paramagnetic region yielded exchange constants J for the chloride compounds in the range 16.7-18.8 K, and a value of 25.3 K for the bromide compound. For these fits the
Non-ferromagnetic overburden casing
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.
NASA Astrophysics Data System (ADS)
Liu, Zhen; Wei, Xinyuan; Wang, Jiajia; Pan, Hong; Ji, Fuhao; Ye, Mao; Yang, Zhongqin; Qiao, Shan
2015-09-01
The local atomic and electronic structures around the dopants in Cr-doped (BixSb1 -x )2Te3 are studied by x-ray absorption fine structure (XAFS) measurements and first-principles calculations. Both Cr and Bi are confirmed substituting Sb sites (CrSb and BiSb). The six nearest Te atoms around Cr move towards Cr and shorten the Cr-Te bond lengths to 2.76 Å and 2.77 Å for x =0.1 and x =0.2 , respectively. Importantly, we reveal the hybridization between the Sb/Te p states and Cr d states by the presence of a pre-edge peak at Cr K -absorption edge, which is also supported by our ab initio calculations. These findings provide important clues to understand the mechanism of ferromagnetic order in this system with quantum anomalous Hall effect.
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.
Hierarchy of Hofstadter states and replica quantum Hall ferromagnetism in graphene superlattices
NASA Astrophysics Data System (ADS)
Yu, G. L.; Gorbachev, R. V.; Tu, J. S.; Kretinin, A. V.; Cao, Y.; Jalil, R.; Withers, F.; Ponomarenko, L. A.; Piot, B. A.; Potemski, M.; Elias, D. C.; Chen, X.; Watanabe, K.; Taniguchi, T.; Grigorieva, I. V.; Novoselov, K. S.; Fal'Ko, V. I.; Geim, A. K.; Mishchenko, A.
2014-07-01
Self-similarity and fractals have fascinated researchers across various disciplines. In graphene placed on boron nitride and subjected to a magnetic field, self-similarity appears in the form of numerous replicas of the original Dirac spectrum, and their quantization gives rise to a fractal pattern of Landau levels, referred to as the Hofstadter butterfly. Here we employ capacitance spectroscopy to probe directly the density of states (DoS) and energy gaps in this spectrum. Without a magnetic field, replica spectra are seen as pronounced DoS minima surrounded by van Hove singularities. The Hofstadter butterfly shows up as recurring Landau fan diagrams in high fields. Electron-electron interactions add another twist to the self-similar behaviour. We observe suppression of quantum Hall ferromagnetism, a reverse Stoner transition at commensurable fluxes and additional ferromagnetism within replica spectra. The strength and variety of the interaction effects indicate a large playground to study many-body physics in fractal Dirac systems.
NASA Astrophysics Data System (ADS)
Huxley, Andrew D.
2015-07-01
The co-existence of superconductivity and ferromagnetism is of potential interest for spintronics and high magnetic field applications as well as a fascinating fundamental state of matter. The recent focus of research is on a family of ferromagnetic superconductors that are superconducting well below their Curie temperature, the first example of which was discovered in 2000. Although there is a 'standard' theoretical model for how magnetic pairing might bring about such a state, why it has only been seen in a few materials that at first sight appear to be very closely related has yet to be fully explained. This review covers the current state of knowledge of the magnetic and superconducting properties of these materials with emphasis on how they conform and differ from the behaviour expected from the 'standard' model and from each other.
Observation of Topological Links Associated with Hopf Insulators in a Solid-State Quantum Simulator
NASA Astrophysics Data System (ADS)
Yuan, X.-X.; He, L.; Wang, S.-T.; Deng, D.-L.; Wang, F.; Lian, W.-Q.; Wang, X.; Zhang, C.-H.; Zhang, H.-L.; Chang, X.-Y.; Duan, L.-M.
2017-06-01
Hopf insulators are intriguing three-dimensional topological insulators characterized by an integer topological invariant. They originate from the mathematical theory of Hopf fibration and epitomize the deep connection between knot theory and topological phases of matter, which distinguishes them from other classes of topological insulators. Here, we implement a model Hamiltonian for Hopf insulators in a solid-state quantum simulator and report the first experimental observation of their topological properties, including fascinating topological links associated with the Hopf fibration and the integer-valued topological invariant obtained from a direct tomographic measurement. Our observation of topological links and Hopf fibration in a quantum simulator opens the door to probe rich topological properties of Hopf insulators in experiments. The quantum simulation and probing methods are also applicable to the study of other intricate three-dimensional topological model Hamiltonians.
Ground state study of the thin ferromagnetic nano-islands for artificial spin ice arrays
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.
NASA Astrophysics Data System (ADS)
Swain, Rashmirekha; Sahu, Sivabrata; Rout, G. C.
2017-05-01
We report here a microscopic tight binding theoretical model study of ferromagnetism in graphene taking into account of substrate effect and Coulomb interaction in both the sub-lattices of the honeycomb lattice. The Coulomb interaction is treated here within mean-field approximation giving rise to the ferromagnetic magnetization under different electron occupancies of graphene. The temperature dependent ferromagnetic magnetization is calculated from the electron co-relations obtained from the electron Green's functions. It is observed that ferromagnetic gap displaces sudden change for different electron occupancies separating paramagnetic from the ferromagnetic phase. The effect of on-site Coulomb interaction energy, electron occupancy and temperature on ferromagnetic gap is investigated and is reported in this communication.
Evolution of ferromagnetic interactions from cluster spin glass state in Co-Ga alloy
NASA Astrophysics Data System (ADS)
Mohammad Yasin, Sk.; Saha, Ritwik; Srinivas, V.; Kasiviswanathan, S.; Nigam, A. K.
2016-11-01
Low temperature magnetic properties of binary CoxGa100-x (x=54-57) alloy have been investigated. Analysis of frequency dependence of ac susceptibility provided a conclusive evidence for the existence of cluster spin glass like behavior with the freezing temperature ~8, 14 K for x=54, 55.5 respectively. The parameters for conventional 'slowing down' of the spin dynamics have been extracted from the acs data, which confirm the presence of glassy phase. The magnitude of Mydosh parameter obtained from the fits is larger than that reported for typical canonical spin glasses and smaller than those for non-interacting ideal superparamagnetic systems but comparable to those of known cluster-glass systems. Memory phenomena using specific cooling protocols also support the spin-glass features in Co55.5Ga44.5 composition. Further the development of ferromagnetic clusters from the cluster spin glass state has been observed in x=57 composition.
Influence of the Sn oxidation state in ferromagnetic Sn-doped In2O3 nanowires
NASA Astrophysics Data System (ADS)
Maloney, Francis Scott; Wang, Wenyong
2016-12-01
Sn-doped indium oxide nanowires were grown using a vapor-liquid-solid technique (VLS). The Sn content of the nanowires was tunable based on the source powder ratios used in the VLS process. The oxidation state of the Sn ions was examined using x-ray photoelectron spectroscopy. It was found that Sn2+ was the dominant ionic species in samples over 6% (atomic percentage) Sn. The nanowires were found to be ferromagnetic at room temperature, and their saturation magnetization increased with increasing Sn concentration, which could be associated with the spin-splitting of a defect band that was encouraged by the imbalance of Sn2+ to Sn4+ species at high Sn concentrations.
Surface-State Spin Textures and Mirror Chern Numbers in Topological Kondo Insulators.
Legner, Markus; Rüegg, Andreas; Sigrist, Manfred
2015-10-09
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.
Mahanti, S D; Jha, Sudhanshu S
2007-12-01
We obtain the best upper bound for the ground-state energy of a system of chargeless fermions of mass m, spin s=1/2 , and magnetic moment mus[over ] as a function of its density in the fully spin-polarized Hartree-Fock determinantal state, specified by a prolate spheroidal plane-wave single-particle occupation function n_(k[over ]) , by minimizing the total energy E at each density with respect to the variational spheroidal deformation parameter beta(2),0< or =beta(2)< or =1 . We find that at high densities, this spheroidal ferromagnetic state is the most likely ground state of the system, but it is still unstable towards the infinite-density collapse. This optimized ferromagnetic state is shown to be a stable ground state of the dipolar system at high densities, if one has an additional repulsive short-range hardcore interaction of sufficient strength and nonvanishing range.
Schor; O'Carroll
2000-08-01
We obtain different properties of general d dimensional lattice ferromagnetic spin systems with nearest neighbor interactions in the high temperature region (beta<1). Each model is characterized by a single site a priori spin distribution, taken to be even. We state our results in terms of the parameter alpha=-3(2) where denotes the kth moment of the a priori distribution. Associated with the model is a lattice quantum field theory that is known to contain particles. We show that for alpha>0, beta small, there exists a bound state with mass below the two-particle threshold. For alpha<0, bound states do not exist. The existence of the bound state has implications on the decay of correlations, i.e., the four-point function decays at a slower rate than twice that of the two-point function. These results are obtained using a lattice version of the Bethe-Salpeter equation in the ladder approximation. The existence and nonexistence results generalize to N-component models with rotationally invariant a priori spin distributions.
Ferromagnetic superexchange in insulating Cr_{2}MoO_{6} by controlling orbital hybridization
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 Cr_{2}MoO_{6}. Despite the same crystal symmetry and similar bond-lengths and bond-angles to Cr_{2}TeO_{6}, Cr_{2}MoO_{6} possesses a magnetic structure of the Cr_{2}MoO_{6} type, different from that seen in Cr_{2}TeO_{6}. 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.
Interaction-induced insulating state in thick multilayer graphene
NASA Astrophysics Data System (ADS)
Nam, Youngwoo; Ki, Dong-Keun; Koshino, Mikito; McCann, Edward; Morpurgo, Alberto F.
2016-12-01
Close to charge neutrality, the low-energy properties of high-quality suspended devices based on atomically thin graphene layers are determined by electron-electron interactions. Bernal-stacked layers, in particular, have shown a remarkable even-odd effect with mono- and tri-layers remaining gapless conductors, and bi- and tetra-layers becoming gapped insulators. These observations—at odds with the established notion that (Bernal) trilayers and thicker multilayers are semi-metals—have resulted in the proposal of a physical scenario leading to a surprising prediction, namely that even-layered graphene multilayers remain insulating irrespective of their thickness. Here, we present data from two devices that conform ideally to this hypothesis, exhibiting the behavior expected for Bernal-stacked hexa- and octa-layer graphene. Despite their large thickness, these multilayers are insulating for carrier density |n| < 2-3 × 1010 cm-2, possess an energy gap of approximately 1.5 meV at charge neutrality—in virtually perfect agreement with what is observed in bi- and tetra-layer graphene—and exhibit the expected integer quantum Hall effect. These findings indicate the soundness of our basic insights on the effect of electron interactions in Bernal graphene multilayers, show that graphene multilayers exhibit unusual and interesting physics that remains to be understood, and pose ever more pressing questions as to the microscopic mechanisms behind the semimetallic behavior of bulk graphite.
NASA Astrophysics Data System (ADS)
Gu, Yingfei; Lee, Ching Hua; Wen, Xueda; Cho, Gil Young; Ryu, Shinsei; Qi, Xiao-Liang
2016-09-01
In this paper, we study (2 +1 ) -dimensional quantum anomalous Hall states, i.e., band insulators with quantized Hall conductance, using exact holographic mapping. Exact holographic mapping is an approach to holographic duality which maps the quantum anomalous Hall state to a different state living in (3 +1 ) -dimensional hyperbolic space. By studying topological response properties and the entanglement spectrum, we demonstrate that the holographic dual theory of a quantum anomalous Hall state is a (3 +1 ) -dimensional topological insulator. The dual description enables a characterization of topological properties of a system by the quantum entanglement between degrees of freedom at different length scales.
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.
Quantum Hall states stabilized in semi-magnetic bilayers of topological insulators
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
Persistent coherence and spin polarization of topological surface states on topological insulators
NASA Astrophysics Data System (ADS)
Pan, Z.-H.; Vescovo, E.; Fedorov, A. V.; Gu, G. D.; Valla, T.
2013-07-01
Gapless surface states on topological insulators are protected from elastic scattering on nonmagnetic impurities, which makes them promising candidates for low-power electronic applications. However, for widespread applications, these states should remain coherent and significantly spin polarized at ambient temperatures. Here, we studied the coherence and spin structure of the topological states on the surface of a model topological insulator, Bi2Se3, at elevated temperatures in spin- and angle-resolved photoemission spectroscopy. We found an extremely weak broadening and essentially no decay of spin polarization of the topological surface state up to room temperature. Our results demonstrate that the topological states on surfaces of topological insulators could serve as a basis for room-temperature electronic devices.
Quantum Hall states stabilized in semi-magnetic bilayers of topological insulators.
Yoshimi, R; Yasuda, K; Tsukazaki, A; Takahashi, K S; Nagaosa, N; Kawasaki, M; Tokura, Y
2015-10-26
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.
High spin polarization and the origin of unique ferromagnetic ground state in CuFeSb
Sirohi, Anshu; Saha, Preetha; Gayen, Sirshendu; Gaurav, Abhishek; Jyotsna, Shubhra; Sheet, Goutam; Singh, Chandan K.; Kabir, Mukul; Thakur, Gohil S.; Haque, Zeba; Gupta, L. C.; Ganguli, Ashok K.
2016-06-13
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.
2011-01-01
Developing Concepts for New Device Funcionalities Giti Khodaparast Virginia Poly Technique and State University (Va. Tech) JANUARY 2011 Final...effort in this area was made primarily on GaMnAs, other ferromagnetic III-Mn-V alloys have also been developed, including the narrow gap ferromagnetic
Edge states and integer quantum Hall effect in topological insulator thin films.
Zhang, Song-Bo; Lu, Hai-Zhou; Shen, Shun-Qing
2015-08-25
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.
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).
Two-dimensional lattice model for the surface states of topological insulators
NASA Astrophysics Data System (ADS)
Zhou, Yan-Feng; Jiang, Hua; Xie, X. C.; Sun, Qing-Feng
2017-06-01
The surface states in three-dimensional (3D) topological insulators can be described by a two-dimensional (2D) continuous Dirac Hamiltonian. However, there exists the fermion doubling problem when putting the continuous 2D Dirac equation into a lattice model. In this paper, we introduce a Wilson term with a zero bare mass into the 2D lattice model to overcome the difficulty. By comparing with a 3D Hamiltonian, we show that the modified 2D lattice model can faithfully describe the low-energy electrical and transport properties of surface states of 3D topological insulators. So this 2D lattice model provides a simple and cheap way to numerically simulate the surface states of 3D topological-insulator nanostructures. Based on the 2D lattice model, we also establish the wormhole effect in a topological-insulator nanowire by a magnetic field along the wire and show the surface states being robust against disorder. The proposed 2D lattice model can be extensively applied to study the various properties and effects, such as the transport properties, Hall effect, universal conductance fluctuations, localization effect, etc. So, it paves a way to study the surface states of the 3D topological insulators.
Wang, Ying; Luo, Guoyu; Liu, Junwei; Sankar, R; Wang, Nan-Lin; Chou, Fangcheng; Fu, Liang; Li, Zhiqiang
2017-08-28
Topological crystalline insulators possess metallic surface states protected by crystalline symmetry, which are a versatile platform for exploring topological phenomena and potential applications. However, progress in this field has been hindered by the challenge to probe optical and transport properties of the surface states owing to the presence of bulk carriers. Here, we report infrared reflectance measurements of a topological crystalline insulator, (001)-oriented Pb1-x Sn x Se in zero and high magnetic fields. We demonstrate that the far-infrared conductivity is unexpectedly dominated by the surface states as a result of their unique band structure and the consequent small infrared penetration depth. Moreover, our experiments yield a surface mobility of 40,000 cm(2) V(-1) s(-1), which is one of the highest reported values in topological materials, suggesting the viability of surface-dominated conduction in thin topological crystalline insulator crystals. These findings pave the way for exploring many exotic transport and optical phenomena and applications predicted for topological crystalline insulators.Probing optical and transport properties of the surface states in topological crystalline insulators remains a challenge. Here, Wang et al. demonstrate that the far-infrared conductivity of Pb1-x Sn x Se (x = 0.23-0.25) single crystals is dominated by the surface states where carriers show a high surface mobility of 40,000 cm(2) V(-1) s(-1).
NMR probe of metallic states in nanoscale topological insulators.
Koumoulis, Dimitrios; Chasapis, Thomas C; Taylor, Robert E; Lake, Michael P; King, Danny; Jarenwattananon, Nanette N; Fiete, Gregory A; Kanatzidis, Mercouri G; Bouchard, Louis-S
2013-01-11
A 125Te NMR study of bismuth telluride nanoparticles as a function of particle size revealed that the spin-lattice relaxation is enhanced below 33 nm, accompanied by a transition of NMR spectra from the single to the bimodal regime. The satellite peak features a negative Knight shift and higher relaxivity, consistent with core polarization from p-band carriers. Whereas nanocrystals follow a Korringa law in the range 140-420 K, micrometer particles do so only below 200 K. The results reveal increased metallicity of these nanoscale topological insulators in the limit of higher surface-to-volume ratios.
Evidence of spontaneous vortex ground state in an iron-based ferromagnetic superconductor
NASA Astrophysics Data System (ADS)
Jiao, Wen-He; Tao, Qian; Ren, Zhi; Liu, Yi; Cao, Guang-Han
2017-09-01
Spontaneous vortex phase (SVP) is an exotic quantum matter in which quantized superconducting vortices form in the absence of external magnetic field. Although being predicted theoretically nearly 40 years ago, its rigorous experimental verification still appears to be lacking. Here we present low-field magnetic measurements on single crystals of the iron-based ferromagnetic superconductor Eu(Fe0.91Rh0.09)2As2 which undergoes a superconducting transition at Tsc = 19.6 K followed by a magnetic transition at Tm = 16.8 K. We observe a characteristic first-order transition from a Meissner state within Tm < T < Tsc to an SVP below Tm, under a magnetic field approaching zero. Additional isothermal magnetization and ac magnetic susceptibility measurements at T ≪Tsc confirm that the system is intrinsically in a spontaneous-vortex ground state. The unambiguous demonstration of SVP in the title material lays a solid foundation for future imaging and spectroscopic studies on this intriguing quantum matter.
Nature of the insulating ground state of the 5d postperovskite CaIrO3
Kim, Sun -Woo; Liu, Chen; Kim, Hyun -Jung; ...
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
NASA Astrophysics Data System (ADS)
Mukherjee, Shantanu; Lee, Wei-Cheng
2015-12-01
The quasiparticle interferences (QPIs) of the featureless Mott insulators are investigated by a T -matrix formalism implemented with the dynamical mean field theory (T -DMFT). In the Mott insulating state, due to the singularity at zero frequency in the real part of the electron self-energy [Re Σ (ω )˜η /ω ] predicted by DMFT, where η can be considered as the "order parameter" for the Mott insulating state, QPIs are completely washed out at small bias voltages. However, a further analysis shows that Re Σ (ω ) serves as an energy-dependent chemical potential shift. As a result, the effective bias voltage seen by the system is e V'=e V -Re Σ (e V ) , which leads to a critical bias voltage e Vc˜√{η } satisfying e V'=0 if and only if η is nonzero. Consequently, the same QPI patterns produced by the noninteracting Fermi surfaces appear at this critical bias voltage e Vc in the Mott insulating state. We propose that this reentry of noninteracting QPI patterns at e Vc could serve as an experimental signature of the Mott insulating state, and the order parameter can be experimentally measured as η ˜(eVc) 2 .
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.
NASA Astrophysics Data System (ADS)
Morán, O.; Saldarriaga, W.; Baca, E.
2010-01-01
Current transport through thin antiferromagnetic (AF) barriers of the perovskite manganite La 1/3Ca 2/3MnO 3 (LCMO) was studied with respect to its dependence on temperature and voltage. Planar-type La 2/3Ca 1/3MnO 3(∼80 nm)/La 1/3Ca 2/3MnO 3(∼7 nm)/YBa 2Cu 3O 7-δ(∼100 nm) heterojunctions were used as basic structures. The current-voltage ( I- V) measurements were carried out on test junctions with a standard area of 20 × 40 μm 2 in a four-terminal configuration. In spite of the carefully controlled growth conditions, barriers with the same nominal thickness showed different electrical behavior varying from elastic tunneling to Mott variable range hopping (VRH) via localized states. Fitting the VRH model to the experimental data, allowed for estimating important physical parameters of the barrier as the density of states at the Fermi level N( EF) and with this the average distance between two localized states ℓ0. The different transport characteristics seem to be related to intrinsic difference in microstructure as the average surface roughness of the constituent layers may already be larger than the thickness of the barrier itself. Independent of the barrier quality, the active presence of the diamagnetic and ferromagnetic phases in the heterostructure was corroborated by transport measurements in magnetic fields and in-plane/out-of-plane magnetization hysteresis loops below the superconducting critical temperature, Tc (∼80 K). The values of the critical magnetic field Hc1 estimated from these experiments were in good accordance with those reported in the literature.
NASA Astrophysics Data System (ADS)
Sato, T.; Koswattage, K. R.; Nakayama, Y.; Ishii, H.
2017-03-01
Although the contact electrification of insulating polymers has been widely used in various technologies, the mechanism of electrification is still not well understood and several models have been proposed to explain the mechanism. Some of the models assume the existence of bandgap states that can store or release electrons to charge the polymer; however, the density of states in the bandgap region is not well examined. In this study, an approach to directly measure the density of state of insulating polymers using hν-dependent high-sensitivity ultraviolet photoemission spectroscopy is proposed. Demonstration of the approach to a representative insulating polymer, nylon-6,6, is reported with the estimation of the charge density and charge penetration depth as a function of the work function difference.
Ground state properties of anti-ferromagnetic spinor Bose gases in one dimension
NASA Astrophysics Data System (ADS)
Hao, Yajiang
2017-03-01
We investigate the ground state properties of anti-ferromagnetic spin-1 Bose gases in one dimensional harmonic potential from the weak repulsion regime to the strong repulsion regime. The Hamiltonian is diagonalized in the Hilbert space composed of the single particle wavefunctions and spin components. With the numerical diagonalization method, the density distributions, magnetization distribution, one body density matrix, and momentum distribution for each component are obtained. It is shown that the spinor Bose gases of different magnetization exhibit the same total density profiles in the full interaction regime, which evolve from the single peak structure embodying the properties of Bose gases to the fermionized shell structure of spin-polarized fermions. But each component displays different density profiles, and magnetic domains emerge in the strong interaction limit for M = 0.25. In the strong interaction limit, one body density matrix and the momentum distributions exhibit the same behaviour as those of spin-polarized fermions. The fermionization of momentum distribution takes place, in contrast to the δ-function-like distribution of single component Bose gases in the full interaction regime.
Steady-State Thermal Performance Evaluation of Steel-Framed Wall Assembly with Local Foam Insulation
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.
Origin of Transitions between Metallic and Insulating States in Simple Metals
Naumov, Ivan I.; Hemley, Russell J.
2015-04-17
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 not previously recognized. 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 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 novel behavior such as phases having three-dimensional Dirac-like points. 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).
Origin of Transitions between Metallic and Insulating States in Simple Metals
Naumov, Ivan I.; Hemley, Russell J.
2015-04-17
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 not previously recognized. 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 multi-center chemical bondingmore » 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 novel behavior such as phases having three-dimensional Dirac-like points. 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).« less
Origin of Transitions between Metallic and Insulating States in Simple Metals.
Naumov, Ivan I; Hemley, Russell J
2015-04-17
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).
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.
Ferromagnetism beyond Lieb's theorem
NASA Astrophysics Data System (ADS)
Costa, Natanael C.; Mendes-Santos, Tiago; Paiva, Thereza; Santos, Raimundo R. dos; Scalettar, Richard T.
2016-10-01
The noninteracting electronic structures of tight-binding models on bipartite lattices with unequal numbers of sites in the two sublattices have a number of unique features, including the presence of spatially localized eigenstates and flat bands. When a uniform on-site Hubbard interaction U is turned on, Lieb proved rigorously that at half-filling (ρ =1 ) the ground state has a nonzero spin. In this paper we consider a "CuO2 lattice" (also known as "Lieb lattice," or as a decorated square lattice), in which "d orbitals" occupy the vertices of the squares, while "p orbitals" lie halfway between two d orbitals; both d and p orbitals can accommodate only up to two electrons. We use exact determinant quantum Monte Carlo (DQMC) simulations to quantify the nature of magnetic order through the behavior of correlation functions and sublattice magnetizations in the different orbitals as a function of U and temperature; we have also calculated the projected density of states, and the compressibility. We study both the homogeneous (H) case, Ud=Up , originally considered by Lieb, and the inhomogeneous (IH) case, Ud≠Up . For the H case at half-filling, we found that the global magnetization rises sharply at weak coupling, and then stabilizes towards the strong-coupling (Heisenberg) value, as a result of the interplay between the ferromagnetism of like sites and the antiferromagnetism between unlike sites; we verified that the system is an insulator for all U . For the IH system at half-filling, we argue that the case Up≠Ud falls under Lieb's theorem, provided they are positive definite, so we used DQMC to probe the cases Up=0 ,Ud=U and Up=U ,Ud=0 . We found that the different environments of d and p sites lead to a ferromagnetic insulator when Ud=0 ; by contrast, Up=0 leads to to a metal without any magnetic ordering. In addition, we have also established that at density ρ =1 /3 , strong antiferromagnetic correlations set in, caused by the presence of one fermion on each
Wang, Aizhu; Zhang, Xiaoming; Feng, Yuanping; Zhao, Mingwen
2017-08-17
Two-dimensional metal-organic frameworks (2D-MOFs) with exotic electronic structures are drawing increasing attention. Here, using first-principles calculations, we demonstrate a spin-gapless MOF, namely, Mn2C6S12, with the coexistence of a spin-polarized Dirac cone and parabolic degenerate points. The Curie temperature evaluated from Monte Carlo simulations implies Mn2C6S12 possessing stable ferromagnetism at room temperature. Taking the spin-orbit coupling into account, the Dirac cone is gapped and the degenerate points are lifted, giving rise to multiple topologically nontrivial states with nonzero Chern number, which imply the possibility of Mn2C6S12 to be a Chern insulator and a Chern half-metal. Our results offer versatile platforms for achieving spin filtering or a quantum anomalous Hall effect with promising application in spintronics devices.
Wang, Aizhu; Zhang, Xiaoming; Feng, Yuan Ping; Zhao, Mingwen
2017-07-30
Two-dimensional metal-organic frameworks (2D-MOFs) with exotic electronic structures are drawing increasing attention. Here, using first-principles calculations, we demonstrate a spin-gapless MOF, namely Mn2C6S12 with the coexistence of spin-polarized Dirac cone and parabolic degenerate points. The Curie temperature evaluated from Monte Carlo simulations implies Mn2C6S12 possessing stable ferromagnetism at room temperature. Taking the spin-orbit coupling into account, the Dirac cone is gaped and the degenerate points are lifted, giving rise to multiple topologically nontrivial states with nonzero Chern number, which imply the possibility of Mn2C6S12 to be a Chern insulator and a Chern half-metal. Our results offer versatile platforms for achieving spin filtering or quantum anomalous Hall effect with promising application in spintronics devices.
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.
Surface plasmon resonance phenomenon of the insulating state polyaniline
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.
NASA Astrophysics Data System (ADS)
Babu, S. Harinath; Kaleemulla, S.; Rao, N. Madhusudhana; Krishnamoorthi, C.
2016-11-01
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.
NASA Astrophysics Data System (ADS)
Singh, Kirandeep; Kaur, Davinder
2017-07-01
This study illustrates the approach to obtain four logic states of ferromagnetic shape memory alloy based multiferroic tunnel junction (MFTJ). In order to achieve giant tunneling electroresistance (TER) and tunneling magnetoresistance (TMR), Ni-Mn-In and Ni-Mn-Sb layers were chosen as electrodes, as well as the concept of the composite barrier was adopted using the STO/PZT [SrTiO3, Strontium Titanate/PbZr0.52Ti0.48O3 (Lead Zirconate Titanate)] (dielectric/ferroelectric) barrier layer. Equated to MFTJ with a single PZT barrier, the introduction of a STO paraelectric (dielectric) barrier is shown to be effective in improving both the TER and TMR of the MFTJs. Particularly, the TER ratio is greatly enhanced by 168% {" separators="|relative TER change: (ΔT/E R T E R =T/ER 2-TE R 1 TE R 1 ×100 }). This is described in terms of the increased asymmetry in the electrostatic modulation on the barrier potential profile with respect to the Ferroelectric (FE) polarization direction. We show that due to the coupling between FE polarization and magnetization at the junction between the barrier and the electrode of a MFTJ, the spin polarization of the tunneling electrons can be reversibly and remanently flipped by switching the FE polarization of the barrier. In addition to the analysis of memory function, the exchange bias phenomena are also studied. A negative exchange bias field of "HEB" ˜-98 Oe occurred at 300 K in these bilayers.
NASA Astrophysics Data System (ADS)
Huang, Huaqing; Wang, Zhaoyou; Luo, Nannan; Liu, Zhirong; Lü, Rong; Wu, Jian; Duan, Wenhui
2015-08-01
We theoretically investigate the electronic properties of the interface between quantum spin Hall (QSH) and quantum anomalous Hall (QAH) insulators. A robust chiral gapless state, which substantially differs from edge states of QSH or QAH insulators, is predicted at the QSH/QAH interface using an effective Hamiltonian model. We systematically reveal distinctive properties of interface states between QSH and single-valley QAH, multivalley high-Chern-number QAH and valley-polarized QAH insulators based on tight-binding models using the interface Green's function method. As an example, first-principles calculations are conducted for the interface states between fully and semihydrogenated bismuth (111) thin films, verifying the existence of interface states in realistic material systems. Due to the physically protected junction structure, the interface state is expected to be more stable and insensitive than topological boundary states against edge defects and chemical decoration. Hence our results of the interface states provide a promising route towards enhancing the performance and stability of low-dissipation electronics in real environment.
Robust topological surface state in Kondo insulator SmB{sub 6} thin films
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.
NASA Astrophysics Data System (ADS)
Lingos, P. C.; Patz, A.; Li, T.; Barmparis, G. D.; Keliri, A.; Kapetanakis, M. D.; Li, L.; Yan, J.; Wang, J.; Perakis, I. E.
2017-06-01
We describe a mechanism for insulator-to-metal transition triggered by spin canting following femtosecond laser excitation of insulating antiferromagnetic (AFM) states of colossal magnetoresistive (CMR) manganites. We show that photoexcitation of composite fermion quasiparticles dressed by spin fluctuations results in the population of a broad metallic conduction band due to canting of the AFM background spins via strong electron-spin local correlation. By inducing spin canting, photoexcitation can increase the quasiparticle energy dispersion and quench the charge excitation energy gap. This increases the critical Jahn-Teller (JT) lattice displacement required to maintain an insulating state. We present femtosecond-resolved pump-probe measurements showing biexponential relaxation of the differential reflectivity below the AFM transition temperature. We observe a nonlinear dependence of the ratio of the femtosecond and picosecond relaxation component amplitudes at the same pump fluence threshold where we observe femtosecond magnetization photoexcitation. We attribute this correlation between nonlinear femtosecond spin and charge dynamics to spin/charge/lattice coupling and population inversion between the polaronic majority carriers and metallic quasielectron minority carriers as the lattice displacement becomes smaller than the critical value required to maintain an insulating state following laser-induced spin canting.
NASA Astrophysics Data System (ADS)
Cedergren, K.; Kafanov, S.; Smirr, J.-L.; Cole, J. H.; Duty, T.
2015-09-01
We have made a systematic investigation of charge transport in one-dimensional chains of Josephson junctions where the characteristic Josephson energy is much less than the single-junction Cooper-pair charging energy, EJ≪EC P . Such chains are deep in the insulating state, where superconducting phase coherence across the chain is absent, and a voltage threshold for conduction is observed at the lowest temperatures. We find that Cooper-pair tunneling in such chains is completely suppressed. Instead, charge transport is dominated by tunneling of single electrons, which is very sensitive to the presence of BCS quasiparticles on the superconducting islands of the chain. Consequently, we observe a strong parity effect, where the threshold voltage vanishes sharply at a characteristic parity temperature T*, which is significantly lower than the critical temperature Tc. A measurable and thermally activated zero-bias conductance appears above T*, with an activation energy equal to the superconducting gap, confirming the role of thermally excited quasiparticles. Conduction below T* and above the voltage threshold occurs via injection of single electrons/holes into the Cooper-pair insulator, forming a nonequilibrium steady state with a significantly enhanced effective temperature. Our results explicitly show that single-electron transport dominates deep in the insulating state of Josephson junction arrays. This conduction process has mostly been ignored in previous studies of both superconducting junction arrays and granular superconducting films below the superconductor-insulator quantum phase transition.
Edge states and integer quantum Hall effect in topological insulator thin films
NASA Astrophysics Data System (ADS)
Zhang, Song-Bo; Lu, Hai-Zhou; Shen, Shun-Qing
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. In this report, I will talk about the Landau levels and edge states of surface Dirac fermions in topological insulators under a 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 as a result of the interplay of magnetic field and structure inversion asymmetry. We also reveal that the edge states exist only for the integer Hall conductance while no edge-state solution can be found for the ''half-integer'' Hall conductance. The addition of top and bottom surface Dirac fermions always form well-defined edge states, and gives an integer quantum Hall effect. 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.
Skyrmion-induced bound states on the surface of three-dimensional topological insulators
Andrikopoulos, Dimitrios De Boeck, Jo; Sorée, Bart
2016-05-21
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 N{sub Sk}. The probability densities of the bound electrons are also derived where it is shown that they are localized within the skyrmion region.
Effect of Dielectric Materials on the Topological Insulator Bi2 Se 3 Surface States
NASA Astrophysics Data System (ADS)
Chang, Jiwon; Register, Leonard; Banerjee, Sanjay; Sahu, Bhagawan
2011-03-01
We study the effects of crystalline dielectric materials on the electronic surface states of a strong topological band insulator (TI) Bi 2 Se 3 using a density functional based electronic structure method [1]. We will discuss the sensitivity of Dirac point degeneracy and linear band dispersion of the TI with respect to different dielectric surface terminations as well as different relative atom positions of the dielectric and the TI. Both passivated and non-passivated substrate surfaces will be considered. Two representative dielectrics Si O2 and boron nitride will be chosen to understand the physics of interplay of interface potential, linear band dispersion and the chemical environments of the TI surface states. Our findings have implications in interpreting experiments and designing novel nanoelectronics device concepts based on TIs. ``Intrinsic and extrinsic perturbations on the surface states of topological insulator Bi 2 Se 3 ,'' J. Chang, P. Jadaun, L. F. Register, S. K. Banerjee and B. Sahu (In preparation)
Topological origin of edge states in two-dimensional inversion-symmetric insulators and semimetals
NASA Astrophysics Data System (ADS)
van Miert, Guido; Ortix, Carmine; Morais Smith, Cristiane
2017-03-01
Symmetries play an essential role in identifying and characterizing topological states of matter. Here, we classify topologically two-dimensional (2D) insulators and semimetals with vanishing spin-orbit coupling using time-reversal ({ T }) and inversion ({ I }) symmetry. This allows us to link the presence of edge states in { I } and { T } symmetric 2D insulators, which are topologically trivial according to the Altland-Zirnbauer table, to a {{{Z}}}2 topological invariant. This invariant is directly related to the quantization of the Zak phase. It also predicts the generic presence of edge states in Dirac semimetals, in the absence of chiral symmetry. We then apply our findings to bilayer black phosphorus and show the occurrence of a gate-induced topological phase transition, where the {{{Z}}}2 invariant changes.
Quantum Oscillations and Hall Anomaly of Surface States in the Topological Insulator Bi2Te3
NASA Astrophysics Data System (ADS)
Qu, Dong-Xia; Hor, Y. S.; Xiong, Jun; Cava, R. J.; Ong, N. P.
2010-08-01
Topological insulators are insulating materials that display massless, Dirac-like surface states in which the electrons have only one spin degree of freedom on each surface. These states have been imaged by photoemission, but little information on their transport parameters, for example, mobility, is available. We report the observation of Shubnikov-de Haas oscillations arising from the surface states in nonmetallic crystals of Bi2Te3. In addition, we uncovered a Hall anomaly in weak fields, which enables the surface current to be seen directly. Both experiments yield a surface mobility (9000 to 10,000 centimeter2 per volt-second) that is substantially higher than in the bulk. The Fermi velocity of 4 × 105 meters per second obtained from these transport experiments agrees with angle-resolved photoemission experiments.
Robust spin-polarized midgap states at step edges of topological crystalline insulators
NASA Astrophysics Data System (ADS)
Sessi, Paolo; Di Sante, Domenico; Szczerbakow, Andrzej; Glott, Florian; Wilfert, Stefan; Schmidt, Henrik; Bathon, Thomas; Dziawa, Piotr; Greiter, Martin; Neupert, Titus; Sangiovanni, Giorgio; Story, Tomasz; Thomale, Ronny; Bode, Matthias
2016-12-01
Topological crystalline insulators are materials in which the crystalline symmetry leads to topologically protected surface states with a chiral spin texture, rendering them potential candidates for spintronics applications. Using scanning tunneling spectroscopy, we uncover the existence of one-dimensional (1D) midgap states at odd-atomic surface step edges of the three-dimensional topological crystalline insulator (Pb,Sn)Se. A minimal toy model and realistic tight-binding calculations identify them as spin-polarized flat bands connecting two Dirac points. This nontrivial origin provides the 1D midgap states with inherent stability and protects them from backscattering. We experimentally show that this stability results in a striking robustness to defects, strong magnetic fields, and elevated temperature.
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.
Spin-polarized surface state transport in a topological Kondo insulator SmB6 nanowire
NASA Astrophysics Data System (ADS)
Kong, Lingjian; Zhou, Yong; Liu, Song; Lin, Zhu; Zhang, Liang; Lin, Fang; Tang, Dongsheng; Wu, Han-Chun; Liu, Junfeng; Lu, Hai-Zhou; Zhu, Rui; Xu, Jun; Liao, Zhi-Min; Yu, Dapeng
2017-06-01
SmB6 , as a topological Kondo insulator, has spin-momentum-locked surface states and fully insulating bulk, which presents promising spintronic applications. Here, we report on the magnetotransport properties of individual SmB6 nanowires. With decreasing temperature below 10 K, the surface states dominate the transport behavior as reflected by the resistance saturation. At 1.5 K, a transition from negative to positive magnetoresistance occurs with gradually increasing the bias current. The nonlocal measurements indicate that the surface state transport is spin polarized, and the spin diffusion length is as long as 0.5 μm. Bias-current-modulated two-channel transport is employed to explain the observed sign reversal of the magnetoresistance.
Robust spin-polarized midgap states at step edges of topological crystalline insulators.
Sessi, Paolo; Di Sante, Domenico; Szczerbakow, Andrzej; Glott, Florian; Wilfert, Stefan; Schmidt, Henrik; Bathon, Thomas; Dziawa, Piotr; Greiter, Martin; Neupert, Titus; Sangiovanni, Giorgio; Story, Tomasz; Thomale, Ronny; Bode, Matthias
2016-12-09
Topological crystalline insulators are materials in which the crystalline symmetry leads to topologically protected surface states with a chiral spin texture, rendering them potential candidates for spintronics applications. Using scanning tunneling spectroscopy, we uncover the existence of one-dimensional (1D) midgap states at odd-atomic surface step edges of the three-dimensional topological crystalline insulator (Pb,Sn)Se. A minimal toy model and realistic tight-binding calculations identify them as spin-polarized flat bands connecting two Dirac points. This nontrivial origin provides the 1D midgap states with inherent stability and protects them from backscattering. We experimentally show that this stability results in a striking robustness to defects, strong magnetic fields, and elevated temperature. Copyright © 2016, American Association for the Advancement of Science.
Magnetic states, correlation effects and metal-insulator transition in FCC lattice
NASA Astrophysics Data System (ADS)
Timirgazin, M. A.; Igoshev, P. A.; Arzhnikov, A. K.; Irkhin, V. Yu
2016-12-01
The ground-state magnetic phase diagram (including collinear and spiral states) of the single-band Hubbard model for the face-centered cubic lattice and related metal-insulator transition (MIT) are investigated within the slave-boson approach by Kotliar and Ruckenstein. The correlation-induced electron spectrum narrowing and a comparison with a generalized Hartree-Fock approximation allow one to estimate the strength of correlation effects. This, as well as the MIT scenario, depends dramatically on the ratio of the next-nearest and nearest electron hopping integrals {{t}\\prime}/t . In contrast with metallic state, possessing substantial band narrowing, insulator one is only weakly correlated. The magnetic (Slater) scenario of MIT is found to be superior over the Mott one. Unlike simple and body-centered cubic lattices, MIT is the first order transition (discontinuous) for most {{t}\\prime}/t . The insulator state is type-II or type-III antiferromagnet, and the metallic state is spin-spiral, collinear antiferromagnet or paramagnet depending on {{t}\\prime}/t . The picture of magnetic ordering is compared with that in the standard localized-electron (Heisenberg) model.
Magnetic states, correlation effects and metal-insulator transition in FCC lattice.
Timirgazin, M A; Igoshev, P A; Arzhnikov, A K; Yu Irkhin, V
2016-12-21
The ground-state magnetic phase diagram (including collinear and spiral states) of the single-band Hubbard model for the face-centered cubic lattice and related metal-insulator transition (MIT) are investigated within the slave-boson approach by Kotliar and Ruckenstein. The correlation-induced electron spectrum narrowing and a comparison with a generalized Hartree-Fock approximation allow one to estimate the strength of correlation effects. This, as well as the MIT scenario, depends dramatically on the ratio of the next-nearest and nearest electron hopping integrals [Formula: see text]. In contrast with metallic state, possessing substantial band narrowing, insulator one is only weakly correlated. The magnetic (Slater) scenario of MIT is found to be superior over the Mott one. Unlike simple and body-centered cubic lattices, MIT is the first order transition (discontinuous) for most [Formula: see text]. The insulator state is type-II or type-III antiferromagnet, and the metallic state is spin-spiral, collinear antiferromagnet or paramagnet depending on [Formula: see text]. The picture of magnetic ordering is compared with that in the standard localized-electron (Heisenberg) model.
NASA Astrophysics Data System (ADS)
Yoshida, Tsuneya; Kawakami, Norio
2016-08-01
We study a bilayer Kane-Mele-Hubbard model with lattice distortion and interlayer spin exchange interaction under cylinder geometry. Our analysis based on real-space dynamical mean field theory with continuous-time quantum Monte Carlo demonstrates the emergence of a topological edge Mott insulating (TEMI) state which hosts gapless edge modes only in collective spin excitations. This is confirmed by the numerical calculations at finite temperatures for the spin-Hall conductivity and the single-particle excitation spectrum; the spin-Hall conductivity is almost quantized, σspinx y˜2 (e /2 π ) , predicting gapless edge modes carrying the spin current, while the helical edge modes in the single-particle spectrum are gapped out with respecting symmetry. It is clarified how the TEMI state evolves from the ordinary spin-Hall insulating state with increasing the Hubbard interaction at a given temperature and then undergoes a phase transition to a trivial Mott insulating state. With a bosonization approach at zero temperature, we further address which collective modes host gapless edge modes in the TEMI state.
Masai, Hiroshi; Terao, Jun; Makuta, Satoshi; Tachibana, Yasuhiro; Fujihara, Tetsuaki; Tsuji, Yasushi
2014-10-22
Controlling the thermal fluctuations and molecular environment of a phosphorescent polymer backbone is vital to enhancing its phosphorescence intensity in the solid state. Here, we demonstrate enhanced phosphorescence control through a systematic investigation of cyclodextrin-based insulated platinum-acetylide polymers with well-defined coverage areas. Modification of the coverage areas revealed two unprecedented effects of macrocyclic insulation on phosphorescence behavior. First, the insulation of particular areas suppresses the thermal relaxation processes of the triplet species because of the restriction of structural fluctuations. Cyclic insulation fixes a polymer chain and concomitantly enhances the phosphorescence intensity in both the solution and solid states. Second, complete three-dimensional insulation protects the polymer from interactions with other platinum and acetylide units, and even oxygen molecules. Notably, these polymers display identical phosphorescence behaviors in both the solution and solid states, essentially achieving "unimolecular phosphorescence."
Non-diffracting states in one-dimensional Floquet photonic topological insulators
NASA Astrophysics Data System (ADS)
Bellec, M.; Michel, C.; Zhang, H.; Tzortzakis, S.; Delplace, P.
2017-07-01
One-dimensional laser-written modulated photonic lattices are known to be particularly suitable for diffraction management purposes. Here, we address the connection between discrete non-diffracting states and topological properties in such devices through the experimental observation and identification of three classes of non-diffracting states. The first one corresponds to topologically protected edge states, recently predicted in Floquet topological insulators, while the second and the third are both bulk modes. One of them testifies a topological transition, although presenting topological features different from those of the edge states, while the other one results from specific band structure engineering.
Room-temperature dilute ferromagnetic dislocations in S r1 -xM nxTi O3 -δ
NASA Astrophysics Data System (ADS)
Ishikawa, Ryo; Shimbo, Yoichi; Sugiyama, Issei; Lugg, Nathan R.; Shibata, Naoya; Ikuhara, Yuichi
2017-07-01
Room-temperature dilute ferromagnetism has been reported for many semiconducting or insulating materials, which are usually in the forms of bulk or thin film. Here, we successfully fabricated dilute ferromagnetic nanowires by using dislocations—one-dimensional lattice defects—embedded between optically transparent, nonmagnetic SrTi O3 single crystals. At the dislocation cores, we have both locally codoped magnetic M n2 + ions and electron donors. The structure, chemistry, and ferromagnetism of dislocations were studied by atomic-resolution scanning transmission electron microscopy combined with magnetic force microscopy. We discuss the origin of dilute ferromagnetism at the dislocations in terms of the percolation of bound magnetic polarons along the dislocation cores, where antiferromagnetic coupling between the high spin state of M n2 + ions and electron donors leads to the long-range Mn-Mn ferromagnetic exchange interaction.
Tunneling Planar Hall Effect in Topological Insulators: Spin Valves and Amplifiers
NASA Astrophysics Data System (ADS)
Scharf, Benedikt; Matos-Abiague, Alex; Han, Jong E.; Hankiewicz, Ewelina M.; Žutić, Igor
2016-10-01
We investigate tunneling across a single ferromagnetic barrier on the surface of a three-dimensional topological insulator. In the presence of a magnetization component along the bias direction, a tunneling planar Hall conductance (TPHC), transverse to the applied bias, develops. Electrostatic control of the barrier enables a giant Hall angle, with the TPHC exceeding the longitudinal tunneling conductance. By changing the in-plane magnetization direction, it is possible to change the sign of both the longitudinal and transverse differential conductance without opening a gap in the topological surface state. The transport in a topological-insulator-ferromagnet junction can, thus, be drastically altered from a simple spin valve to an amplifier.
Tunneling Planar Hall Effect in Topological Insulators: Spin Valves and Amplifiers.
Scharf, Benedikt; Matos-Abiague, Alex; Han, Jong E; Hankiewicz, Ewelina M; Žutić, Igor
2016-10-14
We investigate tunneling across a single ferromagnetic barrier on the surface of a three-dimensional topological insulator. In the presence of a magnetization component along the bias direction, a tunneling planar Hall conductance (TPHC), transverse to the applied bias, develops. Electrostatic control of the barrier enables a giant Hall angle, with the TPHC exceeding the longitudinal tunneling conductance. By changing the in-plane magnetization direction, it is possible to change the sign of both the longitudinal and transverse differential conductance without opening a gap in the topological surface state. The transport in a topological-insulator-ferromagnet junction can, thus, be drastically altered from a simple spin valve to an amplifier.
NASA Astrophysics Data System (ADS)
Kokado, Satoshi; Sakuraba, Yuya; Tsunoda, Masakiyo
2016-10-01
We derive a simple relational expression between the spin polarization ratio of resistivity, Pρ, and the anisotropic magnetoresistance ratio Δρ/ρ, and that between the spin polarization ratio of the density of states at the Fermi energy, PDOS, and Δρ/ρ for nearly half-metallic ferromagnets. We find that Pρ and PDOS increase with increasing |Δρ/ρ| from 0 to a maximum value. In addition, we roughly estimate Pρ and PDOS for a Co2FeGa0.5Ge0.5 Heusler alloy by substituting its experimentally observed Δρ/ρ into the respective expressions.
Superferromagnetic domain state of a discontinuous metal insulator multilayer
Bedanta, S.; Petracic, O.; Kleemann, W.; Kentzinger, E.; Ruecker, U.; Paul, A.; Brueckel, Th.; Cardoso, S.; Freitas, P.P.
2005-07-01
Polarized neutron reflectivity (PNR) and magnetometry studies have been performed on the granular multilayer [Co{sub 80}Fe{sub 20}(1.3 nm)/Al{sub 2}O{sub 3}(3 nm)]{sub 10}. Due to strong interparticle interactions, a collective superferromagnetic state is encountered. Cole-Cole plots drawn from the complex ac susceptibility are measured as functions of frequency, temperature, and field amplitudes that hint at the relaxation, creep, sliding, and switching regimes of pinned domain walls that are in close agreement with results obtained from simulations. Very slow switching with exponential relaxation under near-coercive fields is confirmed by PNR measurements. The complete absence of spin-flip scattering confirms that the magnetization reversal is achieved merely by domain nucleation and growth.
Anomalous Aharonov-Bohm conductance oscillations from topological insulator surface states.
Zhang, Yi; Vishwanath, Ashvin
2010-11-12
We study Aharonov-Bohm (AB) conductance oscillations arising from the surface states of a topological insulator nanowire, when a magnetic field is applied along its length. With strong surface disorder, these oscillations are predicted to have a component with anomalous period Φ(0)=hc/e, twice the conventional period. The conductance maxima are achieved at odd multiples of 1/2Φ(0), implying that a π AB phase for electrons strengthens the metallic nature of surface states. This effect is special to topological insulators, and serves as a defining transport property. A key ingredient, the surface curvature induced Berry phase, is emphasized here. We discuss similarities and differences from recent experiments on Bi2Se3 nanoribbons, and optimal conditions for observing this effect.
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 .
Gaudet, J.; Ross, K. A.; Kermarrec, E.; ...
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
Quantum spin fluctuations in the bulk insulating state of pure and Fe-doped SmB6
NASA Astrophysics Data System (ADS)
Akintola, K.; Pal, A.; Potma, M.; Saha, S. R.; Wang, X. F.; Paglione, J.; Sonier, J. E.
2017-06-01
The intermediate-valence compound SmB6 is a well-known Kondo insulator, in which the hybridization of itinerant 5 d electrons with localized 4 f electrons leads to a transition from metallic to insulating behavior at low temperatures. Recent studies suggest that SmB6 is a topological insulator, with topological metallic surface states emerging from a fully insulating hybridized bulk band structure. Here, we locally probe the bulk magnetic properties of pure and 0.5 % Fe-doped SmB6 by muon spin rotation/relaxation methods. Below 6 K, the Fe impurity induces simultaneous changes in the bulk local magnetism and the electrical conductivity. In the low-T insulating bulk state we observe a temperature-independent dynamic relaxation rate indicative of low-lying magnetic excitations driven primarily by quantum fluctuations.
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
2012-03-06
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, Bi(2)Se(3) and Bi(2)Te(3), 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.
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
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.
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.
Spin-patterned plasmonics: towards optical access to topological-insulator surface states.
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.
Insulating state in tetralayers reveals an even-odd interaction effect in multilayer graphene
NASA Astrophysics Data System (ADS)
Grushina, Anya L.; Ki, Dong-Keun; Koshino, Mikito; Nicolet, Aurelien A. L.; Faugeras, Clément; McCann, Edward; Potemski, Marek; Morpurgo, Alberto F.
2015-03-01
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.
Zeljkovic, Ilija; Walkup, Daniel; Assaf, Badih A; Scipioni, Kane L; Sankar, R; Chou, Fangcheng; Madhavan, Vidya
2015-10-01
The unique crystalline protection of the surface states in topological crystalline insulators has led to a series of predictions of strain-generated phenomena, from the appearance of pseudo-magnetic fields and helical flat bands to the tunability of Dirac surface states by strain that may be used to construct 'straintronic' nanoswitches. However, the practical realization of this exotic phenomenology via strain engineering is experimentally challenging and is yet to be achieved. Here, we have designed an experiment to not only generate and measure strain locally, but also to directly measure the resulting effects on Dirac surface states. We grew heteroepitaxial thin films of topological crystalline insulator SnTe in situ and measured them using high-resolution scanning tunnelling microscopy to determine picoscale changes in the atomic positions, which reveal regions of both tensile and compressive strain. Simultaneous Fourier-transform scanning tunnelling spectroscopy was then used to determine the effects of strain on the Dirac electrons. We find that strain continuously tunes the momentum space position of the Dirac points, consistent with theoretical predictions. Our work demonstrates the fundamental mechanism necessary for using topological crystalline insulators in strain-based applications.
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.
Dirac cones in the gapless interface states between two topological insulators
NASA Astrophysics Data System (ADS)
Takahashi, Ryuji; Murakami, Shuichi
2012-02-01
When two topological insulators are attached together, the states on the interface become gapped due to the hybridization between the surface states. We have shown that if the two topological insulators have the opposite signs for the Dirac velocities, there exist gapless interface states [1]. In the last March meeting we showed a general proof for the existence of the gapless states using the mirror Chern number, which fixes the chirality of the surface states. In this presentation, we report the dispersions of these gapless interface states. They are in general a collection of Dirac cones. For example, if the system has threefold rotational symmetry, the interface states have six Dirac cones. By using the Fu-Kane-Mele model, which is the tight-binding model on the diamond lattice with the spin-orbit interaction, we calculate the dispersion of this gapless interface states, and discuss the relationship with the mirror Chern number.[4pt] [1] R. Takahashi, S. Murakami, Phys. Rev. Lett. 107,166805 (2011).
NASA Astrophysics Data System (ADS)
Huo, Y.; Zhou, C.; Sun, L.; Chui, S. T.; Wu, Y. Z.
2016-11-01
Magnetization excitation in micron sized FeNi disks with different diameters is studied by broadband ferromagnetic resonance (FMR) measurement. Except the main FMR peak, additional adsorption peaks with lower energies are observed. Both micromagnetic simulation and quantum spin wave calculation confirm that the low-energy excitation states are attributed to backward volume magnetostatic (BVM) spin waves. The size dependence of the low-energy states is systematically studied in 50-nm-thick Py disks with diameters larger than 500 nm, and the linewidth of the first BVM state is found to be obviously smaller than that of the FMR absorption peak. Through a quantitative comparison with experimental results, the quantum spin wave calculation is proven to be a reliable method to get the susceptibility and is much faster than the classical micromagnetic simulations.
Edge state reconstruction from strong correlations in quantum spin Hall insulators
NASA Astrophysics Data System (ADS)
Amaricci, A.; Privitera, L.; Petocchi, F.; Capone, M.; Sangiovanni, G.; Trauzettel, B.
2017-05-01
We study the fate of helical edge states in a quantum spin Hall insulators when the whole system is exposed to strong Coulomb interactions. Using dynamical mean-field theory, we show that the dispersion relation of the edge states is strongly affected by Coulomb interactions. In fact, the formerly gapless edge modes become gapped at a critical interaction strength. Interestingly, this critical interaction strength is significantly smaller at the edge than its counterpart in the bulk. Thus, the bulk remains in a topologically nontrivial state at intermediate interaction strengths where the edge states are already gapped out. This peculiar scenario leads to the reconstruction of gapless helical states at the new boundary between the topological bulk and the trivial (Mott insulating) edge. Further increasing the interaction strength triggers the progressive localization on the new boundary, the shrinking of the quantum spin Hall region, and the migration of the helical edge states towards the center of the system. The edge state reconstruction process is eventually interrupted by the Mott localization of the whole sample. Finally, we characterize the topological properties of the system by means of a local Chern marker.
Direct observation of spin-resolved full and empty electron states in ferromagnetic surfaces
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.
Mixed-state ferromagnetism in cubic Ni/ZrO2 nanocomposites by microwave combustion synthesis
NASA Astrophysics Data System (ADS)
Sahoo, Tapas R.; Panda, Sirish R.; Rath, Pragyan P.
2016-09-01
This article shows the magnetic phase diagram for Zr1-xNixO2 compositions synthesized by microwave combustion method. The samples show room temperature ferromagnetism over the entire range of Ni- doping. Ni2+/ZrO2 (<4 at% Ni) indicates a safe substitution limit to show dilute magnetic phase. There is a threshold limit to this Dilute Magnetic Semiconductor (DMS) phase up to ∼ 4%, above which this system serves as a model system for cluster induced magnetism. Microwave combustion method is a convenient and an inexpensive approach to evaluate magnetism in these high temperature phases, which is otherwise possible only by Physical vapor deposition techniques.
Thermodynamically self-consistent non-stochastic micromagnetic model for the ferromagnetic state
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.
A Langmuir Blodgett film presenting a ferromagnetic state below 25 K
NASA Astrophysics Data System (ADS)
Lafuente, C.; Mingotaud, C.; Delhaes, P.
1999-03-01
A positively charged monolayer spread on a sub-phase containing copper hexacyanoferrate leads to hybrid inorganic-organic LB films. Those multilayers present a spin ordering below 25 K. The Curie temperature of the LB films is found to be independent of the multilayer thickness. Their magnetization is clearly proportional to the number of transferred layers, demonstrating that the deposition process is perfectly regular. These results show that a bulk ferromagnetic behavior can be observed in this hybrid material, even if the distance between magnetic layers is considered as large.
Density of States and Magnetic Correlations at a Metal-Mott Insulator Interface
NASA Astrophysics Data System (ADS)
Jiang, Mi; Batrouni, George; Scalettar, Richard
2013-03-01
The possibility of novel behavior at interfaces between strongly and weakly correlated materials has come under increased study recently. In this paper, we use determinant Quantum Monte Carlo to determine the inter-penetration of metallic and Mott insulator physics across an interface in the two dimensional Hubbard Hamiltonian. We quantify the behavior of the density of states at the Fermi level and the short and long range antiferromagnetism as functions of the distance from the interface and with different interaction strength, temperature and hopping across the interface. Induced metallic behavior into the insulator is evident over several lattice spacings, whereas antiferromagnetic correlations remain small on the metallic side. At large interface hopping, singlets form between the two boundary layers, shielding the two systems from each other. We acknowledge support from the National Scence Foundation under grant NSF-PIF-1005502. This work was also supported under ARO Award W911NF0710576 with funds from the DARPA OLE Program a
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
Surface states in a 3D topological insulator: The role of hexagonal warping and curvature
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.
NASA Astrophysics Data System (ADS)
Hamada, Kosuke; Kaneko, Tatsuya; Miyakoshi, Shohei; Ohta, Yukinori
2017-07-01
We comparatively study the excitonic insulator state in the extended Falicov-Kimball model (EFKM, a spinless two-band model) on the two-dimensional square lattice using the variational cluster approximation (VCA) and the cluster dynamical impurity approximation (CDIA). In the latter, the particle-bath sites are included in the reference cluster to take into account the particle-number fluctuations in the correlation sites. We thus calculate the particle-number distribution, order parameter, ground-state phase diagram, anomalous Green's function, and pair coherence length, thereby demonstrating the usefulness of the CDIA in the discussion of the excitonic condensation in the EFKM.
NASA Astrophysics Data System (ADS)
Nakane, R.; Sugahara, S.; Tanaka, M.
2015-04-01
We systematically investigate the structural and magnetic properties of ferromagnetic Fe1-xSix (0.18 ≤ x ≤ 0.33) films formed by rapid thermal annealing (RTA) on silicon-on-insulator (SOI) substrates. During RTA of an Fe film deposited on a SOI substrate (consisting of a top Si layer, a buried oxide SiO2 layer, and a Si substrate), an Fe1-xSix film is synthesized by the thermal reaction of the deposited Fe film and the top Si layer, but the reaction is limited by the buried oxide layer in the SOI substrate, thus the Si concentration x in Fe1-xSix can be controlled by both the initial thicknesses of the Fe film and the top Si layer. A variety of characteristics show that single-phase Fe1-xSix (x = 0.18, 0.22, and 0.25) films with D03 + B2 structure are successfully obtained by choosing the optimum annealing temperature and time. Furthermore, the ordering fraction of D03 and B2 structures in these films is found to be more than 87%, indicating that the crystalline quality of these films is comparable to that of bulk Fe1-xSix materials reported so far. On the other hand, it is found that the Fe1-xSix (x = 0.33) film has Fe3Si and FeSi phases as in the case of bulk Fe1-xSix with x = 0.33. The film production technique and the quality of the ferromagnetic Fe1-xSix presented in this study are very attractive and useful for silicon-based spintronic devices which are compatible with the complementary metal-oxide-semiconductor technology.
NASA Astrophysics Data System (ADS)
Djega-Mariadassou, C.; Decaudin, B.; Bessais, L.; Cizeron, G.
1997-06-01
A method of analysing by Mössbauer spectroscopy a steel ferromagnetic matrix has been developed. It provides the fractions of atomic elements in substitutional or interstitial sites in the Fe lattice, from the comparison between the experimental hyperfine-field distribution 0953-8984/9/23/017/img7 and the calculated one 0953-8984/9/23/017/img8. This method, applied to an M50 steel, in the simple case of ferrite (the annealed state), and extended to the most complex situation of martensites quenched from various temperatures, describes the initial state of the steel before any further tempering treatment. The atomic fractions of Cr, Mo, V, and C in the Fe lattice have been specified.
Glaser, Thorsten; Heidemeier, Maik; Grimme, Stefan; Bill, Eckhard
2004-08-23
The trinuclear Cu(II) complex [(talen)Cu(II)(3)] (1) using the new triplesalen ligand H(6)talen has been synthesized and structurally characterized. The three Cu(II) ions are bridged in a m-phenylene linkage by the phloroglucinol backbone of the ligand. This m-phenylene bridging mode results in ferromagnetic couplings with an S(t) = (3)/(2) spin ground state, which has been analyzed by means of EPR spectroscopy and DFT calculations. The EPR spectrum exhibits an unprecedented pattern of 10 hyperfine lines due to the coupling of three Cu(II) ions (I = (3)/(2)). Resonances around g = 4 in both perpendicular and parallel mode EPR spectra demonstrate a zero-field splitting of D approximately 74 x 10(-4) cm(-1) arising from anisotropic/antisymmetric exchange interactions. The DFT calculations show an alteration in the sign of the spin densities of the central benzene ring corroborating the spin-polarization mechanism as origin for the ferromagnetic coupling.
Dilute ferromagnetic semiconductors: Physics and spintronic structures
NASA Astrophysics Data System (ADS)
Dietl, Tomasz; Ohno, Hideo
2014-01-01
This review compiles results of experimental and theoretical studies on thin films and quantum structures of semiconductors with randomly distributed Mn ions, which exhibit spintronic functionalities associated with collective ferromagnetic spin ordering. Properties of p-type Mn-containing III-V as well as II-VI, IV-VI, V2-VI3, I-II-V, and elemental group IV semiconductors are described, paying particular attention to the most thoroughly investigated system (Ga,Mn)As that supports the hole-mediated ferromagnetic order up to 190 K for the net concentration of Mn spins below 10%. Multilayer structures showing efficient spin injection and spin-related magnetotransport properties as well as enabling magnetization manipulation by strain, light, electric fields, and spin currents are presented together with their impact on metal spintronics. The challenging interplay between magnetic and electronic properties in topologically trivial and nontrivial systems is described, emphasizing the entangled roles of disorder and correlation at the carrier localization boundary. Finally, the case of dilute magnetic insulators is considered, such as (Ga,Mn)N, where low-temperature spin ordering is driven by short-ranged superexchange that is ferromagnetic for certain charge states of magnetic impurities.
NASA Astrophysics Data System (ADS)
Callewaert, Vincent; Shastry, K.; Saniz, Rolando; Makkonen, Ilja; Barbiellini, Bernardo; Assaf, Badih A.; Heiman, Donald; Moodera, Jagadeesh S.; Partoens, Bart; Bansil, Arun; Weiss, A. H.
2016-09-01
Topological insulators are attracting considerable interest due to their potential for technological applications and as platforms for exploring wide-ranging fundamental science questions. In order to exploit, fine-tune, control, and manipulate the topological surface states, spectroscopic tools which can effectively probe their properties are of key importance. Here, we demonstrate that positrons provide a sensitive probe for topological states and that the associated annihilation spectrum provides a technique for characterizing these states. Firm experimental evidence for the existence of a positron surface state near Bi2Te2Se with a binding energy of Eb=2.7 ±0.2 eV is presented and is confirmed by first-principles calculations. Additionally, the simulations predict a significant signal originating from annihilation with the topological surface states and show the feasibility to detect their spin texture through the use of spin-polarized positron beams.
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.
Ground state of underdoped cuprates in vicinity of superconductor-to-insulator transition
Wu, Jie; Bollinger, Anthony T.; Sun, Yujie; Božović, Ivan
2016-08-15
When an insulating underdoped cuprate is doped beyond a critical concentration (x_{c}), high-temperature superconductivity emerges. We have synthesized a series of La_{2–x}Sr_{x}CuO_{4} (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 (ρ_{H}) as a function of temperature down to 300 mK in magnetic fields up to 9 T. At very low temperatures, ρ_{H} 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 the 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.
Ground state of underdoped cuprates in vicinity of superconductor-to-insulator transition
Wu, Jie; Bollinger, Anthony T.; Sun, Yujie; Božović, Ivan
2016-08-15
When an insulating underdoped cuprate is doped beyond a critical concentration (x_{c}), high-temperature superconductivity emerges. We have synthesized a series of La_{2–x}Sr_{x}CuO_{4} (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 (ρ_{H}) as a function of temperature down to 300 mK in magnetic fields up to 9 T. At very low temperatures, ρ_{H} 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 the 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.
Ground state of underdoped cuprates in vicinity of superconductor-to-insulator transition
Wu, Jie; Bollinger, Anthony T.; Sun, Yujie; ...
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
Topological superconductivity induced by ferromagnetic metal chains
NASA Astrophysics Data System (ADS)
Li, Jian; Chen, Hua; Drozdov, Ilya K.; Yazdani, A.; Bernevig, B. Andrei; MacDonald, A. H.
2014-12-01
Recent experiments have provided evidence that one-dimensional (1D) topological superconductivity can be realized experimentally by placing transition-metal atoms that form a ferromagnetic chain on a superconducting substrate. We address some properties of this type of system by using a Slater-Koster tight-binding model to account for important features of the electronic structure of the transition-metal chains on the superconducting substrate. We predict that topological superconductivity is nearly universal when ferromagnetic transition-metal chains form straight lines on superconducting substrates and that it is possible for more complex chain structures. When the chain is weakly coupled to the substrate and is longer than superconducting coherence lengths, its proximity-induced superconducting gap is ˜Δ ESO/J where Δ is the s -wave pair potential on the chain, ESO is the spin-orbit splitting energy induced in the normal chain state bands by hybridization with the superconducting substrate, and J is the exchange splitting of the ferromagnetic chain d bands. Because of the topological character of the 1D superconducting state, Majorana end modes appear within the gaps of finite length chains. We find, in agreement with the experiment, that when the chain and substrate orbitals are strongly hybridized, Majorana end modes are substantially reduced in amplitude when separated from the chain end by less than the coherence length defined by the p -wave superconducting gap. We conclude that Pb is a particularly favorable substrate material for ferromagnetic chain topological superconductivity because it provides both strong s -wave pairing and strong Rashba spin-orbit coupling, but that there is an opportunity to optimize properties by varying the atomic composition and structure of the chain. Finally, we note that in the absence of disorder, a new chain magnetic symmetry, one that is also present in the crystalline topological insulators, can stabilize multiple
A strong ferroelectric ferromagnet created by means of spin-lattice coupling.
Lee, June Hyuk; Fang, Lei; Vlahos, Eftihia; Ke, Xianglin; Jung, Young Woo; Kourkoutis, Lena Fitting; Kim, Jong-Woo; Ryan, Philip J; Heeg, Tassilo; Roeckerath, Martin; Goian, Veronica; Bernhagen, Margitta; Uecker, Reinhard; Hammel, P Chris; Rabe, Karin M; Kamba, Stanislav; Schubert, Jürgen; Freeland, John W; Muller, David A; Fennie, Craig J; Schiffer, Peter; Gopalan, Venkatraman; Johnston-Halperin, Ezekiel; Schlom, Darrell 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(3), was predicted to exhibit strong ferromagnetism (spontaneous magnetization, approximately 7 Bohr magnetons per Eu) and strong ferroelectricity (spontaneous polarization, approximately 10 microC cm(-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
A strong ferroelectric ferromagnet created by means of spin-lattice coupling.
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
Plumb, Nicholas C; Radović, Milan
2017-09-29
Over the last decade, conducting states embedded in insulating transition metal oxides (TMOs) have served as gateways to discovering and probing surprising phenomena that can emerge in complex oxides, while also opening opportunities for engineering advanced devices. These states are commonly realized at thin film interfaces, such as the well-known case of LaAlO3 (LAO) grown on SrTiO3 (STO). In recent years, the use of angle-resolved photoemission spectroscopy (ARPES) to investigate the k-space electronic structure of such materials led to the discovery that metallic states can also be formed on the bare surfaces of certain TMOs. In this topical review, we report on recent studies of low-dimensional metallic states confined at insulating oxide surfaces and interfaces as seen from the perspective of ARPES, which provides a direct view of the occupied band structure. While offering a fairly broad survey of progress in the field, we draw particular attention to STO, whose surface is so far the best-studied, and whose electronic structure is probably of the most immediate interest, given the ubiquitous use of STO substrates as the basis for conducting oxide interfaces. The ARPES studies provide crucial insights into the electronic band structure, orbital character, dimensionality/confinement, spin structure, and collective excitations in STO surfaces and related oxide surface/interface systems. The obtained knowledge increases our understanding of these complex materials and gives new perspectives on how to manipulate their properties.
NASA Astrophysics Data System (ADS)
Choudhari, Tarun; Deo, Nivedita
2017-01-01
A superconductor-topological insulator-superconductor (S/TI/S) junction having normal region at angle θ is studied theoretically to investigate the junction angle dependency of the Andreev reflection and the formation of the Andreev bound states in the step and planar S/TI/S structures. It is found that the Andreev reflection becomes θ dependent only in the presence of the potential barrier at the TI/S interface. In particular, the step and planar TI/S junction have totally different conductive behavior with bias voltage and potential barrier in the regime of retro and specular Andreev reflection. Interestingly, we find that the elliptical cross section of Dirac cone, an important feature of topological insulator with step surface defect, affects the Fabry-Perot resonance of the Andreev reflection induced Andreev bound states (which become Majorana zero energy states at low chemical potential) in the step S/TI/S structure. Unlike the usual planar S/TI/S structures, we find these ellipticity affected Andreev bound states lead to non-monotonic Josephson super-current in the step S/TI/S structure whose non-monotonicity can be controlled with the use of the potential barrier, which may find applications in nanoelectronics.
Linearity of the edge states energy spectrum in the 2D topological insulator
NASA Astrophysics Data System (ADS)
Entin, M. V.; Mahmoodian, M. M.; Magarill, L. I.
2017-06-01
Linearity of the topological insulator edge state spectrum plays a crucial role for various transport phenomena. Previous studies found that this linearity exists near the spectrum crossing point, but did not determine how perfect the linearity is. The purpose of the present study is to answer this question in various edge states models. We examine Volkov and Pankratov (VP) model for the Dirac Hamiltonian and the model BHZ for the Bernevig, Hughes and Zhang (BHZ) Hamiltonian with zero boundary conditions. It is found that both models yield ideally linear edge states. In the BHZ1 model the linearity is conserved up to the spectrum ending points corresponding to the tangency of the edge spectrum with the boundary of 2D states. In contrast, the model BHZ2 with mixed boundary conditions for BHZ Hamiltonian and the 2D tight-binding (TB) model yield weak nonlinearity.
Multiple Coexisting Dirac Surface States in Three-Dimensional Topological Insulator PbBi₆Te₁₀.
Papagno, Marco; Eremeev, Sergey V; Fujii, Jun; Aliev, Ziya S; Babanly, Mahammad B; Mahatha, Sanjoy Kr; Vobornik, Ivana; Mamedov, Nazim T; Pacilé, Daniela; Chulkov, Evgueni V
2016-03-22
By means of angle-resolved photoemission spectroscopy (ARPES) measurements, we unveil the electronic band structure of three-dimensional PbBi6Te10 topological insulator. ARPES investigations evidence multiple coexisting Dirac surface states at the zone-center of the reciprocal space, displaying distinct electronic band dispersion, different constant energy contours, and Dirac point energies. We also provide evidence of Rashba-like split states close to the Fermi level, and deeper M- and V-shaped bands coexisting with the topological surface states. The experimental findings are in agreement with scanning tunneling microscopy measurements revealing different surface terminations according to the crystal structure of PbBi6Te10. Our experimental results are supported by density functional theory calculations predicting multiple topological surface states according to different surface cleavage planes.
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).
Nuclear-spin-induced localization of edge states in two-dimensional topological insulators
NASA Astrophysics Data System (ADS)
Hsu, Chen-Hsuan; Stano, Peter; Klinovaja, Jelena; Loss, Daniel
2017-08-01
We investigate the influence of nuclear spins on the resistance of helical edge states of two-dimensional topological insulators (2DTIs). Via the hyperfine interaction, nuclear spins allow electron backscattering, otherwise forbidden by time-reversal symmetry. We identify two backscattering mechanisms, depending on whether the nuclear spins are ordered or not. Their temperature dependence is distinct but both give resistance, which increases with the edge length, decreasing temperature, and increasing strength of the electron-electron interaction. Overall, we find that the nuclear spins will typically shut down the conductance of the 2DTI edges at zero temperature.
Surface state dominated transport in topological insulator Bi{sub 2}Te{sub 3} nanowires
Hamdou, Bacel Gooth, Johannes; Dorn, August; Nielsch, Kornelius; Pippel, Eckhard
2013-11-04
We report on low temperature magnetoresistance measurements on single-crystalline Bi{sub 2}Te{sub 3} nanowires synthesized via catalytic growth and post-annealing in a Te-rich atmosphere. The observation of Aharonov-Bohm oscillations indicates the presence of topological surface states. Analyses of Subnikov-de Haas oscillations in perpendicular magnetoresistance yield extremely low two-dimensional carrier concentrations and effective electron masses, and very high carrier mobilities. All our findings are in excellent agreement with theoretical predictions of massless Dirac fermions at the surfaces of topological insulators.
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
2015-05-22
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.
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.
Martínez-Velarte, M. Carmen; Kretz, Bernhard; Moro-Lagares, Maria; ...
2017-06-13
Here, we show that the chemical inhomogeneity in ternary three-dimensional topological insulators preserves the topological spin texture of their surface states against a net surface magnetization. The spin texture is that of a Dirac cone with helical spin structure in the reciprocal space, which gives rise to spin-polarized and dissipation-less charge currents. Thanks to the nontrivial topology of the bulk electronic structure, this spin texture is robust against most types of surface defects. However, magnetic perturbations break the time-reversal symmetry, enabling magnetic scattering and loss of spin coherence of the charge carriers. This intrinsic incompatibility precludes the design of magnetoelectronicmore » devices based on the coupling between magnetic materials and topological surface states. We demonstrate that the magnetization coming from individual Co atoms deposited on the surface can disrupt the spin coherence of the carriers in the archetypal topological insulator Bi2Te3, while in Bi2Se2Te the spin texture remains unperturbed. This is concluded from the observation of elastic backscattering events in quasiparticle interference patterns obtained by scanning tunneling spectroscopy. The mechanism responsible for the protection is investigated by energy resolved spectroscopy and ab initio calculations, and it is ascribed to the distorted adsorption geometry of localized magnetic moments due to Se–Te disorder, which suppresses the Co hybridization with the surface states.« less
Intrinsic conduction through topological surface states of insulating Bi2Te3 epitaxial thin films.
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.
Martínez-Velarte, M Carmen; Kretz, Bernhard; Moro-Lagares, María; Aguirre, Myriam H; Riedemann, Trevor M; Lograsso, Thomas A; Morellón, Luis; Ibarra, M Ricardo; Garcia-Lekue, Arán; Serrate, David
2017-07-12
We show that the chemical inhomogeneity in ternary three-dimensional topological insulators preserves the topological spin texture of their surface states against a net surface magnetization. The spin texture is that of a Dirac cone with helical spin structure in the reciprocal space, which gives rise to spin-polarized and dissipation-less charge currents. Thanks to the nontrivial topology of the bulk electronic structure, this spin texture is robust against most types of surface defects. However, magnetic perturbations break the time-reversal symmetry, enabling magnetic scattering and loss of spin coherence of the charge carriers. This intrinsic incompatibility precludes the design of magnetoelectronic devices based on the coupling between magnetic materials and topological surface states. We demonstrate that the magnetization coming from individual Co atoms deposited on the surface can disrupt the spin coherence of the carriers in the archetypal topological insulator Bi2Te3, while in Bi2Se2Te the spin texture remains unperturbed. This is concluded from the observation of elastic backscattering events in quasiparticle interference patterns obtained by scanning tunneling spectroscopy. The mechanism responsible for the protection is investigated by energy resolved spectroscopy and ab initio calculations, and it is ascribed to the distorted adsorption geometry of localized magnetic moments due to Se-Te disorder, which suppresses the Co hybridization with the surface states.
Topological spin texture in a quantum anomalous Hall insulator.
Wu, Jiansheng; Liu, Jie; Liu, Xiong-Jun
2014-09-26
The quantum anomalous Hall (QAH) effect has been recently discovered in an experiment using a thin-film topological insulator with ferromagnetic ordering and strong spin-orbit coupling. Here we investigate the spin degree of freedom of a QAH insulator and uncover the fundamental phenomenon that the edge states exhibit a topologically stable spin texture in the boundary when a chiral-like symmetry is present. This result shows that edge states are chiral in both the orbital and spin degrees of freedom, and the chiral edge spin texture corresponds to the bulk topological states of the QAH insulator. We also study the potential applications of the edge spin texture in designing topological-state-based spin devices, which might be applicable to future spintronic technologies.
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.
Large Fermi Surface of Heavy Electrons at the Border of Mott Insulating State in NiS2
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
Large Fermi Surface of Heavy Electrons at the Border of Mott Insulating State in NiS2
Friedemann, S.; Chang, H.; Gamża, M. B.; ...
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
Large Fermi Surface of Heavy Electrons at the Border of Mott Insulating State in NiS_{2}
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 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 NiS_{2}. 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.
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.
NASA Astrophysics Data System (ADS)
Lin, Wen-Yi; Ho, Chi-Chih; Hsu, Wen-Kuang
2016-02-01
Large arrays of ferromagnetic nanorings are produced by a modified hole-mask colloidal lithography and ring dimension can be modulated to create flux-closed vortex, known as a dipole-free magnetic state with a low crosstalk arising from neighboring entities.
Scaffold State Switching Amplifies, Accelerates, and Insulates Protein Kinase C Signaling*
Greenwald, Eric C.; Redden, John M.; Dodge-Kafka, Kimberly L.; Saucerman, Jeffrey J.
2014-01-01
Scaffold proteins localize two or more signaling enzymes in close proximity to their downstream effectors. A-kinase-anchoring proteins (AKAPs) are a canonical family of scaffold proteins known to bind protein kinase A (PKA) and other enzymes. Several AKAPs have been shown to accelerate, amplify, and specify signal transduction to dynamically regulate numerous cellular processes. However, there is little theory available to mechanistically explain how signaling on protein scaffolds differs from solution biochemistry. In our present study, we propose a novel kinetic mechanism for enzymatic reactions on protein scaffolds to explain these phenomena, wherein the enzyme-substrate-scaffold complex undergoes stochastic state switching to reach an active state. This model predicted anchored enzymatic reactions to be accelerated, amplified, and insulated from inhibition compared with those occurring in solution. We exploited a direct interaction between protein kinase C (PKC) and AKAP7α as a model to validate these predictions experimentally. Using a genetically encoded PKC activity reporter, we found that both the strength and speed of substrate phosphorylation were enhanced by AKAP7α. PKC tethered to AKAP7α was less susceptible to inhibition from the ATP-competitive inhibitor Gö6976 and the substrate-competitive inhibitor PKC 20-28, but not the activation-competitive inhibitor calphostin C. Model predictions and experimental validation demonstrated that insulation is a general property of scaffold tethering. Sensitivity analysis indicated that these findings may be applicable to many other scaffolds as well. Collectively, our findings provide theoretical and experimental evidence that scaffold proteins can amplify, accelerate, and insulate signal transduction. PMID:24302730
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
Kato, Shinya; Inaba, Kensuke; Sugawa, Seiji; Shibata, Kosuke; Yamamoto, Ryuta; Yamashita, Makoto; Takahashi, Yoshiro
2016-04-20
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.
Nontrivial surface state transport in Bi2Se3 topological insulator nanoribbons
NASA Astrophysics Data System (ADS)
Pan, Haiyang; Zhang, Kang; Wei, Zhongxia; Wang, Jue; Han, Min; Song, Fengqi; Wang, Xuefeng; Wang, Baigeng; Zhang, Rong
2017-01-01
Topological insulator nanostructures have the larger surface-to-volume ratios than the bulk materials, which enhances the surface state contribution to the electrical transport. Here, we report on the single-crystalline Bi2Se3 narrow nanoribbons synthesized by the chemical vapor deposition method. The surface state induced Aharonov-Bohm effect was observed in the parallel magnetic field. The weak antilocalization (WAL) at various temperatures can be well fitted by the 1D localization theory, and the fitting coherence length is larger than the cross section size of the nanoribbon. The amplitude of WAL after subtracting the bulk background is only dependent on the vertical component of the magnetic field at various angles, revealing the surface nature of WAL. All these signatures indicate the nontrivial surface state transport in our Bi2Se3 narrow nanoribbons.
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.
Subpicosecond spin dynamics of excited states in the topological insulator Bi2Te3
NASA Astrophysics Data System (ADS)
Sánchez-Barriga, J.; Battiato, M.; Krivenkov, M.; Golias, E.; Varykhalov, A.; Romualdi, A.; Yashina, L. V.; Minár, J.; Kornilov, O.; Ebert, H.; Held, K.; Braun, J.
2017-03-01
Using time-, spin-, and angle-resolved photoemission, we investigate the ultrafast spin dynamics of hot electrons on the surface of the topological insulator Bi2Te3 following optical excitation by femtosecond-infrared pulses. We observe two surface-resonance states above the Fermi level coexisting with a transient population of Dirac fermions that relax in ˜2 ps. One state disperses up to ˜0.4 eV just above the bulk continuum, and the other one at ˜0.8 eV inside a projected bulk band gap. At the onset of the excitation, both states exhibit a reversed spin texture with respect to that of the transient Dirac bands, in agreement with our one-step photoemission calculations. Our data reveal that the high-energy state undergoes spin relaxation within ˜0.5 ps, a process that triggers the subsequent spin dynamics of both the Dirac cone and the low-energy state, which behave as two dynamically locked electron populations. We discuss the origin of this behavior by comparing the relaxation times observed for electrons with opposite spins to the ones obtained from a microscopic Boltzmann model of ultrafast band cooling introduced into the photoemission calculations. Our results demonstrate that the nonequilibrium surface dynamics is governed by electron-electron rather than electron-phonon scattering, with a characteristic time scale unambiguously determined by the complex spin texture of excited states above the Fermi level. Our findings reveal the critical importance of detecting momentum and energy-resolved spin textures with femtosecond resolution to fully understand the subpicosecond dynamics of transient electrons on the surface of topological insulators.
Volume-wise destruction of the antiferromagnetic Mott insulating state through quantum tuning
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
Volume-wise destruction of the antiferromagnetic Mott insulating state through quantum tuning
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.; Luke, G. M.; Chen, B.; Li, W.; Jin, C.; Ding, C; Guo, S.; Ning, F.; Ito, T. U.; Higemoto, W.; Billinge, S. J.L.; Sakamoto, S.; Fujimori, A.; Murakami, T.; Kageyama, H.; Alonso, J. A.; Kotliar, G.; Imada, M.; Uemura, Y. J.
2016-08-17
RENiO_{3} (RE=rare-earth element) and V_{2}O_{3} are archetypal Mott insulator systems. When tuned by chemical substitution (RENiO_{3}) or pressure (V_{2}O_{3}), 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 RENiO_{3} and V_{2}O_{3} 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.
Volume-wise destruction of the antiferromagnetic Mott insulating state through quantum tuning
NASA Astrophysics Data System (ADS)
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.
2016-08-01
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.
Volume-wise destruction of the antiferromagnetic Mott insulating state through quantum tuning
B. A. Frandsen; Liu, L.; Cheung, S. C.; ...
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
Gapless Andreev bound states in the quantum spin Hall insulator HgTe
NASA Astrophysics Data System (ADS)
Bocquillon, Erwann; Deacon, Russell S.; Wiedenmann, Jonas; Leubner, Philipp; Klapwijk, Teunis M.; Brüne, Christoph; Ishibashi, Koji; Buhmann, Hartmut; Molenkamp, Laurens W.
2016-08-01
In recent years, Majorana physics has attracted considerable attention because of exotic new phenomena and its prospects for fault-tolerant topological quantum computation. To this end, one needs to engineer the interplay between superconductivity and electronic properties in a topological insulator, but experimental work remains scarce and ambiguous. Here, we report experimental evidence for topological superconductivity induced in a HgTe quantum well, a 2D topological insulator that exhibits the quantum spin Hall (QSH) effect. The a.c. Josephson effect demonstrates that the supercurrent has a 4π periodicity in the superconducting phase difference, as indicated by a doubling of the voltage step for multiple Shapiro steps. In addition, this response like that of a superconducting quantum interference device to a perpendicular magnetic field shows that the 4π-periodic supercurrent originates from states located on the edges of the junction. Both features appear strongest towards the QSH regime, and thus provide evidence for induced topological superconductivity in the QSH edge states.
Gapless Andreev bound states in the quantum spin Hall insulator HgTe
NASA Astrophysics Data System (ADS)
Bocquillon, Erwann; Deacon, Russell S.; Wiedenmann, Jonas; Leubner, Philipp; Klapwijk, Teunis M.; Brüne, Christoph; Ishibashi, Koji; Buhmann, Hartmut; Molenkamp, Laurens W.
2017-02-01
In recent years, Majorana physics has attracted considerable attention because of exotic new phenomena and its prospects for fault-tolerant topological quantum computation. To this end, one needs to engineer the interplay between superconductivity and electronic properties in a topological insulator, but experimental work remains scarce and ambiguous. Here, we report experimental evidence for topological superconductivity induced in a HgTe quantum well, a 2D topological insulator that exhibits the quantum spin Hall (QSH) effect. The a.c. Josephson effect demonstrates that the supercurrent has a 4π periodicity in the superconducting phase difference, as indicated by a doubling of the voltage step for multiple Shapiro steps. In addition, this response like that of a superconducting quantum interference device to a perpendicular magnetic field shows that the 4π-periodic supercurrent originates from states located on the edges of the junction. Both features appear strongest towards the QSH regime, and thus provide evidence for induced topological superconductivity in the QSH edge states.
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.
Charge driven metal-insulator transitions in LaMnO3|SrTiO3 (111) superlattices
NASA Astrophysics Data System (ADS)
Cossu, F.; Tahini, H. A.; Singh, N.; Schwingenschlögl, U.
2017-06-01
Interfaces of perovskite oxides, due to the strong interplay between the lattice, charge and spin degrees of freedom, can host various phase transitions, which is particularly interesting if these transitions can be tuned by external fields. Recently, ferromagnetism was found together with a seemingly insulating state in superlattices of manganites and titanates. We therefore study the (111) oriented (\\text{LaMnO}_3)6-x\\vert(\\text{SrTiO}_3)6+x~(x = -0.5, 0, 0.5) superlattices by means of ab initio calculations, predicting a ferromagnetic ground state due to double exchange in all cases. We shed light on the ferromagnetic coupling in the LaMnO3 region and at the interfaces. The insulating states of specific superlattices can be understood on the basis of Jahn-Teller modes and electron/hole doping.
Phase segregation of superconductivity and ferromagnetism at the LaAlO3/SrTiO3 interface.
Mohanta, N; Taraphder, A
2014-01-15
The highly conductive two-dimensional electron gas formed at the interface between insulating SrTiO3 and LaAlO3 shows low-temperature superconductivity coexisting with inhomogeneous ferromagnetism. The Rashba spin-orbit interaction with the in-plane Zeeman field of the system favors p(x) ± ip(y)-wave superconductivity at finite momentum. Owing to the intrinsic disorder at the interface, the role of spatial inhomogeneity in the superconducting and ferromagnetic states becomes important. We find that, for strong disorder, the system breaks up into mutually excluded regions of superconductivity and ferromagnetism. This inhomogeneity-driven electronic phase separation accounts for the unusual coexistence of superconductivity and ferromagnetism observed at the interface.
Effective Hamiltonian for surface states of topological insulator thin films with hexagonal warping
Siu, Zhuo Bin; Jalil, Mansoor B. A.; Tan, Seng Ghee
2016-05-15
The effective Hamiltonian of the surface states on semi-infinite slabs of the topological insulators (TI) Bi{sub 2}Te{sub 3} and Bi{sub 2}Se{sub 3} 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 Bi{sub 2}Se{sub 3} 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.
Hung, Hsiang-Hsuan; Wu, Jiansheng; Sun, Kuei; Chiu, Ching-Kai
2017-06-14
We study a vortex chain in a thin film of a topological insulator with proximity-induced superconductivity-a promising platform to realize Majorana zero modes (MZMs)-by modeling it as a two-leg Majorana ladder. While each pair of MZMs hybridizes through vortex tunneling, we hereby show that MZMs can be stabilized on the ends of the ladder with the presence of tilted external magnetic field and four-Majorana interaction. Furthermore, a fruitful phase diagram is obtained by controlling the direction of magnetic field and the thickness of the sample. We reveal many-body Majorana states and interaction-induced topological phase transitions and also identify trivial-superconducting and commensurate/incommensurate charge-density-wave states in the phase diagram.
The surface-state of the topological insulator Bi2Se3 revealed by cyclotron resonance
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.
Shift charge and spin photocurrents in Dirac surface states of topological insulator
NASA Astrophysics Data System (ADS)
Kim, Kun Woo; Morimoto, Takahiro; Nagaosa, Naoto
2017-01-01
The generation of photocurrent in condensed matter is of main interest for photovoltaic and optoelectronic applications. Shift current, a nonlinear photoresponse, has attracted recent intensive attention as a dominant player of bulk photovoltaic effect in ferroelectric materials. In three-dimensional topological insulators Bi2X3 (X =Te , Se), we find that Dirac surface states with a hexagonal warping term support shift current by linearly polarized light. Moreover, we study "shift spin current" that arises in Dirac surface states by introducing time-reversal symmetry breaking perturbation. The estimate for the magnitudes of the shift charge and spin current densities are 0.13 I0 and 0.40 I0 (nA/m) for Bi2Te3 with the intensity of light I0 measured in (W/m2) , respectively, which can offer a useful method to generate these currents efficiently.
Mapping the effect of defect-induced strain disorder on the Dirac states of topological insulators
NASA Astrophysics Data System (ADS)
Storz, Oliver; Cortijo, Alberto; Wilfert, Stefan; Kokh, K. A.; Tereshchenko, O. E.; Vozmediano, María A. H.; Bode, Matthias; Guinea, Francisco; Sessi, Paolo
2016-09-01
We provide a detailed microscopic characterization of the influence of defect-induced disorder on the Dirac spectrum of three-dimensional topological insulators. By spatially resolved Landau-level spectroscopy measurements, we reveal the existence of nanoscale fluctuations of both the Dirac point energy as well as of the Dirac-fermion velocity which is found to spatially change in opposite direction for electrons and holes, respectively. These results evidence a scenario which goes beyond the existing picture based on chemical potential fluctuations. The findings are consistently explained by considering the microscopic effects of local stain introduced by defects, which our model calculations show to effectively couple to topological states, reshaping their Dirac-like dispersion over a large energy range. In particular, our results indicate that the presence of microscopic spatially varying stain, inevitably present in crystals because of the random distribution of defects, effectively couple to topological states and should be carefully considered for correctly describing the effects of disorder.
Geometric Effect on Quantum Anomalous Hall State in Magnetic Topological Insulator
NASA Astrophysics Data System (ADS)
Xing, Yanxia
An intriguing observation on the quantum anomalous Hall (QAH) effect in a magnetic topological insulator (MTI) is the dissipative edge states. With the aid of non-equilibrium Green's functions,the QAH effect in an MTI with a three dimensional effective tight-binding model is studied.We predict that due to geometric structure in the third dimension z,the unideal contact between terminal leads and central scattering region induces the backscattering in the central Hall bar,as the function of split gates. Such backscattering leads to a nonzero longitudinal resistance and quantized Hall resistance, which would explain the dissipative edge states in experiments.A further numerical simulation prove above prediction as well.These results are rewarding on future experimental observations and transport calculations based on first principe.
Nature of the insulating ground state of the 5d postperovskite CaIrO_{3}
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 CaIrO_{3} 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 t_{2g} 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 Ir^{4+} spins. Furthermore, the spin-orbit coupling (SOC) hybridizes the t_{2g} states to open an insulating gap. These results indicate that CaIrO_{3} 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.
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.
Double perovskite heterostructures: magnetism, Chern bands, and Chern insulators.
Cook, Ashley M; Paramekanti, Arun
2014-08-15
Experiments demonstrating the controlled growth of oxide heterostructures have raised the prospect of realizing topologically nontrivial states of correlated electrons in low dimensions. Here, we study heterostructures consisting of {111} bilayers of double perovskites separated by inert band insulators. In bulk, these double perovskites have well-defined local moments interacting with itinerant electrons leading to high temperature ferromagnetism. Incorporating spin-orbit coupling in the two-dimensional honeycomb geometry of a {111} bilayer, we find a rich phase diagram with tunable ferromagnetic order, topological Chern bands, and a C=±2 Chern insulator regime. Our results are of broad relevance to oxide materials such as Sr_{2}FeMoO_{6}, Ba_{2}FeReO_{6}, and Sr_{2}CrWO_{6}.
NASA Astrophysics Data System (ADS)
Belovs, M.; Cěbers, A.
2009-05-01
The self-propelling motion of the flexible ferromagnetic swimmer is described. Necessary symmetry breaking is achieved by the buckling instability at field inversion. The characteristics of self-propulsion are in good agreement with the numerical calculations of the Floquet multipliers for the ferromagnetic filament under the action of ac magnetic field. In the low frequency range the power stroke of self-propelling motion is similar to that used by the unicellular green algae chlamydomonas and in the high frequency region the self-propulsion is due to the undulation waves propagating from the free ends perpendicularly to ac magnetic field.
Belovs, M; Cēbers, A
2009-05-01
The self-propelling motion of the flexible ferromagnetic swimmer is described. Necessary symmetry breaking is achieved by the buckling instability at field inversion. The characteristics of self-propulsion are in good agreement with the numerical calculations of the Floquet multipliers for the ferromagnetic filament under the action of ac magnetic field. In the low frequency range the power stroke of self-propelling motion is similar to that used by the unicellular green algae chlamydomonas and in the high frequency region the self-propulsion is due to the undulation waves propagating from the free ends perpendicularly to ac magnetic field.
Disorder-induced room temperature ferromagnetism in glassy chromites.
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
2014-04-15
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.
Disorder-induced Room Temperature Ferromagnetism in Glassy Chromites
NASA Astrophysics Data System (ADS)
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.
2014-04-01
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.
NASA Astrophysics Data System (ADS)
Xu, Li; Li, Zhi-Jian; Hou, Hai-Yan; Niu, Pengbin; Nie, Yi-Hang
2016-10-01
We theoretically analyze the thermoelectric properties of the single-spin state based on the resonant tunneling of electron in the ferromagnetic-normal junction with artificial magnetic impurities. The thermoelectric coefficients, such as electrical conductance G, thermal conductance K, thermopower S and effective figure of merit Y, have been calculated using the nonequilibrium Green function in the linear regime. It is found that the thermoelectric coefficients can achieve considerable values by adjusting key parameters of the hybrid mesoscopic structure, such as the level detuning, the interdot hopping coefficient, the external magnetic field and the angle θ. When the level detuning changes, the spectra of electrical conductance and thermal conductance exhibit the electronic Dicke-like effect in the low temperature. Two valleys of electrical conductance and thermal conductance are always located at the single-spin level of QD2 ({{\\varepsilon}2\\uparrow} and ~{{\\varepsilon}2\\downarrow} ), and can achieve the antiresonant point by adjusting the interdot hopping coefficient. Thermoelectric coefficients can achieve considerable values near valleys because the Wiedemann-Franz law is strongly violated. Thermopower S and effective figure of merit Y can get larger values in the vicinity of {{\\varepsilon}2\\uparrow} by adjusting key parameters of the hybrid mesoscopic structure, such as the level detuning, the interdot hopping coefficient and the polarization. But the thermoelectric effect is reversed by changing the angle θ. When the angle θ increases, S and Y are suppressed in the vicinity of {{\\varepsilon}2\\uparrow}, meanwhile, S and Y are enhanced in the vicinity of {{\\varepsilon}2\\downarrow}. {χ+}=\\cos \\fracθ{2}|\\uparrow >+\\sin \\fracθ{2}|\\downarrow > shows that an electron in the state {χ+} can virtually tunnel into the spin-up (or spin-down) state of the ferromagnet. The amplitude of electron tunneling is \\cos \\fracθ{2} (or \\sin \\fracθ{2
Insulating states of a broken-gap two-dimensional electron-hole system
NASA Astrophysics Data System (ADS)
Takashina, K.; Nicholas, R. J.; Kardynal, B.; Mason, N. J.; Maude, D. K.; Portal, J. C.
2003-12-01
It has recently been found that an InAs/GaSb based electron-hole system exhibits insulating behavior with unusual properties when the numbers of occupied electron and hole Landau levels are equal [R. J. Nicholas, K. Takashina, M. Lakrimi, B. Kardynal, S. Khym, N. J. Mason, D. M. Symons, D. K. Maude, and J. C. Portal, Phys. Rev. Lett. 85, 2364 (2000)]. In this insulating state, the Hall resistance becomes symmetric (even) under field reversal [Rxy(B)=Rxy(-B)], and both the Hall and longitudinal resistances display reproducible fluctuations. In this paper we present experimental studies of the geometry dependence of this phenomenon. In particular, we show conclusively that the conduction responsible for the reproducible fluctuations and the symmetric Hall resistance occur due to the presence of the mesa edge. Further investigations of the edge conductance are presented. We show that as a function of magnetic field, the edge conduction shows a qualitatively opposite behavior to the conductivity in the sample interior. This is confirmed through measurements with an in-plane component of magnetic field. Also, the size of the conductance fluctuations is found to have a monotonic relationship with the absolute value of the conductance. A model based on counterpropagating edge channels is presented which qualitatively accounts for the observed behavior.
NASA Astrophysics Data System (ADS)
Magarill, L. I.; Entin, M. V.
2016-12-01
The electron absorption and the edge photocurrent of a 2D topological insulator are studied for transitions between edge states to 2D states. The circular polarized light is found to produce the edge photocurrent, the direction of which is determined by light polarization and edge orientation. It is shown that the edge-state current is found to exceed the 2D current owing to the topological protection of the edge states.
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.
Pressure-induced insulator-metal transition in EuMnO3
NASA Astrophysics Data System (ADS)
Qiu, R.; Bousquet, E.; Cano, A.
2017-08-01
We study the influence of external pressure on the electronic and magnetic structure of EuMnO3 from first-principles calculations. We find a pressure-induced insulator-metal transition at which the magnetic order changes from A-type antiferromagnetic to ferromagnetic with a strong interplay with Jahn-Teller distortions. In addition, we find that the non-centrosymmetric E *-type antiferromagnetic order can become nearly degenerate with the ferromagnetic ground state in the high-pressure metallic state. This situation can be exploited to promote a magnetically-driven realization of a non-centrosymmetric (ferroelectric-like) metal.
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.
Angular and Linear Momentum of Excited Ferromagnets
NASA Astrophysics Data System (ADS)
Yan, Peng; Kamra, Akashdeep; Cao, Yunshan; Bauer, Gerrit
2014-03-01
The angular momentum vector of a Heisenberg ferromagnet with isotropic exchange interaction is conserved, while under uniaxial crystalline anisotropy the projection of the total spin along the easy axis is a constant of motion. Using Noether's theorem, we prove that these conservation laws persist in the presence of dipole-dipole interactions. However, spin and orbital angular momentum are not conserved separately anymore. We also define the linear momentum of ferromagnetic textures. We illustrate the general principles with special reference to spin transfer torques and identify the emergence of a non-adiabatic effective field acting on domain walls in ferromagnetic insulators
NASA Astrophysics Data System (ADS)
Wang, Guo-Xiang; Sha, Yang; Li, Yan-Ying
2015-11-01
We study a tight-binding model of the supercubane-like lattice with spin-orbit coupling. By evaluating the Z2 topological indices, we find that the supercubane-like lattice can support strong topological insulator and the phase diagrams of the lattice with different filling fractions are present. Strong topological insulators with Z2 invariants (1 ; 000) and (1 ; 111) can be realized for 1/8 filling fraction and semimetals can be obtained for 1/8, quarter and half filling fractions. We analyze and discuss the characteristics of these topological insulators and their surface states. Spin textures of surface states are also evaluated for 1 1 bar 1 slab geometry.
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
2015-12-09
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.
Koirala, Nikesh; Han, Myung -Geun; Brahlek, Matthew; ...
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.
Observation of phononic helical edge states in a mechanical topological insulator
NASA Astrophysics Data System (ADS)
Süsstrunk, Roman; Huber, Sebastian D.
2015-07-01
A topological insulator, as originally proposed for electrons governed by quantum mechanics, is characterized by a dichotomy between the interior and the edge of a finite system: The bulk has an energy gap, and the edges sustain excitations traversing this gap. However, it has remained an open question whether the same physics can be observed for systems obeying Newton’s equations of motion. We conducted experiments to characterize the collective behavior of mechanical oscillators exhibiting the phenomenology of the quantum spin Hall effect. The phononic edge modes are shown to be helical, and we demonstrate their topological protection via the stability of the edge states against imperfections. Our results may enable the design of topological acoustic metamaterials that can capitalize on the stability of the surface phonons as reliable wave guides.
Sum-Rule Constraints on the Surface State Conductance of Topological Insulators
NASA Astrophysics Data System (ADS)
Post, K. W.; Chapler, B. C.; Liu, M. K.; Wu, J. S.; Stinson, H. T.; Goldflam, M. D.; Richardella, A. R.; Lee, J. S.; Reijnders, A. A.; Burch, K. S.; Fogler, M. M.; Samarth, N.; Basov, D. N.
2015-09-01
We report the Drude oscillator strength D and the magnitude of the bulk band gap Eg of the epitaxially grown, topological insulator (Bi ,Sb )2Te3 . The magnitude of Eg, in conjunction with the model independent f -sum rule, allows us to establish an upper bound for the magnitude of D expected in a typical Dirac-like system composed of linear bands. The experimentally observed D is found to be at or below this theoretical upper bound, demonstrating the effectiveness of alloying in eliminating bulk charge carriers. Moreover, direct comparison of the measured D to magnetoresistance measurements of the same sample supports assignment of the observed low-energy conduction to topological surface states.
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 In_{2}Se_{3}/(Bi_{0.5}In_{0.5})_{2}Se_{3} heterostructure, we introduce a quantum generation of Bi_{2}Se_{3} 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 Bi_{2}Se_{3}.
Sum-rule constraints on the surface state conductance of topological insulators.
Post, K W; Chapler, B C; Liu, M K; Wu, J S; Stinson, H T; Goldflam, M D; Richardella, A R; Lee, J S; Reijnders, A A; Burch, K S; Fogler, M M; Samarth, N; Basov, D N
2015-09-11
We report the Drude oscillator strength D and the magnitude of the bulk band gap E_{g} of the epitaxially grown, topological insulator (Bi,Sb)_{2}Te_{3}. The magnitude of E_{g}, in conjunction with the model independent f-sum rule, allows us to establish an upper bound for the magnitude of D expected in a typical Dirac-like system composed of linear bands. The experimentally observed D is found to be at or below this theoretical upper bound, demonstrating the effectiveness of alloying in eliminating bulk charge carriers. Moreover, direct comparison of the measured D to magnetoresistance measurements of the same sample supports assignment of the observed low-energy conduction to topological surface states.
High-Temperature Ferromagnetism in Low-Carrier Density Semimetals
NASA Astrophysics Data System (ADS)
Young, David P.
2000-03-01
Experimental results are presented on the magnetic and transport properties of lightly-doped alkaline earth hexaborides. At very low doping levels ( ~0.5%) weak ferromagnetism is observed at high temperature, which is an unexpected result considering that none of the constituent elements is normally associated with magnetic ordering. This phenomenon is fairly robust, with a Curie temperature near 600 K, which is also close to the value of the Fermi temperature. Two important aspects of this ferromagnetism are the following: (1)the magnetic ordering is only observed over a small range of doping concentrations, and (2)the maximum magnetic moment is about 0.1 μB per charge carrier. Two different theoretical descriptions have been proposed to explain the ferromagnetism: polarization of the free 3D electron gas at low density and spontaneous magnetization arising from a spin-triplet state in a doped excitonic insulator. This work is due to a collaborative effort with M.E. Torelli, D.W. Hall, A.D. Bianchi, F. Drymiotis, and Z. Fisk, NHMFL and Florida State University, Tallahassee, FL; R. Zysler, CA Bariloche, Argentina; R.G. Goodrich, Louisiana State University, Baton Rouge, LA; S. Oseroff, San Diego State University, San Diego, CA; J.D. Thompson and J.L. Sarrao, LANL, Los Alamos, NM; and H.-R. Ott, ETH, Zurich.
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
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.
A General Theorem Relating the Bulk Topological Number to Edge States in Two-dimensional Insulators
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.
Semiconductor-to-insulator transition of undoped-BaTiO3 in quenched state
NASA Astrophysics Data System (ADS)
Kwon, Hyung-Soon; Yoo, Han-Ill; Kim, Chang-Hoon; Hur, Kang-Heon
2010-04-01
It has been believed that the defect structure of "undoped" BaTi1-ηO3-δ be governed by background acceptor impurities, most likely AlTi', in its near stoichiometry regime (δ ≈0). Its electrical conductivity versus oxygen activity should, thus, be expected to be similar to that of Al-doped BaTiO3 not only in equilibrium state at elevated temperatures, but also in quenched state at low temperatures. In quenched state, however, the "undoped" BaTiO3 from various sources totally betray this expectation: A landmark of the quenched-state defect structure, semiconductor-to-insulator transition falls at an oxygen activity >10 orders lower than that of the Al-doped and it is rather similar to that of variable-valent Mn-doped BaTiO3. The transition is found to systematically depend on the nonmolecularity (η), suggesting the defect structure being governed by the multiply-charged cation vacancies. The hole-trapping energies of VBa″ and VTi⁗ are estimated and compared with those for the fixed-valent AlTi' and variable-valent MnTi″.
Plasma-Wave Terahertz Detection Mediated by Topological Insulators Surface States.
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.
Decoupling of the antiferromagnetic and insulating states in Tb-doped Sr2IrO4
Wang, J. C.; Aswartham, S.; Ye, Feng; ...
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
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.
Quantum transport in ferromagnetic graphene: Role of Berry curvature
Chowdhury, Debashree; Basu, Banasri
2014-12-10
The magnetic effects in ferromagnetic graphene basically depend on the principle of exchange interaction when ferromagntism is induced by depositing an insulator layer on graphene. Here we deal with the consequences of non-uniformity in the exchange coupling strength of the ferromagnetic graphene. We discuss how the in- homogeneity in the coordinate and momentum of the exchange vector field can provide interesting results in the conductivity analysis of the ferromagnetic graphene. Our analysis is based on the Kubo formalism of quantum transport.
Svanidze, E.; Liu, L.; Frandsen, B.; ...
2015-03-01
A quantum critical point (QCP) occurs upon chemical doping of the weak itinerant ferromagnet Sc₃̣₁ In. Remarkable for a system with no local moments, the QCP is accompanied by non-Fermi liquid behavior, manifested in the logarithmic divergence of the specific heat both in the ferro-and the paramagnetic states, as well as linear temperature dependence of the low-temperature resistivity. With doping, critical scaling is observed close to the QCP, as the critical exponents δ, γ, and β have weak composition dependence, with δ nearly twice and β almost half of their respective mean-field values. The unusually large paramagnetic moment μPM ~more » 1.3μB/F:U: is nearly composition independent. Evidence for strong spin fluctuations, accompanying the QCP at xc – 0.035 ± 0.005, may be ascribed to the reduced dimensionality of Sc₃̣₁ In, associated with the nearly one-dimensional Sc-In chains.« less
NASA Astrophysics Data System (ADS)
Svanidze, E.; Liu, L.; Frandsen, B.; White, B. D.; Besara, T.; Goko, T.; Medina, T.; Munsie, T. J. S.; Luke, G. M.; Zheng, D.; Jin, C. Q.; Siegrist, T.; Maple, M. B.; Uemura, Y. J.; Morosan, E.
2015-01-01
A quantum critical point (QCP) occurs upon chemical doping of the weak itinerant ferromagnet Sc3.1In . Remarkable for a system with no local moments, the QCP is accompanied by non-Fermi liquid behavior, manifested in the logarithmic divergence of the specific heat both in the ferro-and the paramagnetic states, as well as linear temperature dependence of the low-temperature resistivity. With doping, critical scaling is observed close to the QCP, as the critical exponents δ , γ , and β have weak composition dependence, with δ nearly twice and β almost half of their respective mean-field values. The unusually large paramagnetic moment μPM˜1.3 μB/F .U . is nearly composition independent. Evidence for strong spin fluctuations, accompanying the QCP at xc=0.035 ±0.005 , may be ascribed to the reduced dimensionality of Sc3.1In , associated with the nearly one-dimensional Sc-In chains.
Magnetically Defined Qubits on 3D Topological Insulators
NASA Astrophysics Data System (ADS)
Ferreira, Gerson J.; Loss, Daniel
2013-09-01
We explore potentials that break time-reversal symmetry to confine the surface states of 3D topological insulators into quantum wires and quantum dots. A magnetic domain wall on a ferromagnet insulator cap layer provides interfacial states predicted to show the quantum anomalous Hall effect (QAHE). Here, we show that confinement can also occur at magnetic domain heterostructures, with states extended in the inner domain, as well as interfacial QAHE states at the surrounding domain walls. The proposed geometry allows the isolation of the wire and dot from spurious circumventing surface states. For the quantum dots, we find that highly spin-polarized quantized QAHE states at the dot edge constitute a promising candidate for quantum computing qubits.
Topological crystalline insulators.
Fu, Liang
2011-03-11
The recent discovery of topological insulators has revived interest in the band topology of insulators. In this Letter, we extend the topological classification of band structures to include certain crystal point group symmetry. We find a class of three-dimensional "topological crystalline insulators" which have metallic surface states with quadratic band degeneracy on high symmetry crystal surfaces. These topological crystalline insulators are the counterpart of topological insulators in materials without spin-orbit coupling. Their band structures are characterized by new topological invariants. We hope this work will enlarge the family of topological phases in band insulators and stimulate the search for them in real materials.
State-of-the-Art Highly Insulating Window Frames - Research and Market Review
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
Nature of the Insulating Ground State of the Two-Dimensional Sn Atom Lattice on SiC(0001).
Yi, Seho; Lee, Hunpyo; Choi, Jin-Ho; Cho, Jun-Hyung
2016-07-28
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).
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.
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.
NASA Astrophysics Data System (ADS)
Kitagawa, Shunsaku; Sekiya, Taishi; Araki, Shingo; Kobayashi, Tatsuo C.; Ishida, Kenji; Kambe, Takashi; Kimura, Takumi; Nishimoto, Naoki; Kudo, Kazutaka; Nohara, Minoru
2015-09-01
The mineral tetrahedrite Cu12Sb4S13 exhibits a first-order metal-insulator transition (MIT) at TMI = 85 K and ambient pressure. We measured the 63Cu-NMR at ambient pressure and the resistivity and magnetic susceptibility at high pressures. 63Cu-NMR results indicate a nonmagnetic insulating ground state in this compound. The MIT is monotonically suppressed by pressure and disappears at ˜1.0 GPa. Two other anomalies are observed in the resistivity measurements, and the pressure-temperature phase diagram of Cu12Sb4S13 is constructed.
NASA Astrophysics Data System (ADS)
Baidya, Santu; Waghmare, Umesh V.; Paramekanti, Arun; Saha-Dasgupta, Tanusri
2016-10-01
Towards the goal of realizing topological phases in thin films of correlated oxide and heterostructures, we propose here a quantum anomalous Hall insulator (QAHI) in ultrathin films of double perovskites based on mixed 3 d -5 d or 3 d -4 d transition-metal ions, grown along the [111] direction. Considering the specific case of ultrathin Ba2FeReO6 , we present a theoretical analysis of an effective Hamiltonian derived from first principles. We establish that a strong spin-orbit coupling at the Re site, t2 g symmetry of the low-energy d bands, polarity of its [111] orientation of perovskite structure, and mixed 3 d -5 d chemistry results in room temperature magnetism with a robust QAHI state of Chern number C =1 and a large band gap. We uncover and highlight a nonrelativistic orbital Rashba-type effect in addition to the spin-orbit coupling, that governs this QAHI state. With a band gap of ˜100 meV in electronic structure and magnetic transition temperature Tc˜300 K estimated by Monte Carlo simulations, our finding of the QAHI state in ultrathin Ba2FeReO6 is expected to stimulate experimental verification along with possible practical applications of its dissipationless edge currents.
NASA Astrophysics Data System (ADS)
Siu, Zhuo Bin; Chowdhury, Debashree; Basu, Banasri; Jalil, Mansoor B. A.
2017-08-01
A topological insulator (TI) thin film differs from the more typically studied thick TI system in that the former has both a top and a bottom surface where the states localized at both surfaces can couple to one other across the finite thickness. An out-of-plane magnetic field leads to the formation of discrete Landau level states in the system, whereas an in-plane magnetization breaks the angular momentum symmetry of the system. In this work, we study the spin accumulation induced by the application of an in-plane electric field to the TI thin film system where the Landau level states and inter-surface coupling are simultaneously present. We show, via Kubo formula calculations, that the in-plane spin accumulation perpendicular to the magnetization due to the electric field vanishes for a TI thin film with symmetric top and bottom surfaces. A finite in-plane spin accumulation perpendicular to both the electric field and magnetization emerges upon applying either a differential magnetization coupling or a potential difference between the two film surfaces. This spin accumulation results from the breaking of the antisymmetry of the spin accumulation around the k-space equal-energy contours.
Parente, Vincenzo; Campagnano, Gabriele; Giuliano, Domenico; Tagliacozzo, Arturo; Guinea, Francisco
2014-03-04
The scattering of Dirac electrons by topological defects could be one of the most relevant sources of resistance in graphene and at the boundary surfaces of a three-dimensional topological insulator (3D TI). In the long wavelength, continuous limit of the Dirac equation, the topological defect can be described as a distortion of the metric in curved space, which can be accounted for by a rotation of the Gamma matrices and by a spin connection inherited with the curvature. These features modify the scattering properties of the carriers. We discuss the self-energy of defect formation with this approach and the electron cross-section for intra-valley scattering at an edge dislocation in graphene, including corrections coming from the local stress. The cross-section contribution to the resistivity, ρ, is derived within the Boltzmann theory of transport. On the same lines, we discuss the scattering of a screw dislocation in a two-band 3D TI, like Bi1-xSbx, and we present the analytical simplified form of the wavefunction for gapless helical states bound at the defect. When a 3D TI is sandwiched between two even-parity superconductors, Dirac boundary states acquire superconductive correlations by proximity. In the presence of a magnetic vortex piercing the heterostructure, two Majorana states are localized at the two interfaces and bound to the vortex core. They have a half integer total angular momentum each, to match with the unitary orbital angular momentum of the vortex charge.
High-field magnetic behavior and forced-ferromagnetic state in an ErF e11TiH single crystal
NASA Astrophysics Data System (ADS)
Kostyuchenko, N. V.; Zvezdin, A. K.; Tereshina, E. A.; Skourski, Y.; Doerr, M.; Drulis, H.; Pelevin, I. A.; Tereshina, I. S.
2015-09-01
The crystal-field and exchange parameters are determined for the single-crystalline hydride ErF e11TiH compound by analyzing the experimental magnetization curves obtained in magnetic fields of up to 60 T. By using the calculated parameters we succeeded in modeling theoretical magnetization curves for ErF e11TiH up to 200 S and to study in detail the transition from ferrimagnetic to a ferromagnetic state in the applied magnetic field.
NASA Astrophysics Data System (ADS)
Agrapidis, Cliò Efthimia; Drechsler, Stefan-Ludwig; van den Brink, Jeroen; Nishimoto, Satoshi
2017-06-01
Motivated by the magnetic properties of the spin-chain compounds LiCuSbO4≡LiSbCuO4 and Rb2Cu2Mo3O12 , we study the ground state of the Heisenberg chain with dimerized nearest-neighbor ferromagnetic (FM) (J1,J1'<0 ) and next-nearest-neighbor antiferromagnetic (J2>0 ) couplings. Using the density-matrix renormalization group technique and spin-wave theory, we find a first-order transition between a fully polarized FM and an incommensurate spiral state at 2 α =β /(1 +β ) , where α is the frustration ratio J2/|J1| and β the degree of dimerization J1'/J1 . In the singlet spiral state the spin-gap is vanishingly small in the vicinity of the FM transition, corresponding to a situation of LiCuSbO4. For larger α , corresponding to Rb2Cu2Mo3O12 , and smaller β there is a crossover from this frustration induced incommensurate state to an Affleck-Lieb-Kennedy-Tasaki-type valence-bond solid state with substantial spin gaps.
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.
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.
Magnetically Defined Qubits on 3D Topological Insulators
NASA Astrophysics Data System (ADS)
Ferreira, Gerson J.; Loss, Daniel
2014-03-01
We explore potentials that break time-reversal symmetry to confine the surface states of 3D topological insulators into quantum wires and quantum dots. A magnetic domain wall on a ferromagnet insulator cap layer provides interfacial states predicted to show the quantum anomalous Hall effect. Here, we show that confinement can also occur at magnetic domain heterostructures, with states extended in the inner domain, as well as interfacial QAHE states at the surrounding domain walls. The proposed geometry allows the isolation of the wire and dot from spurious circumventing surface states. For the quantum dots, we find that highly spin-polarized quantized QAHE states at the dot edge constitute a promising candidate for quantum computing qubits. See [Ferreira and Loss, Phys. Rev. Lett. 111, 106802 (2013)]. We explore potentials that break time-reversal symmetry to confine the surface states of 3D topological insulators into quantum wires and quantum dots. A magnetic domain wall on a ferromagnet insulator cap layer provides interfacial states predicted to show the quantum anomalous Hall effect. Here, we show that confinement can also occur at magnetic domain heterostructures, with states extended in the inner domain, as well as interfacial QAHE states at the surrounding domain walls. The proposed geometry allows the isolation of the wire and dot from spurious circumventing surface states. For the quantum dots, we find that highly spin-polarized quantized QAHE states at the dot edge constitute a promising candidate for quantum computing qubits. See [Ferreira and Loss, Phys. Rev. Lett. 111, 106802 (2013)]. We acknowledge support from the Swiss NSF, NCCR Nanoscience, NCCR QSIT, and the Brazillian Research Support Center Initiative (NAP Q-NANO) from Pró-Reitoria de Pesquisa (PRP/USP).
Non-destructive testing of biaxial stress state in ferromagnetic materials
NASA Astrophysics Data System (ADS)
Vengrinovich, V. L.; Vintov, D. A.; Dmitrovich, D. V.
2014-02-01
The technique for biaxial stress state quantitative non destructive testing using magnetic, namely Barkhausen Noise, measurements is developed and checked experimentally. The main elaboration concerns the application of uni-axial calibration data for bi-axial stress measurement in the material which treatment pre-history is not definitely known. The article is aimed to get over difficulties, accompanying factual nondestructive stress evaluation, implied from its tensor nature. The developed technique of stress calibration and measurement assumes the bi-axial stress components recovery from uni-axial magnetic and Barkhausen noise measurement results. The complete technology, based on new calibration procedure with grid diagrams is considered in the article.
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.
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.
Magnetic impurities on the surface of a topological insulator
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.
Surface states scattering from a step defect in the topological insulator Bi2Te3
NASA Astrophysics Data System (ADS)
An, Jin; Ting, C. S.
2012-10-01
We study theoretically, using a quantum-mechanical approach, the general scattering problem of a straight step defect on the surface of the topological insulator Bi2Te3 with a strong warping effect. At high energy where the warping effect is large, an electron incident on a step defect running along Γ-M may exhibit perfect transmission whereas an electron incident on a defect running along Γ-K has a finite probability of being reflected and may even exhibit resonant total reflection. The transmission in the latter case is also sensitive to whether there is particle-hole symmetry in the system. Although backscattering is prohibited by time-reversal symmetry, an electron incident nearly normally on a defect running along Γ-K has a finite reflection. This is interpreted as the consequence of the existence of decaying modes localized at the defect, acting as a magnetic barrier for the propagating modes. The predicted Friedel oscillations and the power-law decay behavior of the local density of states (LDOS) near the defect are in good agreement with recent scanning tunneling microscopy experiments on Bi2Te3. The high-energy LDOS of the surface states is also found to show multiperiodic Friedel oscillations, caused by the competing characteristic scattering processes.
Evidence of topological insulator state in the semimetal LaBi
NASA Astrophysics Data System (ADS)
Lou, R.; Fu, B.-B.; Xu, Q. N.; Guo, P.-J.; Kong, L.-Y.; Zeng, L.-K.; Ma, J.-Z.; Richard, P.; Fang, C.; Huang, Y.-B.; Sun, S.-S.; Wang, Q.; Wang, L.; Shi, Y.-G.; Lei, H. C.; Liu, K.; Weng, H. M.; Qian, T.; Ding, H.; Wang, S.-C.
2017-03-01
By employing angle-resolved photoemission spectroscopy combined with first-principles calculations, we performed a systematic investigation on the electronic structure of LaBi, which exhibits extremely large magnetoresistance (XMR), and is theoretically predicted to possess band anticrossing with nontrivial topological properties. Here, the observations of the Fermi-surface topology and band dispersions are similar to previous studies on LaSb [L.-K. Zeng, R. Lou, D.-S. Wu, Q. N. Xu, P.-J. Guo, L.-Y. Kong, Y.-G. Zhong, J.-Z. Ma, B.-B. Fu, P. Richard, P. Wang, G. T. Liu, L. Lu, Y.-B. Huang, C. Fang, S.-S. Sun, Q. Wang, L. Wang, Y.-G. Shi, H. M. Weng, H.-C. Lei, K. Liu, S.-C. Wang, T. Qian, J.-L. Luo, and H. Ding, Phys. Rev. Lett. 117, 127204 (2016), 10.1103/PhysRevLett.117.127204], a topologically trivial XMR semimetal, except the existence of a band inversion along the Γ -X direction, with one massless and one gapped Dirac-like surface state at the X and Γ points, respectively. The odd number of massless Dirac cones suggests that LaBi is analogous to the time-reversal Z2 nontrivial topological insulator. These findings open up a new series for exploring novel topological states and investigating their evolution from the perspective of topological phase transition within the family of rare-earth monopnictides.
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
A Low-Noise Solid-State Nanopore Platform Based on a Highly Insulating Substrate
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
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).
Exotic surface states in hybrid structures of topological insulators and Weyl semimetals
NASA Astrophysics Data System (ADS)
Juergens, Stefan; Trauzettel, Björn
2017-02-01
Topological insulators (TIs) and Weyl semimetals (WSMs) are two realizations of topological matter usually appearing separately in nature. However, they are directly related to each other via a topological phase transition. In this paper, we investigate the question whether these two topological phases can exist together at the same time, with a combined, hybrid surface state at the joint boundaries. We analyze effective models of a three-dimensional TI and an inversion symmetric WSM and couple them in a way that certain symmetries, like inversion, are preserved. A tunnel coupling approach enables us to obtain the hybrid surface state Hamiltonian analytically. This offers the possibility of a detailed study of its dispersion relation depending on the investigated couplings. For spin-symmetric coupling, we find that two Dirac nodes can emerge out of the combination of a single Dirac node and a Fermi arc. For spin-asymmetric coupling, the dispersion relation is gapped and the former Dirac node gets spin-polarized. We propose different experimental realization of the hybrid system, including compressively strained HgTe as well as heterostructures of TI and WSM materials.
Spin-orbit driven magnetic insulating state with Jeff=1/2 character in a 4d oxide
Calder, S.; Li, Ling; Okamoto, Satoshi; ...
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
Substitution-induced spin-splitted surface states in topological insulator (Bi1−xSbx)2Te3
He, Xiaoyue; Li, Hui; Chen, Lan; Wu, Kehui
2015-01-01
We present a study on surface states of topological insulator (Bi1−xSbx)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
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.
Ramp-edge structured tunneling devices using ferromagnet electrodes
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.
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.
Quantum anomalous Hall effect in magnetic insulator heterostructure.
Xu, Gang; Wang, Jing; Felser, Claudia; Qi, Xiao-Liang; Zhang, Shou-Cheng
2015-03-11
On the basis of ab initio calculations, we predict that a monolayer of Cr-doped (Bi,Sb)2Te3 and GdI2 heterostructure is a quantum anomalous Hall insulator with a nontrivial band gap up to 38 meV. The principle behind our prediction is that the band inversion between two topologically trivial ferromagnetic insulators can result in a nonzero Chern number, which offers a better way to realize the quantum anomalous Hall state without random magnetic doping. In addition, a simple effective model is presented to describe the basic mechanism of spin polarized band inversion in this system. Moreover, we predict that 3D quantum anomalous Hall insulator could be realized in (Bi2/3Cr1/3)2Te3 /GdI2 superlattice.
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
The public health benefits of insulation retrofits in existing housing in the United States
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
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.
Park, Wan Kyu; Sun, Lunan; Noddings, Alexander; Kim, Dae-Jeong; Fisk, Zachary; Greene, Laura H.
2016-01-01
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
Parente, Vincenzo; Campagnano, Gabriele; Giuliano, Domenico; Tagliacozzo, Arturo; Guinea, Francisco
2014-01-01
The scattering of Dirac electrons by topological defects could be one of the most relevant sources of resistance in graphene and at the boundary surfaces of a three-dimensional topological insulator (3D TI). In the long wavelength, continuous limit of the Dirac equation, the topological defect can be described as a distortion of the metric in curved space, which can be accounted for by a rotation of the Gamma matrices and by a spin connection inherited with the curvature. These features modify the scattering properties of the carriers. We discuss the self-energy of defect formation with this approach and the electron cross-section for intra-valley scattering at an edge dislocation in graphene, including corrections coming from the local stress. The cross-section contribution to the resistivity, ρ, is derived within the Boltzmann theory of transport. On the same lines, we discuss the scattering of a screw dislocation in a two-band 3D TI, like Bi1−xSbx, and we present the analytical simplified form of the wavefunction for gapless helical states bound at the defect. When a 3D TI is sandwiched between two even-parity superconductors, Dirac boundary states acquire superconductive correlations by proximity. In the presence of a magnetic vortex piercing the heterostructure, two Majorana states are localized at the two interfaces and bound to the vortex core. They have a half integer total angular momentum each, to match with the unitary orbital angular momentum of the vortex charge. PMID:28788537
Wu, Liang; Tse, Wang-Kong; Brahlek, M; Morris, C M; Aguilar, R Valdés; Koirala, N; Oh, S; Armitage, N P
2015-11-20
We have utilized time-domain magnetoterahertz spectroscopy to investigate the low-frequency optical response of the topological insulator Cu_{0.02}Bi_{2}Se_{3} and Bi_{2}Se_{3} films. With both field and frequency dependence, such experiments give sufficient information to measure the mobility and carrier density of multiple conduction channels simultaneously. We observe sharp cyclotron resonances (CRs) in both materials. The small amount of Cu incorporated into the Cu_{0.02}Bi_{2}Se_{3} induces a true bulk insulator with only a single type of conduction with a total sheet carrier density of ~4.9×10^{12}/cm^{2} and mobility as high as 4000 cm^{2}/V·s. This is consistent with conduction from two virtually identical topological surface states (TSSs) on the top and bottom of the film with a chemical potential ~145 meV above the Dirac point and in the bulk gap. The CR broadens at high fields, an effect that we attribute to an electron-phonon interaction. This assignment is supported by an extended Drude model analysis of the zero-field Drude conductance. In contrast, in normal Bi_{2}Se_{3} films, two conduction channels were observed, and we developed a self-consistent analysis method to distinguish the dominant TSSs and coexisting trivial bulk or two-dimensional electron gas states. Our high-resolution Faraday rotation spectroscopy on Cu_{0.02}Bi_{2}Se_{3} paves the way for the observation of quantized Faraday rotation under experimentally achievable conditions to push the chemical potential in the lowest Landau level.
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.
Spintronics Based on Topological Insulators
NASA Astrophysics Data System (ADS)
Fan, Yabin; Wang, Kang L.
2016-10-01
Spintronics using topological insulators (TIs) as strong spin-orbit coupling (SOC) materials have emerged and shown rapid progress in the past few years. Different from traditional heavy metals, TIs exhibit very strong SOC and nontrivial topological surface states that originate in the bulk band topology order, which can provide very efficient means to manipulate adjacent magnetic materials when passing a charge current through them. In this paper, we review the recent progress in the TI-based magnetic spintronics research field. In particular, we focus on the spin-orbit torque (SOT)-induced magnetization switching in the magnetic TI structures, spin-torque ferromagnetic resonance (ST-FMR) measurements in the TI/ferromagnet structures, spin pumping and spin injection effects in the TI/magnet structures, as well as the electrical detection of the surface spin-polarized current in TIs. Finally, we discuss the challenges and opportunities in the TI-based spintronics field and its potential applications in ultralow power dissipation spintronic memory and logic devices.
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
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 Pb_{1-x}Sn_{x}Te 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.
NASA Astrophysics Data System (ADS)
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
2015-05-01
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.
Zhong, Ruidan; He, Xugang; Schneeloch, J. A.; ...
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
Zhang, Qianfan; Zhang, Zhiyong; Zhu, Zhiyong; Schwingenschlögl, Udo; Cui, Yi
2012-03-27
Topological insulator is a new state of matter attracting tremendous interest due to its gapless linear dispersion and spin momentum locking topological states located near the surface. Heterostructures, which have traditionally been powerful in controlling the electronic properties of semiconductor devices, are interesting for topological insulators. Here, we studied the spatial distribution of the topological state in Sb(2)Se(3)-Bi(2)Se(3) heterostructures by first-principle simulation and discovered that an exotic topological state exists. Surprisingly, the state migrates from the nontrivial Bi(2)Se(3) into the trivial Sb(2)Se(3) region and spreads across the entire Sb(2)Se(3) slab, extending beyond the concept of "surface" state while preserving all of the topological surface state characteristics. This unusual topological state arises from the coupling between different materials and the modification of electronic structure near Fermi energy. Our study demonstrates that heterostructures can open up opportunities for controlling the real-space distribution of the topological state and inducing quantum phase transitions between topologically trivial and nontrivial states.
Odd-frequency superconductivity induced in topological insulators with and without hexagonal warping
NASA Astrophysics Data System (ADS)
Vasenko, A. S.; Golubov, A. A.; Silkin, V. M.; Chulkov, E. V.
2017-07-01
We study the effect of the Fermi surface anisotropy on the odd-frequency spin-triplet pairing component of the induced pair potential. We consider a superconductor/ ferromagnetic insulator (S/FI) hybrid structure formed on the 3D topological insulator (TI) surface. In this case three ingredients ensure the possibility of the odd-frequency pairing: (1) the topological surface states, (2) the induced pair potential, and (3) the magnetic moment of a nearby ferromagnetic insulator. We take into account the strong anisotropy of the Dirac point in topological insulators when the chemical potential lies well above the Dirac cone and its constant energy contour has a snowflake shape. Within this model, we propose that the S/FI boundary should be properly aligned with respect to the snowflake constant energy contour to have an odd-frequency symmetry of the corresponding pairing component and to insure the Majorana bound state at the S/FI boundary. For arbitrary orientation of the boundary, the Majorana bound state is absent. This provides a selection rule to the realization of Majorana modes in S/FI hybrid structures, formed on the topological insulator surface.
Vasenko, A S; Golubov, A A; Silkin, V M; Chulkov, E V
2017-07-26
We study the effect of the Fermi surface anisotropy on the odd-frequency spin-triplet pairing component of the induced pair potential. We consider a superconductor/ ferromagnetic insulator (S/FI) hybrid structure formed on the 3D topological insulator (TI) surface. In this case three ingredients ensure the possibility of the odd-frequency pairing: (1) the topological surface states, (2) the induced pair potential, and (3) the magnetic moment of a nearby ferromagnetic insulator. We take into account the strong anisotropy of the Dirac point in topological insulators when the chemical potential lies well above the Dirac cone and its constant energy contour has a snowflake shape. Within this model, we propose that the S/FI boundary should be properly aligned with respect to the snowflake constant energy contour to have an odd-frequency symmetry of the corresponding pairing component and to insure the Majorana bound state at the S/FI boundary. For arbitrary orientation of the boundary, the Majorana bound state is absent. This provides a selection rule to the realization of Majorana modes in S/FI hybrid structures, formed on the topological insulator surface.
Surface Kondo effect and non-trivial metallic state of the Kondo insulator YbB12
NASA Astrophysics Data System (ADS)
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
2016-08-01
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.
Surface Kondo effect and non-trivial metallic state of the Kondo insulator YbB12
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
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.
Topological properties of ferromagnetic superconductors
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
Topological properties of ferromagnetic superconductors
Cheung, Alfred K. C.; Raghu, S.
2016-04-27
Here, a variety of heavy fermion superconductors, such as UCoGe, UGe^{2}, 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.
Topological properties of ferromagnetic superconductors
Cheung, Alfred K. C.; Raghu, S.
2016-04-27
Here, a variety of heavy fermion superconductors, such as UCoGe, UGe^{2}, 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.
Origin of bulk quantum oscillations in the bulk Kondo insulating ground state of SmB6
NASA Astrophysics Data System (ADS)
Sebastian, Suchitra; Tan, B. S.; Hsu, Y.-T.; Zeng, B.; Ciomaga Hatnean, M.; Harrison, N.; Zhu, Z.; Hartstein, M.; Kiourlappou, M.; Srivastava, M.; Johannes, M. D.; Murphy, T. P.; Park, J.-H.; Balicas, L.; Shitsevalova, N.; Lonzarich, G. G.; Balakrishnan, G.
I will discuss our recent observation of quantum oscillations corresponding to a bulk Fermi surface in the Kondo insulator SmB6, and consider their possible origin. New complementary experimental results will be presented which raise the interesting question of whether the underlying ground state corresponds to a novel Kondo regime in which the spin channel is gapless while the charge channel is gapped.
Bhunia, Hrishikesh; Bar, Abhijit; Bera, Abhijit; Pal, Amlan J
2017-04-12
A 2D form of Bi2Se3 which acts as a topological insulator was grown through colloidal synthesis method. The surface-states and edge-states of the nanoplates were simultaneously probed through scanning tunneling spectroscopy (STS). At the interior, density of states (DOS) revealed the location of conduction and valence band edges. The DOS at the edges, on the other hand, brought out gapless conducting states along with a Dirac point at a non-zero value below the Fermi energy representing the Dirac cone of a 2D topological insulator. In differential tunnel conductance (dI/dV), images are recorded at different voltages and the two sections of the topological insulator can be viewed selectively or simultaneously with a clear contrast in illumination. Upon increasing the 2D-nanoplates thickness, the material turned into a 3D topological insulator with gapless surface states.
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.
A new ''surface-states'' mode of switching in M-thin insulator-n-p + devices
NASA Astrophysics Data System (ADS)
Habib, Serag E.-D.; Eltoukhy, A. A.
1981-04-01
The metal-insulator (tunnel) -n-p device (MISS) is known to display current-controlled negative resistance in its I-V characteristics. This negative resistance behavior originates from a positive feedback interaction between the p-n junction and the tunnel metal insulator semiconductor (MIS) parts of the device. The simple structure of the MISS and its amenability to integration has so far been masked by the incomplete understanding of the physics of operation of the device. In this paper, the effect of the surface states at the semiconductor-insulator interface on the MISS behavior is considered. It is shown that the bias dependence of the charge stored in the surface states reduces the switching voltage Vw below the punch-through or avalanche breakdown limits. A simplified model of the MISS is developed taking into account the different roles of the surface states as charge storage centers, recombination centers, and tunnel-current carrying sites. This model is believed to bridge the gap between the previous theoretical models and experimental works.
The direct magnetoelectric effect in ferroelectric-ferromagnetic epitaxial heterostructures
NASA Astrophysics Data System (ADS)
Fina, I.; Dix, N.; Rebled, J. M.; Gemeiner, P.; Martí, X.; Peiró, F.; Dkhil, B.; Sánchez, F.; Fàbrega, L.; Fontcuberta, J.
2013-08-01
Ferroelectric (FE) and ferromagnetic (FM) materials engineered in horizontal heterostructures allow interface-mediated magnetoelectric coupling. The so-called converse magnetoelectric effect (CME) has been already demonstrated by electric-field poling of the ferroelectric layers and subsequent modification of the magnetic state of adjacent ferromagnetic layers by strain effects and/or free-carrier density tuning. Here we focus on the direct magnetoelectric effect (DME) where the dielectric state of a ferroelectric thin film is modified by a magnetic field. Ferroelectric BaTiO3 (BTO) and ferromagnetic CoFe2O4 (CFO) oxide thin films have been used to create epitaxial FE/FM and FM/FE heterostructures on SrTiO3(001) substrates buffered with metallic SrRuO3. It will be shown that large ferroelectric polarization and DME can be obtained by appropriate selection of the stacking order of the FE and FM films and their relative thicknesses. The dielectric permittivity, at the structural transitions of BTO, is strongly modified (up to 36%) when measurements are performed under a magnetic field. Due to the insulating nature of the ferromagnetic layer and the concomitant absence of the electric-field effect, the observed DME effect solely results from the magnetostrictive response of CFO elastically coupled to the BTO layer. These findings show that appropriate architecture and materials selection allow overcoming substrate-induced clamping in multiferroic multi-layered films.Ferroelectric (FE) and ferromagnetic (FM) materials engineered in horizontal heterostructures allow interface-mediated magnetoelectric coupling. The so-called converse magnetoelectric effect (CME) has been already demonstrated by electric-field poling of the ferroelectric layers and subsequent modification of the magnetic state of adjacent ferromagnetic layers by strain effects and/or free-carrier density tuning. Here we focus on the direct magnetoelectric effect (DME) where the dielectric state of a
NASA Astrophysics Data System (ADS)
Zheng, Guolin; Wang, Ning; Yang, Jiyong; Wang, Weike; Du, Haifeng; Ning, Wei; Yang, Zhaorong; Lu, Hai-Zhou; Zhang, Yuheng; Tian, Mingliang
2016-02-01
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.
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
NASA Astrophysics Data System (ADS)
He, James Jun; Zhou, Tong; Gu, Z. C.; Law, K. T.
When a magnetic field is applied to a quantum spin Hall insulator (QSHI) without inversion symmetry, the edge states become gapful due to the breaking of time reversal symmetry (TRS) and the QSHI becomes a trivial spin Hall insulator (SHI) whose Chern number is N = 0 . In this work we show that disorder can drive such a SHI to a Chern insulator (CI) with N = 1 which supports a gapless chiral edge state. This CI exists in a finite range of disorder strength. Interestingly, the edge state is protected by the bulk mobility gap instead of an energy gap. For this reason, the new phase is called an Anderson Chern insulator (ACI).
Li, Hai; Zhao, Yuan Yuan
2017-10-02
In the framework of the Bogoliubov-de Gennes equation, we investigate the thermal transport properties in topological-insulator-based superconducting hybrid structures with mixed spin-singlet and spin-triplet pairing states, and emphasize the different manifestations of the spin-singlet and spin-triplet pairing states in the thermal transport signatures. It is revealed that the temperature-dependent differential thermal conductance strongly depends on the components of the pairing state, and the negative differential thermal conductance only occurs in the spin-singlet pairing state dominated regime. It is also found that the thermal conductance is profoundly sensitive to the components of the pairing state. In the spin-singlet pairing state controlled regime, the thermal conductance obviously oscillates with the phase difference and junction length. With increasing the proportion of the spin-triplet pairing state, the oscillating characteristic of the thermal conductance fades out distinctly. These results suggest an alternative route for distinguishing the components of pairing states in topological-insulator-based superconducting hybrid structures. © 2017 IOP Publishing Ltd.
Quantum anomalous Hall effect in magnetic topological insulators
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.
Quantum anomalous Hall effect in magnetic topological insulators
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
Magnetism and Metal-Insulator Transition in Fe(Sb1−xTex)2
Petrovic, C.; Hu, R.; Mitrovic, V.F.
2009-02-09
We have investigated structural, magnetic, and transport properties of Fe(Sb{sub 1-x}Te{sub x}){sub 2} single crystals. Whereas metallic ground state is induced for x = 0.001, canted antiferromagnetism is observed for 0.1 {le} x {le} 0.4 with an intermediate ferromagnetic phase for x = 0.2. With higher Te doping, semiconducting behavior is restored and the variable range hopping conduction mechanism dominates at low temperatures for 0.4 {le} x {le} 0.6. We discuss our results within the framework of inverted metal to insulator in correlated electron insulators.
Emergent Momentum-Space Skyrmion Texture on the Surface of Topological Insulators.
Mohanta, Narayan; Kampf, Arno P; Kopp, Thilo
2017-04-05
The quantum anomalous Hall effect has been theoretically predicted and experimentally verified in magnetic topological insulators. In addition, the surface states of these materials exhibit a hedgehoglike "spin" texture in momentum space. Here, we apply the previously formulated low-energy model for Bi2Se3, a parent compound for magnetic topological insulators, to a slab geometry in which an exchange field acts only within one of the surface layers. In this sample set up, the hedgehog transforms into a skyrmion texture beyond a critical exchange field. This critical field marks a transition between two topologically distinct phases. The topological phase transition takes place without energy gap closing at the Fermi level and leaves the transverse Hall conductance unchanged and quantized to e(2)/2h. The momentum-space skyrmion texture persists in a finite field range. It may find its realization in hybrid heterostructures with an interface between a three-dimensional topological insulator and a ferromagnetic insulator.
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.
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.