Surface Plasmons in 3D Topological Insulators
NASA Astrophysics Data System (ADS)
Kogar, Anshul; Vig, Sean; Cho, Gil; Thaler, Alexander; Xiao, Yiran; Hughes, Taylor; Wong, Man-Hong; Chiang, Tai-Chang; MacDougall, Greg; Abbamonte, Peter
2015-03-01
Most studies of three-dimensional (3D) topological insulators have concentrated on their one-electron properties as exhibited by angle-resolved photoemission spectroscopy (ARPES) or by scanning tunneling microscopy (STM). Many-body interactions are often neglected in the treatment of models of topological insulators, such as in the Kane-Mele and Bernevig-Hughes-Zhang models. Using angle-resolved inelastic electron scattering from the surface, I will present data on the collective mode that owes its existence to the presence of many-body interactions, the surface plasmon (SP), in two known 3D topological insulators, Bi2Se3 and Bi0.5Sb1.5Se1 . 5 + xTe1 . 5 - x. Surprisingly, the SP was prominent even after depressing the Fermi energy into the bulk band gap. Having studied the SP as a function of doping, momentum transfer and its aging properties, I will present evidence to suggest that bulk-surface coupling is crucial in explaining many of its properties. A simple model with dynamic bulk screening will be presented showing qualitative agreement with the observations. Lastly, the relation of the observed surface plasmon to the predicted spin-plasmon mode and to the kinks seen in the electronic dispersion as measured by ARPES will be discussed. The work was supported as part of the Center for Emergent Superconductivity, an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science.
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.
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).
Spin-dependent Peltier effect in 3D topological insulators
NASA Astrophysics Data System (ADS)
Sengupta, Parijat; Kubis, Tillmann; Povolotskyi, Michael; Klimeck, Gerhard
2013-03-01
The Peltier effect represents the heat carrying capacity of a certain material when current passes through it. When two materials with different Peltier coefficients are placed together, the Peltier effect causes heat to flow either towards or away from the interface between them. This work utilizes the spin-polarized property of 3D topological insulator (TI) surface states to describe the transport of heat through the spin-up and spin-down channels. It has been observed that the spin channels are able to carry heat independently of each other. Spin currents can therefore be employed to supply or extract heat from an interface between materials with spin-dependent Peltier coefficients. The device is composed of a thin film of Bi2Se3 sandwiched between two layers of Bi2Te3. The thin film of Bi2Se3serves both as a normal and topological insulator. It is a normal insulator when its surfaces overlap to produce a finite band-gap. Using an external gate, Bi2Se3 film can be again tuned in to a TI. Sufficiently thick Bi2Te3 always retain TI behavior. Spin-dependent Peltier coefficients are obtained and the spin Nernst effect in TIs is shown by controlling the temperature gradient to convert charge current to spin current.
New Class of 3D Topological Insulator in Double Perovskite.
Pi, Shu-Ting; Wang, Hui; Kim, Jeongwoo; Wu, Ruqian; Wang, Yin-Kuo; Lu, Chi-Ken
2017-01-19
We predict a new class of 3D topological insulators (TIs) in which the spin-orbit coupling (SOC) can more effectively generate band gap. Band gap of conventional TI is mainly limited by two factors, the strength of SOC and, from electronic structure perspective, the band gap when SOC is absent. While the former is an atomic property, the latter can be minimized in a generic rock-salt lattice model in which a stable crossing of bands at the Fermi level along with band character inversion occurs in the absence of SOC. Thus large-gap TIs or TIs composed of lighter elements can be expected. In fact, we find by performing first-principles calculations that the model applies to a class of double perovskites A2BiXO6 (A = Ca, Sr, Ba; X = Br, I) and the band gap is predicted up to 0.55 eV. Besides, surface Dirac cones are robust against the presence of dangling bond at boundary.
Samarium Hexaboride: The First True 3D Topological Insulator?
NASA Astrophysics Data System (ADS)
Wolgast, Steven G.
The recent theoretical prediction of a topologically protected surface state in the mixed-valent insulator SmB6 has motivated a series of charge transport studies, which are presented here. It is first studied using a specialized configuration designed to distinguish bulk-dominated conduction from surface-dominated conduction. As the material is cooled below 4 K, it exhibits a crossover from thermally activated bulk transport to metallic surface conduction with a fully insulating bulk. The robustness and magnitude of the surface conductivity, as is manifest in the literature of SmB6, is strong evidence for the topological insulator (TI) metallic surface states predicted for this material. This resolves a decades-old puzzle surrounding the low-temperature behavior of SmB6. Next, the magnetotransport properties of the surface are investigated using a Corbino disk geometry, which can directly measure the conductivity of individual surfaces. Both (011) and (001) crystal surfaces show a strong negative magnetoresistance at all magnetic field angles, due primarily to changes in the carrier density. The low mobility value accounts for the failure so far to observe Shubnikov-de Haas oscillations below 95 T. Small variations in the mobility and temperature dependence suggest a suppression of Kondo scattering from native oxide-layer magnetic moments. At low fields, a dynamical field-sweep-rate-dependent hysteretic behavior is observed. It persists at the slowest sweep rates, and cannot be explained by quantum interference corrections; it is likely due to extrinsic effects such as the magnetocaloric effect or glassy ordering of the native oxide moments. Pulsed magnetic field measurements up to 60 T at temperatures throughout the crossover regime clearly distinguish the surface magnetoresistance from the bulk magnetoresistance. The bulk magnetoresistance is due to a reduction in the bulk gap with increasing magnetic field. Finally, small subsurface cracks formed in SmB6 via
Quantized Faraday and Kerr rotation and axion electrodynamics of a 3D topological insulator.
Wu, Liang; Salehi, M; Koirala, N; Moon, J; Oh, S; Armitage, N P
2016-12-02
Topological insulators have been proposed to be best characterized as bulk magnetoelectric materials that show response functions quantized in terms of fundamental physical constants. Here, we lower the chemical potential of three-dimensional (3D) Bi2Se3 films to ~30 meV above the Dirac point and probe their low-energy electrodynamic response in the presence of magnetic fields with high-precision time-domain terahertz polarimetry. For fields higher than 5 tesla, we observed quantized Faraday and Kerr rotations, whereas the dc transport is still semiclassical. A nontrivial Berry's phase offset to these values gives evidence for axion electrodynamics and the topological magnetoelectric effect. The time structure used in these measurements allows a direct measure of the fine-structure constant based on a topological invariant of a solid-state system.
Quantized Faraday and Kerr rotation and axion electrodynamics of a 3D topological insulator
NASA Astrophysics Data System (ADS)
Wu, Liang; Salehi, M.; Koirala, N.; Moon, J.; Oh, S.; Armitage, N. P.
2016-12-01
Topological insulators have been proposed to be best characterized as bulk magnetoelectric materials that show response functions quantized in terms of fundamental physical constants. Here, we lower the chemical potential of three-dimensional (3D) Bi2Se3 films to ~30 meV above the Dirac point and probe their low-energy electrodynamic response in the presence of magnetic fields with high-precision time-domain terahertz polarimetry. For fields higher than 5 tesla, we observed quantized Faraday and Kerr rotations, whereas the dc transport is still semiclassical. A nontrivial Berry’s phase offset to these values gives evidence for axion electrodynamics and the topological magnetoelectric effect. The time structure used in these measurements allows a direct measure of the fine-structure constant based on a topological invariant of a solid-state system.
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.
Strong interband Faraday rotation in 3D topological insulator Bi2Se3.
Ohnoutek, L; Hakl, M; Veis, M; Piot, B A; Faugeras, C; Martinez, G; Yakushev, M V; Martin, R W; Drašar, Č; Materna, A; Strzelecka, G; Hruban, A; Potemski, M; Orlita, M
2016-01-11
The Faraday effect is a representative magneto-optical phenomenon, resulting from the transfer of angular momentum between interacting light and matter in which time-reversal symmetry has been broken by an externally applied magnetic field. Here we report on the Faraday rotation induced in the prominent 3D topological insulator Bi2Se3 due to bulk interband excitations. The origin of this non-resonant effect, extraordinarily strong among other non-magnetic materials, is traced back to the specific Dirac-type Hamiltonian for Bi2Se3, which implies that electrons and holes in this material closely resemble relativistic particles with a non-zero rest mass.
Interfacing 2D and 3D Topological Insulators: Bi(111) Bilayer on Bi2Te3
NASA Astrophysics Data System (ADS)
Hirahara, Toru; Bihlmayer, Gustav; Sakamoto, Yusuke; Yamada, Manabu; Miyazaki, Hidetoshi; Kimura, Shin-Ichi; Blügel, Stefan; Hasegawa, Shuji
2012-02-01
Topological insulators (TI) are insulating materials but have metallic edge states that carry spin currents and are robust against nonmagnetic impurities [1]. While there have been a large number of reports on three-dimensional (3D) TI, only few works have been done in terms of two-dimensional (2D) TI. In the present paper, we report the successful formation of bilayer Bi, which was theoretically predicted to be a 2D TI [2]. We deposited bilayer Bi on a 3D TI Bi2Te3, which the lattice mismatch is very small. From angle-resolved photoemission spectroscopy measurements and ab initio calculations, the electronic structure of the system can be understood as an overlap of the band dispersions of bilayer Bi and Bi2Te3. Our results show that the Dirac cone is actually robust against nonmagnetic perturbations and imply a unique situation where the topologically protected one- and two-dimensional edge states are coexisting at the surface [3]. [0pt] [1] M. Z. Hasan and C. L. Kane, Rev. Mod. Phys. 82, 3045 (2010).[0pt] [2] S. Murakami, Phys. Rev. Lett. 97, 236805 (2006).[0pt] [3] T. Hirahara et al., Phys. Rev. Lett. 107, 166801 (2011).
Giant Faraday effect due to Pauli exclusion principle in 3D topological insulators.
Paudel, Hari P; Leuenberger, Michael N
2014-02-26
Experiments using ARPES, which is based on the photoelectric effect, show that the surface states in 3D topological insulators (TI) are helical. Here we consider Weyl interface fermions due to band inversion in narrow-bandgap semiconductors, such as Pb1-xSnxTe. The positive and negative energy solutions can be identified by means of opposite helicity in terms of the spin helicity operator in 3D TI as ĥ(TI) = (1/ |p|_ |) β (σ|_ x p|_ ) · z^, where β is a Dirac matrix and z^ points perpendicular to the interface. Using the 3D Dirac equation and bandstructure calculations we show that the transitions between positive and negative energy solutions, giving rise to electron-hole pairs, obey strict optical selection rules. In order to demonstrate the consequences of these selection rules, we consider the Faraday effect due to the Pauli exclusion principle in a pump-probe setup using a 3D TI double interface of a PbTe/Pb₀.₃₁Sn₀.₆₉Te/PbTe heterostructure. For that we calculate the optical conductivity tensor of this heterostructure, which we use to solve Maxwell's equations. The Faraday rotation angle exhibits oscillations as a function of probe wavelength and thickness of the heterostructure. The maxima in the Faraday rotation angle are of the order of mrds.
NASA Astrophysics Data System (ADS)
Oh, Seongshik
Topological insulator (TI) is one of the rare systems in the history of condensed matter physics that is initiated by theories and followed by experiments. Although this theory-driven advance helped move the field quite fast despite its short history, apparently there exist significant gaps between theories and experiments. Many of these discrepancies originate from the very fact that the worlds readily accessible to theories are often far from the real worlds that are available in experiments. For example, the very paradigm of topological protection of the surface states on Z2 TIs such as Bi2Se3, Bi2Te3, Sb2Te3, etc, is in fact valid only if the sample size is infinite and the crystal momentum is well-defined in all three dimensions. On the other hand, many widely studied forms of TIs such as thin films and nano-wires have significant confinement in one or more of the dimensions with varying level of disorders. In other words, many of the real world topological systems have some important parameters that are not readily captured by theories, and thus it is often questionable how far the topological theories are valid to real systems. Interestingly, it turns out that this very uncertainty of the theories provides additional control knobs that allow us to explore hidden topological territories. In this talk, I will discuss how these additional knobs in thin film topological insulators reveal surprising, at times beautiful, landscapes at the boundaries between order and disorder, 2D and 3D, normal and topological phases. This work is supported by Gordon and Betty Moore Foundation's EPiQS Initiative (GBMF4418).
Strong interband Faraday rotation in 3D topological insulator Bi2Se3
Ohnoutek, L.; Hakl, M.; Veis, M.; Piot, B. A.; Faugeras, C.; Martinez, G.; Yakushev, M. V.; Martin, R. W.; Drašar, Č.; Materna, A.; Strzelecka, G.; Hruban, A.; Potemski, M.; Orlita, M.
2016-01-01
The Faraday effect is a representative magneto-optical phenomenon, resulting from the transfer of angular momentum between interacting light and matter in which time-reversal symmetry has been broken by an externally applied magnetic field. Here we report on the Faraday rotation induced in the prominent 3D topological insulator Bi2Se3 due to bulk interband excitations. The origin of this non-resonant effect, extraordinarily strong among other non-magnetic materials, is traced back to the specific Dirac-type Hamiltonian for Bi2Se3, which implies that electrons and holes in this material closely resemble relativistic particles with a non-zero rest mass. PMID:26750455
Weakly-coupled quasi-1D helical modes in disordered 3D topological insulator quantum wires
Dufouleur, J.; Veyrat, L.; Dassonneville, B.; Xypakis, E.; Bardarson, J. H.; Nowka, C.; Hampel, S.; Schumann, J.; Eichler, B.; Schmidt, O. G.; Büchner, B.; Giraud, R.
2017-01-01
Disorder remains a key limitation in the search for robust signatures of topological superconductivity in condensed matter. Whereas clean semiconducting quantum wires gave promising results discussed in terms of Majorana bound states, disorder makes the interpretation more complex. Quantum wires of 3D topological insulators offer a serious alternative due to their perfectly-transmitted mode. An important aspect to consider is the mixing of quasi-1D surface modes due to the strong degree of disorder typical for such materials. Here, we reveal that the energy broadening γ of such modes is much smaller than their energy spacing Δ, an unusual result for highly-disordered mesoscopic nanostructures. This is evidenced by non-universal conductance fluctuations in highly-doped and disordered Bi2Se3 and Bi2Te3 nanowires. Theory shows that such a unique behavior is specific to spin-helical Dirac fermions with strong quantum confinement, which retain ballistic properties over an unusually large energy scale due to their spin texture. Our result confirms their potential to investigate topological superconductivity without ambiguity despite strong disorder. PMID:28374744
NASA Astrophysics Data System (ADS)
Sochnikov, Ilya; Kirtley, John R.; Moler, Kathryn A.; Maier, Luis; Bruene, Christoph; Buhmann, Hartmut; Molenkamp, Laurens W.
2014-03-01
Inducing superconductivity on the surface of a 3D topological insulator is important for novel broken symmetry phases. However, it is difficult to assess the existence of the surface superconductivity with a single experimental technique. We have used a scanning SQUID microscope to characterize the magnetic properties of hybrid structures made of the 3D topological insulator HgTe and superconducting Nb. The magnetic response of superconducting rings with exotic Josephson junctions reveals the current-phase relation, while measurements of bilayer HgTe/Nb disks reveal the total superfluid density of the hybrid structure. We analyze the degree of skew in the current-phase relation to determine the relative contribution of surface states, and discuss other contributions to the current-phase relation. This work sets an agenda for discussion of the prospects for detection of new broken symmetry phases in 3D topological insulators.
Extremely large, gate tunable spin Hall angle in 3D Topological Insulator pn junction
NASA Astrophysics Data System (ADS)
Habib, K. M. Masum; Sajjad, Redwan; Ghosh, Avik
2015-03-01
The band structure of the surface states of a three dimensional Topological Insulator (3D TI) is similar to that of graphene featuring massless Dirac Fermions. We show that due to this similarity, the chiral tunneling of electron in a graphene pn junction also appears in 3D TI. Electrons with very small incident angle (modes) are allowed to transmit through a TI pn junction (TIPNJ) due to the chiral tunneling. The rest of the electrons are reflected. As a result, the charge current in a TIPNJ is suppressed. Due to the spin momentum locking, all the small angle modes are spin-down states. Therefore, the transmitted end of the TIPNJ becomes highly spin polarized. On the other hand, the spin of the reflected electron is flipped due to spin momentum locking. This enhances the spin current at the injection end. Thus, the interplay between the chiral tunneling and spin momentum locking reduces the charge current but enhances the spin current at the same time, leading to an extremely large (~20) spin Hall angle. Since the chiral tunneling can be controlled by an external electric field, the spin Hall angle is gate tunable. The spin current generated by a TIPNJ can be used for energy-efficient switching of nanoscaled ferromagnets, which is an essential part of spintronic devices. This work is supported by the NRI INDEX center.
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.
Effects of Surface Morphology on the 3D Topological Insulator Samarium Hexaboride
NASA Astrophysics Data System (ADS)
Wolgast, Steven; Eo, Yun Suk; Kurdak, Cagliyan; Kim, Dae-Jeong; Fisk, Zachary
2015-03-01
The recent verification of a topologically-protected surface state in SmB6 at low temperatures has led to several transport studies of the surface states. This task is complicated because current can flow on all surfaces of a topological insulator, each of which can have different transport characteristics. Our own measurements using a Corbino disc geometry overcome this difficulty, limiting the conduction to individual surfaces. However, the sheet conductivities of our samples counter-intuitively decrease with finer surface polishing. We therefore investigate surface and sub-surface morphology as a factor affecting the surface conductivity. Specifically, surface cracks may themselves harbor surface states and contribute to the total electrical conduction, yielding a higher measured sheet conductivity than that of a flat surface. This situation may contribute to the (sometimes unphysically) large surface conductivities already observed in SmB6. Performed in part at the Lurie Nanofabrication Facility and the Electron Microbeam Analysis Laboratory. Funded by NSF Grant #DMR-1006500 and DMR-1441965. Thanks to Gang Li and Fan Yu for optical imaging.
Sensing Coulomb impurities with 1/f noise in 3D Topological Insulator
NASA Astrophysics Data System (ADS)
Bhattacharyya, Semonti; Banerjee, Mitali; Nhalil, Hariharan; Elizabeth, Suja; Ghosh, Arindam
2015-03-01
Electrical transport in the non-trivial surface states of bulk Topological Insulator (TI) reveal several intriguing properties ranging from bipolar field effect transistor action, weak antilocalization in quantum transport, to the recently discovered quantum anomalous Hall effect. Many of these phenomena depend crucially on the nature of disorder and its screening by the Dirac Fermions at the TI surface. We have carried out a systematic study of low-frequency 1/f noise in Bi1.6Sb0.4Te2Se1 single crystals, to explore the dominant source of scattering of surface electrons and monitor relative contributions of the surface and bulk channels. Our results reveal that while trapped coulomb impurities at the substrate-TI interface are dominating source of scattering for thin (10 nm) TI, charged crystal disorder contribute strongly in thick TI (110 nm) channels. An unexpected maximum at 25K in noise from thick TI devices indicate scattering of the surface states by a cooperative charge dynamics in the bulk of the TI, possibly associated with the Selenium vacancies. Our experiment demonstrates, for the first time, impact of the bulk charge distribution on the surface state transport in TIs that could be crucial to the implementation of these materials in electronic applications.
NASA Astrophysics Data System (ADS)
Wang, Na; Wang, JianFeng; Si, Chen; Gu, Bing-Lin; Duan, WenHui
2016-08-01
The introduction of magnetism in SnTe-class topological crystalline insulators is a challenging subject with great importance in the quantum device applications. Based on the first-principles calculations, we have studied the defect energetics and magnetic properties of 3 d transition-metal (TM)-doped SnTe. We find that the doped TM atoms prefer to stay in the neutral states and have comparatively high formation energies, suggesting that the uniform TMdoping in SnTe with a higher concentration will be difficult unless clustering. In the dilute doping regime, all the magnetic TMatoms are in the high-spin states, indicating that the spin splitting energy of 3 d TM is stronger than the crystal splitting energy of the SnTe ligand. Importantly, Mn-doped SnTe has relatively low defect formation energy, largest local magnetic moment, and no defect levels in the bulk gap, suggesting that Mn is a promising magnetic dopant to realize the magnetic order for the theoretically-proposed large-Chern-number quantum anomalous Hall effect (QAHE) in SnTe.
Synthesis and characterization of 3D topological insulators: a case TlBi(S1−xSex)2
Segawa, Kouji
2015-01-01
In this article, practical methods for synthesizing Tl-based ternary III-V-VI2 chalcogenide TlBi(SSex)2 are described in detail, along with characterization by x-ray diffraction and charge transport properties. The TlBi(SSex)2 system is interesting because it shows a topological phase transition, where a topologically nontrivial phase changes to a trivial phase without changing the crystal structure qualitatively. In addition, Dirac semimetals whose bulk band structure shows a Dirac-like dispersion are considered to exist near the topological phase transition. The technique shown here is also generally applicable for other chalcogenide topological insulators, and will be useful for studying topological insulators and related materials. PMID:27877743
Pressure evolution of electrical transport in the 3D topological insulator (Bi,Sb)2(Te,Se)3
NASA Astrophysics Data System (ADS)
Jeffries, Jason; Butch, N. P.; Vohra, Y. K.; Weir, S. T.
2014-03-01
The group V-VI compounds--like Bi2Se3, Sb2Te3, or Bi2Te3--have been widely studied in recent years for their bulk topological properties. The high-Z members of this series form with the same crystal structure, and are therefore amenable to isostructural substitution studies. It is possible to tune the Bi-Sb and Te-Se ratios such that the material exhibits insulating behavior, thus providing an excellent platform for understanding how a topological insulator evolves with applied pressure. We report our observations of the pressure-dependent electrical transport and compare that behavior with other binary V-VI compounds under pressure. Lawrence Livermore National Laboratory is operated by Lawrence Livermore National Security, LLC, for the U.S. Department of Energy, National Nuclear Security Administration under Contract DE-AC52-07NA27344.
Transport properties of a 3D topological insulator based on a strained high-mobility HgTe film.
Kozlov, D A; Kvon, Z D; Olshanetsky, E B; Mikhailov, N N; Dvoretsky, S A; Weiss, D
2014-05-16
We investigate the magnetotransport properties of strained 80 nm thick HgTe layers featuring a high mobility of μ ∼ 4 × 10(5) cm(2)/V · s. By means of a top gate, the Fermi energy is tuned from the valence band through the Dirac-type surface states into the conduction band. Magnetotransport measurements allow us to disentangle the different contributions of conduction band electrons, holes, and Dirac electrons to the conductivity. The results are in line with previous claims that strained HgTe is a topological insulator with a bulk gap of ≈ 15 meV and gapless surface states.
Pressure evolution of electrical transport in the 3D topological insulator (Bi,Sb) 2 (Se,Te) 3
Jeffries, J. R.; Butch, N. P.; Vohra, Y. K.; ...
2015-03-18
The group V-VI compounds|like Bi2Se3, Sb2Te3, or Bi2Te3|have been widely studied in recent years for their bulk topological properties. The high-Z members of this series form with the same crystal structure, and are therefore amenable to isostructural substitution studies. It is possible to tune the Bi-Sb and Te-Se ratios such that the material exhibits insulating behavior, thus providing an excellent platform for understanding how a topological insulator evolves with applied pressure. We report our observations of the pressure-dependent electrical transport and crystal structure of a pseudobinary (Bi,Sb)2(Te,Se)3 compound. Similar to some of its sister compounds, the (Bi,Sb)2(Te,Se)3 pseudobinary compound undergoesmore » multiple, pressure-induced phase transformations that result in metallization, the onset of a close-packed crystal structure, and the development of distinct superconducting phases.« less
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.
Topological insulators: Engineered heterostructures
NASA Astrophysics Data System (ADS)
Hesjedal, Thorsten; Chen, Yulin
2017-01-01
The combination of topological properties and magnetic order can lead to new quantum states and exotic physical phenomena. In particular, the coupling between topological insulators and antiferromagnets enables magnetic and electronic structural engineering.
NASA Astrophysics Data System (ADS)
Liu, Zhen; Wei, Xinyuan; Wang, Jiajia; Pan, Hong; Ji, Fuhao; Xi, Fuchun; Zhang, Jing; Hu, Tiandou; Zhang, Shuo; Jiang, Zheng; Wen, Wen; Huang, Yuying; Ye, Mao; Yang, Zhongqin; Qiao, Shan
2014-09-01
Transition-metal-doped Bi2Se3 crystals, X0.05Bi2Se3 (X =Cr, Co, Ni, and Cu), are prepared and studied by powder x-ray diffraction and extended x-ray absorption fine structure (EXAFS) measurements to get the local structures around the dopants. Cr atoms substitute Bi atoms and Co atoms may substitute Bi atoms or form Co3Se4 (C2/m) clusters. Cu is also found taking the Bi substitutional site, which differs from the reported superconductor phase of Cu-doped Bi2Se3, suggesting the dependence of site occupancy of Cu in Bi2Se3 on the process of sample growing. For Ni0.05Bi2Se3, Ni atoms form Ni3Se4 (C2/m) clusters. The nearest neighbors of X (X =Cr, Co, and Cu) are Se atoms, and the X-Se bond lengths are extracted from EXAFS as 2.50 Å for Cr-Se, 2.40 Å for Co-Se, and 2.38 Å for Cu-Se, which show the direct evidences of dramatic structural relaxations around 3d dopants. The bond information of local structures around dopants is valuable for subsequent theoretical studies, and can hardly be obtained from other techniques.
Topology dictionary for 3D video understanding.
Tung, Tony; Matsuyama, Takashi
2012-08-01
This paper presents a novel approach that achieves 3D video understanding. 3D video consists of a stream of 3D models of subjects in motion. The acquisition of long sequences requires large storage space (2 GB for 1 min). Moreover, it is tedious to browse data sets and extract meaningful information. We propose the topology dictionary to encode and describe 3D video content. The model consists of a topology-based shape descriptor dictionary which can be generated from either extracted patterns or training sequences. The model relies on 1) topology description and classification using Reeb graphs, and 2) a Markov motion graph to represent topology change states. We show that the use of Reeb graphs as the high-level topology descriptor is relevant. It allows the dictionary to automatically model complex sequences, whereas other strategies would require prior knowledge on the shape and topology of the captured subjects. Our approach serves to encode 3D video sequences, and can be applied for content-based description and summarization of 3D video sequences. Furthermore, topology class labeling during a learning process enables the system to perform content-based event recognition. Experiments were carried out on various 3D videos. We showcase an application for 3D video progressive summarization using the topology dictionary.
Inversion-symmetric topological insulators
NASA Astrophysics Data System (ADS)
Hughes, Taylor L.; Prodan, Emil; Bernevig, B. Andrei
2011-06-01
We analyze translationally invariant insulators with inversion symmetry that fall outside the current established classification of topological insulators. These insulators exhibit no edge or surface modes in the energy spectrum and hence they are not edge metals when the Fermi level is in the bulk gap. However, they do exhibit protected modes in the entanglement spectrum localized on the cut between two entangled regions. Their entanglement entropy cannot be made to vanish adiabatically, and hence the insulators can be called topological. There is a direct connection between the inversion eigenvalues of the Hamiltonian band structure and the midgap states in the entanglement spectrum. The classification of protected entanglement levels is given by an integer N, which is the difference between the negative inversion eigenvalues at inversion symmetric points in the Brillouin zone, taken in sets of 2. When the Hamiltonian describes a Chern insulator or a nontrivial time-reversal invariant topological insulator, the entirety of the entanglement spectrum exhibits spectral flow. If the Chern number is zero for the former, or time reversal is broken in the latter, the entanglement spectrum does not have spectral flow, but, depending on the inversion eigenvalues, can still exhibit protected midgap bands similar to impurity bands in normal semiconductors. Although spectral flow is broken (implying the absence of real edge or surface modes in the original Hamiltonian), the midgap entanglement bands cannot be adiabatically removed, and the insulator is “topological.” We analyze the linear response of these insulators and provide proofs and examples of when the inversion eigenvalues determine a nontrivial charge polarization, a quantum Hall effect, an anisotropic three-dimensional (3D) quantum Hall effect, or a magnetoelectric polarization. In one dimension, we establish a link between the product of the inversion eigenvalues of all occupied bands at all inversion
Notes on topological insulators
NASA Astrophysics Data System (ADS)
Kaufmann, Ralph M.; Li, Dan; Wehefritz-Kaufmann, Birgit
2016-11-01
This paper is a survey of the ℤ2-valued invariant of topological insulators used in condensed matter physics. The ℤ-valued topological invariant, which was originally called the TKNN invariant in physics, has now been fully understood as the first Chern number. The ℤ2 invariant is more mysterious; we will explain its equivalent descriptions from different points of view and provide the relations between them. These invariants provide the classification of topological insulators with different symmetries in which K-theory plays an important role. Moreover, we establish that both invariants are realizations of index theorems which can also be understood in terms of condensed matter physics.
Reconfigurable Microwave Photonic Topological Insulator
NASA Astrophysics Data System (ADS)
Goryachev, Maxim; Tobar, Michael E.
2016-12-01
Using full 3D finite-element simulation and underlining Hamiltonian models, we demonstrate reconfigurable photonic analogues of topological insulators on a regular lattice of tunable posts in a reentrant 3D lumped element-type system. The tunability allows a dynamical in situ change of media chirality and other properties via the alteration of the same parameter for all posts, and as a result, great flexibility in the choice of bulk-edge configurations. Additionally, one-way photon transport without an external magnetic field is demonstrated. The ideas are illustrated by using both full finite-element simulation as well as simplified harmonic oscillator models. Dynamical reconfigurability of the proposed systems paves the way to a class of systems that can be employed for random access, topological signal processing, and sensing.
Pressure evolution of electrical transport in the 3D topological insulator (Bi,Sb) _{2} (Se,Te) _{3}
Jeffries, J. R.; Butch, N. P.; Vohra, Y. K.; Weir, S. T.
2015-03-18
The group V-VI compounds|like Bi_{2}Se_{3}, Sb_{2}Te_{3}, or Bi_{2}Te_{3}|have been widely studied in recent years for their bulk topological properties. The high-Z members of this series form with the same crystal structure, and are therefore amenable to isostructural substitution studies. It is possible to tune the Bi-Sb and Te-Se ratios such that the material exhibits insulating behavior, thus providing an excellent platform for understanding how a topological insulator evolves with applied pressure. We report our observations of the pressure-dependent electrical transport and crystal structure of a pseudobinary (Bi,Sb)_{2}(Te,Se)_{3} compound. Similar to some of its sister compounds, the (Bi,Sb)_{2}(Te,Se)_{3} pseudobinary compound undergoes multiple, pressure-induced phase transformations that result in metallization, the onset of a close-packed crystal structure, and the development of distinct superconducting phases.
NASA Astrophysics Data System (ADS)
Slagle, Kevin
2015-03-01
Using determinant quantum Monte Carlo simulations, we demonstrate that an extended Hubbard model on a bilayer honeycomb lattice has two novel quantum phase transitions, each with connections to symmetry protected topological states. 1) The first is a continuous phase transition between the weakly interacting gapless Dirac fermion phase and a strongly interacting fully gapped and symmetric trivial phase. Because there is no spontaneous symmetry breaking, this transition cannot be described by the standard Gross-Neveu model. We argue that this phase transition is related to the Z16 classification of the topological superconductor 3He-B phase with interactions. 2) The second is a quantum critical point between a quantum spin Hall insulator with spin Sz conservation and the previously mentioned strongly interacting gapped phase. At the critical point the single particle excitations remain gapped, while spin and charge gaps close. We argue that this transition is described by a bosonic O(4) nonlinear sigma model field theory with a topological Θ-term.
Klimovskikh, I. I.; Sostina, D.; Petukhov, A.; Rybkin, A. G.; Eremeev, S. V.; Chulkov, E. V.; Tereshchenko, O. E.; Kokh, K. A.; Shikin, A. M.
2017-01-01
Two- and three-dimensional topological insulators are the key materials for the future nanoelectronic and spintronic devices and quantum computers. By means of angle- and spin-resolved photoemission spectroscopy we study the electronic and spin structure of the Bi-bilayer/3D topological insulator in quantum tunneling regime formed under the short annealing of Bi2Te2.4Se0.6. Owing to the temperature-induced restructuring of the topological insulator’s surface quintuple layers, the hole-like spin-split Bi-bilayer bands and the parabolic electronic-like state are observed instead of the Dirac cone. Scanning Tunneling Microscopy and X-ray Photoemission Spectroscopy measurements reveal the appearance of the Bi2 terraces at the surface under the annealing. The experimental results are supported by density functional theory calculations, predicting the spin-polarized Bi-bilayer bands interacting with the quintuple-layers-derived states. Such an easily formed heterostructure promises exciting applications in spin transport devices and low-energy electronics. PMID:28378826
The birth of topological insulators.
Moore, Joel E
2010-03-11
Certain insulators have exotic metallic states on their surfaces. These states are formed by topological effects that also render the electrons travelling on such surfaces insensitive to scattering by impurities. Such topological insulators may provide new routes to generating novel phases and particles, possibly finding uses in technological applications in spintronics and quantum computing.
Singh, Rahul; Shukla, K K; Kumar, A; Okram, G S; Singh, D; Ganeshan, V; Lakhani, Archana; Ghosh, A K; Chatterjee, Sandip
2016-09-21
Magnetoresistance (MR), thermo power, magnetization and Hall effect measurements have been performed on Co-doped Bi2Se3 topological insulators. The undoped sample shows that the maximum MR as a destructive interference due to a π-Berry phase leads to a decrease of MR. As the Co is doped, the linearity in MR is increased. The observed MR of Bi2Se3 can be explained with the classical model. The low temperature MR behavior of Co doped samples cannot be explained with the same model, but can be explained with the quantum linear MR model. Magnetization behavior indicates the establishment of ferromagnetic ordering with Co doping. Hall effect data also supports the establishment of ferromagnetic ordering in Co-doped Bi2Se3 samples by showing the anomalous Hall effect. Furthermore, when spectral weight suppression is insignificant, Bi2Se3 behaves as a dilute magnetic semiconductor. Moreover, the maximum power factor is observed when time reversal symmetry (TRS) is maintained. As the TRS is broken the power factor value is decreased, which indicates that with the rise of Dirac cone above the Fermi level the anomalous Hall effect and linearity in MR increase and the power factor decreases.
Floquet topological insulators for sound
Fleury, Romain; Khanikaev, Alexander B; Alù, Andrea
2016-01-01
The unique conduction properties of condensed matter systems with topological order have recently inspired a quest for the similar effects in classical wave phenomena. Acoustic topological insulators, in particular, hold the promise to revolutionize our ability to control sound, allowing for large isolation in the bulk and broadband one-way transport along their edges, with topological immunity against structural defects and disorder. So far, these fascinating properties have been obtained relying on moving media, which may introduce noise and absorption losses, hindering the practical potential of topological acoustics. Here we overcome these limitations by modulating in time the acoustic properties of a lattice of resonators, introducing the concept of acoustic Floquet topological insulators. We show that acoustic waves provide a fertile ground to apply the anomalous physics of Floquet topological insulators, and demonstrate their relevance for a wide range of acoustic applications, including broadband acoustic isolation and topologically protected, nonreciprocal acoustic emitters. PMID:27312175
Floquet topological insulators for sound
NASA Astrophysics Data System (ADS)
Fleury, Romain; Khanikaev, Alexander B.; Alù, Andrea
2016-06-01
The unique conduction properties of condensed matter systems with topological order have recently inspired a quest for the similar effects in classical wave phenomena. Acoustic topological insulators, in particular, hold the promise to revolutionize our ability to control sound, allowing for large isolation in the bulk and broadband one-way transport along their edges, with topological immunity against structural defects and disorder. So far, these fascinating properties have been obtained relying on moving media, which may introduce noise and absorption losses, hindering the practical potential of topological acoustics. Here we overcome these limitations by modulating in time the acoustic properties of a lattice of resonators, introducing the concept of acoustic Floquet topological insulators. We show that acoustic waves provide a fertile ground to apply the anomalous physics of Floquet topological insulators, and demonstrate their relevance for a wide range of acoustic applications, including broadband acoustic isolation and topologically protected, nonreciprocal acoustic emitters.
Thermoelectric transport in topological insulators
NASA Astrophysics Data System (ADS)
Takahashi, Ryuji; Murakami, Shuichi
2012-12-01
Thermoelectric transport in topological insulators (TIs) is theoretically studied. TIs have gapless edge states in two dimensions, and do surface states in three dimensions. Both of the states have backscattering-free nature, and they remain gapless in the presence of nonmagnetic impurities. In particular, the edge states in two-dimensional TIs form perfect conducting channels. In this study, we calculate system-size dependence of thermoelectric properties in two-dimensional TIs, and evaluate the inelastic scattering length of the edge states by phonons, which affects the thermoelectric properties sensitively. We also study thermoelectric transport in three-dimensional (3D) TIs and compare with two dimensions. In both two- and three-dimensional TIs, there is a competition between the surface/edge and bulk transports in the thermoelectric phenomena. The surface transport in 3D TIs is relatively weak compared with the bulk transport due to impurities. Furthermore, we also study gapped 3D TIs in thin slab geometry and show large values of the figure of merit in the gapped system. This result is consistent with the previous work.
Topological BF field theory description of topological insulators
Cho, Gil Young; Moore, Joel E.
2011-06-15
Research Highlights: > We show that a BF theory is the effective theory of 2D and 3D topological insulators. > The non-gauge-invariance of the bulk theory yields surface terms for a bosonized Dirac fermion. > The 'axion' term in electromagnetism is correctly obtained from gapped surfaces. > Generalizations to possible fractional phases are discussed in closing. - Abstract: Topological phases of matter are described universally by topological field theories in the same way that symmetry-breaking phases of matter are described by Landau-Ginzburg field theories. We propose that topological insulators in two and three dimensions are described by a version of abelian BF theory. For the two-dimensional topological insulator or quantum spin Hall state, this description is essentially equivalent to a pair of Chern-Simons theories, consistent with the realization of this phase as paired integer quantum Hall effect states. The BF description can be motivated from the local excitations produced when a {pi} flux is threaded through this state. For the three-dimensional topological insulator, the BF description is less obvious but quite versatile: it contains a gapless surface Dirac fermion when time-reversal-symmetry is preserved and yields 'axion electrodynamics', i.e., an electromagnetic E . B term, when time-reversal symmetry is broken and the surfaces are gapped. Just as changing the coefficients and charges of 2D Chern-Simons theory allows one to obtain fractional quantum Hall states starting from integer states, BF theory could also describe (at a macroscopic level) fractional 3D topological insulators with fractional statistics of point-like and line-like objects.
Topological Insulators from Electronic Superstructures
NASA Astrophysics Data System (ADS)
Sugita, Yusuke; Motome, Yukitoshi
2016-07-01
The possibility of realizing topological insulators by the spontaneous formation of electronic superstructures is theoretically investigated in a minimal two-orbital model including both the spin-orbit coupling and electron correlations on a triangular lattice. Using the mean-field approximation, we show that the model exhibits several different types of charge-ordered insulators, where the charge disproportionation forms a honeycomb or kagome superstructure. We find that the charge-ordered insulators in the presence of strong spin-orbit coupling can be topological insulators showing quantized spin Hall conductivity. Their band gap is dependent on electron correlations as well as the spin-orbit coupling, and even vanishes while showing the massless Dirac dispersion at the transition to a trivial charge-ordered insulator. Our results suggest a new route to realize and control topological states of quantum matter by the interplay between the spin-orbit coupling and electron correlations.
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
Disorder induced Floquet Topological Insulators
NASA Astrophysics Data System (ADS)
Bhattacharjee, Paraj; Lindner, Netanel; Rechtsman, Mikael; Refael, Gil
2014-03-01
We investigate the possibility of realizing a disorder induced topological state in two dimensional periodically driven systems. This phenomenon is akin to the topological Anderson insulator (TAI) in equilibrium systems. We focus on graphene band structures, where in the presence of the driving electromagnetic field, but in the absence of disorder, the system starts off in a trivial state due to the presence of a sublattice potential. We show that by adding on-site disorder a topological state is induced in this system. We numerically compute the average Bott index (the analog of the Chern number for disordered systems) to show that starting from a trivial phase, topological behavior can be observed at finite disorder strength. In the topological phase, we detect chiral edge states by a numerical time evolution of wavepackets at the edge of the system. We propose an experimental set-up in photonic lattices to observe this phenomenon.
Topological Insulators at Room Temperature
Zhang, Haijun; Liu, Chao-Xing; Qi, Xiao-Liang; Dai, Xi; Fang, Zhong; Zhang, Shou-Cheng; /Stanford U., Phys. Dept.
2010-03-25
Topological insulators are new states of quantum matter with surface states protected by the time-reversal symmetry. In this work, we perform first-principle electronic structure calculations for Sb{sub 2}Te{sub 3}, Sb{sub 2}Se{sub 3}, Bi{sub 2}Te{sub 3} and Bi{sub 2}Se{sub 3} crystals. Our calculations predict that Sb{sub 2}Te{sub 3}, Bi{sub 2}T e{sub 3} and Bi{sub 2}Se{sub 3} are topological insulators, while Sb{sub 2}Se{sub 3} is not. In particular, Bi{sub 2}Se{sub 3} has a topologically non-trivial energy gap of 0.3eV , suitable for room temperature applications. We present a simple and unified continuum model which captures the salient topological features of this class of materials. These topological insulators have robust surface states consisting of a single Dirac cone at the {Lambda} point.
Topology-driven magnetic quantum phase transition in topological insulators.
Zhang, Jinsong; Chang, Cui-Zu; Tang, Peizhe; Zhang, Zuocheng; Feng, Xiao; Li, Kang; Wang, Li-Li; Chen, Xi; Liu, Chaoxing; Duan, Wenhui; He, Ke; Xue, Qi-Kun; Ma, Xucun; Wang, Yayu
2013-03-29
The breaking of time reversal symmetry in topological insulators may create previously unknown quantum effects. We observed a magnetic quantum phase transition in Cr-doped Bi2(SexTe1-x)3 topological insulator films grown by means of molecular beam epitaxy. Across the critical point, a topological quantum phase transition is revealed through both angle-resolved photoemission measurements and density functional theory calculations. We present strong evidence that the bulk band topology is the fundamental driving force for the magnetic quantum phase transition. The tunable topological and magnetic properties in this system are well suited for realizing the exotic topological quantum phenomena in magnetic topological insulators.
3D Modeling from Photos Given Topological Information.
Kim, Young Min; Cho, Junghyun; Ahn, Sang Chul
2016-09-01
Reconstructing 3D models given a single-view 2D information is inherently an ill-posed problem and requires additional information such as shape prior or user input.We introduce a method to generate multiple 3D models of a particular category given corresponding photographs when the topological information is known. While there is a wide range of shapes for an object of a particular category, the basic topology usually remains constant.In consequence, the topological prior needs to be provided only once for each category and can be easily acquired by consulting an existing database of 3D models or by user input. The input of topological description is only connectivity information between parts; this is in contrast to previous approaches that have required users to interactively mark individual parts. Given the silhouette of an object and the topology, our system automatically finds a skeleton and generates a textured 3D model by jointly fitting multiple parts. The proposed method, therefore, opens the possibility of generating a large number of 3D models by consulting a massive number of photographs. We demonstrate examples of the topological prior and reconstructed 3D models using photos.
Method for 3D Airway Topology Extraction
Grothausmann, Roman; Kellner, Manuela; Heidrich, Marko; Lorbeer, Raoul-Amadeus; Ripken, Tammo; Meyer, Heiko; Kuehnel, Mark P.; Ochs, Matthias; Rosenhahn, Bodo
2015-01-01
In lungs the number of conducting airway generations as well as bifurcation patterns varies across species and shows specific characteristics relating to illnesses or gene variations. A method to characterize the topology of the mouse airway tree using scanning laser optical tomography (SLOT) tomograms is presented in this paper. It is used to test discrimination between two types of mice based on detected differences in their conducting airway pattern. Based on segmentations of the airways in these tomograms, the main spanning tree of the volume skeleton is computed. The resulting graph structure is used to distinguish between wild type and surfactant protein (SP-D) deficient knock-out mice. PMID:25767561
Quantum Capacitance in Topological Insulators
Xiu, Faxian; Meyer, Nicholas; Kou, Xufeng; He, Liang; Lang, Murong; Wang, Yong; Yu, Xinxin; Fedorov, Alexei V.; Zou, Jin; Wang, Kang L.
2012-01-01
Topological insulators show unique properties resulting from massless, Dirac-like surface states that are protected by time-reversal symmetry. Theory predicts that the surface states exhibit a quantum spin Hall effect with counter-propagating electrons carrying opposite spins in the absence of an external magnetic field. However, to date, the revelation of these states through conventional transport measurements remains a significant challenge owing to the predominance of bulk carriers. Here, we report on an experimental observation of Shubnikov-de Haas oscillations in quantum capacitance measurements, which originate from topological helical states. Unlike the traditional transport approach, the quantum capacitance measurements are remarkably alleviated from bulk interference at high excitation frequencies, thus enabling a distinction between the surface and bulk. We also demonstrate easy access to the surface states at relatively high temperatures up to 60 K. Our approach may eventually facilitate an exciting exploration of exotic topological properties at room temperature. PMID:22993694
Transmission in graphene-topological insulator heterostructures
NASA Astrophysics Data System (ADS)
De Beule, C.; Zarenia, M.; Partoens, B.
2017-03-01
We investigate scattering of the topological surface state of a three-dimensional time-reversal invariant topological insulator when graphene is deposited on the topological-insulator surface. Specifically, we consider the (111) surface of a Bi2Se3 -like topological insulator. We present a low-energy model for the graphene-topological insulator heterostructure and we calculate the transmission probability at zigzag and armchair edges of the deposited graphene, and the conductance through graphene nanoribbon barriers, and show that its features can be understood from antiresonances in the transmission probability.
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.
3D dimeron as a stable topological object
NASA Astrophysics Data System (ADS)
Yang, Shijie; Liu, Yongkai
2015-03-01
Searching for novel topological objects is always an intriguing task for scientists in various fields. We study a new three-dimensional (3D) topological structure called 3D dimeron in the trapped two-component Bose-Einstein condensates. The 3D dimeron differs to the conventional 3D skyrmion for the condensates hosting two interlocked vortex-rings. We demonstrate that the vortex-rings are connected by a singular string and the complexity constitutes a vortex-molecule. The stability is investigated through numerically evolving the Gross-Pitaevskii equations, giving a coherent Rabi coupling between the two components. Alternatively, we find that the stable 3D dimeron can be naturally generated from a vortex-free Gaussian wave packet via incorporating a synthetic non-Abelian gauge potential into the condensates. This work is supported by the NSF of China under Grant No. 11374036 and the National 973 program under Grant No. 2012CB821403.
Quantum anomalous Hall effect in topological insulator memory
Jalil, Mansoor B. A.; Tan, S. G.; Siu, Z. B.
2015-05-07
We theoretically investigate the quantum anomalous Hall effect (QAHE) in a magnetically coupled three-dimensional-topological insulator (3D-TI) system. We apply the generalized spin-orbit coupling Hamiltonian to obtain the Hall conductivity σ{sup xy} of the system. The underlying topology of the QAHE phenomenon is then analyzed to show the quantization of σ{sup xy} and its relation to the Berry phase of the system. Finally, we analyze the feasibility of utilizing σ{sup xy} as a memory read-out in a 3D-TI based memory at finite temperatures, with comparison to known magnetically doped 3D-TIs.
Topological Insulator and Thermoelectric Effects
NASA Astrophysics Data System (ADS)
Xu, Yong
The recent discovery of topological insulator (TI) offers new opportunities for the development of thermoelectricity, because many TIs (like Bi2Te3) are excellent thermoelectric materials. In this talk, I will first introduce our theoretical predictions of anomalous Seebeck effect and strong size effect in TI [PRL 112, 226801 (2014)]. Then I will report our recent proof experiments, which find in TI thin films that (i) the hole-type Seebeck effect and the electron-type Hall effect coexist in the same TI sample for all the measured temperatures (up to 300 K), and (ii) the thermoelectric properties depend sensitively on the film thickness. The unconventional phenomena are revealed to be closely related to the topological nature of the material. These findings may inspire new ideas for designing TI-based high-efficiency thermoelectric devices.
Topological Insulator Nanowires and Nanoribbons
Kong, D.S.
2010-06-02
Recent theoretical calculations and photoemission spectroscopy measurements on the bulk Bi{sub 2}Se{sub 3} material show that it is a three-dimensional topological insulator possessing conductive surface states with nondegenerate spins, attractive for dissipationless electronics and spintronics applications. Nanoscale topological insulator materials have a large surface-to-volume ratio that can manifest the conductive surface states and are promising candidates for devices. Here we report the synthesis and characterization of high quality single crystalline Bi{sub 2}Se{sub 3} nanomaterials with a variety of morphologies. The synthesis of Bi{sub 2}Se{sub 3} nanowires and nanoribbons employs Au-catalyzed vapor-liquid-solid (VLS) mechanism. Nanowires, which exhibit rough surfaces, are formed by stacking nanoplatelets along the axial direction of the wires. Nanoribbons are grown along [11-20] direction with a rectangular crosssection and have diverse morphologies, including quasi-one-dimensional, sheetlike, zigzag and sawtooth shapes. Scanning tunneling microscopy (STM) studies on nanoribbons show atomically smooth surfaces with {approx}1 nm step edges, indicating single Se-Bi-Se-Bi-Se quintuple layers. STM measurements reveal a honeycomb atomic lattice, suggesting that the STM tip couples not only to the top Se atomic layer, but also to the Bi atomic layer underneath, which opens up the possibility to investigate the contribution of different atomic orbitals to the topological surface states. Transport measurements of a single nanoribbon device (four terminal resistance and Hall resistance) show great promise for nanoribbons as candidates to study topological surface states.
Experimental realization of new topological phases of matter beyond topological insulators
NASA Astrophysics Data System (ADS)
Neupane, Madhab
A three-dimensional (3D) Z2 topological insulator (TI) is a crystalline solid, which is an insulator in the bulk but features spin-polarized Dirac electron states on its surface. In 2007, the first 3D TI was discovered in a bismuth-based compound. The discovery of the first TI tremendously accelerated research into phases of matter characterized by non-trivial topological invariants. Not only did the 3D Z2 TI itself attract great research interest, it also inspired the prediction of a range of new topological phases of matter. The primary examples are the topological Kondo insulator, the topological 3D Dirac and Weyl semimetals, the topological crystalline insulator, topological nodal line semimetal and the topological superconductor. Each of these phases was predicted to exhibit surface states with unique properties protected by a non-trivial topological invariant. In this talk, I will discuss the experimental realization of these new phases of matter in real materials by momentum and time-resolved photoemission spectroscopy. Special attention will be given to the experimental discovery of Dirac semimetal phase in Cd3As2 and topological nodal-line phase in PbTaSe2. The unusual properties of the protected topological surface states can lead to potential future applications in spintronics and quantum information, which hold promise to revolutionize our electronics and energy industries. This work is supported by start-up funds from University of Central Florida (MN) andLos Alamos National Laboratory LDRD program. The work at Princeton and Princeton-led ARPES measurements are supported by the Gordon and Betty Moore Foundations EPiQS Initiative through grant GBMF4547 (Hasan) and by U.S. Department of Energy DE-FG-02-05ER46200.
Topological Quantum Information in a 3D Neutral Atom Array
2015-01-02
AFRL-OSR-VA-TR-2015-0051 TOPOLOGICAL QUANTUM INFORMATION IN A 3D NEUTRAL ATOM ARRAY David Weiss PENNSYLVANIA STATE UNIVERSITY Final Report 01/02/2015...v Prescribed by ANSI Std. Z39.18 12-23-2014 Final 12-01-2008-9-30-2014 (DARPA) TOPOLOGICAL QUANTUM INFORMATION IN A 3D NEUTRAL ATOM ARRAY FA9550-09...using neutral atoms in an optical lattice, with the ultimate end to execute a version of the Kitaev toric code Hamiltonian model . Toward that end we
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.
Thermoelectric effects and topological insulators
NASA Astrophysics Data System (ADS)
Xu, Yong
2016-11-01
The recent discovery of topological insulators (TIs) offers new opportunities for the development of thermoelectrics, because many TIs (like Bi2Te3) are excellent thermoelectric (TE) materials. In this review, we will first describe the general TE properties of TIs and show that the coexistence of the bulk and boundary states in TIs introduces unusual TE properties, including strong size effects and an anomalous Seebeck effect. Importantly, the TE figure of merit zT of TIs is no longer an intrinsic property, but depends strongly on the geometric size. The geometric parameters of two-dimensional TIs can be tuned to enhance zT to be significantly greater than 1. Then a few proof-of-principle experiments on three-dimensional TIs will be discussed, which observed unconventional TE phenomena that are closely related to the topological nature of the materials. However, current experiments indicate that the metallic surface states, if their advantage of high mobility is not fully utilized, would be detrimental to TE performance. Finally, we provide an outlook for future work on topological materials, which offers great possibilities to discover exotic TE effects and may lead to significant breakthroughs in improving zT. Project supported by the National Thousand-Young-Talents Program, China and Tsinghua University Initiative Scientific Research Program, China.
Topological insulators: A romance with many dimensions
NASA Astrophysics Data System (ADS)
Manoharan, Hari C.
2010-07-01
Electric charges on the boundaries of certain insulators are programmed by topology to keep moving forward when they encounter an obstacle, rather than scattering backwards and increasing the resistance of the system. This is just one reason why topological insulators are one of the hottest topics in physics right now.
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
Theory of topological quantum phase transitions in 3D noncentrosymmetric systems.
Yang, Bohm-Jung; Bahramy, Mohammad Saeed; Arita, Ryotaro; Isobe, Hiroki; Moon, Eun-Gook; Nagaosa, Naoto
2013-02-22
We construct a general theory describing the topological quantum phase transitions in 3D systems with broken inversion symmetry. While the consideration of the system's codimension generally predicts the appearance of a stable metallic phase between the normal and topological insulators, it is shown that a direct topological phase transition between two insulators is also possible when an accidental band crossing occurs along directions with high crystalline symmetry. At the quantum critical point, the energy dispersion becomes quadratic along one direction while the dispersions along the other two orthogonal directions are linear, which manifests the zero chirality of the band touching point. Because of the anisotropic dispersion at quantum critical point, various thermodynamic and transport properties show unusual temperature dependence and anisotropic behaviors.
Bulk Topological Insulators and Superconductors: Discovery and the new Frontiers
NASA Astrophysics Data System (ADS)
Hasan, M. Zahid
2011-03-01
While most known phases of matter are characterized by broken symmetries, the discovery of quantum Hall effects (1980s) revealed that there exists an organizational principle based on topology rather than broken symmetry. In the past few years, theory and experiments have suggested that new types of topological states of matter exist in certain bulk insulators without any applied magnetic field. These topological insulators are characterized by a full band gap in their bulk and gap-less conducting edge or surface states protected by time-reversal symmetry. Unlike the quantum Hall systems, the bulk 3D topological insulators can be doped into superconductors and magnets revealing the interplay between topological-order and broken-symmetry-order [Rev. Mod. Phys 82, 3045 (2010)]. In this talk, I will highlight the experimental observations and focus on recent experimental developments on bulk topological insulators. I will then draw connections between the topological physics and their potential applications in electronics and the emergent new frontiers in fundamental physics. Work in collaboration with D. Hsieh, Y. Xia, L. Wray, D. Qian, C.L. Kane, H. Lin, A. Bansil, D. Grauer, R.J. Cava, Y.S. Hor, J. H. Dil, F. Meier, L. Patthey, J. Osterwalder, A.V. Fedorov.
Topological insulators and superconductors from string theory
Ryu, Shinsei; Takayanagi, Tadashi
2010-10-15
Topological insulators and superconductors in different spatial dimensions and with different discrete symmetries have been fully classified recently, revealing a periodic structure for the pattern of possible types of topological insulators and superconductors, both in terms of spatial dimensions and in terms of symmetry classes. It was proposed that K theory is behind the periodicity. On the other hand, D-branes, a solitonic object in string theory, are also known to be classified by K theory. In this paper, by inspecting low-energy effective field theories realized by two parallel D-branes, we establish a one-to-one correspondence between the K-theory classification of topological insulators/superconductors and D-brane charges. In addition, the string theory realization of topological insulators and superconductors comes naturally with gauge interactions, and the Wess-Zumino term of the D-branes gives rise to a gauge field theory of topological nature, such as ones with the Chern-Simons term or the {theta} term in various dimensions. This sheds light on topological insulators and superconductors beyond noninteracting systems, and the underlying topological field theory description thereof. In particular, our string theory realization includes the honeycomb lattice Kitaev model in two spatial dimensions, and its higher-dimensional extensions. Increasing the number of D-branes naturally leads to a realization of topological insulators and superconductors in terms of holography (AdS/CFT).
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.
Dirac Fermions without bulk backscattering in rhombohedral topological insulators
NASA Astrophysics Data System (ADS)
Mera Acosta, Carlos; Lima, Matheus; Seixas, Leandro; da Silva, Antônio; Fazzio, Adalberto
2015-03-01
The realization of a spintronic device using topological insulators is not trivial, because there are inherent difficulties in achieving the surface transport regime. The majority of 3D topological insulators materials (3DTI) despite of support helical metallic surface states on an insulating bulk, forming topological Dirac fermions protected by the time-reversal symmetry, exhibit electronic scattering channels due to the presence of residual continuous bulk states near the Dirac-point. From ab initio calculations, we studied the microscopic origin of the continuous bulk states in rhombohedral topological insulators materials with the space group D3d 5 (R 3 m) , showing that it is possible to understand the emergence of residual continuous bulk states near the Dirac-point into a six bands effective model, where the breaking of the R3 symmetry beyond the Γ point has an important role in the hybridization of the px, py and pz atomic orbitals. Within these model, the mechanisms known to eliminate the bulk scattering, for instance: the stacking faults (SF), electric field and alloy, generated the similar effect in the effective states of the 3DTI. Finally, we show how the surface electronic transport is modified by perturbations of bulk with SF. We would like to thank the financial support by Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP).
Materials and Theory of Topological Insulators
2012-10-30
NUMBER 5f. WORK UNIT NUMBER 5c. PROGRAM ELEMENT NUMBER 5b. GRANT NUMBER 5a. CONTRACT NUMBER W911NF-09-1-0508 611102 Form Approved OMB NO. 0704-0188...2013 5.00 X. Zhang, H. Zhang, J. Wang, C. Felser, S.-C. Zhang. Actinide Topological Insulator Materials with Strong Interaction, Science, (03 2012...new classes of materials, including the Heusler alloys, the ternary compounds with honeycomb lattice structures and actinide topological insulators
Electrical control of spin in topological insulators
NASA Astrophysics Data System (ADS)
Chang, Kai
2012-02-01
All-electrical manipulation of electron spin in solids becomes a central issue of quantum information processing and quantum computing. The many previous proposals are based on spin-orbit interactions in semiconductors. Topological insulator, a strong spin-orbit coupling system, make it possible to control the spin transport electrically. Recent calculations proved that external electric fields can drive a HgTe quantum well from normal band insulator phase to topological insulator phase [1]. Since the topological edge states are robust against local perturbation, the controlling of edge states using local fields is a challenging task. We demonstrate that a p-n junction created electrically in HgTe quantum wells with inverted band structure exhibits interesting intraband and interband tunneling processes. We find a perfect intraband transmission for electrons injected perpendicularly to the interface of the p-n junction. The opacity and transparency of electrons through the p-n junction can be tuned by changing the incidence angle, the Fermi energy and the strength of the Rashba spin-orbit interaction (RSOI). The occurrence of a conductance plateau due to the formation of topological edge states in a quasi-one-dimensional p-n junction can be switched on and off by tuning the gate voltage. The spin orientation can be substantially rotated when the samples exhibit a moderately strong RSOI [2]. An electrical switching of the edge-state transport can also be realized using quantum point contacts in quantum spin Hall bars. The switch-on/off of the edge channel is caused by the finite size effect of the quantum point contact and therefore can be manipulated by tuning the voltage applied on the split gate [3,4]. The magnetic ions doped on the surface of 3D TI can be correlated through the helical electrons. The RKKY interaction mediated by the helical Dirac electrons consists of the Heisenberg-like, Ising-like, and Dzyaloshinskii-Moriya (DM)-like terms, which can be tuned
Observation of photonic anomalous Floquet topological insulators
NASA Astrophysics Data System (ADS)
Maczewsky, Lukas J.; Zeuner, Julia M.; Nolte, Stefan; Szameit, Alexander
2017-01-01
Topological insulators are a new class of materials that exhibit robust and scatter-free transport along their edges -- independently of the fine details of the system and of the edge -- due to topological protection. To classify the topological character of two-dimensional systems without additional symmetries, one commonly uses Chern numbers, as their sum computed from all bands below a specific bandgap is equal to the net number of chiral edge modes traversing this gap. However, this is strictly valid only in settings with static Hamiltonians. The Chern numbers do not give a full characterization of the topological properties of periodically driven systems. In our work, we implement a system where chiral edge modes exist although the Chern numbers of all bands are zero. We employ periodically driven photonic waveguide lattices and demonstrate topologically protected scatter-free edge transport in such anomalous Floquet topological insulators.
Observation of photonic anomalous Floquet topological insulators
Maczewsky, Lukas J.; Zeuner, Julia M.; Nolte, Stefan; Szameit, Alexander
2017-01-01
Topological insulators are a new class of materials that exhibit robust and scatter-free transport along their edges — independently of the fine details of the system and of the edge — due to topological protection. To classify the topological character of two-dimensional systems without additional symmetries, one commonly uses Chern numbers, as their sum computed from all bands below a specific bandgap is equal to the net number of chiral edge modes traversing this gap. However, this is strictly valid only in settings with static Hamiltonians. The Chern numbers do not give a full characterization of the topological properties of periodically driven systems. In our work, we implement a system where chiral edge modes exist although the Chern numbers of all bands are zero. We employ periodically driven photonic waveguide lattices and demonstrate topologically protected scatter-free edge transport in such anomalous Floquet topological insulators. PMID:28051080
3D Surface Topology Guides Stem Cell Adhesion and Differentiation
Viswanathan, Priyalakshmi; Ondeck, Matthew G.; Chirasatitsin, Somyot; Nghamkham, Kamolchanok; Reilly, Gwendolen C.; Engler, Adam J.; Battaglia, Giuseppe
2015-01-01
Polymerized high internal phase emulsion (polyHIPE) foams are extremely versatile materials for investigating cell-substrate interactions in vitro. Foam morphologies can be controlled by polymerization conditions to result in either open or closed pore structures with different levels of connectivity, consequently enabling the comparison between 2D and 3D matrices using the same substrate with identical surface chemistry conditions. Additionally, here we achieve the control of pore surface topology (i.e. how different ligands are clustered together) using amphiphilic block copolymers as emulsion stabilisers. We demonstrate that adhesion of human mesenchymal progenitor (hES-MP) cells cultured on polyHIPE foams is dependent on foam surface topology and chemistry but is independent of porosity and interconnectivity. We also demonstrate that the interconnectivity, architecture and surface topology of the foams has an effect on the osteogenic differentiation potential of hES-MP cells. Together these data demonstrate that the adhesive heterogeneity of a 3D scaffold could regulate not only mesenchymal stem cell attachment but also cell behavior in the absence of soluble growth factors. PMID:25818420
NASA Astrophysics Data System (ADS)
Gu, Genda
The discovery of 3D topological insulator materials and topological superconductor open up a new research field in the condensed matter physics. In order to search for the ideal topological insulator, topological crystalline insulator and topological superconductor, we have grown a large number of the single crystals of Pb-system (Pb-Sn-In-Te) topological crystalline insulator and their topological superconductor . We have measured the physical properties on these single crystals by various techniques. We have studied the effect of crystal growth condition, impurity and composition on the bulk electrical conductivity of these single crystals. We try to find out which composition and crystal growth condition is the best for the ideal topological insulator, topological crystalline insulator and topological superconductor. We have got the bulk topological superconductor with Tc = 5 K . Work was supported by the Office of Basic Energy Sciences (BES), Division of Materials Sciences and Engineering, U.S. Department of Energy (DOE), through Contract No. DE-SC00112704.
Topology of nonsymmorphic crystalline insulators and superconductors
NASA Astrophysics Data System (ADS)
Shiozaki, Ken; Sato, Masatoshi; Gomi, Kiyonori
2016-05-01
Topological classification in our previous paper [K. Shiozaki and M. Sato, Phys. Rev. B 90, 165114 (2014), 10.1103/PhysRevB.90.165114] is extended to nonsymmorphic crystalline insulators and superconductors. Using the twisted equivariant K theory, we complete the classification of topological crystalline insulators and superconductors in the presence of additional order-two nonsymmorphic space-group symmetries. The order-two nonsymmorphic space groups include half-lattice translation with Z2 flip, glide, twofold screw, and their magnetic space groups. We find that the topological periodic table shows modulo-2 periodicity in the number of flipped coordinates under the order-two nonsymmorphic space group. It is pointed out that the nonsymmorphic space groups allow Z2 topological phases even in the absence of time-reversal and/or particle-hole symmetries. Furthermore, the coexistence of the nonsymmorphic space group with time-reversal and/or particle-hole symmetries provides novel Z4 topological phases, which have not been realized in ordinary topological insulators and superconductors. We present model Hamiltonians of these new topological phases and analytic expressions of the Z2 and Z4 topological invariants. The half-lattice translation with Z2 spin flip and glide symmetry are compatible with the existence of boundaries, leading to topological surface gapless modes protected by the order-two nonsymmorphic symmetries. We also discuss unique features of these gapless surface modes.
Sizable band gap in organometallic topological insulator
NASA Astrophysics Data System (ADS)
Derakhshan, V.; Ketabi, S. A.
2017-01-01
Based on first principle calculation when Ceperley-Alder and Perdew-Burke-Ernzerh type exchange-correlation energy functional were adopted to LSDA and GGA calculation, electronic properties of organometallic honeycomb lattice as a two-dimensional topological insulator was calculated. In the presence of spin-orbit interaction bulk band gap of organometallic lattice with heavy metals such as Au, Hg, Pt and Tl atoms were investigated. Our results show that the organometallic topological insulator which is made of Mercury atom shows the wide bulk band gap of about ∼120 meV. Moreover, by fitting the conduction and valence bands to the band-structure which are produced by Density Functional Theory, spin-orbit interaction parameters were extracted. Based on calculated parameters, gapless edge states within bulk insulating gap are indeed found for finite width strip of two-dimensional organometallic topological insulators.
Tunable Dirac fermion dynamics in topological insulators.
Chen, Chaoyu; Xie, Zhuojin; Feng, Ya; Yi, Hemian; Liang, Aiji; He, Shaolong; Mou, Daixiang; He, Junfeng; Peng, Yingying; Liu, Xu; Liu, Yan; Zhao, Lin; Liu, Guodong; Dong, Xiaoli; Zhang, Jun; Yu, Li; Wang, Xiaoyang; Peng, Qinjun; Wang, Zhimin; Zhang, Shenjin; Yang, Feng; Chen, Chuangtian; Xu, Zuyan; Zhou, X J
2013-01-01
Three-dimensional topological insulators are characterized by insulating bulk state and metallic surface state involving relativistic Dirac fermions which are responsible for exotic quantum phenomena and potential applications in spintronics and quantum computations. It is essential to understand how the Dirac fermions interact with other electrons, phonons and disorders. Here we report super-high resolution angle-resolved photoemission studies on the Dirac fermion dynamics in the prototypical Bi2(Te,Se)3 topological insulators. We have directly revealed signatures of the electron-phonon coupling and found that the electron-disorder interaction dominates the scattering process. The Dirac fermion dynamics in Bi2(Te3-xSex) topological insulators can be tuned by varying the composition, x, or by controlling the charge carriers. Our findings provide crucial information in understanding and engineering the electron dynamics of the Dirac fermions for fundamental studies and potential applications.
NASA Astrophysics Data System (ADS)
Gu, Genda; Yang, Alina; Schneeloch, J.; Zhong, R. D.; Xu, Z. J.; Tranquada, J. M.; Pan, Z. H.; Si, W. D.; Shi, X. Y.; Li, Q.; Valla, T.
2013-03-01
The discovery of 3D topological insulator materials and topological superconductor opens up a new research field in the condensed matter physics. We have grown a number of Bi-Sb-Te-Se topological insulator, and Cu-Bi-Se and Sn-In-Te topological superconductor single crystals. We have measured the physical properties on these single crystals. We have studied the effect of growth condition and impurity on the bulk electrical conductivity of these single crystals. We try to answer two questions for the topological insulator materials if it is possible to grow the bulk-insulating topological insulator single crystals and Which maximum resistivity of these topological insulator single crystals we can grow. For the topological superconductor, we have got the bulk superconducting single crystals with a maximum Tc =4.5K. DOE under Contract No. DE-AC02-98CH10886 and the DOE Center for Emergent Superconductivity.
Chern insulators without band inversion in Mo S2 monolayers with 3 d adatoms
NASA Astrophysics Data System (ADS)
Wei, Xinyuan; Zhao, Bao; Zhang, Jiayong; Xue, Yang; Li, Yun; Yang, Zhongqin
2017-02-01
Electronic and topological properties of Mo S2 monolayers endowed with 3 d transition metal (TM) adatoms (V-Fe) are explored by using ab initio methods and k .p models. Without the consideration of the Hubbard U interaction, the V, Cr, and Fe adatoms tend to locate on the top of the Mo atoms, while the most stable site for the Mn atom is at the hollow position of the Mo-S hexagon. After the Hubbard U is applied, the most stable sites of all the systems become the top of the Mo atoms. Chern insulators without band inversion are achieved in these systems. The V and Fe adsorption systems are the best candidates to produce the topological states. The k .p model calculations indicate that these topological states are determined by the TM magnetism, the C3 v crystal field from the Mo S2 substrate, and the TM atomic spin-orbit coupling (SOC). The special two-meron pseudospin texture is found to contribute to the topology. The apparent difference between the Berry curvatures for the V and Fe adsorption systems is also explored. Our results widen the understanding of the Chern insulators and are helpful for the applications of the Mo S2 monolayers in the future electronics and spintronics.
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.
Strain-Induced Ferroelectric Topological Insulator.
Liu, Shi; Kim, Youngkuk; Tan, Liang Z; Rappe, Andrew M
2016-03-09
Ferroelectricity and band topology are two extensively studied yet distinct properties of insulators. Nonetheless, their coexistence has never been observed in a single material. Using first-principles calculations, we demonstrate that a noncentrosymmetric perovskite structure of CsPbI3 allows for the simultaneous presence of ferroelectric and topological orders with appropriate strain engineering. Metallic topological surface states create an intrinsic short-circuit condition, helping stabilize bulk polarization. Exploring diverse structural phases of CsPbI3 under pressure, we identify that the key structural feature for achieving a ferroelectric topological insulator is to suppress PbI6 cage rotation in the perovskite structure, which could be obtained via strain engineering. Ferroelectric control over the density of topological surface states provides a new paradigm for device engineering, such as perfect-focusing Veselago lens and spin-selective electron collimator. Our results suggest that CsPbI3 is a simple model system for ferroelectric topological insulators, enabling future studies exploring the interplay between conventional symmetry-breaking and topological orders and their novel applications in electronics and spintronics.
Three-dimensional topological insulator based nanospaser
NASA Astrophysics Data System (ADS)
Paudel, Hari P.; Apalkov, Vadym; Stockman, Mark I.
2016-04-01
After the discovery of the spaser (surface plasmon amplification by stimulated emission of radiation), first proposed by Bergman and Stockman in 2003, it has become possible to deliver optical energy beyond the diffraction limit and generate an intense source of an optical field. The spaser is a nanoplasmonic counterpart of a laser. One of the major advantages of the spaser is its size: A spaser is a truly nanoscopic device whose size can be made smaller than the skin depth of a material to a size as small as the nonlocality radius (˜1 nm). Recently, an electrically pumped graphene based nanospaser has been proposed that operates in the midinfrared region and utilizes a nanopatch of graphene as a source of plasmons and a quantum-well cascade as its gain medium. Here we propose an optically pumped nanospaser based on three-dimensional topological insulator (3D TI) materials, such as Bi2Se3 , that operates at an energy close to the bulk band-gap energy ˜0.3 eV and uses the surface as a source for plasmons and its bulk as a gain medium. Population inversion is obtained in the bulk and the radiative energy of the exciton recombination is transferred to the surface plasmons of the same material to stimulate spasing action. This is truly a nanoscale spaser as it utilizes the same material for dual purposes. We show theoretically the possibility of achieving spasing with a 3D TI. As the spaser operates in the midinfrared spectral region, it can be a useful device for a number of applications, such as nanoscopy, nanolithography, nanospectroscopy, and semiclassical information processing.
The winding road to topological insulators
NASA Astrophysics Data System (ADS)
Mele, Eugene J.
2015-12-01
This note gives a brief discussion of the discovery of topological insulators from a consideration of the low energy properties of single layer graphene. Topological band theoretic classification of insulating states in two and three-dimensions and experimental realizations are briefly discussed. Note to readers: This is a short summary of a talk that was given at the Nobel Symposium ‘New Forms of Matter: Topological Insulators and Superconductors’ held in Stockholm in June 2014. The talk was in the spirit of an overview talk but focusing on the background and early history of the field rather than reviewing the substantial (and growing) technical literature on the subject. Readers interested in technical details will surely be disappointed and should read no further, but others may be interested in some of the developments recounted here.
Topological Insulator Realized with Piezoelectric Resonators
NASA Astrophysics Data System (ADS)
McHugh, S.
2016-07-01
We propose a realization of a two-dimensional topological insulator using an array of microwave piezoelectric resonators. The resonators are coupled electrically, but acoustically isolated. The inter-resonator electromagnetic coupling required to reproduce an effective mechanical topological insulator is found explicitly. Both the acoustic and electric response show the essential features of topological insulator, e.g., helical edge states. The helical edge states may be useful for engineering nonreciprocal electronic devices like isolators and circulators. These components do not often appear in the radios of modern mobile phones since they traditionally require bulky magnetic material. However, a nonreciprocal device based on piezoelectric resonators may meet the demands of phone manufacturers due to their small size, high-linearity, and ease of fabrication.
A quantum dot in topological insulator nanofilm.
Herath, Thakshila M; Hewageegana, Prabath; Apalkov, Vadym
2014-03-19
We introduce a quantum dot in topological insulator nanofilm as a bump at the surface of the nanofilm. Such a quantum dot can localize an electron if the size of the dot is large enough, ≳5 nm. The quantum dot in topological insulator nanofilm has states of two types, which belong to two ('conduction' and 'valence') bands of the topological insulator nanofilm. We study the energy spectra of such defined quantum dots. We also consider intraband and interband optical transitions within the dot. The optical transitions of the two types have the same selection rules. While the interband absorption spectra have multi-peak structure, each of the intraband spectra has one strong peak and a few weak high frequency satellites.
Plutonium Hexaboride is a Correlated Topological Insulator
NASA Astrophysics Data System (ADS)
Deng, Xiaoyu; Haule, Kristjan; Kotliar, Gabriel
2013-10-01
We predict that plutonium hexaboride (PuB6) is a strongly correlated topological insulator, with Pu in an intermediate valence state of Pu2.7+. Within the combination of dynamical mean field theory and density functional theory, we show that PuB6 is an insulator in the bulk, with nontrivial Z2 topological invariants. Its metallic surface states have a large Fermi pocket at the X¯ point and the Dirac cones inside the bulk derived electronic states, causing a large surface thermal conductivity. PuB6 has also a very high melting temperature; therefore, it has ideal solid state properties for a nuclear fuel material.
Plutonium hexaboride is a correlated topological insulator.
Deng, Xiaoyu; Haule, Kristjan; Kotliar, Gabriel
2013-10-25
We predict that plutonium hexaboride (PuB(6)) is a strongly correlated topological insulator, with Pu in an intermediate valence state of Pu(2.7+). Within the combination of dynamical mean field theory and density functional theory, we show that PuB(6) is an insulator in the bulk, with nontrivial Z(2) topological invariants. Its metallic surface states have a large Fermi pocket at the X[over ¯] point and the Dirac cones inside the bulk derived electronic states, causing a large surface thermal conductivity. PuB(6) has also a very high melting temperature; therefore, it has ideal solid state properties for a nuclear fuel material.
Plutonium hexaboride is a correlated topological insulator
NASA Astrophysics Data System (ADS)
Deng, Xiaoyu; Haule, Kristjan; Kotliar, Gabriel; Department of Physics and Astronomy, Rutgers University Team
2014-03-01
We predict that plutonium hexaboride (PuB6) is a strongly correlated topological insulator, with Pu in an intermediate valence state of Pu2 . 7 +. Within the combination of dynamical mean field theory and density functional theory, we show that PuB6 is an insulator in the bulk, with non-trivial Z2 topological invariants. Its metallic surface states have large Fermi pocket at X point and the Dirac cones inside the bulk derived electronic states causing a large surface thermal conductivity. PB6 has also a very high melting temperature therefore it has ideal solid state properties for a nuclear fuel material.
Nonequilibrium Floquet States in Topological Kondo Insulators
2016-02-04
proposed state: the non-equilibrium Floquet topological metal. The main idea relies on the knowledge that the low - temperature insulating state of SmB6...is readily transformed to a metallic state by application of external pressure [Cooley 1995]. With low - temperature topological conduction occurring...reflecting on years of experience in performing low - temperature ultrasound measurements on single-crystal samples, both the Sapporo and UMD groups agree
Floquet Topological Insulators in Uranium Compounds
NASA Astrophysics Data System (ADS)
Pi, Shu-Ting; Savrasov, Sergey
2014-03-01
A major issue regarding the Uranium based nuclear fuels is to conduct the heat from the core area to its outer area. Unfortunately, those materials are notorious for their extremely low thermal conductivity due to the phonon-dominated-heat-transport properties in insulating states. Although metallic Uranium compounds are helpful in increasing the thermal conductivity, their low melting point still make those efforts in vain. In this report, we will figure out potential Uranium based Floquet topological insulators where the insulating bulk states accompanied with metallic surface states is achieved by applying periodic electrical fields which makes the coexistence of both benefits possible.
Band structure engineering in topological insulator based heterostructures.
Menshchikova, T V; Otrokov, M M; Tsirkin, S S; Samorokov, D A; Bebneva, V V; Ernst, A; Kuznetsov, V M; Chulkov, E V
2013-01-01
The ability to engineer an electronic band structure of topological insulators would allow the production of topological materials with tailor-made properties. Using ab initio calculations, we show a promising way to control the conducting surface state in topological insulator based heterostructures representing an insulator ultrathin films on the topological insulator substrates. Because of a specific relation between work functions and band gaps of the topological insulator substrate and the insulator ultrathin film overlayer, a sizable shift of the Dirac point occurs resulting in a significant increase in the number of the topological surface state charge carriers as compared to that of the substrate itself. Such an effect can also be realized by applying the external electric field that allows a gradual tuning of the topological surface state. A simultaneous use of both approaches makes it possible to obtain a topological insulator based heterostructure with a highly tunable topological surface state.
Optoelectronic devices, plasmonics, and photonics with topological insulators
NASA Astrophysics Data System (ADS)
Politano, Antonio; Viti, Leonardo; Vitiello, Miriam S.
2017-03-01
Topological insulators are innovative materials with semiconducting bulk together with surface states forming a Dirac cone, which ensure metallic conduction in the surface plane. Therefore, topological insulators represent an ideal platform for optoelectronics and photonics. The recent progress of science and technology based on topological insulators enables the exploitation of their huge application capabilities. Here, we review the recent achievements of optoelectronics, photonics, and plasmonics with topological insulators. Plasmonic devices and photodetectors based on topological insulators in a wide energy range, from terahertz to the ultraviolet, promise outstanding impact. Furthermore, the peculiarities, the range of applications, and the challenges of the emerging fields of topological photonics and thermo-plasmonics are discussed.
Low-Dimensional Topological Crystalline Insulators.
Wang, Qisheng; Wang, Feng; Li, Jie; Wang, Zhenxing; Zhan, Xueying; He, Jun
2015-09-01
Topological crystalline insulators (TCIs) are recently discovered topological phase with robust surface states residing on high-symmetry crystal surfaces. Different from conventional topological insulators (TIs), protection of surface states on TCIs comes from point-group symmetry instead of time-reversal symmetry in TIs. The distinct properties of TCIs make them promising candidates for the use in novel spintronics, low-dissipation quantum computation, tunable pressure sensor, mid-infrared detector, and thermoelectric conversion. However, similar to the situation in TIs, the surface states are always suppressed by bulk carriers, impeding the exploitation of topology-induced quantum phenomenon. One effective way to solve this problem is to grow low-dimensional TCIs which possess large surface-to-volume ratio, and thus profoundly increase the carrier contribution from topological surface states. Indeed, through persistent effort, researchers have obtained unique quantum transport phenomenon, originating from topological surface states, based on controllable growth of low-dimensional TCIs. This article gives a comprehensive review on the recent progress of controllable synthesis and topological surface transport of low-dimensional TCIs. The possible future direction about low-dimensional TCIs is also briefly discussed at the end of this paper.
Topological crystalline insulator SnTe nanoribbons
NASA Astrophysics Data System (ADS)
Dahal, Bishnu R.; Dulal, Rajendra P.; Pegg, Ian L.; Philip, John
2017-03-01
Topological crystalline insulators are systems in which a band inversion that is protected by crystalline mirror symmetry gives rise to nontrivial topological surface states. SnTe is a topological crystalline insulator. It exhibits p-type conductivity due to Sn vacancies and Te antisites, which leads to high carrier density in the bulk. Thus growth of high quality SnTe is a prerequisite for understanding the topological crystalline insulating behavior. We have grown SnTe nanoribbons using a solution method. The width of the SnTe ribbons varies from 500 nm to 2 μm. They exhibit rock salt crystal structure with a lattice parameter of 6.32 Å. The solution method that we have adapted uses low temperature, so the Sn vacancies can be controlled. The solution grown SnTe nanoribbons exhibit strong semiconducting behavior with an activation energy of 240 meV. This activation energy matches with the calculated band gap for SnTe with a lattice parameter of 6.32 Å, which is higher than that reported for bulk SnTe. The higher activation energy makes the thermal excitation of bulk charges very difficult on the surface. As a result, the topological surfaces will be free from the disturbance caused by the thermal excitations
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.
Fractional Topological Insulators in Three Dimensions
Maciejko, Joseph; Zhang Shoucheng; Qi Xiaoliang; Karch, Andreas
2010-12-10
Topological insulators can be generally defined by a topological field theory with an axion angle {theta} of 0 or {pi}. In this work, we introduce the concept of fractional topological insulator defined by a fractional axion angle and show that it can be consistent with time reversal T invariance if ground state degeneracies are present. The fractional axion angle can be measured experimentally by the quantized fractional bulk magnetoelectric polarization P{sub 3}, and a 'halved' fractional quantum Hall effect on the surface with Hall conductance of the form {sigma}{sub H}=(p/q)(e{sup 2}/2h) with p, q odd. In the simplest of these states the electron behaves as a bound state of three fractionally charged 'quarks' coupled to a deconfined non-Abelian SU(3) 'color' gauge field, where the fractional charge of the quarks changes the quantization condition of P{sub 3} and allows fractional values consistent with T invariance.
Comparative study of Weyl semimetal and topological/Chern insulators: Thin-film point of view
NASA Astrophysics Data System (ADS)
Yoshimura, Yukinori; Onishi, Wataru; Kobayashi, Koji; Ohtsuki, Tomi; Imura, Ken-Ichiro
2016-12-01
Regarding three-dimensional (3D) topological insulators and semimetals as a stack of constituent two-dimensional (2D) topological (or sometimes nontopological) systems is a useful viewpoint. Here, we perform a comparative study of the paradigmatic 3D topological phases: Weyl semimetal (WSM), strong and weak topological insulators (STI/WTI), and Chern insulator (CI). By calculating the Z and Z2 indices for the thin films of such 3D topological phases, we follow dimensional evolution of topological properties from 2D to 3D. It is shown that the counterparts of STI and WTI in the time-reversal symmetry broken CI system are, respectively, WSM and CI phases. The number ND of helical Dirac cones emergent on the surface of a topological insulator is shown to be identical to the number NW of the pairs of Weyl cones in the corresponding WSM phase: ND=NW . To test the robustness of this scenario against disorder, we have studied the transport property of disordered WSM thin films, taking into account both the bulk and surface contributions.
Room temperature giant and linear magnetoresistance in topological insulator Bi2Te3 nanosheets.
Wang, Xiaolin; Du, Yi; Dou, Shixue; Zhang, Chao
2012-06-29
Topological insulators, a new class of condensed matter having bulk insulating states and gapless metallic surface states, have demonstrated fascinating quantum effects. However, the potential practical applications of the topological insulators are still under exploration worldwide. We demonstrate that nanosheets of a Bi(2)Te(3) topological insulator several quintuple layers thick display giant and linear magnetoresistance. The giant and linear magnetoresistance achieved is as high as over 600% at room temperature, with a trend towards further increase at higher temperatures, as well as being weakly temperature-dependent and linear with the field, without any sign of saturation at measured fields up to 13 T. Furthermore, we observed a magnetic field induced gap below 10 K. The observation of giant and linear magnetoresistance paves the way for 3D topological insulators to be useful for practical applications in magnetoelectronic sensors such as disk reading heads, mechatronics, and other multifunctional electromagnetic applications.
Room Temperature Giant and Linear Magnetoresistance in Topological Insulator Bi2Te3 Nanosheets
NASA Astrophysics Data System (ADS)
Wang, Xiaolin; Du, Yi; Dou, Shixue; Zhang, Chao
2012-06-01
Topological insulators, a new class of condensed matter having bulk insulating states and gapless metallic surface states, have demonstrated fascinating quantum effects. However, the potential practical applications of the topological insulators are still under exploration worldwide. We demonstrate that nanosheets of a Bi2Te3 topological insulator several quintuple layers thick display giant and linear magnetoresistance. The giant and linear magnetoresistance achieved is as high as over 600% at room temperature, with a trend towards further increase at higher temperatures, as well as being weakly temperature-dependent and linear with the field, without any sign of saturation at measured fields up to 13 T. Furthermore, we observed a magnetic field induced gap below 10 K. The observation of giant and linear magnetoresistance paves the way for 3D topological insulators to be useful for practical applications in magnetoelectronic sensors such as disk reading heads, mechatronics, and other multifunctional electromagnetic applications.
Quantum interference in topological insulator Josephson junctions
NASA Astrophysics Data System (ADS)
Song, Juntao; Liu, Haiwen; Liu, Jie; Li, Yu-Xian; Joynt, Robert; Sun, Qing-feng; Xie, X. C.
2016-05-01
Using nonequilibrium Green's functions, we studied numerically the transport properties of a Josephson junction, superconductor-topological insulator-superconductor hybrid system. Our numerical calculation shows first that proximity-induced superconductivity is indeed observed in the edge states of a topological insulator adjoining two superconducting leads and second that the special characteristics of topological insulators endow the edge states with an enhanced proximity effect with a superconductor but do not forbid the bulk states to do the same. In a size-dependent analysis of the local current, it was found that a few residual bulk states can lead to measurable resistance, whereas because these bulk states spread over the whole sample, their contribution to the interference pattern is insignificant when the sample size is in the micrometer range. Based on these numerical results, it is concluded that the apparent disappearance of residual bulk states in the superconducting interference process as described by Hart et al. [Nat. Phys. 10, 638 (2014), 10.1038/nphys3036] is just due to the effects of size: the contribution of the topological edge states outweighs that of the residual bulk states.
NASA Astrophysics Data System (ADS)
Xing, Xu-Feng; Abolfazl Mostafavia, Mir; Wang, Chen
2016-06-01
Topological relations are fundamental for qualitative description, querying and analysis of a 3D scene. Although topological relations for 2D objects have been extensively studied and implemented in GIS applications, their direct extension to 3D is very challenging and they cannot be directly applied to represent relations between components of complex 3D objects represented by 3D B-Rep models in R3. Herein we present an extended Region Connection Calculus (RCC) model to express and formalize topological relations between planar regions for creating 3D model represented by Boundary Representation model in R3. We proposed a new dimension extended 9-Intersection model to represent the basic relations among components of a complex object, including disjoint, meet and intersect. The last element in 3*3 matrix records the details of connection through the common parts of two regions and the intersecting line of two planes. Additionally, this model can deal with the case of planar regions with holes. Finally, the geometric information is transformed into a list of strings consisting of topological relations between two planar regions and detailed connection information. The experiments show that the proposed approach helps to identify topological relations of planar segments of point cloud automatically.
Topological mirror insulators in one dimension
NASA Astrophysics Data System (ADS)
Lau, Alexander; van den Brink, Jeroen; Ortix, Carmine
2016-10-01
We demonstrate the existence of topological insulators in one dimension (1D) protected by mirror and time-reversal symmetries. They are characterized by a nontrivial Z2 topological invariant defined in terms of the "partial" polarizations, which we show to be quantized in the presence of a 1D mirror point. The topological invariant determines the generic presence or absence of integer boundary charges at the mirror-symmetric boundaries of the system. We check our findings against spin-orbit coupled Aubry-André-Harper models that can be realized, e.g., in cold-atomic Fermi gases loaded in one-dimensional optical lattices or in density- and Rashba spin-orbit-modulated semiconductor nanowires. In this setup, in-gap end-mode Kramers doublets appearing in the topologically nontrivial state effectively constitute a double-quantum dot with spin-orbit coupling.
From a normal insulator to a topological insulator in plumbene
NASA Astrophysics Data System (ADS)
Yu, Xiang-Long; Huang, Li; Wu, Jiansheng
2017-03-01
Plumbene, similar to silicene, has a buckled honeycomb structure with a large band gap (˜400 meV). All previous studies have shown that it is a normal insulator. Here, we perform first-principles calculations and employ a sixteen-band tight-binding model with nearest-neighbor and next-nearest-neighbor hopping terms to investigate electronic structures and topological properties of the plumbene monolayer. We find that it can become a topological insulator with a large bulk gap (˜200 meV) through electron doping, and the nontrivial state is very robust with respect to external strain. Plumbene can be an ideal candidate for realizing the quantum spin Hall effect at room temperature. By investigating effects of external electric and magnetic fields on electronic structures and transport properties of plumbene, we present two rich phase diagrams with and without electron doping and propose a theoretical design for a four-state spin-valley filter.
Topological oxide insulator in cubic perovskite structure.
Jin, Hosub; Rhim, Sonny H; Im, Jino; Freeman, Arthur J
2013-01-01
The emergence of topologically protected conducting states with the chiral spin texture is the most prominent feature at the surface of topological insulators. On the application side, large band gap and high resistivity to distinguish surface from bulk degrees of freedom should be guaranteed for the full usage of the surface states. Here, we suggest that the oxide cubic perovskite YBiO3, more than just an oxide, defines itself as a new three-dimensional topological insulator exhibiting both a large bulk band gap and a high resistivity. Based on first-principles calculations varying the spin-orbit coupling strength, the non-trivial band topology of YBiO3 is investigated, where the spin-orbit coupling of the Bi 6p orbital plays a crucial role. Taking the exquisite synthesis techniques in oxide electronics into account, YBiO3 can also be used to provide various interface configurations hosting exotic topological phenomena combined with other quantum phases.
NASA Astrophysics Data System (ADS)
Gu, Genda; Yang, Alina; Schneeloch, John; Xu, Zhijun; Tranquada, John; Zhoa, J. G.; Pan, Z. H.; Yang, H. B.; Si, W. D.; Valla, T.
2012-02-01
The discovery of 3D topological insulator and topological superconductor materials opens up a new research field in the condensed matter physics. In order to exploit the novel surface properties of these topological insulators, it is crucial to achieve a bulk-insulating state in these topological insulator crystals. Unfortunately, all available topological insulator crystals are not bulk-insulating. We have grown a number of Bi-Se, Bi-Te, Sb-Te-Se, Bi-Sb-Se, Bi-Sb-Te-Se and Bi-Sb-Te-Se-S topological insulator single crystals by using 5N and 6N pure elements. We have measured the physical properties on these single crystals. We have studied the effect of growth condition and impurity on the bulk electrical conductivity of these single crystals. We try to answer two questions if it is possible to grow the bulk-insulating topological insulator single crystals and which maximum resistivity of these topological insulator single crystals we can grow. We have also grown a number of CuxBi2Se3 topological superconductor single crystals.
Interface electronic structure at the topological insulator-ferrimagnetic insulator junction.
Kubota, Y; Murata, K; Miyawaki, J; Ozawa, K; Onbasli, M C; Shirasawa, T; Feng, B; Yamamoto, Sh; Liu, R-Y; Yamamoto, S; Mahatha, S K; Sheverdyaeva, P; Moras, P; Ross, C A; Suga, S; Harada, Y; Wang, K L; Matsuda, I
2017-02-08
An interface electron state at the junction between a three-dimensional topological insulator film, Bi2Se3, and a ferrimagnetic insulator film, Y3Fe5O12 (YIG), was investigated by measurements of angle-resolved photoelectron spectroscopy and x-ray absorption magnetic circular dichroism. The surface state of the Bi2Se3 film was directly observed and localized 3d spin states of the Fe(3+) in the YIG film were confirmed. The proximity effect is likely described in terms of the exchange interaction between the localized Fe 3d electrons in the YIG film and delocalized electrons of the surface and bulk states in the Bi2Se3 film.
Interface electronic structure at the topological insulator-ferrimagnetic insulator junction
NASA Astrophysics Data System (ADS)
Kubota, Y.; Murata, K.; Miyawaki, J.; Ozawa, K.; Onbasli, M. C.; Shirasawa, T.; Feng, B.; Yamamoto, Sh; Liu, R.-Y.; Yamamoto, S.; Mahatha, S. K.; Sheverdyaeva, P.; Moras, P.; Ross, C. A.; Suga, S.; Harada, Y.; Wang, K. L.; Matsuda, I.
2017-02-01
An interface electron state at the junction between a three-dimensional topological insulator film, Bi2Se3, and a ferrimagnetic insulator film, Y3Fe5O12 (YIG), was investigated by measurements of angle-resolved photoelectron spectroscopy and x-ray absorption magnetic circular dichroism. The surface state of the Bi2Se3 film was directly observed and localized 3d spin states of the Fe3+ in the YIG film were confirmed. The proximity effect is likely described in terms of the exchange interaction between the localized Fe 3d electrons in the YIG film and delocalized electrons of the surface and bulk states in the Bi2Se3 film.
Topological Quantum Information Processing Mediated Via Hybrid Topological Insulator Structures
2013-11-13
manipulation, entanglement and detection ofMajorana fermions in diamond-topological insulator-superconductor heterojunctions. Furthennore, we propose to...particles which obey non-Abelian statistics. The simplest of these particles, Majorana fermions , are believed to exist as excitations in exotic...materials under extreme conditions. Additionally, Majorana fermions have been proposed to exist in a new class of materials commonly referred to as
Junction between surfaces of two topological insulators
NASA Astrophysics Data System (ADS)
Sen, Diptiman; Deb, Oindrila
2012-02-01
We study scattering from a line junction which separates the surfaces of two three-dimensional topological insulators; some aspects of this problem were recently studied in Takahashi and Murakami, Phys. Rev. Lett. 107, 166805 (2011). The velocities of the Dirac electrons on the two surfaces may be unequal and may even have opposite signs; in the latter case, we find that the electrons must, in general, go into the two-dimensional interface separating the two topological insulators. We also study what happens if the two surfaces are at an angle φ with respect to each other. We find in this case that there are bound states which propagate along the line junction with a velocity and direction of spin which depend on the bending angle φ.
Topological insulator in a helicoidal magnetization field
NASA Astrophysics Data System (ADS)
Stagraczyński, S.; Chotorlishvili, L.; Dugaev, V. K.; Jia, C.-L.; Ernst, A.; Komnik, A.; Berakdar, J.
2016-11-01
A key feature of topological insulators is the robustness of the electron energy spectrum. At a surface of a topological insulator, the Dirac point is protected by the characteristic symmetry of the system. The breaking of the symmetry opens a gap in the energy spectrum. Therefore, topological insulators are very sensitive to magnetic fields, which can open a gap in the electronic spectrum. Concerning "internal" magnetic effects, for example, the situation with doped magnetic impurities, is not trivial. A single magnetic impurity is not enough to open the band gap, while in the case of a ferromagnetic chain of deposited magnetic impurities the Dirac point is lifted. However, a much more interesting case is when localized magnetic impurities form a chiral spin order. Our first principle density functional theory calculations have shown that this is the case for Fe deposited on the surface of a Bi2Se3 topological insulator. But not only magnetic impurities can form a chiral helicoidal spin texture. An alternative way is to use chiral multiferroics (prototype material is LiCu2O2 ) that induce a proximity effect. The theoretical approach we present here is valid for both cases. We observed that opposite to a ferromagnetically ordered case, a chiral spin order does not destroy the Dirac point. We also observed that the energy gap appears at the edges of the new Brillouin zone. Another interesting result concerns the spin dynamics. We derived an equation for the spin density dynamics with a spin current and relaxation terms. We have shown that the motion of the conductance electron generates a magnetic torque and exerts a certain force on the helicoidal texture.
Disorder effects in correlated topological insulators
NASA Astrophysics Data System (ADS)
Hung, Hsiang-Hsuan; Barr, Aaron; Prodan, Emil; Fiete, Gregory A.
2016-12-01
Using exact diagonalization and quantum Monte Carlo calculations we investigate the effects of disorder on the phase diagram of both noninteracting and interacting models of two-dimensional topological insulators. In the fermion sign problem-free interacting models we study, electron-electron interactions are described by an on-site repulsive Hubbard interaction and disorder is included via the one-body hopping operators. In both the noninteracting and interacting models we make use of recent advances in highly accurate real-space numerical evaluation of topological invariants to compute phase boundaries and in the noninteracting models determine critical exponents of the transitions. We find different models exhibit distinct stability conditions of the topological phase with respect to interactions and disorder. We provide a general analytical theory that accurately predicts these trends.
Interacting weak topological insulators and their transition to Dirac semimetal phases
NASA Astrophysics Data System (ADS)
Sangiovanni, Giorgio; Hanke, Werner; Li, Gang; Trauzettel, Bjoern
Topological insulators in the presence of strong Coulomb interaction constitute novel phases of matter. Transitions between these phases can be driven by single-particle or many-body effects. On the basis of ab-initio calculations, we identify a concrete material, i.e. Ca2PtO4, that turns out to be a hole-doped weak topological insulator. Interestingly, the Pt- d orbitals in this material are relevant for the band inversion that gives rise to the topological phase. Therefore, Coulomb interaction should be of importance in Ca2PtO4. To study the influence of interactions on the weak topological insulating phase, we look at a toy model corresponding to a layer-stacked 3D version of the Bernevig-Hughes-Zhang model with local interactions. For small to intermediate interaction strength, we discover novel interaction-driven topological phase transitions between the weak topological insulator and two Dirac semimetal phases. The latter correspond to gapless topological phases. For strong interactions, the system eventually becomes a Mott insulator. DFG Grant No. Ha 1537/23-1 within the Forschergruppe FOR 1162, SPP Grant Ha 1537/24-2, SFB 1170 ``ToCoTronics'', SPP 1666, the Helmholtz Foundation (VITI), the ``Elitenetzwerk Bayern'' (ENB graduate school on ``Topological insulators'').
Contribution of Topological Domains and Loop Formation to 3D Chromatin Organization.
Ea, Vuthy; Baudement, Marie-Odile; Lesne, Annick; Forné, Thierry
2015-07-27
Recent investigations on 3D chromatin folding revealed that the eukaryote genomes are both highly compartmentalized and extremely dynamic. This review presents the most recent advances in topological domains' organization of the eukaryote genomes and discusses the relationship to chromatin loop formation. CTCF protein appears as a central factor of these two organization levels having either a strong insulating role at TAD borders, or a weaker architectural role in chromatin loop formation. TAD borders directly impact on chromatin dynamics by restricting contacts within specific genomic portions thus confining chromatin loop formation within TADs. We discuss how sub-TAD chromatin dynamics, constrained into a recently described statistical helix conformation, can produce functional interactions by contact stabilization.
Topological Quantum Information Processing Mediated Via Hybrid Topogical Insulator Structures
2014-03-28
formation, manipulation, entanglement and detection of Majorana fermions in diamond-topological insulator- superconductor heterojunctions. Furthermore...between Superconductors and Topological Insulators Recent advances have revealed a new type of information processing, topological quantum...vortex lines6 and lattices7 in TI – superconductor heterostructures. Some of our most impactful work in this area has come through collaborations with
NASA Astrophysics Data System (ADS)
Kim, Youngseok; Philip, Timothy M.; Park, Moon Jip; Gilbert, Matthew J.
2016-12-01
As a promising candidate system to realize topological superconductivity, the system of a 3D topological insulator (TI) grown on top of the s -wave superconductor has been extensively studied. To access the topological superconductivity experimentally, the 3D TI sample must be thin enough to allow for Cooper pair tunneling to the exposed surface of TI. The use of magnetically ordered dopants to break time-reversal symmetry may allow the surface of a TI to host Majorana fermion, which are believed to be a signature of topological superconductivity. In this work, we study a magnetically-doped thin film TI-superconductor hybrid system. Considering the proximity induced order parameter in thin film of TI, we analyze the gap closing points of the Hamiltonian and draw the phase diagram as a function of relevant parameters: the hybridization gap, Zeeman energy, and chemical potential of the TI system. Our findings provide a useful guide in choosing relevant parameters to facilitate the observation of topological superconductivity in thin film TI-superconductor hybrid systems. In addition, we further perform numerical analysis on a TI proximity coupled to an s -wave superconductor and find that, due to the spin-momentum locked nature of the surface states in TI, the induced s -wave order parameter of the surface states persists even at large magnitude of the Zeeman energy.
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.
Quantum interference and Aharonov-Bohm oscillations in topological insulators
NASA Astrophysics Data System (ADS)
Bardarson, Jens H.; Moore, Joel E.
2013-05-01
Topological insulators (TIs) have an insulating bulk but a metallic surface. In the simplest case, the surface electronic structure of a three-dimensional (3D) TI is described by a single two-dimensional (2D) Dirac cone. A single 2D Dirac fermion cannot be realized in an isolated 2D system with time-reversal symmetry, but rather owes its existence to the topological properties of the 3D bulk wavefunctions. The transport properties of such a surface state are of considerable current interest; they have some similarities with graphene, which also realizes Dirac fermions, but have several unique features in their response to magnetic fields. In this review we give an overview of some of the main quantum transport properties of TI surfaces. We focus on the efforts to use quantum interference phenomena, such as weak anti-localization and the Aharonov-Bohm effect, to verify in a transport experiment the Dirac nature of the surface state and its defining properties. In addition to explaining the basic ideas and predictions of the theory, we provide a survey of recent experimental work.
Quantum interference and Aharonov-Bohm oscillations in topological insulators.
Bardarson, Jens H; Moore, Joel E
2013-05-01
Topological insulators (TIs) have an insulating bulk but a metallic surface. In the simplest case, the surface electronic structure of a three-dimensional (3D) TI is described by a single two-dimensional (2D) Dirac cone. A single 2D Dirac fermion cannot be realized in an isolated 2D system with time-reversal symmetry, but rather owes its existence to the topological properties of the 3D bulk wavefunctions. The transport properties of such a surface state are of considerable current interest; they have some similarities with graphene, which also realizes Dirac fermions, but have several unique features in their response to magnetic fields. In this review we give an overview of some of the main quantum transport properties of TI surfaces. We focus on the efforts to use quantum interference phenomena, such as weak anti-localization and the Aharonov-Bohm effect, to verify in a transport experiment the Dirac nature of the surface state and its defining properties. In addition to explaining the basic ideas and predictions of the theory, we provide a survey of recent experimental work.
Topological Field Theory of Time-Reversal Invariant Insulators
Qi, Xiao-Liang; Hughes, Taylor; Zhang, Shou-Cheng; /Stanford U., Phys. Dept.
2010-03-19
We show that the fundamental time reversal invariant (TRI) insulator exists in 4 + 1 dimensions, where the effective field theory is described by the 4 + 1 dimensional Chern-Simons theory and the topological properties of the electronic structure is classified by the second Chern number. These topological properties are the natural generalizations of the time reversal breaking (TRB) quantum Hall insulator in 2 + 1 dimensions. The TRI quantum spin Hall insulator in 2 + 1 dimensions and the topological insulator in 3 + 1 dimension can be obtained as descendants from the fundamental TRI insulator in 4 + 1 dimensions through a dimensional reduction procedure. The effective topological field theory, and the Z{sub 2} topological classification for the TRI insulators in 2+1 and 3+1 dimensions are naturally obtained from this procedure. All physically measurable topological response functions of the TRI insulators are completely described by the effective topological field theory. Our effective topological field theory predicts a number of novel and measurable phenomena, the most striking of which is the topological magneto-electric effect, where an electric field generates a magnetic field in the same direction, with an universal constant of proportionality quantized in odd multiples of the fine structure constant {alpha} = e{sup 2}/hc. Finally, we present a general classification of all topological insulators in various dimensions, and describe them in terms of a unified topological Chern-Simons field theory in phase space.
An edge index for topological insulators
NASA Astrophysics Data System (ADS)
Prodan, Emil
2009-03-01
Topological insulators display dissipationless currents flowing at the edges of the samples. These currents are associated to chiral edge modes, whose existence is intrinsically linked to the topology of the electronic states of the bulk. The edge modes can be easily investigated when the edges are smooth and have a periodicity, but as soon as the periodicity is absent, the problem becomes un-traceable by purely theoretical means. In my talk I will exemplify the use of non-commutative calculus to explore the properties, especially the stability of the edge modes. For example, using such techniques one can give a fairly elementary proof that the edge modes in Chern insulators survive even for a rough (random) edge. Similarly, for the Spin-Hall effect, one can define an observable and its associated current whose conductance remains quantized during various deformations of the Hamiltonian system. It turns out that in all cases, the edge conductance is given by the index of a Fredholm operator, which provides a new topological invariant linked directly to the edge rather than the bulk.
A scheme for a topological insulator field effect transistor
NASA Astrophysics Data System (ADS)
Vali, Mehran; Dideban, Daryoosh; Moezi, Negin
2015-05-01
We propose a scheme for a topological insulator field effect transistor. The idea is based on the gate voltage control of the Dirac fermions in a ferromagnetic topological insulator channel with perpendicular magnetization connecting to two metallic topological insulator leads. Our theoretical analysis shows that the proposed device displays a switching effect with high on/off current ratio and a negative differential conductance with a good peak to valley ratio.
The topological insulator in a fractal space
Song, Zhi-Gang; Zhang, Yan-Yang; Li, Shu-Shen
2014-06-09
We investigate the band structures and transport properties of a two-dimensional model of topological insulator, with a fractal edge or a fractal bulk. A fractal edge does not affect the robust transport even when the fractal pattern has reached the resolution of the atomic-scale, because the bulk is still well insulating against backscattering. On the other hand, a fractal bulk can support the robust transport only when the fractal resolution is much larger than a critical size. Smaller resolution of bulk fractal pattern will lead to remarkable backscattering and localization, due to strong couplings of opposite edge states on narrow sub-edges which appear almost everywhere in the fractal bulk.
Infrared studies of topological insulator systems
NASA Astrophysics Data System (ADS)
Post, Kirk; Chapler, Brian; Schafgans, Alex; Liu, Mengkun; Wu, Jih-Sheng; Richardella, Anthony; Lee, Joon Sue; Reijnders, Anjan; Lee, Yun Sang; He, Liang; Kou, Xufeng; Novak, Mario; Taskin, Alexey; Segawa, Kouji; Goldflam, Michael; Stinson, H. Theodore; Qi, Xiao Liang; Burch, Kenneth; Wang, Kang; Fogler, Michael; Samarth, Nitin; Ando, Yoichi; Basov, Dimitri
The theoretical prediction, and subsequent experimental realization, of topological insulator (TI) systems, has vaulted this new class of materials to the vanguard of condensed matter physics. Since their discovery, we have carried out a number of infrared studies on various TI systems, including Bi2Se3, Bi1-xSbx, and Bi2-xSbxTe3-ySey crystals as well as Bi2Se3 and (Bi,Sb)2Te3 thin films. A key element of these works is the revelation that the infrared response of Bi1-xSbx crystals and (Bi,Sb)2Te3 thin films possess a significant, or even dominant, component from the topologically protected surface states. I will review these works and discuss future prospects of measuring the surface state response through optical spectroscopy techniques
Thermoelectric figure of merit in topological insulators
NASA Astrophysics Data System (ADS)
Takahashi, Ryuji; Murakami, Shuichi
2011-07-01
Transport behavior of two-dimensional topological insulators is theoretically studied in narrow ribbon geometry. The system has perfectly conducting edge channels, which are free from backscattering. At high temperature, the bulk states dominate in the transport phenomenon. However, at low temperature the conducting channels along the edges become dominant. It causes a bulk-to-edge crossover. Namely, by lowering temperature, the figure of merit first decreases by a competition between the bulk and the edge transport, and then increases again because the edge transport become larger.
Distinctive features of transport in topological insulators
NASA Astrophysics Data System (ADS)
Sacksteder, Vincent; Wu, Quansheng; Arnardottir, Kristin; Shelykh, Ivan; Kettemann, Stefan
2015-03-01
The surface states of a topological insulator in a fine-tuned magnetic field are ideal candidates for realizing a topological metal which is protected against disorder. Its signatures are (1) a conductance plateau in long wires and (2) a conductivity which always increases with sample size. We numerically show that the bulk substantially accelerates the conductance plateaus's decay in a magnetic field. It also reduces the effects of surface disorder and causes the magnitude of the surface conductivity and the magnetoconductivity to depend systematically on sample details such as doping and disorder strength. In addition, we predict a new signature of the topological state: at low temperatures the magnetoresistance will deviate strongly from the Hikami-Larkin-Nagaoka (HLN) formula. In this regime the magnetoresistance is dominated by scattering processes which wrap around the TI sample. The HLN formula's shoulder is replaced by a feature with a larger critical field magnetic strength that is caused by wrapping. Inside the wrapping regime the magnetoconductance will lose its dependence on temperature. This new topological signature should be visible in the same samples and temperatures where the Altshuler-Aronov-Spivak (AAS) effect has already been observed.
Correlation effects in two-dimensional topological insulators.
Hohenadler, M; Assaad, F F
2013-04-10
Topological insulators have become one of the most active research areas in condensed matter physics. This article reviews progress on the topic of electronic correlation effects in the two-dimensional case, with a focus on systems with intrinsic spin-orbit coupling and numerical results. Topics addressed include an introduction to the noninteracting case, an overview of theoretical models, correlated topological band insulators, interaction-driven phase transitions, topological Mott insulators and fractional topological states, correlation effects on helical edge states, and topological invariants of interacting systems.
Persistent optical gating of a topological insulator
Yeats, Andrew L.; Pan, Yu; Richardella, Anthony; Mintun, Peter J.; Samarth, Nitin; Awschalom, David D.
2015-01-01
The spin-polarized surface states of topological insulators (TIs) are attractive for applications in spintronics and quantum computing. A central challenge with these materials is to reliably tune the chemical potential of their electrons with respect to the Dirac point and the bulk bands. We demonstrate persistent, bidirectional optical control of the chemical potential of (Bi,Sb)2Te3 thin films grown on SrTiO3. By optically modulating a space-charge layer in the SrTiO3 substrates, we induce a persistent field effect in the TI films comparable to electrostatic gating techniques but without additional materials or processing. This enables us to optically pattern arbitrarily shaped p- and n-type regions in a TI, which we subsequently image with scanning photocurrent microscopy. The ability to optically write and erase mesoscopic electronic structures in a TI may aid in the investigation of the unique properties of the topological insulating phase. The gating effect also generalizes to other thin-film materials, suggesting that these phenomena could provide optical control of chemical potential in a wide range of ultrathin electronic systems. PMID:26601300
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
Topological effects on the magnetoconductivity in topological insulators
NASA Astrophysics Data System (ADS)
Sacksteder, Vincent E.; Arnardottir, Kristin Bjorg; Kettemann, Stefan; Shelykh, Ivan A.
2014-12-01
Three-dimensional strong topological insulators (TIs) guarantee the existence of a two-dimensional (2-D) conducting surface state which completely covers the surface of the TI. The TI surface state necessarily wraps around the TI's top, bottom, and two sidewalls, and is therefore topologically distinct from ordinary 2-D electron gases (2-DEGs) which are planar. This has several consequences for the magnetoconductivity Δ σ , a frequently studied measure of weak antilocalization which is sensitive to the quantum coherence time τϕ and to temperature. We show that conduction on the TI sidewalls systematically reduces Δ σ , multiplying it by a factor which is always less than one and decreases in thicker samples. In addition, we present both an analytical formula and numerical results for the tilted-field magnetoconductivity which has been measured in several experiments. Lastly, we predict that as the temperature is reduced Δ σ will enter a wrapped regime where it is sensitive to diffusion processes which make one or more circuits around the TI. In this wrapped regime the magnetoconductivity's dependence on temperature, typically 1 /T2 in 2-DEGs, disappears. We present numerical and analytical predictions for the wrapped regime at both small and large field strengths. The wrapped regime and topological signatures discussed here should be visible in the same samples and at the same temperatures where the Altshuler-Aronov-Spivak (AAS) effect has already been observed, when the measurements are repeated with the magnetic field pointed perpendicularly to the TI's top face.
A first theoretical realization of honeycomb topological magnon insulator
NASA Astrophysics Data System (ADS)
Owerre, S. A.
2016-09-01
It has been recently shown that in the Heisenberg (anti)ferromagnet on the honeycomb lattice, the magnons (spin wave quasipacticles) realize a massless two-dimensional (2D) Dirac-like Hamiltonian. It was shown that the Dirac magnon Hamiltonian preserves time-reversal symmetry defined with the sublattice pseudo spins and the Dirac points are robust against magnon-magnon interactions. The Dirac points also occur at nonzero energy. In this paper, we propose a simple realization of nontrivial topology (magnon edge states) in this system. We show that the Dirac points are gapped when the inversion symmetry of the lattice is broken by introducing a next-nearest neighbour Dzyaloshinskii-Moriya (DM) interaction. Thus, the system realizes magnon edge states similar to the Haldane model for quantum anomalous Hall effect in electronic systems. However, in contrast to electronic spin current where dissipation can be very large due to Ohmic heating, noninteracting topological magnons can propagate for a long time without dissipation as magnons are uncharged particles. We observe the same magnon edge states for the XY model on the honeycomb lattice. Remarkably, in this case the model maps to interacting hardcore bosons on the honeycomb lattice. Quantum magnetic systems with nontrivial magnon edge states are called topological magnon insulators. They have been studied theoretically on the kagome lattice and recently observed experimentally on the kagome magnet Cu(1-3, bdc) with three magnon bulk bands. Our results for the honeycomb lattice suggests an experimental procedure to search for honeycomb topological magnon insulators within a class of 2D quantum magnets and ultracold atoms trapped in honeycomb optical lattices. In 3D lattices, Dirac and Weyl points were recently studied theoretically, however, the criteria that give rise to them were not well-understood. We argue that the low-energy Hamiltonian near the Weyl points should break time-reversal symmetry of the pseudo spins
A first theoretical realization of honeycomb topological magnon insulator.
Owerre, S A
2016-09-28
It has been recently shown that in the Heisenberg (anti)ferromagnet on the honeycomb lattice, the magnons (spin wave quasipacticles) realize a massless two-dimensional (2D) Dirac-like Hamiltonian. It was shown that the Dirac magnon Hamiltonian preserves time-reversal symmetry defined with the sublattice pseudo spins and the Dirac points are robust against magnon-magnon interactions. The Dirac points also occur at nonzero energy. In this paper, we propose a simple realization of nontrivial topology (magnon edge states) in this system. We show that the Dirac points are gapped when the inversion symmetry of the lattice is broken by introducing a next-nearest neighbour Dzyaloshinskii-Moriya (DM) interaction. Thus, the system realizes magnon edge states similar to the Haldane model for quantum anomalous Hall effect in electronic systems. However, in contrast to electronic spin current where dissipation can be very large due to Ohmic heating, noninteracting topological magnons can propagate for a long time without dissipation as magnons are uncharged particles. We observe the same magnon edge states for the XY model on the honeycomb lattice. Remarkably, in this case the model maps to interacting hardcore bosons on the honeycomb lattice. Quantum magnetic systems with nontrivial magnon edge states are called topological magnon insulators. They have been studied theoretically on the kagome lattice and recently observed experimentally on the kagome magnet Cu(1-3, bdc) with three magnon bulk bands. Our results for the honeycomb lattice suggests an experimental procedure to search for honeycomb topological magnon insulators within a class of 2D quantum magnets and ultracold atoms trapped in honeycomb optical lattices. In 3D lattices, Dirac and Weyl points were recently studied theoretically, however, the criteria that give rise to them were not well-understood. We argue that the low-energy Hamiltonian near the Weyl points should break time-reversal symmetry of the pseudo spins
Systematic control of surface Dirac fermion density on topological insulator Bi2Te3
NASA Astrophysics Data System (ADS)
Xu, Suyang; Xia, Yuqi; Grauer, David; Hor, Yewsan; Cava, Robert; Hasan, Zahid
2010-03-01
Three dimensional (3D) topological insulators are quantum materials with a spin-orbit induced bulk insulating gap that exhibit quantum-Hall-like phenomena in the absence of applied magnetic fields. They feature surface states that are topologically protected against scattering by time reversal symmetry. The proposed applications of topological insulators in device geometries rely on the ability to tune the chemical potential on their surfaces in the vicinity of the Dirac node. Here, we demonstrate a suite of surface control methods based on a combination of photo-doping and molecular-doping to tune the Dirac fermion density on the topological (111) surface of Bi2Te3. Their efficacy is demonstrated via direct electronic structure measurements using high resolution angle-resolved photoemission spectroscopy. These results open up new opportunities for probing topological behavior of Dirac electrons in Bi2Te3. At least one of the methods demonstrated here can be successfully applied to other topological insulators (Bi1-xSbx, Sb2Te3 and Bi2Se3). More importantly, our methods of topological surface state manipulation demonstrated here are highly suitable for future spectroscopic studies of topological phenomena which will complement the transport results gained from the traditional electrical gating techniques.
Heterostructures of Topological Insulators and Superconductors
NASA Astrophysics Data System (ADS)
Lababidi, Mahmoud
Topological insulators (TI), such as Bi2Se 3, are a new class of quantum materials discovered recently. They are insulating in the bulk but can conduct on the surfaces. The robust surface states of three-dimensional strong TIs form a unique two-dimensional system of massless electrons, known as a helical metal, with a linear energy-momentum dispersion and spin-momentum locking. While these surface modes alone have spurred great interest, their interaction with superconductors (S) in close proximity has opened up opportunities to engineer topological superconductivity using TI-S heterostructures. This thesis is a microscopic, self-consistent theoretical investigation of the interplay between TI and superconductors. Three types of TI-based heterostructures with increasing complexity are studied in detail. We first present a detailed study of the coupling between a metal and a topological insulator. We compute the spin-active scattering matrix for electrons coming from the metal incident on the metal-TI interface. We find that there exists a critical incident angle, where perfect spin-flip occurs as the incoming electron is reflected. We discuss the origin of this phenomena and its potential implications in spintronics. We then compute the local spectrum at the metal-TI interface, and examine its evolution from the tunneling limit (bad contact) to the strong coupling limit (good contact). The calculations are done using two complementary approaches; in a continuum model based on a k·p Hamiltonian a wave function matching approach is taken and the lattice model requires the use of lattice Green's functions. The study of metal-TI interface lays the foundation for our subsequent theory of S-TI interface. Next we carry out microscopic, self-consistent calculations of the superconducting order parameter and pairing correlations near a S-TI interface, where S is an s-wave superconductor. We discuss the suppression of the order parameter by the topological insulator and
Rotating and Precessing Dissipative-Optical-Topological-3D Solitons
NASA Astrophysics Data System (ADS)
Veretenov, N. A.; Rosanov, N. N.; Fedorov, S. V.
2016-10-01
We predict and study a new type of three-dimensional soliton: asymmetric rotating and precessing stable topological-dissipative-optical localized structures in homogeneous media with saturable amplification and absorption. The crucial factor determining their dynamics is the ratio of the diffusion coefficients characterizing the frequency dispersion and angular selectivity (dichroism) of the scheme. These vortex solitons exist and are stable for overcritical values of the selectivity coefficients and can be realized in lasers of large sizes with saturable absorption.
Quantum phase transitions of topological insulators without gap closing.
Rachel, Stephan
2016-10-12
We consider two-dimensional Chern insulators and time-reversal invariant topological insulators and discuss the effect of perturbations breaking either particle-number conservation or time-reversal symmetry. The appearance of trivial mass terms is expected to cause quantum phase transitions into trivial phases when such a perturbation overweighs the topological term. These phase transitions are usually associated with a bulk-gap closing. In contrast, the chiral Chern insulator is unaffected by particle-number breaking perturbations. Moreover, the [Formula: see text] topological insulator undergoes phase transitions into topologically trivial phases without bulk-gap closing in the presence of any of such perturbations. In certain cases, these phase transitions can be circumvented and the protection restored by another U(1) symmetry, e.g. due to spin conservation. These findings are discussed in the context of interacting topological insulators.
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
Associating Specific Materials with Topological Insulation Behavior
NASA Astrophysics Data System (ADS)
Zhang, Xiuwen
2014-03-01
The first-principles (a) total-energy/stability calculations combined with (b) electronic structure calculations of band inversion, spin-polarization and topological invariants (Z2) has led to the design and prediction of specific materials that are topological insulators in this study. We classify bulk materials into four types of band-inversion behaviors (TI-1, TI-2, BI-3, BI-4), based on the number of band inversions and their distributions on various time reversal invariant k points. Depending on the inversion type in bulk, the corresponding surface states have different protections e.g., protected by time reversal symmetry (in TI-1 materials), spatial symmetry (in TI-2), or not protected (in BI-3, BI-4). Subject 1 Discovery of new TI by screening materials for a Z2 metric: Such high-throughput search in the framework of Inverse Design methodology predicts a few previously undocumented materials that are TI-1 in their ground state crystal structure. We also predict dozens of materials that are TI-1 however in structures that are not ground states (e.g. perovskite structure of II-Bi-O3). Subject 2 Design Principle to increase the gap of TI-1 materials: In HgTe-like cubic topological materials, the insulating gap is zero since the spin-orbit splitting is positive and so a 4-fold half-filled p-like band is near the Fermi level. By design of hybridization of d-orbitals into the p-like bands, one can create negative spin-orbit splitting and so a finite insulating gap. Subject 3 Unconventional spin textures of TI surface states: Despite the fact that one of our predicted TI-1 KBaBi has inversion symmetry in the bulk-a fact that that would preclude bulk spin polarization-we find a Dresselhaus-like spin texture with non-helical spin texture. This originates from the local spin polarization, anchored on the atomic sites with inversion asymmetric point groups, that is compensated due to global inversion symmetry in bulk. In collaboration with: Jun-Wei Luo, Qihang Liu
A topological framework for interactive queries on 3D models in the Web.
Figueiredo, Mauro; Rodrigues, José I; Silvestre, Ivo; Veiga-Pires, Cristina
2014-01-01
Several technologies exist to create 3D content for the web. With X3D, WebGL, and X3DOM, it is possible to visualize and interact with 3D models in a web browser. Frequently, three-dimensional objects are stored using the X3D file format for the web. However, there is no explicit topological information, which makes it difficult to design fast algorithms for applications that require adjacency and incidence data. This paper presents a new open source toolkit TopTri (Topological model for Triangle meshes) for Web3D servers that builds the topological model for triangular meshes of manifold or nonmanifold models. Web3D client applications using this toolkit make queries to the web server to get adjacent and incidence information of vertices, edges, and faces. This paper shows the application of the topological information to get minimal local points and iso-lines in a 3D mesh in a web browser. As an application, we present also the interactive identification of stalactites in a cave chamber in a 3D web browser. Several tests show that even for large triangular meshes with millions of triangles, the adjacency and incidence information is returned in real time making the presented toolkit appropriate for interactive Web3D applications.
Experimental demonstration of anomalous Floquet topological insulator for sound.
Peng, Yu-Gui; Qin, Cheng-Zhi; Zhao, De-Gang; Shen, Ya-Xi; Xu, Xiang-Yuan; Bao, Ming; Jia, Han; Zhu, Xue-Feng
2016-11-11
Time-reversal invariant topological insulator is widely recognized as one of the fundamental discoveries in condensed matter physics, for which the most fascinating hallmark is perhaps a spin-based topological protection, the absence of scattering of conduction electrons with certain spins on matter surface. Recently, it has created a paradigm shift for topological insulators, from electronics to photonics, phononics and mechanics as well, bringing about not only involved new physics but also potential applications in robust wave transport. Despite the growing interests in topologically protected acoustic wave transport, T-invariant acoustic topological insulator has not yet been achieved. Here we report experimental demonstration of anomalous Floquet topological insulator for sound: a strongly coupled metamaterial ring lattice that supports one-way propagation of pseudo-spin-dependent edge states under T-symmetry. We also demonstrate the formation of pseudo-spin-dependent interface states due to lattice dislocations and investigate the properties of pass band and band gap states.
Experimental demonstration of anomalous Floquet topological insulator for sound
NASA Astrophysics Data System (ADS)
Peng, Yu-Gui; Qin, Cheng-Zhi; Zhao, De-Gang; Shen, Ya-Xi; Xu, Xiang-Yuan; Bao, Ming; Jia, Han; Zhu, Xue-Feng
2016-11-01
Time-reversal invariant topological insulator is widely recognized as one of the fundamental discoveries in condensed matter physics, for which the most fascinating hallmark is perhaps a spin-based topological protection, the absence of scattering of conduction electrons with certain spins on matter surface. Recently, it has created a paradigm shift for topological insulators, from electronics to photonics, phononics and mechanics as well, bringing about not only involved new physics but also potential applications in robust wave transport. Despite the growing interests in topologically protected acoustic wave transport, T-invariant acoustic topological insulator has not yet been achieved. Here we report experimental demonstration of anomalous Floquet topological insulator for sound: a strongly coupled metamaterial ring lattice that supports one-way propagation of pseudo-spin-dependent edge states under T-symmetry. We also demonstrate the formation of pseudo-spin-dependent interface states due to lattice dislocations and investigate the properties of pass band and band gap states.
``Holographic'' treatment of surface disorder in topological insulators
NASA Astrophysics Data System (ADS)
Kim, Kun Woo; Mong, Roger; Franz, Marcel; Refael, Gil
2014-03-01
What is the effect of surface-only disorder on the electronic states of a 3d TI? The layers in the clean bulk parallel to surface probe the surface impurities as they hop in and out of the surface layer. A recursive treatment of the impurity effects is made possible through successive elimination of the lattice layer by layer. This leads to non-linear renormalization group flow of an effective surface impurity potential. We found an exact mapping between the recursion relation and Schrodinger equation along the layers, therefore the modified self energy due to surface impurity could be simply obtained from the transfer matrix method. As a concrete example of 2d topological insulator, we found the exact expression of on-layer self energy for a clean system and an asymptotic expression that captures a general behavior of layers deep in the bulk.
Opto-electronic characterization of three dimensional topological insulators
NASA Astrophysics Data System (ADS)
Plank, H.; Danilov, S. N.; Bel'kov, V. V.; Shalygin, V. A.; Kampmeier, J.; Lanius, M.; Mussler, G.; Grützmacher, D.; Ganichev, S. D.
2016-10-01
We demonstrate that the terahertz/infrared radiation induced photogalvanic effect, which is sensitive to the surface symmetry and scattering details, can be applied to study the high frequency conductivity of the surface states in (Bi1-xSbx)2Te3 based three dimensional (3D) topological insulators (TIs). In particular, measuring the polarization dependence of the photogalvanic current and scanning with a micrometre sized beam spot across the sample, provides access to (i) topographical inhomogeneities in the electronic properties of the surface states and (ii) the local domain orientation. An important advantage of the proposed method is that it can be applied to study TIs at room temperature and even in materials with a high electron density of bulk carriers.
Thermoelectric effects in topological crystalline insulators
NASA Astrophysics Data System (ADS)
Rameshti, Babak Zare; Asgari, Reza
2016-11-01
We investigate the electrical conductivity and thermoelectric effects in topological crystalline insulators in the presence of short- and long-range impurity interactions. We employ the generalized Boltzmann formalism for anisotropic Fermi surface systems. The conductivity exhibits a local minimum as doping varies owing to the Van Hove singularity in the density of states originated from the saddle point in the surface states' band structure. Suppression of the interband scattering of the charge carriers at high-energy Dirac points results in a maximum in the electrical conductivity. Whenever the Fermi level passes an extremum in the conductivity, the Seebeck coefficient changes sign. In addition, it is revealed that profound thermoelectric effects can be attained around these extrema points.
Anomalous Josephson current in superconducting topological insulator
NASA Astrophysics Data System (ADS)
Yamakage, Ai; Sato, Masatoshi; Yada, Keiji; Kashiwaya, Satoshi; Tanaka, Yukio
2013-03-01
We investigate the effect of helical Majorana fermions at the surface of superconducting topological insulators (STIs) on the Josephson current by referring to possible pairing states of Cu-doped Bi2Se3. The surface state in the present STI has a spin helicity because the directions of spin and momentum are locked to each other. The Josephson current-phase relation in an STI/s-wave superconductor junction shows robust sin(2φ) owing to mirror symmetry, where φ denotes the macroscopic phase difference between the two superconductors. In contrast, the maximum Josephson current in an STI/STI junction exhibits a nonmonotonic temperature dependence depending on the relative spin helicity of the two surface states. Detecting these features qualifies as distinct experimental evidence for the identification of the helical Majorana fermion in STIs.
Anomalous magnetoresistance in magnetized topological insulator cylinders
Siu, Zhuo Bin; Jalil, Mansoor B. A.
2015-05-07
The close coupling between the spin and momentum degrees of freedom in topological insulators (TIs) presents the opportunity for the control of one to manipulate the other. The momentum can, for example, be confined on a curved surface and the spin influenced by applying a magnetic field. In this work, we study the surface states of a cylindrical TI magnetized in the x direction perpendicular to the cylindrical axis lying along the z direction. We show that a large magnetization leads to an upwards bending of the energy bands at small |k{sub z}|. The bending leads to an anomalous magnetoresistance where the transmission between two cylinders magnetized in opposite directions is higher than when the cylinders are magnetized at intermediate angles with respect to each other.
Hg-Based Epitaxial Materials for Topological Insulators
2014-07-01
systems with foreseen applications ranging from topological superconductors and spintronics to quantum computation. Recent trends in TI consist in...topological superconductors and spintronics to quantum computation (e.g. see C.L.Kane and J.E.Moore "Topological Insulators" Physics World (2011) 24
Unusual spin dynamics in topological insulators
Dóra, Balázs; Simon, Ferenc
2015-01-01
The dynamic spin susceptibility (DSS) has a ubiquitous Lorentzian form around the Zeeman energy in conventional materials with weak spin orbit coupling, whose spectral width characterizes the spin relaxation rate. We show that DSS has an unusual non-Lorentzian form in topological insulators, which are characterized by strong SOC, and the anisotropy of the DSS reveals the orientation of the underlying spin texture of topological states. At zero temperature, the high frequency part of DSS is universal and increases in certain directions as ωd−1 with d = 2 and 3 for surface states and Weyl semimetals, respectively, while for helical edge states, the interactions renormalize the exponent as d = 2K − 1 with K the Luttinger-liquid parameter. As a result, spin relaxation rate cannot be deduced from the DSS in contrast to the case of usual metals, which follows from the strongly entangled spin and charge degrees of freedom in these systems. PMID:26439629
Zhang, Xiaoguang; McGuire, Michael A.; Chen, Yong P.; Li, An -Ping; Durand, Corentin; Hus, Saban M.; Ma, Chuanxu; Hu, Yang; Cao, Helin; Miotkowski, Ireneusz
2016-03-08
Topological insulators, with characteristic topological surface states, have emerged as a new state of matter with rich potentials for both fundamental physics and device applications. However, the experimental detection of the surface transport has been hampered by the unavoidable extrinsic conductivity associated with the bulk crystals. Here we show that a four-probe transport spectroscopy in a multi-probe scanning tunneling microscopy system can be used to differentiate conductivities from the surface states and the coexisting bulk states in topological insulators. We derive a scaling relation of measured resistance with respect to varying inter-probe spacing for two interconnected conduction channels, which allows quantitative determination of conductivities from both channels. Using this method, we demonstrate the separation of 2D and 3D conduction in topological insulators by comparing the conductance scaling of Bi_{2}Se_{3}, Bi_{2}Te_{2}Se, and Sb-doped Bi_{2}Se_{3} with that of a pure 2D conductance of graphene on SiC substrate. We also report the 2D conductance enhancement due to the surface doping effect in topological insulators. This technique can be applied to reveal 2D to 3D crossover of conductance in other complex systems.
Zhang, Xiaoguang; McGuire, Michael A.; Chen, Yong P.; ...
2016-03-08
Topological insulators, with characteristic topological surface states, have emerged as a new state of matter with rich potentials for both fundamental physics and device applications. However, the experimental detection of the surface transport has been hampered by the unavoidable extrinsic conductivity associated with the bulk crystals. Here we show that a four-probe transport spectroscopy in a multi-probe scanning tunneling microscopy system can be used to differentiate conductivities from the surface states and the coexisting bulk states in topological insulators. We derive a scaling relation of measured resistance with respect to varying inter-probe spacing for two interconnected conduction channels, which allowsmore » quantitative determination of conductivities from both channels. Using this method, we demonstrate the separation of 2D and 3D conduction in topological insulators by comparing the conductance scaling of Bi2Se3, Bi2Te2Se, and Sb-doped Bi2Se3 with that of a pure 2D conductance of graphene on SiC substrate. We also report the 2D conductance enhancement due to the surface doping effect in topological insulators. This technique can be applied to reveal 2D to 3D crossover of conductance in other complex systems.« less
Anomalous photoelectric effect of a polycrystalline topological insulator film.
Zhang, Hongbin; Yao, Jiandong; Shao, Jianmei; Li, Hai; Li, Shuwei; Bao, Dinghua; Wang, Chengxin; Yang, Guowei
2014-07-29
A topological insulator represents a new state of quantum matter that possesses an insulating bulk band gap as well as a spin-momentum-locked Dirac cone on the surface that is protected by time-reversal symmetry. Photon-dressed surface states and light-induced surface photocurrents have been observed in topological insulators. Here, we report experimental observations of an anomalous photoelectric effect in thin films of Bi2Te3, a polycrystalline topological insulator. Under illumination with non-polarised light, transport measurements reveal that the resistance of the topological surface states suddenly increases when the polycrystalline film is illuminated. The resistance variation is positively dependent on the light intensity but has no relation to the applied electric field; this finding can be attributed to the gap opening of the surface Dirac cone. This observation of an anomalous photoelectric effect in polycrystalline topological insulators offers exciting opportunities for the creation of photodetectors with an unusually broad spectral range. Moreover, polycrystalline topological insulator films provide an attractive material platform for exploring the nature and practical application of topological insulators.
Anomalous Photoelectric Effect of a Polycrystalline Topological Insulator Film
Zhang, Hongbin; Yao, Jiandong; Shao, Jianmei; Li, Hai; Li, Shuwei; Bao, Dinghua; Wang, Chengxin; Yang, Guowei
2014-01-01
A topological insulator represents a new state of quantum matter that possesses an insulating bulk band gap as well as a spin-momentum-locked Dirac cone on the surface that is protected by time-reversal symmetry. Photon-dressed surface states and light-induced surface photocurrents have been observed in topological insulators. Here, we report experimental observations of an anomalous photoelectric effect in thin films of Bi2Te3, a polycrystalline topological insulator. Under illumination with non-polarised light, transport measurements reveal that the resistance of the topological surface states suddenly increases when the polycrystalline film is illuminated. The resistance variation is positively dependent on the light intensity but has no relation to the applied electric field; this finding can be attributed to the gap opening of the surface Dirac cone. This observation of an anomalous photoelectric effect in polycrystalline topological insulators offers exciting opportunities for the creation of photodetectors with an unusually broad spectral range. Moreover, polycrystalline topological insulator films provide an attractive material platform for exploring the nature and practical application of topological insulators. PMID:25069391
Current-Induced Spin Polarization in Topological Insulator-Graphene Heterostructures.
Vaklinova, Kristina; Hoyer, Alexander; Burghard, Marko; Kern, Klaus
2016-04-13
Further development of the field of all-electric spintronics requires the successful integration of spin transport channels with spin injector/generator elements. While with the advent of graphene and related 2D materials high performance spin channel materials are available, the use of nanostructured spin generators remains a major challenge. Especially promising for the latter purpose are 3D topological insulators, whose 2D surface states host massless Dirac Fermions with spin-momentum locking. Here, we demonstrate injection of spin-polarized current from a topological insulator into graphene, enabled by its intimate coupling to an ultrathin Bi2Te2Se nanoplatelet within a van der Waals epitaxial heterostructure. The spin switching signal, whose magnitude scales inversely with temperature, is detectable up to ∼15 K. Our findings establish topological insulators as prospective future components of spintronic devices wherein spin manipulation is achieved by purely electrical means.
Current-Induced Spin Polarization in Topological Insulator-Graphene Heterostructures
NASA Astrophysics Data System (ADS)
Vaklinova, Kristina; Hoyer, Alexander; Burghard, Marko; Kern, Klaus
2016-04-01
Further development of the field of all-electric spintronics requires the successful integration of spin transport channels with spin injector/generator elements. While with the advent of graphene and related 2D materials high performance spin channel materials are available, the use of nanostructured spin generators remains a major challenge. Especially promising for the latter purpose are 3D topological insulators, whose 2D surface states host massless Dirac fermions with spin-momentum locking. Here, we demonstrate injection of spin-polarized current from a topological insulator into graphene, enabled by its intimate coupling to an ultrathin Bi2Te2Se nanoplatelet within a van der Waals epitaxial heterostructure. The spin switching signal, whose magnitude scales inversely with temperature, is detectable up to ~15 K. Our findings establish topological insulators as prospective future components of spintronic devices wherein spin manipulation is achieved by purely electrical means.
Time periodic perturbations and transport signatures in Floquet topological insulators
NASA Astrophysics Data System (ADS)
Farrell, Aaron; Pereg-Barnea, T.
A Floquet topological insulator represents an out-of-equilibrium topological state. The topology of these systems emerges only when a time periodic perturbation is carefully applied. In time-periodic systems one can define quasi-energy states, which replace equilibrium stationary states (i.e. energy eigenstates). For appropriate time periodic potential strengths, a Floquet topological insulator exhibits edge localized quasi-energy states which lie in a gap of its quasi-energy spectrum. These edge states are a non-equilibrium analogue of the topologically protected edge-states in equilibrium topological insulators. In an equilibrium system such edge states lead to robust ``signature'' transport properties, e.g. a two-terminal conductivity of 2e2 / h . This talk will explore how these transport signatures translate to Floquet topological insulators. We will do this by developing and generalizing intuition borrowed from the field of photon-assited tunneling. We show that ultimately the signature transport properties of an equilibrium topological insulator do not directly hold in the out-of-equilibrium setting. This fact notwithstanding, we will provide an indirect way of retaining these transport signatures by using a so-called Floquet sum rule.
Baruselli, Pier Paolo; Vojta, Matthias
2015-10-09
SmB_{6} was recently proposed to be both a strong topological insulator and a topological crystalline insulator. For this and related cubic topological Kondo insulators, we prove the existence of four different topological phases, distinguished by the sign of mirror Chern numbers. We characterize these phases in terms of simple observables, and we provide concrete tight-binding models for each phase. Based on theoretical and experimental results for SmB_{6} we conclude that it realizes the phase with C_{k_{z}=0}^{+}=+2, C_{k_{z}=π}^{+}=+1, C_{k_{x}=k_{y}}^{+}=-1, and we propose a corresponding minimal model.
Automatic extraction of insulators from 3D LiDAR data of an electrical substation
NASA Astrophysics Data System (ADS)
Arastounia, M.; Lichti, D. D.
2013-10-01
A considerable percentage of power outages are caused by animals that come into contact with conductive elements of electrical substations. These can be prevented by insulating conductive electrical objects, for which a 3D as-built plan of the substation is crucial. This research aims to create such a 3D as-built plan using terrestrial LiDAR data while in this paper the aim is to extract insulators, which are key objects in electrical substations. This paper proposes a segmentation method based on a new approach of finding the principle direction of points' distribution. This is done by forming and analysing the distribution matrix whose elements are the range of points in 9 different directions in 3D space. Comparison of the computational performance of our method with PCA (principal component analysis) shows that our approach is 25% faster since it utilizes zero-order moments while PCA computes the first- and second-order moments, which is more time-consuming. A knowledge-based approach has been developed to automatically recognize points on insulators. The method utilizes known insulator properties such as diameter and the number and the spacing of their rings. The results achieved indicate that 24 out of 27 insulators could be recognized while the 3 un-recognized ones were highly occluded. Check point analysis was performed by manually cropping all points on insulators. The results of check point analysis show that the accuracy, precision and recall of insulator recognition are 98%, 86% and 81%, respectively. It is concluded that automatic object extraction from electrical substations using only LiDAR data is not only possible but also promising. Moreover, our developed approach to determine the directional distribution of points is computationally more efficient for segmentation of objects in electrical substations compared to PCA. Finally our knowledge-based method is promising to recognize points on electrical objects as it was successfully applied for
Topological insulators/superconductors: Potential future electronic materials
Hor, Y. S.
2014-03-05
A new material called topological insulator has been discovered and becomes one of the fastest growing field in condensed matter physics. Topological insulator is a new quantum phase of matter which has Dirac-like conductivity on its surface, but bulk insulator through its interior. It is considered a challenging problem for the surface transport measurements because of dominant internal conductance due to imperfections of the existing crystals of topological insulators. By a proper method, the internal bulk conduction can be suppressed in a topological insulator, and permit the detection of the surface currents which is necessary for future fault-tolerant quantum computing applications. Doped topological insulators have depicted a large variety of bulk physical properties ranging from magnetic to superconducting behaviors. By chemical doping, a TI can change into a bulk superconductor. Nb{sub x}Bi{sub 2}Se{sub 3} is shown to be a superconductor with T{sub c} ∼ 3.2 K, which could be a potential candidate for a topological superconductor.
Aharonov-Bohm interference in topological insulator nanoribbons.
Peng, Hailin; Lai, Keji; Kong, Desheng; Meister, Stefan; Chen, Yulin; Qi, Xiao-Liang; Zhang, Shou-Cheng; Shen, Zhi-Xun; Cui, Yi
2010-03-01
Topological insulators represent unusual phases of quantum matter with an insulating bulk gap and gapless edges or surface states. The two-dimensional topological insulator phase was predicted in HgTe quantum wells and confirmed by transport measurements. Recently, Bi(2)Se(3) and related materials have been proposed as three-dimensional topological insulators with a single Dirac cone on the surface, protected by time-reversal symmetry. The topological surface states have been observed by angle-resolved photoemission spectroscopy experiments. However, few transport measurements in this context have been reported, presumably owing to the predominance of bulk carriers from crystal defects or thermal excitations. Here we show unambiguous transport evidence of topological surface states through periodic quantum interference effects in layered single-crystalline Bi(2)Se(3) nanoribbons, which have larger surface-to-volume ratios than bulk materials and can therefore manifest surface effects. Pronounced Aharonov-Bohm oscillations in the magnetoresistance clearly demonstrate the coherent propagation of two-dimensional electrons around the perimeter of the nanoribbon surface, as expected from the topological nature of the surface states. The dominance of the primary h/e oscillation, where h is Planck's constant and e is the electron charge, and its temperature dependence demonstrate the robustness of these states. Our results suggest that topological insulator nanoribbons afford promising materials for future spintronic devices at room temperature.
Electromagnetic effect on disordered surface of topological insulators
NASA Astrophysics Data System (ADS)
Taguchi, Katsuhisa; Shintani, Kunitaka; Tanaka, Yukio
2016-02-01
We theoretically study electromagnetic effects due to magnetization on disordered surface of topological insulators with attaching a ferromagnetic insulator junction by using the result of the magnetization dynamics induced charge flow. We find that the electric polarization is induced by not only the magnetization but also the spatial derivative of the magnetization with the diffusion on the disordered surface.
NASA Astrophysics Data System (ADS)
Lun, Huijie; Li, Xuefei; Wang, Xiao; Li, Haiyan; Li, Yamin; Bai, Yan
2017-01-01
A new coordination polymer, {[Mn(HPIDC)(H2O)]·2H2O}n (1) (H3PIDC = 2-(pyridin-4-yl)-1H-imidazole-4,5-dicarboxylic acid), has been obtained by hydrothermal method and structurally characterized by X-ray single crystal diffraction, elemental analysis and thermogravimetric analysis (TGA). X-ray single crystal diffraction reveals that compound 1 crystallizing in acentric Pna21 space group, exhibits an ultimate racemic three-dimension framework with rare noninterpenetrated (10,3)-d (or utp) topology due to the alternate array of left- and right-handed helixes. Moreover, compound 1 also features ferroelectric, nonlinear optical (NLO) and antiferromagnetic behaviors.
Control over topological insulator photocurrents with light polarization.
McIver, J W; Hsieh, D; Steinberg, H; Jarillo-Herrero, P; Gedik, N
2011-12-04
Three-dimensional topological insulators represent a new quantum phase of matter with spin-polarized surface states that are protected from backscattering. The static electronic properties of these surface states have been comprehensively imaged by both photoemission and tunnelling spectroscopies. Theorists have proposed that topological surface states can also exhibit novel electronic responses to light, such as topological quantum phase transitions and spin-polarized electrical currents. However, the effects of optically driving a topological insulator out of equilibrium have remained largely unexplored experimentally, and no photocurrents have been measured. Here, we show that illuminating the topological insulator Bi(2)Se(3) with circularly polarized light generates a photocurrent that originates from topological helical Dirac fermions, and that reversing the helicity of the light reverses the direction of the photocurrent. We also observe a photocurrent that is controlled by the linear polarization of light and argue that it may also have a topological surface state origin. This approach may allow the probing of dynamic properties of topological insulators and lead to novel opto-spintronic devices.
Magnon waveguide with nanoscale confinement constructed from topological magnon insulators
NASA Astrophysics Data System (ADS)
Mook, Alexander; Henk, Jürgen; Mertig, Ingrid
2015-05-01
Topological magnon insulators host spatially confined edge magnons brought about by the Dzyaloshinskii-Moriya interaction. Bringing two topological magnon insulators into contact results in topologically protected unidirectional interface magnons. These interface modes decay rapidly toward the bulk regions of the sample. As a result, heat and spin currents associated with these magnons are as well unidirectional and strongly confined to a few-nanometer-wide strip along the interface. On top of this, these interface currents follow any geometry owing to the topological nature of the magnons. In this theoretical study, we propose and analyze two recipes for composing magnon waveguides with nanoscale confinement, one from topologically different phases, another from identical phases. We further identify material classes to construct these magnon waveguides and propose an experiment to verify their topological nature.
Magnetic quantum dot in two-dimensional topological insulators
NASA Astrophysics Data System (ADS)
Li, Guo; Zhu, Jia-Lin; Yang, Ning
2017-03-01
Magnetic quantum dots in two-dimensional band and topological insulators are studied by solving the modified Dirac model under nonuniform magnetic fields. The Landau levels split into discrete states with certain angular momentum. The states splitting from the zero Landau levels lie in the energy gap for topological insulators but are out of the gap for band insulators. It is found that the ground states oscillate between the spin-up and spin-down states when the magnetic field or the dot size changes. The oscillation manifests itself as changes of sign and strength of charge currents near the dot's edge.
Park, Byung Cheol; Kim, Tae-Hyeon; Sim, Kyung Ik; Kang, Boyoun; Kim, Jeong Won; Cho, Beongki; Jeong, Kwang-Ho; Cho, Mann-Ho; Kim, Jae Hoon
2015-03-16
Strong spin-orbit interaction and time-reversal symmetry in topological insulators generate novel quantum states called topological surface states. Their study provides unique opportunities to explore exotic phenomena such as spin Hall effects and topological phase transitions, relevant to the development of quantum devices for spintronics and quantum computation. Although ultrahigh-vacuum surface probes can identify individual topological surface states, standard electrical and optical experiments have so far been hampered by the interference of bulk and quantum well states. Here, with terahertz time-domain spectroscopy of ultrathin Bi₂Se₃ films, we give evidence for topological phase transitions, a single conductance quantum per topological surface state, and a quantized terahertz absorbance of 2.9% (four times the fine structure constant). Our experiment demonstrates the feasibility to isolate, detect and manipulate topological surface states in the ambient at room temperature for future fundamental research on the novel physics of topological insulators and their practical applications.
Optical conductivity of topological insulator thin films
Li, L. L.; Xu, W.; Peeters, F. M.
2015-05-07
We present a detailed theoretical study on the optoelectronic properties of topological insulator thin film (TITFs). The k·p approach is employed to calculate the energy spectra and wave functions for both the bulk and surface states in the TITF. With these obtained results, the optical conductivities induced by different electronic transitions among the bulk and surface states are evaluated using the energy-balance equation derived from the Boltzmann equation. We find that for Bi{sub 2}Se{sub 3}-based TITFs, three characteristic regimes for the optical absorption can be observed. (i) In the low radiation frequency regime (photon energy ℏω<200 meV), the free-carrier absorption takes place due to intraband electronic transitions. An optical absorption window can be observed. (ii) In the intermediate radiation frequency regime (200<ℏω<300 meV), the optical absorption is induced mainly by interband electronic transitions from surface states in the valance band to surface states in the conduction band and an universal value σ{sub 0}=e{sup 2}/(8ℏ) for the optical conductivity can be obtained. (iii) In the high radiation frequency regime (ℏω>300 meV), the optical absorption can be achieved via interband electronic transitions from bulk and surface states in the valance band to bulk and surface states in the conduction band. A strong absorption peak can be observed. These interesting findings indicate that optical measurements can be applied to identify the energy regimes of bulk and surface states in the TITF.
A non-commutative framework for topological insulators
NASA Astrophysics Data System (ADS)
Bourne, C.; Carey, A. L.; Rennie, A.
2016-04-01
We study topological insulators, regarded as physical systems giving rise to topological invariants determined by symmetries both linear and anti-linear. Our perspective is that of non-commutative index theory of operator algebras. In particular, we formulate the index problems using Kasparov theory, both complex and real. We show that the periodic table of topological insulators and superconductors can be realized as a real or complex index pairing of a Kasparov module capturing internal symmetries of the Hamiltonian with a spectral triple encoding the geometry of the sample’s (possibly non-commutative) Brillouin zone.
Quantum Criticality of Quasi-One-Dimensional Topological Anderson Insulators
NASA Astrophysics Data System (ADS)
Altland, Alexander; Bagrets, Dmitry; Fritz, Lars; Kamenev, Alex; Schmiedt, Hanno
2014-05-01
We present an analytic theory of quantum criticality in the quasi-one-dimensional topological Anderson insulators of class AIII and BDI. We describe the systems in terms of two parameters (g, χ) representing localization and topological properties, respectively. Surfaces of half-integer valued χ define phase boundaries between distinct topological sectors. Upon increasing system size, the two parameters exhibit flow similar to the celebrated two-parameter flow describing the class A quantum Hall insulator. However, unlike the quantum Hall system, an exact analytical description of the entire phase diagram can be given. We check the quantitative validity of our theory by comparison to numerical transfer matrix computations.
Coulomb impurities in two-dimensional topological insulators
NASA Astrophysics Data System (ADS)
Zhu, Jia-Lin; Li, Guo; Yang, Ning
2017-03-01
Introducing a powerful method, we obtain the exact solutions for a Coulomb impurity in two-dimensional infinite and finite topological insulators. The level order and zero-energy degeneracy of the spectra are found to be quite different between topological trivial and nontrivial phases. For quantum dots of topological insulator, the variation of the edge and Coulomb states with dot size, Coulomb potential, and magnetic field are clearly shown. It is found that for small dots the edge states can be strongly coupled with the Coulomb states and for large dots the edge states are insensitive to the Coulomb fields but sensitive to the magnetic fields.
Dislocation conduction in Bi-Sb topological insulators
NASA Astrophysics Data System (ADS)
Hamasaki, Hiromu; Tokumoto, Yuki; Edagawa, Keiichi
2017-02-01
Previous theoretical works have predicted that when a specific condition is satisfied, dislocations in three-dimensional topological insulators form one-dimensional gapless states, which are topologically protected against disorder scattering. Here, the predicted dislocation conduction is experimentally investigated in Bi-Sb topological insulators. High-density dislocations with the Burgers vector satisfying the conductivity condition are introduced into Bi-Sb single crystals by plastic deformation. Conductivity measurements for deformed and undeformed samples and those for the deformed samples in different orientations show excess conductivity due to dislocation conduction.
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.
Tailoring exchange couplings in magnetic topological-insulator/antiferromagnet heterostructures
NASA Astrophysics Data System (ADS)
He, Qing Lin; Kou, Xufeng; Grutter, Alexander J.; Yin, Gen; Pan, Lei; Che, Xiaoyu; Liu, Yuxiang; Nie, Tianxiao; Zhang, Bin; Disseler, Steven M.; Kirby, Brian J.; Ratcliff, William, II; Shao, Qiming; Murata, Koichi; Zhu, Xiaodan; Yu, Guoqiang; Fan, Yabin; Montazeri, Mohammad; Han, Xiaodong; Borchers, Julie A.; Wang, Kang L.
2017-01-01
Magnetic topological insulators such as Cr-doped (Bi,Sb)2Te3 provide a platform for the realization of versatile time-reversal symmetry-breaking physics. By constructing heterostructures exhibiting Néel order in an antiferromagnetic CrSb and ferromagnetic order in Cr-doped (Bi,Sb)2Te3, we realize emergent interfacial magnetic phenomena which can be tailored through artificial structural engineering. Through deliberate geometrical design of heterostructures and superlattices, we demonstrate the use of antiferromagnetic exchange coupling in manipulating the magnetic properties of magnetic topological insulators. Proximity effects are shown to induce an interfacial spin texture modulation and establish an effective long-range exchange coupling mediated by antiferromagnetism, which significantly enhances the magnetic ordering temperature in the superlattice. This work provides a new framework on integrating topological insulators with antiferromagnetic materials and unveils new avenues towards dissipationless topological antiferromagnetic spintronics.
Spin texture of an irradiated warped topological insulator surface
NASA Astrophysics Data System (ADS)
Sinha, Debabrata
2016-08-01
Topological insulator is a new state of matter which exhibits exotic surface electronic properties. Determining the spin texture of this class of materials is of paramount importance for understanding its topological order and can lead to potential applications in spintronics. Here, we have investigated the nature of the surface state of the topological insulator with hexagonal warping subjected to an off-resonant circularly polarized light. The resulting electronic ground state exhibits a novel feature of spin texture breaking the conventional spin-momentum locking present on a topological insulator surface. The observed spin texture is shown to be a consequence of the symmetry group of the underlying crystal. The generalisation of our method to the other 2D graphene-like systems is straightforward. Our calculation traces a simple experimental route for a realisation of the non trivial spin textures.
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.
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.
Evidence for a Strong Topological Insulator Phase in ZrTe5
NASA Astrophysics Data System (ADS)
Manzoni, G.; Gragnaniello, L.; Autès, G.; Kuhn, T.; Sterzi, A.; Cilento, F.; Zacchigna, M.; Enenkel, V.; Vobornik, I.; Barba, L.; Bisti, F.; Bugnon, Ph.; Magrez, A.; Strocov, V. N.; Berger, H.; Yazyev, O. V.; Fonin, M.; Parmigiani, F.; Crepaldi, A.
2016-12-01
The complex electronic properties of ZrTe5 have recently stimulated in-depth investigations that assigned this material to either a topological insulator or a 3D Dirac semimetal phase. Here we report a comprehensive experimental and theoretical study of both electronic and structural properties of ZrTe5, revealing that the bulk material is a strong topological insulator (STI). By means of angle-resolved photoelectron spectroscopy, we identify at the top of the valence band both a surface and a bulk state. The dispersion of these bands is well captured by ab initio calculations for the STI case, for the specific interlayer distance measured in our x-ray diffraction study. Furthermore, these findings are supported by scanning tunneling spectroscopy revealing the metallic character of the sample surface, thus confirming the strong topological nature of ZrTe5.
Topological Crystalline Insulator in a New Bi Semiconducting Phase
Munoz, F.; Vergniory, M. G.; Rauch, T.; Henk, J.; Chulkov, E. V.; Mertig, I.; Botti, S.; Marques, M. A. L.; Romero, A. H.
2016-01-01
Topological crystalline insulators are a type of topological insulators whose topological surface states are protected by a crystal symmetry, thus the surface gap can be tuned by applying strain or an electric field. In this paper we predict by means of ab initio calculations a new phase of Bi which is a topological crystalline insulator characterized by a mirror Chern number nM = −2, but not a strong topological insulator. This system presents an exceptional property: at the (001) surface its Dirac cones are pinned at the surface high-symmetry points. As a consequence they are also protected by time-reversal symmetry and can survive against weak disorder even if in-plane mirror symmetry is broken at the surface. Taking advantage of this dual protection, we present a strategy to tune the band-gap based on a topological phase transition unique to this system. Since the spin-texture of these topological surface states reduces the back-scattering in carrier transport, this effective band-engineering is expected to be suitable for electronic and optoelectronic devices with reduced dissipation. PMID:26905601
Autore, Marta; Giorgianni, Flavio; D' Apuzzo, Fausto; Di Gaspare, Alessandra; Lo Vecchio, Irene; Brahlek, Matthew; Koirala, Nikesh; Oh, Seongshik; Schade, Urlich; Ortolani, Michele; Lupi, Stefano
2016-02-28
A 3D Topological Insulator (TI) is an intrinsically stratified electronic material characterized by an insulating bulk and Dirac free electrons at the interface with vacuum or another dielectric. In this paper, we investigate, through terahertz (THz) spectroscopy, the plasmon excitation of Dirac electrons on thin films of (Bi1-xInx)2Se3 TI patterned in the form of a micro-ribbon array, across a Quantum Phase Transition (QPT) from the topological to a trivial insulating phase. The latter is achieved by In doping onto the Bi-site and is characterized by massive electrons at the surface. While the plasmon frequency is nearly independent of In content, the plasmon width undergoes a sudden broadening across the QPT, perfectly mirroring the single particle (free electron) behavior as measured on the same films. This strongly suggests that the topological protection from backscattering characterizing Dirac electrons in the topological phase extends also to their plasmon excitations.
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 effects and quantum phase transitions in topological insulators
Varney, Christopher N.; Sun Kai; Galitski, Victor; Rigol, Marcos
2010-09-15
We study strong correlation effects in topological insulators via the Lanczos algorithm, which we utilize to calculate the exact many-particle ground-state wave function and its topological properties. We analyze the simple, noninteracting Haldane model on a honeycomb lattice with known topological properties and demonstrate that these properties are already evident in small clusters. Next, we consider interacting fermions by introducing repulsive nearest-neighbor interactions. A first-order quantum phase transition was discovered at finite interaction strength between the topological band insulator and a topologically trivial Mott insulating phase by use of the fidelity metric and the charge-density-wave structure factor. We construct the phase diagram at T=0 as a function of the interaction strength and the complex phase for the next-nearest-neighbor hoppings. Finally, we consider the Haldane model with interacting hard-core bosons, where no evidence for a topological phase is observed. An important general conclusion of our work is that despite the intrinsic nonlocality of topological phases their key topological properties manifest themselves already in small systems and therefore can be studied numerically via exact diagonalization and observed experimentally, e.g., with trapped ions and cold atoms in optical lattices.
Lee, Yejin; Hong, Kyunghi; Hong, Sung-Ae
2007-05-01
Garment fit and resultant air volume is a crucial factor in thermal insulation, and yet, it has been difficult to quantify the air volume of clothing microclimate and relate it to the thermal insulation value just using the information on the size of clothing pattern without actual 3D volume measurement in wear condition. As earlier methods for the computation of air volume in clothing microclimate, vacuum over suit and circumference model have been used. However, these methods have inevitable disadvantages in terms of cost or accuracy due to the limitations of measurement equipment. In this paper, the phase-shifting moiré topography was introduced as one of the 3D scanning tools to measure the air volume of clothing microclimate quantitatively. The purpose of this research is to adopt a non-contact image scanning technology, phase-shifting moiré topography, to ascertain relationship between air volume and insulation value of layered clothing systems in wear situations where the 2D fabric creates new conditions in 3D spaces. The insulation of vests over shirts as a layered clothing system was measured with a thermal manikin in the environmental condition of 20 degrees C, 65% RH and air velocity of 0.79 m/s. As the pattern size increased, the insulation of the clothing system was increased. But beyond a certain limit, the insulation started to decrease due to convection and ventilation, which is more apparent when only the vest was worn over the torso of manikin. The relationship between clothing air volume and insulation was difficult to predict with a single vest due to the extreme openings which induced active ventilation. But when the vest was worn over the shirt, the effects of thickness of the fabrics on insulation were less pronounced compared with that of air volume. In conclusion, phase-shifting moiré topography was one of the efficient and accurate ways of quantifying air volume and its distribution across the clothing microclimate. It is also noted
Photonic topological insulator with broken time-reversal symmetry.
He, Cheng; Sun, Xiao-Chen; Liu, Xiao-Ping; Lu, Ming-Hui; Chen, Yulin; Feng, Liang; Chen, Yan-Feng
2016-05-03
A topological insulator is a material with an insulating interior but time-reversal symmetry-protected conducting edge states. Since its prediction and discovery almost a decade ago, such a symmetry-protected topological phase has been explored beyond electronic systems in the realm of photonics. Electrons are spin-1/2 particles, whereas photons are spin-1 particles. The distinct spin difference between these two kinds of particles means that their corresponding symmetry is fundamentally different. It is well understood that an electronic topological insulator is protected by the electron's spin-1/2 (fermionic) time-reversal symmetry [Formula: see text] However, the same protection does not exist under normal circumstances for a photonic topological insulator, due to photon's spin-1 (bosonic) time-reversal symmetry [Formula: see text] In this work, we report a design of photonic topological insulator using the Tellegen magnetoelectric coupling as the photonic pseudospin orbit interaction for left and right circularly polarized helical spin states. The Tellegen magnetoelectric coupling breaks bosonic time-reversal symmetry but instead gives rise to a conserved artificial fermionic-like-pseudo time-reversal symmetry, Tp ([Formula: see text]), due to the electromagnetic duality. Surprisingly, we find that, in this system, the helical edge states are, in fact, protected by this fermionic-like pseudo time-reversal symmetry Tp rather than by the bosonic time-reversal symmetry Tb This remarkable finding is expected to pave a new path to understanding the symmetry protection mechanism for topological phases of other fundamental particles and to searching for novel implementations for topological insulators.
Photonic topological insulator with broken time-reversal symmetry
He, Cheng; Sun, Xiao-Chen; Liu, Xiao-Ping; Lu, Ming-Hui; Chen, Yulin; Feng, Liang; Chen, Yan-Feng
2016-01-01
A topological insulator is a material with an insulating interior but time-reversal symmetry-protected conducting edge states. Since its prediction and discovery almost a decade ago, such a symmetry-protected topological phase has been explored beyond electronic systems in the realm of photonics. Electrons are spin-1/2 particles, whereas photons are spin-1 particles. The distinct spin difference between these two kinds of particles means that their corresponding symmetry is fundamentally different. It is well understood that an electronic topological insulator is protected by the electron’s spin-1/2 (fermionic) time-reversal symmetry Tf2=−1. However, the same protection does not exist under normal circumstances for a photonic topological insulator, due to photon’s spin-1 (bosonic) time-reversal symmetry Tb2=1. In this work, we report a design of photonic topological insulator using the Tellegen magnetoelectric coupling as the photonic pseudospin orbit interaction for left and right circularly polarized helical spin states. The Tellegen magnetoelectric coupling breaks bosonic time-reversal symmetry but instead gives rise to a conserved artificial fermionic-like-pseudo time-reversal symmetry, Tp (Tp2=−1), due to the electromagnetic duality. Surprisingly, we find that, in this system, the helical edge states are, in fact, protected by this fermionic-like pseudo time-reversal symmetry Tp rather than by the bosonic time-reversal symmetry Tb. This remarkable finding is expected to pave a new path to understanding the symmetry protection mechanism for topological phases of other fundamental particles and to searching for novel implementations for topological insulators. PMID:27092005
Multifunctional electronic structure in a topological insulator class
NASA Astrophysics Data System (ADS)
Xu, Suyang; Hasan, Zahid
2011-03-01
The discovery of topological properties in three dimensional bulk solids have opened up many new research avenues in condensed matter physics. Only a very few compounds have been identified to be topological insulators to this date. However, none of them is proven to be suitable for the majority of experimental configurations including giant magnetoelectric and anomalous optical rotation, unusual exciton condensation, or the neutral half-fermions and interface superconductivity. In fact the realization of even any one of these proposals requires a number of multiply-connected topological compounds with modulated surface band dispersions and naturally tuned in-gap Fermi level, as well as spin variations in the presence of long life-time of the surface states. Here, using conventional and spin-sensitive probes, we report the discovery of several classes of positive band-gap high figure of merit topological insulators with critically important functional properties such as high degree of bulk resistivity and insulation, electronic structure with both in-gap Dirac point and Fermi level crossing, long surface state life-times, as well as chirality inversion through the Dirac node. The unprecedented combinations of electronic, spin, life-time and resistive bulk transport featured by the topological insulators uncovered here not only provide a new platform for research on topological quantum phenomena but also pave the way for functional devices.
Experimental demonstration of anomalous Floquet topological insulator for sound
Peng, Yu-Gui; Qin, Cheng-Zhi; Zhao, De-Gang; Shen, Ya-Xi; Xu, Xiang-Yuan; Bao, Ming; Jia, Han; Zhu, Xue-Feng
2016-01-01
Time-reversal invariant topological insulator is widely recognized as one of the fundamental discoveries in condensed matter physics, for which the most fascinating hallmark is perhaps a spin-based topological protection, the absence of scattering of conduction electrons with certain spins on matter surface. Recently, it has created a paradigm shift for topological insulators, from electronics to photonics, phononics and mechanics as well, bringing about not only involved new physics but also potential applications in robust wave transport. Despite the growing interests in topologically protected acoustic wave transport, T-invariant acoustic topological insulator has not yet been achieved. Here we report experimental demonstration of anomalous Floquet topological insulator for sound: a strongly coupled metamaterial ring lattice that supports one-way propagation of pseudo-spin-dependent edge states under T-symmetry. We also demonstrate the formation of pseudo-spin-dependent interface states due to lattice dislocations and investigate the properties of pass band and band gap states. PMID:27834375
NASA Astrophysics Data System (ADS)
Xia, Yuqi
Topological insulators are materials with a bulk band gap, which carry conducting surface states that are protected against disorder. In three dimensions, the insulators carry 2D Dirac fermions on their surfaces. The opening of a magnetic surface gap can exhibit a topological magnetoelectric effect, and support Majorana fermions which can be manipulated for quantum computation. Previous spin and angle-resolved photoemission studies have shown that Bi 1-xSbx alloy belongs to this class of materials, with a characteristic number nu 0 = 1. Some materials challenges with Bi1-x Sbx alloy however are the significant degree of bulk disorder and a small band gap. Both problems make gating difficult for the manipulation and control of the charge carriers. While ordinary materials such as superconductors and liquid crystals can be described by an order parameter, topological insulators are not associated with a local order parameter resulting from a spontaneous broken symmetry. Rather, they manifest a topological order which requires a direct probe of how their energy bands are connected. Measurement techniques designed to detect a particular order parameter are therefore insufficient to identify the topological character of a material. Alternatively, one can look for properties analogous to the quantum Hall effect as a signature of a topologically ordered system. However, using transport probes to isolate the surface states of the topological insulator requires a pristine bulk with minimal charge carrier density. While advances have been made recently in this direction, a good candidate for such measurements has been elusive. In this thesis, we describe a systematic study of a new topological insulator class with a large band gap and a single surface state Fermi surface. Using synchrochon-based angle-resolved photoemission spectroscopy (ARPES), we measured the topological character of these materials by observing the dispersion of their metallic electronic states confined
Efficient THz emission from a topological insulator surface
NASA Astrophysics Data System (ADS)
Zhu, Li-Guo; Kubera, Brian; Mak, Kin Fai; Shan, Jie
2012-02-01
Bi2Se3 is a 3D topological insulator (TI) recently confirmed by the ARPES.ootnotetextHsieh et al. Nature 460, 1101 (2009). Direct optical probe of its metallic surface states is, however, hindered by the remnant Drude response of the bulk material. Second-order nonlinear optical techniques with their surface specificity provide unique opportunities for studying surface electronic transitions in TIs such as Bi2Se3 with bulk inversion symmetry.ootnotetextHsieh et al. Phys. Rev. Lett. 106, 057401 (2011). Here we demonstrate efficient THz emission from the surface of Bi2Se3 under the excitation of a femtosecond optical pulse. The emission arises from optical rectification of the optical pulse at the TI surface and the transient current within the surface depletion region. By spectrally resolving the emission under different pump and emission polarizations, we separate the different contributions. Effects arising from just a few atomic layers of the sample surface due to resonance enhancement of the quasi-real optical transitions between the surface electronic states will be discussed.
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.
Topological crystalline insulators in the SnTe material class
NASA Astrophysics Data System (ADS)
Hsieh, Timothy H.; Lin, Hsin; Liu, Junwei; Duan, Wenhui; Bansil, Arun; Fu, Liang
2012-07-01
Topological crystalline insulators are new states of matter in which the topological nature of electronic structures arises from crystal symmetries. Here we predict the first material realization of topological crystalline insulator in the semiconductor SnTe by identifying its non-zero topological index. We predict that as a manifestation of this non-trivial topology, SnTe has metallic surface states with an even number of Dirac cones on high-symmetry crystal surfaces such as {001}, {110} and {111}. These surface states form a new type of high-mobility chiral electron gas, which is robust against disorder and topologically protected by reflection symmetry of the crystal with respect to {110} mirror plane. Breaking this mirror symmetry via elastic strain engineering or applying an in-plane magnetic field can open up a continuously tunable band gap on the surface, which may lead to wide-ranging applications in thermoelectrics, infra-red detection and tunable electronics. Closely related semiconductors PbTe and PbSe also become topological crystalline insulators after band inversion by pressure, strain and alloying.
Ion beam modification of topological insulator bismuth selenide
NASA Astrophysics Data System (ADS)
Sharma, P. A.; Lima Sharma, A. L.; Hekmaty, M.; Hattar, K.; Stavila, V.; Goeke, R.; Erickson, K.; Medlin, D. L.; Brahlek, M.; Koirala, N.; Oh, S.
2014-12-01
We demonstrate chemical doping of a topological insulator Bi2Se3 using ion implantation. Ion beam-induced structural damage was characterized using grazing incidence X-ray diffraction and transmission electron microscopy. Ion damage was reversed using a simple thermal annealing step. Carrier-type conversion was achieved using ion implantation followed by an activation anneal in Bi2Se3 thin films. These two sets of experiments establish the feasibility of ion implantation for chemical modification of Bi2Se3, a prototypical topological insulator. Ion implantation can, in principle, be used for any topological insulator. The direct implantation of dopants should allow better control over carrier concentrations for the purposes of achieving low bulk conductivity. Ion implantation also enables the fabrication of inhomogeneously doped structures, which in turn should make possible new types of device designs.
Terahertz plasmonic excitations in Bi2Se3 topological insulator.
Autore, M; Di Pietro, P; Di Gaspare, A; D'Apuzzo, F; Giorgianni, F; Brahlek, Matthew; Koirala, Nikesh; Oh, Seangshik; Lupi, S
2017-05-10
After the discovery of Dirac electrons in condensed matter physics, more specifically in graphene and its derivatives, their potentialities in the fields of plasmonics and photonics have been readily recognized, leading to a plethora of applications in active and tunable optical devices. Massless Dirac carriers have been further found in three-dimensional topological insulators. These exotic quantum systems have an insulating gap in the bulk and intrinsic Dirac metallic states at any surface, sustaining not only single-particle excitations but also plasmonic collective modes. In this paper we will review the plasmon excitations in different microstructures patterned on Bi2Se3 topological insulator thin films as measured by terahertz spectroscopy. We discuss the dependence of the plasmon absorption versus the microstructure shape, wavevector, and magnetic field. Finally we will discuss the topological protection of both the Dirac single-particle and plasmon excitations.
Ion beam modification of topological insulator bismuth selenide
Sharma, P. A. Lima Sharma, A. L.; Hattar, K.; Goeke, R.; Hekmaty, M.; Stavila, V.; Erickson, K.; Medlin, D. L.; Brahlek, M.; Koirala, N.; Oh, S.
2014-12-15
We demonstrate chemical doping of a topological insulator Bi{sub 2}Se{sub 3} using ion implantation. Ion beam-induced structural damage was characterized using grazing incidence X-ray diffraction and transmission electron microscopy. Ion damage was reversed using a simple thermal annealing step. Carrier-type conversion was achieved using ion implantation followed by an activation anneal in Bi{sub 2}Se{sub 3} thin films. These two sets of experiments establish the feasibility of ion implantation for chemical modification of Bi{sub 2}Se{sub 3}, a prototypical topological insulator. Ion implantation can, in principle, be used for any topological insulator. The direct implantation of dopants should allow better control over carrier concentrations for the purposes of achieving low bulk conductivity. Ion implantation also enables the fabrication of inhomogeneously doped structures, which in turn should make possible new types of device designs.
Ion beam modification of topological insulator bismuth selenide
Sharma, Peter Anand; Sharma, A. L. Lima; Hekmaty, Michelle A.; Hattar, Khalid Mikhiel; Stavila, Vitalie; Goeke, Ronald S.; Erickson, K.; Medlin, Douglas L.; Brahlek, M.; Oh, S.; Koirala, N.
2014-12-17
In this study, we demonstrate chemical doping of a topological insulator Bi_{2}Se_{3} using ion implantation. Ion beam-induced structural damage was characterized using grazing incidence X-ray diffraction and transmission electron microscopy. Ion damage was reversed using a simple thermal annealing step. Carrier-type conversion was achieved using ion implantation followed by an activation anneal in Bi_{2}Se_{3} thin films. These two sets of experiments establish the feasibility of ion implantation for chemical modification of Bi_{2}Se_{3}, a prototypical topological insulator. Ion implantation can, in principle, be used for any topological insulator. The direct implantation of dopants should allow better control over carrier concentrations for the purposes of achieving low bulk conductivity. Ion implantation also enables the fabrication of inhomogeneously doped structures, which in turn should make possible new types of device designs.
Ion beam modification of topological insulator bismuth selenide
Sharma, Peter Anand; Sharma, A. L. Lima; Hekmaty, Michelle A.; ...
2014-12-17
In this study, we demonstrate chemical doping of a topological insulator Bi2Se3 using ion implantation. Ion beam-induced structural damage was characterized using grazing incidence X-ray diffraction and transmission electron microscopy. Ion damage was reversed using a simple thermal annealing step. Carrier-type conversion was achieved using ion implantation followed by an activation anneal in Bi2Se3 thin films. These two sets of experiments establish the feasibility of ion implantation for chemical modification of Bi2Se3, a prototypical topological insulator. Ion implantation can, in principle, be used for any topological insulator. The direct implantation of dopants should allow better control over carrier concentrations formore » the purposes of achieving low bulk conductivity. Ion implantation also enables the fabrication of inhomogeneously doped structures, which in turn should make possible new types of device designs.« less
Disorder enabled band structure engineering of a topological insulator surface
Xu, Yishuai; Chiu, Janet; Miao, Lin; He, Haowei; Alpichshev, Zhanybek; Kapitulnik, A.; Biswas, Rudro R.; Wray, L. Andrew
2017-01-01
Three-dimensional topological insulators are bulk insulators with Z2 topological electronic order that gives rise to conducting light-like surface states. These surface electrons are exceptionally resistant to localization by non-magnetic disorder, and have been adopted as the basis for a wide range of proposals to achieve new quasiparticle species and device functionality. Recent studies have yielded a surprise by showing that in spite of resisting localization, topological insulator surface electrons can be reshaped by defects into distinctive resonance states. Here we use numerical simulations and scanning tunnelling microscopy data to show that these resonance states have significance well beyond the localized regime usually associated with impurity bands. At native densities in the model Bi2X3 (X=Bi, Te) compounds, defect resonance states are predicted to generate a new quantum basis for an emergent electron gas that supports diffusive electrical transport. PMID:28155858
Disorder enabled band structure engineering of a topological insulator surface
NASA Astrophysics Data System (ADS)
Xu, Yishuai; Chiu, Janet; Miao, Lin; He, Haowei; Alpichshev, Zhanybek; Kapitulnik, A.; Biswas, Rudro R.; Wray, L. Andrew
2017-02-01
Three-dimensional topological insulators are bulk insulators with Z2 topological electronic order that gives rise to conducting light-like surface states. These surface electrons are exceptionally resistant to localization by non-magnetic disorder, and have been adopted as the basis for a wide range of proposals to achieve new quasiparticle species and device functionality. Recent studies have yielded a surprise by showing that in spite of resisting localization, topological insulator surface electrons can be reshaped by defects into distinctive resonance states. Here we use numerical simulations and scanning tunnelling microscopy data to show that these resonance states have significance well beyond the localized regime usually associated with impurity bands. At native densities in the model Bi2X3 (X=Bi, Te) compounds, defect resonance states are predicted to generate a new quantum basis for an emergent electron gas that supports diffusive electrical transport.
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.
Disorder enabled band structure engineering of a topological insulator surface
Xu, Yishuai; Chiu, Janet; Miao, Lin; ...
2017-02-03
Three-dimensional topological insulators are bulk insulators with Z2 topological electronic order that gives rise to conducting light-like surface states. These surface electrons are exceptionally resistant to localization by non-magnetic disorder, and have been adopted as the basis for a wide range of proposals to achieve new quasiparticle species and device functionality. Recent studies have yielded a surprise by showing that in spite of resisting localization, topological insulator surface electrons can be reshaped by defects into distinctive resonance states. Here we use numerical simulations and scanning tunnelling microscopy data to show that these resonance states have significance well beyond the localizedmore » regime usually associated with impurity bands. Lastly, at native densities in the model Bi2X3 (X=Bi, Te) compounds, defect resonance states are predicted to generate a new quantum basis for an emergent electron gas that supports diffusive electrical transport.« less
Observation of chiral currents at the magnetic domain boundary of a topological insulator
NASA Astrophysics Data System (ADS)
Wang, Yihua
2015-03-01
The broken time-reversal symmetry (TRS) states on the surface of a three-dimensional topological insulator (3D-TI) promise many exotic quantum phenomena. Breaking TRS opens a band gap on the surface Dirac cone and transforms the metallic surface into a Chern insulator. The TRS-broken surface states coupled to a superconductor are predicted to lead to Majorana fermions, which are the fundamental ingredients of topological quantum computation. Just as the surface Dirac cone is a signature of the non-trivial topological bulk band structure of a time-reversal invariant 3D-TI, bulk-boundary correspondence dictates that the TRS-broken surface states with a nonzero Chern number is manifested by a gapless chiral edge state (CES) at the domain boundary. In the special case where the domain boundary is the edge of the sample surface, CES along the edge leads to a quantized anomalous Hall conductance, which was recently measured in a magnetically doped 3D-TI. More generally, a magnetic domain boundary on the surface of TI hosts a CES, which is yet to be directly demonstrated because any local change of conductivity due to the CES does not affect conductance globally. Here we use a scanning superconducting quantum interference device (SQUID) to show that in a uniformly magnetized topological insulator - ferromagnetic insulator (TI-FMI) heterostructure current flows at the edge of the surface of the topological insulator when the Fermi level is gate-tuned to the surface band gap. We further induce micron-scale magnetic structures using the field coil of the SQUID and show that there emerges a chiral edge current at the magnetic domain boundary. In both cases the magnitude of the chiral edge current depends on the chemical potential rather than the applied current. Such magnetic nano-structures, which can be readily created on a TI in an arbitrary geometry, provide a versatile platform for detecting topological magnetoelectric effects and may allow the engineering of
3D van der Waals σ-model and its topological excitations
NASA Astrophysics Data System (ADS)
Bulgadaev, S. A.
2001-09-01
It is shown that the 3D vector van der Waals nonlinear σ-model (NSM) on a sphere S2 has two types of topological excitations: reminiscent vortices and instantons of 2D NSM. The first ones, the hedgehogs, are described by the homotopic group π2(S2) = Z and have logarithmic energies. They are an analog of 2D vortices. The second ones, corresponding to 2D instantons, are the hopfions. They are described by the homotopic group π3(S2) = Z, or the Hopf invariant HinZ, and have finite energy. The possibility of a topological phase transition in this model and its applications are briefly discussed.
Metal-Insulator Transition and Topological Properties of Pyrochlore Iridates
NASA Astrophysics Data System (ADS)
Zhang, Hongbin; Haule, Kristjan; Vanderbilt, David
2017-01-01
Combining density functional theory (DFT) and embedded dynamical mean-field theory (DMFT) methods, we study the metal-insulator transition in R2Ir2 O7 (R =Y , Eu, Sm, Nd, Pr, and Bi) and the topological nature of the insulating compounds. Accurate free energies evaluated using the charge self-consistent DFT +DMFT method reveal that the metal-insulator transition occurs for an A -cation radius between that of Nd and Pr, in agreement with experiments. The all-in-all-out magnetic phase, which is stable in the Nd compound but not the Pr one, gives rise to a small Ir4 + magnetic moment of ≈0.4 μB and opens a sizable correlated gap. We demonstrate that within this state-of-the-art theoretical method, the insulating bulk pyrochlore iridates are topologically trivial.
Acoustic topological insulator and robust one-way sound transport
NASA Astrophysics Data System (ADS)
He, Cheng; Ni, Xu; Ge, Hao; Sun, Xiao-Chen; Chen, Yan-Bin; Lu, Ming-Hui; Liu, Xiao-Ping; Chen, Yan-Feng
2016-12-01
Topological design of materials enables topological symmetries and facilitates unique backscattering-immune wave transport. In airborne acoustics, however, the intrinsic longitudinal nature of sound polarization makes the use of the conventional spin-orbital interaction mechanism impossible for achieving band inversion. The topological gauge flux is then typically introduced with a moving background in theoretical models. Its practical implementation is a serious challenge, though, due to inherent dynamic instabilities and noise. Here we realize the inversion of acoustic energy bands at a double Dirac cone and provide an experimental demonstration of an acoustic topological insulator. By manipulating the hopping interaction of neighbouring ’atoms’ in this new topological material, we successfully demonstrate the acoustic quantum spin Hall effect, characterized by robust pseudospin-dependent one-way edge sound transport. Our results are promising for the exploration of new routes for experimentally studying topological phenomena and related applications, for example, sound-noise reduction.
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.
Spintronics and pseudospintronics in graphene and topological insulators.
Pesin, Dmytro; MacDonald, Allan H
2012-04-23
The two-dimensional electron systems in graphene and in topological insulators are described by massless Dirac equations. Although the two systems have similar Hamiltonians, they are polar opposites in terms of spin-orbit coupling strength. We briefly review the status of efforts to achieve long spin-relaxation times in graphene with its weak spin-orbit coupling, and to achieve large current-induced spin polarizations in topological-insulator surface states that have strong spin-orbit coupling. We also comment on differences between the magnetic responses and dilute-moment coupling properties of the two systems, and on the pseudospin analogue of giant magnetoresistance in bilayer graphene.
Topological insulating phases from two-dimensional nodal loop semimetals
NASA Astrophysics Data System (ADS)
Li, Linhu; Araújo, Miguel A. N.
2016-10-01
Starting from a minimal model for a two-dimensional nodal loop semimetal, we study the effect of chiral mass gap terms. The resulting Dirac loop anomalous Hall insulator's Chern number is the phase-winding number of the mass gap terms on the loop. We provide simple lattice models, analyze the topological phases, and generalize a previous index characterizing topological transitions. The responses of the Dirac loop anomalous Hall and quantum spin Hall insulators to a magnetic field's vector potential are also studied both in weak- and strong-field regimes, as well as the edge states in a ribbon geometry.
Topological Effect to Surface Plasmon Excitation in Topological Insulator Nanowires
NASA Astrophysics Data System (ADS)
Li, Mingda; Cui, Wenping; Li, Ju; Zhu, Yimei; Wu, Lijun; Meng, Qingping; Liu, Weishu; Ren, Zhifeng; Katmis, Ferhat; Wei, Peng; Moodera, Jagadeesh; Zhang, Yong; Li Group, MIT Team; CFN, BNL Collaboration; FBML, MIT Collaboration; CMSE, MIT Collaboration; Ren Group, BC Collaboration
2013-03-01
We present a theoretical investigation of the surface plasmon at the interface between topologically-non-trivial cylindrical core and topological-trivial surrounding material, from the axion electrodynamics and modified constitutive relations. We find that the topological effect lowers the SP energy in any case, while as the diameter of the core becomes smaller, the topological modification to SP energy is reduced. A qualitative picture based on perturbation theory of shifted boundary is given to explain these phenomena, from which we also infer that in order to amplify the topological effect, the difference between the inverse of dielectric constants of two materials must be increased. We also find that when the surrounding material goes magnetic, the magnetism overcomes topological effect, makes the latter seemingly suppressed. What's more, bulk plasmon energy at 17.5 +/- 0.2eV for semiconducting Bi2Se3 nanoparticle is observed from high-resolution Electron Energy Loss Spectrum Image measurements. High-resolution EELS measurement
Crack identification by 3D time-domain elastic or acoustic topological sensitivity
NASA Astrophysics Data System (ADS)
Bellis, Cédric; Bonnet, Marc
2009-03-01
The topological sensitivity analysis, based on the asymptotic behavior of a cost functional associated with the creation of a small trial flaw in a defect-free solid, provides a computationally-fast, non-iterative approach for identifying flaws embedded in solids. This concept is here considered for crack identification using time-dependent measurements on the external boundary. The topological derivative of a cost function under the nucleation of a crack of infinitesimal size is established, in the framework of time-domain elasticity or acoustics. The simplicity and efficiency of the proposed formulation is enhanced by the recourse to an adjoint solution. Numerical results obtained on a 3-D elastodynamic example using the conventional FEM demonstrate the usefulness of the topological derivative as a crack indicator function. To cite this article: C. Bellis, M. Bonnet, C. R. Mecanique 337 (2009).
Quantum anomalous Hall effect and tunable topological states in 3d transition metals doped silicene.
Zhang, Xiao-Long; Liu, Lan-Feng; Liu, Wu-Ming
2013-10-09
Silicene is an intriguing 2D topological material which is closely analogous to graphene but with stronger spin orbit coupling effect and natural compatibility with current silicon-based electronics industry. Here we demonstrate that silicene decorated with certain 3d transition metals (Vanadium) can sustain a stable quantum anomalous Hall effect using both analytical model and first-principles Wannier interpolation. We also predict the quantum valley Hall effect and electrically tunable topological states could be realized in certain transition metal doped silicene where the energy band inversion occurs. Our findings provide new scheme for the realization of quantum anomalous Hall effect and platform for electrically controllable topological states which are highly desirable for future nanoelectronics and spintronics application.
Z2 antiferromagnetic topological insulators with broken C4 symmetry
NASA Astrophysics Data System (ADS)
Bègue, Frédéric; Pujol, Pierre; Ramazashvili, Revaz
2017-04-01
A two-dimensional topological insulator may arise in a centrosymmetric commensurate Néel antiferromagnet (AF), where staggered magnetization breaks both the elementary translation and time reversal, but retains their product as a symmetry. Fang et al. [6] proposed an expression for a Z2 topological invariant to characterize such systems. Here, we show that this expression does not allow to detect all the existing phases if a certain lattice symmetry is lacking. We implement numerical techniques to diagnose topological phases of a toy Hamiltonian, and verify our results by computing the Chern numbers of degenerate bands, and also by explicitly constructing the edge states, thus illustrating the efficiency of the method.
Nonlinear optical and optoelectronic studies of topological insulator surfaces
NASA Astrophysics Data System (ADS)
McIver, James W.
Since their experimental discovery in 2008, topological insulators have been catapulted to the forefront of condensed matter physics research owing to their potential to realize both exciting new technologies as well as novel electronic phases that are inaccessible in any other material class. Their exotic properties arise from a rare quantum organization of its electrons called "topological order,'' which evades the conventional broken symmetry based-classification scheme used to categorize nearly every other state of ordered matter. Instead, topologically ordered phases are classified by topological invariants, which characterize the phase of an electron's wavefunction as it moves through momentum space. When a topologically ordered phase is interfaced with an ordinary phase, such as the vacuum, a novel metallic state appears at their shared boundary. In topological insulators, this results in the formation of a two-dimensional metallic state that spans all of its surfaces. The surface state electronic spectrum is characterized by a single linearly dispersing and helically spin-polarized Dirac cone that is robust against disorder. The helical nature of the surface Dirac cone is highly novel because the Dirac electrons carry a net magnetic moment and are capable of transporting 100% spin-polarized electrical currents, which are the long-sought electronic properties needed for many spin-based electronic applications. However, owing to the small bulk band gap and intrinsic electronic doping inherent to these materials, isolating the surface electronic response from the bulk has proven to be a major experimental obstacle. In this thesis, we demonstrate the means by which light can be used to isolate and study the surface electronic response of topological insulators using optoelectronic and nonlinear optical techniques. In chapter 1, we overview the physics of topological order and topological insulators. In chapter 2, we show how polarized light can be used to
NASA Astrophysics Data System (ADS)
Martín-Ruiz, A.; Cambiaso, M.; Urrutia, L. F.
2016-10-01
A general technique to analyze the classical interaction between ideal topological insulators, and electromagnetic sources and fields, has been previously elaborated. Nevertheless it is not immediately applicable in the laboratory as it fails to describe real ponderable media. In this work we provide a description of real topologically insulating materials taking into account their dielectric and magnetic properties. For inhomogeneous permittivity and permeability, the problem of finding the Green's function must be solved in an ad hoc manner. Nevertheless, the physically feasible cases of piecewise constant ɛ , μ and θ make the problem tractable, where θ encodes the topological magnetoelectric polarizability properties of the medium. To this end we employ the Green's function method to find the fields resulting from the interaction between these materials and electromagnetic sources. Furthermore we exploit the fact that in the cases here studied, the full Green's function can be successfully found if the Green's function of the corresponding ponderable media with θ =0 is known. Our results satisfactorily reproduce previously existing ones and also generalize some others. The method here elaborated can be exploited to determine the electromagnetic fields for more general configurations aiming to measure the interaction between real 3D topological insulators and electromagnetic fields.
Experimental Discovery of Topological Insulators and Related Superconductors
Hasan, M Zahid
2010-09-15
Most quantum states of condensed matter are categorized by the symmetries they break. The remarkable discovery of charge Quantum Hall effects (1980s) revealed that there exists an organizational principle of matter based only on the topological distinctions, but in the presence of time-reversal symmetry breaking. In the past few years, theoretical developments suggest that new classes of topological states of matter might exist that are purely topological in nature in the sense that they do not break time-reversal symmetry, and hence can be realized without any applied magnetic field: "Quantum Hall-like effects without Magnetic Fields." This talk describes our discovery of new topologically ordered states of matter (topological insulators) and discusses the unusual electro-magnetic, spin, and superconducting properties this novel phase of quantum matter might exhibit and their potential applications.
Topological insulating phases in oxide multilayers using perovskites and rutiles
NASA Astrophysics Data System (ADS)
Pardo, Victor; Lado, Jose L.; Baldomir, Daniel
2014-03-01
Ab initio calculations combined with tight-binding modelling have been performed in 5d-electron-based perovskite multilayers in the large spin-orbit coupling limit. The topological properties of the systems (SrTiO3)7/(SrIrO3)2andisoelectronic(KTaO3)7/(KPtO3)2 grown along the (111) direction have been analyzed as a function of on-site Coulomb repulsion U, parity asymmetry and uniaxial strain. The former is found to be a topological semimetal and the latter is a topological insulator describable as the high-U limit of the other one. This high-U phase can be driven to a trivial insulating phase by a perpendicular external electric field. In the talk, we will describe how to proceed in a similar way with rutile-based multilayered structures, where a 4d/5d electron dioxide with rutile structure, sandwiched by a band insulator like TiO2 or SnO2 can lead to topologically non-trivial properties if band filling and strain are tuned. We discuss also the possibility of obtaining similar topological states using isoelectronic fluorides. The electronic structure and properties of free-standing thin films will be also briefly discussed. We acknowledge support through the Ramon y Cajal Program and Xunta de Galicia through project no. EM2013/037.
Unconventional quantum Hall effect in Floquet topological insulators.
Tahir, M; Vasilopoulos, P; Schwingenschlögl, U
2016-09-28
We study an unconventional quantum Hall effect for the surface states of ultrathin Floquet topological insulators in a perpendicular magnetic field. The resulting band structure is modified by photon dressing and the topological property is governed by the low-energy dynamics of a single surface. An exchange of symmetric and antisymmetric surface states occurs by reversing the light's polarization. We find a novel quantum Hall state in which the zeroth Landau level undergoes a phase transition from a trivial insulator state, with Hall conductivity [Formula: see text] at zero Fermi energy, to a Hall insulator state with [Formula: see text]. These findings open new possibilities for experimentally realizing nontrivial quantum states and unusual quantum Hall plateaus at [Formula: see text].
Quantum transport in magnetic topological insulator thin films.
Lu, Hai-Zhou; Zhao, An; Shen, Shun-Qing
2013-10-04
The experimental observation of the long-sought quantum anomalous Hall effect was recently reported in magnetically doped topological insulator thin films [Chang et al., Science 340, 167 (2013)]. An intriguing observation is a rapid decrease from the quantized plateau in the Hall conductance, accompanied by a peak in the longitudinal conductance as a function of the gate voltage. Here, we present a quantum transport theory with an effective model for magnetic topological insulator thin films. The good agreement between theory and experiment reveals that the measured transport originates from a topologically nontrivial conduction band which, near its band edge, has concentrated Berry curvature and a local maximum in group velocity. The indispensable roles of the broken structure inversion and particle-hole symmetries are also revealed. The results are instructive for future experiments and transport studies based on first-principles calculations.
NASA Astrophysics Data System (ADS)
Sato, T.; Segawa, Kouji; Kosaka, K.; Souma, S.; Nakayama, K.; Eto, K.; Minami, T.; Ando, Yoichi; Takahashi, T.
2011-11-01
The three-dimensional (3D) topological insulator is a novel quantum state of matter where an insulating bulk hosts a linearly dispersing surface state, which can be viewed as a sea of massless Dirac fermions protected by the time-reversal symmetry (TRS). Breaking the TRS by a magnetic order leads to the opening of a gap in the surface state, and consequently the Dirac fermions become massive. It has been proposed theoretically that such a mass acquisition is necessary to realize novel topological phenomena, but achieving a sufficiently large mass is an experimental challenge. Here we report an unexpected discovery that the surface Dirac fermions in a solid-solution system TlBi(S1-xSex)2 acquire a mass without explicitly breaking the TRS. We found that this system goes through a quantum phase transition from the topological to the non-topological phase, and, by tracing the evolution of the electronic states using the angle-resolved photoemission, we observed that the massless Dirac state in TlBiSe2 switches to a massive state before it disappears in the non-topological phase. This result suggests the existence of a condensed-matter version of the `Higgs mechanism' where particles acquire a mass through spontaneous symmetry breaking.
Sensitivity of a 3D fully-gapped topological superconductor to non-magnetic impurities
NASA Astrophysics Data System (ADS)
Ota, Yukihiro; Nagai, Yuki; Machida, Masahiko
2015-03-01
Topological superconductors (TSC) are notable materials, owing to the mathematical curiosity and the application potential. The bulk TSC can emerge by copper intercalation into topological insulator Bi2Se3. In this paper, we theoretically study the non-magnetic impurity effects in the mean-field model of CuxBi2Se3, focusing on the odd-parity fully-gapped superconducting state. Calculating the density of states with a self-consistent T-matrix approach, we test the presence of mid-gap states, leading to pair-breaking effects. Remarkably, the sensitivity to non-magnetic impurities strongly depend on a normal-state dispersion character, i.e., either non-relativistic or relativistic dispersion relations. We show unification picture for understanding this intriguing result, deriving a low-energy effective superconducting theory.
Spatially-protected Topology and Group Cohomology in Band Insulators
NASA Astrophysics Data System (ADS)
Alexandradinata, A.
This thesis investigates band topologies which rely fundamentally on spatial symmetries. A basic geometric property that distinguishes spatial symmetry regards their transformation of the spatial origin. Point groups consist of spatial transformations that preserve the spatial origin, while un-split extensions of the point groups by spatial translations are referred to as nonsymmorphic space groups. The first part of the thesis addresses topological phases with discretely-robust surface properties: we introduce theories for the Cnv point groups, as well as certain nonsymmorphic groups that involve glide reflections. These band insulators admit a powerful characterization through the geometry of quasimomentum space; parallel transport in this space is represented by the Wilson loop. The non-symmorphic topology we study is naturally described by a further extension of the nonsymmorphic space group by quasimomentum translations (the Wilson loop), thus placing real and quasimomentum space on equal footing -- here, we introduce the language of group cohomology into the theory of band insulators. The second part of the thesis addresses topological phases without surface properties -- their only known physical consequences are discrete signatures in parallel transport. We provide two such case studies with spatial-inversion and discrete-rotational symmetries respectively. One lesson learned here regards the choice of parameter loops in which we carry out transport -- the loop must be chosen to exploit the symmetry that protects the topology. While straight loops are popular for their connection with the geometric theory of polarization, we show that bent loops also have utility in topological band theory.
Zhang, Qiangqiang; Hao, Menglong; Xu, Xiang; Xiong, Guoping; Li, Hui; Fisher, Timothy S
2017-04-14
In this article, flyweight three-dimensional (3D) graphene scaffolds (GSs) have been demonstrated with a microinterface barrier-derived thermal insulation and flame retardancy characteristics. Such 3D GSs were fabricated by a modified hydrothermal method and a unidirectional freeze-casting process with hierarchical porous microstructures. Because of high porosity (99.9%), significant phonon scattering, and strong π-π interaction at the interface barriers of multilayer graphene cellular walls, the GSs demonstrate a sequence of multifunctional properties simultaneously, such as lightweight density, thermal insulating characteristics, and outstanding mechanical robustness. At 100 °C, oxidized GSs exhibit a thermal conductivity of 0.0126 ± 0.0010 W/(m K) in vacuum. The thermal conductivity of oxidized GSs remains relatively unaffected despite large-scale deformation-induced densification of the microstructures, as compared to the behavior of reduced GSs (rGSs) whose thermal conductivity increases dramatically under compression. The contrasting behavior of oxidized GSs and rGSs appears to derive from large differences in the intersheet contact resistance and varying intrinsic thermal conductivity between reduced and oxidized graphene sheets. The oxidized GSs also exhibit excellent flame retardant behavior and mechanical robustness, with only 2% strength decay after flame treatment. In a broader context, this work demonstrates a useful strategy to design porous nanomaterials with a tunable heat conduction behavior through interface engineering at the nanoscale.
Design of Chern and Mott insulators in buckled 3 d oxide honeycomb lattices
NASA Astrophysics Data System (ADS)
Doennig, David; Baidya, Santu; Pickett, Warren E.; Pentcheva, Rossitza
2016-04-01
Perovskite (La X O3 )2/(LaAlO3)4(111) superlattices with X spanning the entire 3 d transition-metal series combine the strongly correlated, multiorbital nature of electrons in transition-metal oxides with a honeycomb lattice as a key feature. Based on density functional theory calculations including strong interaction effects, we establish trends in the evolution of electronic states as a function of several control parameters: band filling, interaction strength, spin-orbit coupling (SOC), and lattice instabilities. Competition between local pseudocubic and global trigonal symmetry as well as the additional flexibility provided by the magnetic and spin degrees of freedom of 3 d ions lead to a broad array of distinctive broken-symmetry ground states not accessible for the (001)-growth direction, offering a platform to design two-dimensional electronic functionalities. Constraining the symmetry between the two triangular sublattices causes X =Mn , Co, and Ti to emerge as Chern insulators driven by SOC. For X =Mn we illustrate how interaction strength and lattice distortions can tune these systems between a Dirac semimetal, a Chern and a trivial Mott insulator.
Gigantic Surface Lifetime of an Intrinsic Topological Insulator
Neupane, Madhab; Xu, Su-Yang; Ishida, Yukiaki; ...
2015-09-09
We report that the interaction between light and novel two-dimensional electronic states holds promise to realize new fundamental physics and optical devices. Here, we use pump-probe photoemission spectroscopy to study the optically excited Dirac surface states in the bulk-insulating topological insulator Bi2Te2Se and reveal optical properties that are in sharp contrast to those of bulk-metallic topological insulators. We observe a gigantic optical lifetime exceeding 4 μs (1 μs=10more » $${-}$$6 s) for the surface states in Bi2Te2Se, whereas the lifetime in most topological insulators, such as Bi2Se3, has been limited to a few picoseconds (1 ps=10$${-}$$12 s). Moreover, we discover a surface photovoltage, a shift of the chemical potential of the Dirac surface states, as large as 100 mV. Lastly, our results demonstrate a rare platform to study charge excitation and relaxation in energy and momentum space in a two-dimensional system.« less
Gigantic Surface Lifetime of an Intrinsic Topological Insulator
Neupane, Madhab; Xu, Su-Yang; Ishida, Yukiaki; Jia, Shuang; Fregoso, Benjamin M.; Liu, Chang; Belopolski, Ilya; Bian, Guang; Alidoust, Nasser; Durakiewicz, Tomasz; Galitski, Victor; Shin, Shik; Cava, Robert J.; Hasan, M. Zahid
2015-09-09
We report that the interaction between light and novel two-dimensional electronic states holds promise to realize new fundamental physics and optical devices. Here, we use pump-probe photoemission spectroscopy to study the optically excited Dirac surface states in the bulk-insulating topological insulator Bi_{2}Te_{2}Se and reveal optical properties that are in sharp contrast to those of bulk-metallic topological insulators. We observe a gigantic optical lifetime exceeding 4 μs (1 μs=10^{${-}$6} s) for the surface states in Bi_{2}Te_{2}Se, whereas the lifetime in most topological insulators, such as Bi2Se3, has been limited to a few picoseconds (1 ps=10^{${-}$12} s). Moreover, we discover a surface photovoltage, a shift of the chemical potential of the Dirac surface states, as large as 100 mV. Lastly, our results demonstrate a rare platform to study charge excitation and relaxation in energy and momentum space in a two-dimensional system.
Realization of Negative Capacitance with Topological Insulator Based MOS Capacitor
NASA Astrophysics Data System (ADS)
Yuan, Hui; Zhang, Kai; Zhu, Hao; Li, Haitao; Ioannou, Dimitris; Baumgart, Helmut; Richter, Curt; Li, Qiliang; ECE, George Mason University Team; Semiconductor and Dimensional Metrology Division of NIST Team; ECE, Old Dominion University Team
2013-03-01
Negative capacitance is one of way to achieve steep subthreshold slope exceeding its thermal limit in metal-oxide-semiconductor field effect transistor (MOSFET). The common materials under study for negative capacitance are ferroelectric thin films. However, the integration of regular ferroelectric materials (e.g., PZT) into semiconductor based devices is usually difficult due to the high temperature required for crystallization and precise control of oxygen percentage in ferroelectric materials. In this work, we found that negative capacitance can be achieved by introducing a topological insulator interlayer into a conventional MOS capacitor. Three-dimensional topological insulators inherently contain a insulator/semiconductor bulk and a gapless conducting surface. When an electric field is added to topological insulator interlayer, imbalanced charge carriers (electrons and holes) would be generated and then accumulate on either surface of the film, resulting in a temporary residual polarization. As a result, a ferroelectric-like hysteresis and negative capacitance are achieved. We believe this approach will be very attractive to achieve steep subthreshold using negative capacitance. Supported by NSF Career grant 0846649.
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.
Imperfect two-dimensional topological insulator field-effect transistors
NASA Astrophysics Data System (ADS)
Vandenberghe, William G.; Fischetti, Massimo V.
2017-01-01
To overcome the challenge of using two-dimensional materials for nanoelectronic devices, we propose two-dimensional topological insulator field-effect transistors that switch based on the modulation of scattering. We model transistors made of two-dimensional topological insulator ribbons accounting for scattering with phonons and imperfections. In the on-state, the Fermi level lies in the bulk bandgap and the electrons travel ballistically through the topologically protected edge states even in the presence of imperfections. In the off-state the Fermi level moves into the bandgap and electrons suffer from severe back-scattering. An off-current more than two-orders below the on-current is demonstrated and a high on-current is maintained even in the presence of imperfections. At low drain-source bias, the output characteristics are like those of conventional field-effect transistors, at large drain-source bias negative differential resistance is revealed. Complementary n- and p-type devices can be made enabling high-performance and low-power electronic circuits using imperfect two-dimensional topological insulators.
Imperfect two-dimensional topological insulator field-effect transistors.
Vandenberghe, William G; Fischetti, Massimo V
2017-01-20
To overcome the challenge of using two-dimensional materials for nanoelectronic devices, we propose two-dimensional topological insulator field-effect transistors that switch based on the modulation of scattering. We model transistors made of two-dimensional topological insulator ribbons accounting for scattering with phonons and imperfections. In the on-state, the Fermi level lies in the bulk bandgap and the electrons travel ballistically through the topologically protected edge states even in the presence of imperfections. In the off-state the Fermi level moves into the bandgap and electrons suffer from severe back-scattering. An off-current more than two-orders below the on-current is demonstrated and a high on-current is maintained even in the presence of imperfections. At low drain-source bias, the output characteristics are like those of conventional field-effect transistors, at large drain-source bias negative differential resistance is revealed. Complementary n- and p-type devices can be made enabling high-performance and low-power electronic circuits using imperfect two-dimensional topological insulators.
Imperfect two-dimensional topological insulator field-effect transistors
Vandenberghe, William G.; Fischetti, Massimo V.
2017-01-01
To overcome the challenge of using two-dimensional materials for nanoelectronic devices, we propose two-dimensional topological insulator field-effect transistors that switch based on the modulation of scattering. We model transistors made of two-dimensional topological insulator ribbons accounting for scattering with phonons and imperfections. In the on-state, the Fermi level lies in the bulk bandgap and the electrons travel ballistically through the topologically protected edge states even in the presence of imperfections. In the off-state the Fermi level moves into the bandgap and electrons suffer from severe back-scattering. An off-current more than two-orders below the on-current is demonstrated and a high on-current is maintained even in the presence of imperfections. At low drain-source bias, the output characteristics are like those of conventional field-effect transistors, at large drain-source bias negative differential resistance is revealed. Complementary n- and p-type devices can be made enabling high-performance and low-power electronic circuits using imperfect two-dimensional topological insulators. PMID:28106059
Reduced density matrix and order parameters of a topological insulator
NASA Astrophysics Data System (ADS)
Yu, Wing Chi; Li, Yan Chao; Sacramento, P. D.; Lin, Hai-Qing
2016-12-01
It has been recently proposed that the reduced density matrix may be used to derive the order parameter of a condensed matter system. Here we propose order parameters for the phases of a topological insulator, specifically a spinless Su-Schrieffer-Heeger (SSH) model, and consider the effect of short-range interactions. All the derived order parameters and their possible corresponding quantum phases are verified by the entanglement entropy and electronic configuration analysis results. The order parameter appropriate to the topological regions is further proved by calculating the Berry phase under twisted boundary conditions. It is found that the topological nontrivial phase is robust to the introduction of repulsive intersite interactions and can appear in the topological trivial parameter region when appropriate interactions are added.
Anomalous Phase Shift of Quantum Oscillations in 3D Topological Semimetals.
Wang, C M; Lu, Hai-Zhou; Shen, Shun-Qing
2016-08-12
Berry phase physics is closely related to a number of topological states of matter. Recently discovered topological semimetals are believed to host a nontrivial π Berry phase to induce a phase shift of ±1/8 in the quantum oscillation (+ for hole and - for electron carriers). We theoretically study the Shubnikov-de Haas oscillation of Weyl and Dirac semimetals, taking into account their topological nature and inter-Landau band scattering. For a Weyl semimetal with broken time-reversal symmetry, the phase shift is found to change nonmonotonically and go beyond known values of ±1/8 and ±5/8, as a function of the Fermi energy. For a Dirac semimetal or paramagnetic Weyl semimetal, time-reversal symmetry leads to a discrete phase shift of ±1/8 or ±5/8. Different from the previous works, we find that the topological band inversion can lead to beating patterns in the absence of Zeeman splitting. We also find the resistivity peaks should be assigned integers in the Landau index plot. Our findings may account for recent experiments in Cd_{2}As_{3} and should be helpful for exploring the Berry phase in various 3D systems.
Anomalous Phase Shift of Quantum Oscillations in 3D Topological Semimetals
NASA Astrophysics Data System (ADS)
Wang, C. M.; Lu, Hai-Zhou; Shen, Shun-Qing
2016-08-01
Berry phase physics is closely related to a number of topological states of matter. Recently discovered topological semimetals are believed to host a nontrivial π Berry phase to induce a phase shift of ±1 /8 in the quantum oscillation (+ for hole and - for electron carriers). We theoretically study the Shubnikov-de Haas oscillation of Weyl and Dirac semimetals, taking into account their topological nature and inter-Landau band scattering. For a Weyl semimetal with broken time-reversal symmetry, the phase shift is found to change nonmonotonically and go beyond known values of ±1 /8 and ±5 /8 , as a function of the Fermi energy. For a Dirac semimetal or paramagnetic Weyl semimetal, time-reversal symmetry leads to a discrete phase shift of ±1 /8 or ±5 /8 . Different from the previous works, we find that the topological band inversion can lead to beating patterns in the absence of Zeeman splitting. We also find the resistivity peaks should be assigned integers in the Landau index plot. Our findings may account for recent experiments in Cd2 As3 and should be helpful for exploring the Berry phase in various 3D systems.
NASA Astrophysics Data System (ADS)
Li, An-Ping; Durand, Corentin; Hus, Saban; Zhang, Xiaoguang; McGuire, Michael; Chen, Yong
The direct measurement of the topological surface states (TSS) conductivity is often hard to achieve due to the pronounced contribution from the bulk conduction channel. Here, we show a new method to differentiate conductivities from the surface states and the coexisting bulk states in topological insulators (TI) using a four-probe transport spectroscopy in a multi-probe scanning tunneling microscopy system. In contrast to conventional models that assume two resistors in parallel to count for both the TSS and bulk conductance channels, we derive a scaling relation of measured resistance with respect to varying inter-probe spacing for two interconnected conduction channels, which allows quantitative determination of conductivities from both channels. Using this method, we demonstrate the separation of 2D and 3D conduction in TI by comparing the conductance scaling of Bi2Se3, Bi2Te2Se, and Sb-doped Bi2Se3 with that of a pure 2D conductance of graphene on SiC substrate. We also quantitatively show the effect of surface doping carriers on the 2D conductance enhancement in TI. The method offers an approach to understanding not just the topological insulators but also the 2D to 3D crossover of conductance in other complex systems. This research was conducted at the Center for Nanophase Materials Sciences, which is a DOE Office of Science User Facility.
NASA Astrophysics Data System (ADS)
Kalvin, Alan D.; Cutting, Court B.; Haddad, Betsy; Noz, Marilyn E.
1991-06-01
Three-dimensional (3D) medical imaging deals with the visualization, manipulation, and measuring of objects in 3D medical images. So far, research efforts have concentrated primarily on visualization, using well-developed methods from computer graphics. Very little has been achieved in developing techniques for manipulating medical objects, or for extracting quantitative measurements from them beyond volume calculation (by counting voxels), and computing distances and angles between manually located surface points. A major reason for the slow pace in the development of manipulation and quantification methods lies with the limitations of current algorithms for constructing surfaces from 3D solid objects. We show that current surface construction algorithms either (a) do not construct valid surface descriptions of solid objects or (b) produce surface representations that are not particularly suitable for anything other than visualization. We present ALLIGATOR, a new surface construction algorithm that produces valid, topologically connected surface representations of solid objects. We have developed a modeling system based on the surface representations created by ALLIGATOR that is suitable for developing algorithms to visualize, manipulate, and quantify 3D medical objects. Using this modeling system we have developed a method for efficiently computing principle curvatures and directions on surfaces. These measurements form the basis for a new metric system being developed for morphometrics. The modeling system is also being used in the development of systems for quantitative pre-surgical planning and surgical augmentation.
Topological insulators and C*-algebras: Theory and numerical practice
Hastings, Matthew B.; Loring, Terry A.
2011-07-15
Research Highlights: > We classify topological insulators using C* algebras. > We present new K-theory invariants. > We develop efficient numerical algorithms based on this technique. > We observe unexpected quantum phase transitions using our algorithm. - Abstract: We apply ideas from C*-algebra to the study of disordered topological insulators. We extract certain almost commuting matrices from the free Fermi Hamiltonian, describing band projected coordinate matrices. By considering topological obstructions to approximating these matrices by exactly commuting matrices, we are able to compute invariants quantifying different topological phases. We generalize previous two dimensional results to higher dimensions; we give a general expression for the topological invariants for arbitrary dimension and several symmetry classes, including chiral symmetry classes, and we present a detailed K-theory treatment of this expression for time reversal invariant three dimensional systems. We can use these results to show non-existence of localized Wannier functions for these systems. We use this approach to calculate the index for time-reversal invariant systems with spin-orbit scattering in three dimensions, on sizes up to 12{sup 3}, averaging over a large number of samples. The results show an interesting separation between the localization transition and the point at which the average index (which can be viewed as an 'order parameter' for the topological insulator) begins to fluctuate from sample to sample, implying the existence of an unsuspected quantum phase transition separating two different delocalized phases in this system. One of the particular advantages of the C*-algebraic technique that we present is that it is significantly faster in practice than other methods of computing the index, allowing the study of larger systems. In this paper, we present a detailed discussion of numerical implementation of our method.
Estimation of the thermal conductivity of hemp based insulation material from 3D tomographic images
NASA Astrophysics Data System (ADS)
El-Sawalhi, R.; Lux, J.; Salagnac, P.
2016-08-01
In this work, we are interested in the structural and thermal characterization of natural fiber insulation materials. The thermal performance of these materials depends on the arrangement of fibers, which is the consequence of the manufacturing process. In order to optimize these materials, thermal conductivity models can be used to correlate some relevant structural parameters with the effective thermal conductivity. However, only a few models are able to take into account the anisotropy of such material related to the fibers orientation, and these models still need realistic input data (fiber orientation distribution, porosity, etc.). The structural characteristics are here directly measured on a 3D tomographic image using advanced image analysis techniques. Critical structural parameters like porosity, pore and fiber size distribution as well as local fiber orientation distribution are measured. The results of the tested conductivity models are then compared with the conductivity tensor obtained by numerical simulation on the discretized 3D microstructure, as well as available experimental measurements. We show that 1D analytical models are generally not suitable for assessing the thermal conductivity of such anisotropic media. Yet, a few anisotropic models can still be of interest to relate some structural parameters, like the fiber orientation distribution, to the thermal properties. Finally, our results emphasize that numerical simulations on 3D realistic microstructure is a very interesting alternative to experimental measurements.
Orbital selective spin-texture in a topological insulator
Singh, Bahadur Prasad, R.
2015-05-15
Three-dimensional topological insulators support a metallic non-trivial surface state with unique spin texture, where spin and momentum are locked perpendicular to each other. In this work, we investigate the orbital selective spin-texture associated with the topological surface states in Sb2Te{sub 3}, using the first principles calculations. Sb2Te{sub 3} is a strong topological insulator with a p-p type bulk band inversion at the Γ-point and supports a single topological metallic surface state with upper (lower) Dirac-cone has left (right) handed spin-texture. Here, we show that the topological surface state has an additional locking between the spin and orbitals, leading to an orbital selective spin-texture. The out-of-plane orbitals (p{sub z} orbitals) have an isotropic orbital texture for both the Dirac cones with an associated left and right handed spin-texture for the upper and lower Dirac cones, respectively. In contrast, the in-planar orbital texture (p{sub x} and p{sub y} projections) is tangential for the upper Dirac-cone and is radial for the lower Dirac-cone surface state. The dominant in-planar orbital texture in both the Dirac cones lead to a right handed orbital-selective spin-texture.
The topological Anderson insulator phase in the Kane-Mele model
Orth, Christoph P.; Sekera, Tibor; Bruder, Christoph; Schmidt, Thomas L.
2016-01-01
It has been proposed that adding disorder to a topologically trivial mercury telluride/cadmium telluride (HgTe/CdTe) quantum well can induce a transition to a topologically nontrivial state. The resulting state was termed topological Anderson insulator and was found in computer simulations of the Bernevig-Hughes-Zhang model. Here, we show that the topological Anderson insulator is a more universal phenomenon and also appears in the Kane-Mele model of topological insulators on a honeycomb lattice. We numerically investigate the interplay of the relevant parameters, and establish the parameter range in which the topological Anderson insulator exists. A staggered sublattice potential turns out to be a necessary condition for the transition to the topological Anderson insulator. For weak enough disorder, a calculation based on the lowest-order Born approximation reproduces quantitatively the numerical data. Our results thus considerably increase the number of candidate materials for the topological Anderson insulator phase. PMID:27045779
Disorder Effects in Charge Transport and Spin Response of Topological Insulators
NASA Astrophysics Data System (ADS)
Zhao, Lukas Zhonghua
Topological insulators are a class of solids in which the non-trivial inverted bulk band structure gives rise to metallic surface states that are robust against impurity backscattering. First principle calculations predicted Bi2Te3, Sb2Te3 and Bi2Se3 to be three-dimensional (3D) topological insulators with a single Dirac cone on the surface. The topological surface states were subsequently observed by angle-resolved photoemission (ARPES) and scanning tunneling microscopy (STM). The investigations of charge transport through topological surfaces of 3D topological insulators, however, have faced a major challenge due to large charge carrier densities in the bulk donated by randomly distributed defects such as vacancies and antisites. This bulk disorder intermixes surface and bulk conduction channels, thereby complicating access to the low-energy (Dirac point) charge transport or magnetic response and resulting in the relatively low measured carrier mobilities. Moreover, charge inhomogeneity arising from bulk disorder can result in pronounced nanoscale spatial fluctuations of energy on the surface, leading to the formation of surface `puddles' of different carrier types. Great efforts have been made to combat the undesirable effects of disorder in 3D topological insulators and to reduce bulk carriers through chemical doping, nanostructure fabrication, and electric gating. In this work we have developed a new way to reduce bulk carrier densities using high-energy electron irradiation, thereby allowing us access to the topological surface quantum channels. We also found that disorder in 3D topological insulators can be beneficial. It can play an important part in enabling detection of unusual magnetic response from Dirac fermions and in uncovering new excitations, namely surface superconductivity in Dirac `puddles'. In Chapter 3 we show how by using differential magnetometry we could probe spin rotation in the 3D topological material family (Bi2Se 3, Bi2Te3 and Sb2Te3
Quantum Hall superfluids in topological insulator thin films.
Tilahun, Dagim; Lee, Byounghak; Hankiewicz, E M; MacDonald, A H
2011-12-09
Three-dimensional topological insulators have protected Dirac-cone surface states. In this Letter we argue that gapped excitonic superfluids with spontaneous coherence between top and bottom surfaces can occur in the topological insulator (TI)-thin-film quantum Hall regime. We find that the large dielectric constants of TI materials increase the layer separation range over which coherence survives and decrease the superfluid sound velocity, but have little influence on the superfluid density or on the charge gap. The coherent state at total Landau-level filling factor νT=0 is predicted to be free of edge modes, qualitatively altering its transport phenomenology compared to the widely studied case of νT=1 in GaAs double-quantum wells.
Characterizing the structure of topological insulator thin films
Richardella, Anthony; Kandala, Abhinav; Lee, Joon Sue; Samarth, Nitin
2015-08-01
We describe the characterization of structural defects that occur during molecular beam epitaxy of topological insulator thin films on commonly used substrates. Twinned domains are ubiquitous but can be reduced by growth on smooth InP (111)A substrates, depending on details of the oxide desorption. Even with a low density of twins, the lattice mismatch between (Bi, Sb){sub 2}Te{sub 3} and InP can cause tilts in the film with respect to the substrate. We also briefly discuss transport in simultaneously top and back electrically gated devices using SrTiO{sub 3} and the use of capping layers to protect topological insulator films from oxidation and exposure.
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
Topological insulator in the presence of spatially correlated disorder
NASA Astrophysics Data System (ADS)
Girschik, Adrian; Libisch, Florian; Rotter, Stefan
2013-07-01
We investigate the effect of spatially correlated disorder on two-dimensional topological insulators and on the quantum spin Hall effect which the helical edge states in these systems give rise to. Our work expands the scope of previous investigations which found that uncorrelated disorder can induce a nontrivial phase called the topological Anderson insulator (TAI). In extension of these studies, we find that spatial correlations in the disorder can entirely suppress the emergence of the TAI phase. We show that this phenomenon is associated with a quantum percolation transition and quantify it by generalizing an existing effective medium theory to the case of correlated disorder potentials. The predictions of this theory are in good agreement with our numerics and may be crucial for future experiments.
Aharonov-Bohm oscillations in disordered topological insulator nanowires.
Bardarson, J H; Brouwer, P W; Moore, J E
2010-10-08
A direct signature of electron transport at the metallic surface of a topological insulator is the Aharonov-Bohm oscillation observed in a recent study of Bi2Se3 nanowires [Peng, Nature Mater. 9, 225 (2010)] where conductance was found to oscillate as a function of magnetic flux ϕ through the wire, with a period of one flux quantum ϕ0=h/e and maximum conductance at zero flux. This seemingly agrees neither with diffusive theory, which would predict a period of half a flux quantum, nor with ballistic theory, which in the simplest form predicts a period of ϕ0 but a minimum at zero flux due to a nontrivial Berry phase in topological insulators. We show how h/e and h/2e flux oscillations of the conductance depend on doping and disorder strength, provide a possible explanation for the experiments, and discuss further experiments that could verify the theory.
Topological insulators are tunable waveguides for hyperbolic polaritons
NASA Astrophysics Data System (ADS)
Wu, Jhih-Sheng; Basov, Dimitri; Fogler, Michael
We present a theoretical analysis showing that layered topological insulators, for example, Bi2Se3 are optically hyperbolic materials in a range of THz frequencies. As such, these topological insulators possess deeply subdiffractional, highly directional collective modes: hyperbolic phonon-polaritons. We predict that in thin crystals the dispersion of these modes is split into discrete subbands and is strongly influenced by electron surface states. If the surface states are doped, then hybrid collective modes result from coupling of the phonon-polaritons with surface plasmons. The strength of the hybridization can be controlled by an external gate that varies the chemical potential of the surface states. We also show that momentum-dependence of the plasmon-phonon coupling leads to a polaritonic analog of the Goos-Hänchen effect. Directionality of the polaritonic rays and their tunable Goos-Hänchen shift are observable via THz nanoimaging.
Topological insulators are tunable waveguides for hyperbolic polaritons
NASA Astrophysics Data System (ADS)
Wu, Jhih-Sheng; Basov, D. N.; Fogler, M. M.
2015-11-01
We present a theoretical analysis showing that layered topological insulators, for example, Bi2Se3 are optically hyperbolic materials in the range of terahertz (THz) frequencies. As such, these topological insulators possess deeply subdiffractional, highly directional collective modes: hyperbolic phonon polaritons. We predict that in thin crystals the dispersion of these modes is split into discrete subbands and is strongly influenced by electron surface states. If the surface states are doped, then hybrid collective modes result from coupling of the phonon polaritons with surface plasmons. The strength of the hybridization can be controlled by an external gate that varies the chemical potential of the surface states. We also show that the momentum dependence of the plasmon-phonon coupling leads to a polaritonic analog of the Goos-Hänchen effect. The directionality of the polaritonic rays and their tunable Goos-Hänchen shift is observable via THz nanoimaging.
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.
Sutradhar, Alok; Park, Jaejong; Carrau, Diana; Nguyen, Tam H; Miller, Michael J; Paulino, Glaucio H
2016-07-01
Large craniofacial defects require efficient bone replacements which should not only provide good aesthetics but also possess stable structural function. The proposed work uses a novel multiresolution topology optimization method to achieve the task. Using a compliance minimization objective, patient-specific bone replacement shapes can be designed for different clinical cases that ensure revival of efficient load transfer mechanisms in the mid-face. In this work, four clinical cases are introduced and their respective patient-specific designs are obtained using the proposed method. The optimized designs are then virtually inserted into the defect to visually inspect the viability of the design . Further, once the design is verified by the reconstructive surgeon, prototypes are fabricated using a 3D printer for validation. The robustness of the designs are mechanically tested by subjecting them to a physiological loading condition which mimics the masticatory activity. The full-field strain result through 3D image correlation and the finite element analysis implies that the solution can survive the maximum mastication of 120 lb. Also, the designs have the potential to restore the buttress system and provide the structural integrity. Using the topology optimization framework in designing the bone replacement shapes would deliver surgeons new alternatives for rather complicated mid-face reconstruction.
Transport across a junction of topological insulators and a superconductor
NASA Astrophysics Data System (ADS)
Soori, Abhiram; Deb, Oindrila; Sengupta, K.; Sen, Diptiman
2013-06-01
We study transport across a line junction lying between two orthogonal topological insulator surfaces and a superconductor which can have either s-wave (spin-singlet) or p-wave (spin-triplet) pairing symmetry. The junction can have three time-reversal invariant barriers on three sides. We compute the charge and the spin conductance across such a junction and study their behaviors as a function of the bias voltage applied across the junction and the three parameters used to characterize the barrier. We find that the presence of topological insulators and a superconductor leads to both Dirac- and Schrödinger-like features in charge and spin conductances. We discuss the effect of bound states on the superconducting side of the barrier on the conductance; in particular, we show that for triplet p-wave superconductors, such a junction may be used to determine the spin state of its Cooper pairs. Our study reveals that there is a nonzero spin conductance for some particular spin states of the triplet Cooper pairs; this is an effect of the topological insulators which break the spin rotation symmetry. Finally, we find an unusual satellite peak (in addition to the usual zero bias peak) in the spin conductance for p-wave symmetry of the superconductor order parameter.
Transport through quantum wells and superlattices on topological insulator surfaces.
Song, J-T; Li, Y-X; Sun, Q-F
2014-05-07
We investigate electron transmission coefficients through quantum wells and quantum superlattices on topological insulator surfaces. The quantum well or superlattice is not constituted by general electronic potential barriers but by Fermi velocity barriers which originate in the different topological insulator surfaces. It is found that electron resonant modes can be renormalized by quantum wells and more clearly by quantum superlattices. The depth and width of a quantum well and superlattice, the incident angle of an electron, and the Fermi energy can be used to effectively tune the electron resonant modes. In particular, the number N of periodic structures that constitute a superlattice can further strengthen these regulating effects. These results suggest that a device could be developed to select and regulate electron propagation modes on topological insulator surfaces. Finally, we also study the conductance and the Fano factor through quantum wells and quantum superlattices. In contrast to what has been reported before, the suppression factors of 0.4 in the conductance and 0.85 in the Fano factor are observed in a quantum well, while the transport for a quantum superlattice shows strong oscillating behavior at low energy and reaches the same saturated values as in the case of a quantum well at sufficiently large energies.
Coherent single-spin source based on topological insulators
NASA Astrophysics Data System (ADS)
Xing, Yanxia; Yang, Zhong-Liu; Sun, Qing-Feng; Wang, Jian
2015-03-01
We report on the injection of quantized pure spin current into quantum conductors. In particular, we propose an on-demand single-spin source generated by periodically varying the gate voltages of two quantum dots that are connected to a two-dimensional topological insulator via tunneling barriers. Due to the nature of the helical states of the topological insulator, one or several spin pairs can be pumped out per cycle giving rise to a pure quantized alternating spin current. Depending on the phase difference between two gate voltages, this device can serve as an on-demand single-spin emitter or single-charge emitter. Again, due to the helicity of the topological insulator, the single-spin emitter or charge emitter is dissipationless and immune to disorder. The proposed single-spin emitter can be an important building block of future spintronic devices. We gratefully acknowledge the financial support from from NSF-China under Grant (Nos. 11174032 and 11374246), NBRP of China (2012CB921303), and a RGC Grant (HKU 705212P) from the Government of HKSAR.
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.
Electron localization in ultrathin films of three-dimensional topological insulators
NASA Astrophysics Data System (ADS)
Liao, Jian; Shi, Gang; Liu, Nan; Li, Yongqing
2016-11-01
The recent discovery of three-dimensional (3D) topological insulators (TIs) has provided a fertile ground for obtaining further insights into electron localization in condensed matter systems. In the past few years, a tremendous amount of research effort has been devoted to investigate electron transport properties of 3D TIs and their low dimensional structures in a wide range of disorder strength, covering transport regimes from weak antilocalization to strong localization. The knowledge gained from these studies not only offers sensitive means to probe the surface states of 3D TIs but also forms a basis for exploring novel topological phases. In this article, we briefly review the main experimental progress in the study of the localization in 3D TIs, with a focus on the latest results on ultrathin TI films. Some new transport data will also be presented in order to complement those reported previously in the literature. Project supported by the National Basic Research Program of China (Grant Nos. 2012CB921703 and 2015CB921102), the National Natural Science Foundation of China (Grant Nos. 61425015, 11374337, and 91121003), and the Strategic Priority Research Program of the Chinese Academy of Sciences (Grant No. XDB070202).
Imaging topological radar for 3D imaging in cultural heritage reproduction and restoration
NASA Astrophysics Data System (ADS)
Poggi, Claudio; Guarneri, Massimiliano; Fornetti, Giorgio; Ferri de Collibus, Mario; De Dominicis, Luigi; Paglia, Emiliano; Ricci, Roberto
2005-10-01
We present the last results obtained by using our Imaging Topological Radar (ITR), an high resolution laser scanner aimed at reconstruction 3D digital models of real targets, either single objects or complex scenes. The system, based on amplitude modulation ranging technique, enables to obtain simultaneously a shade-free, high resolution, photographic-like picture and accurate range data in the form of a range image, with resolution depending mainly on the laser modulation frequency (current best performance are ~100μm). The complete target surface is reconstructed from sampled points by using specifically developed software tools. The system has been successfully applied to scan different types of real surfaces (stone, wood, alloy, bones) and is suitable of relevant applications in different fields, ranging from industrial machining to medical diagnostics. We present some relevant examples of 3D reconstruction in the heritage field. Such results were obtained during recent campaigns carried out in situ in various Italian historical and archaeological sites (S. Maria Antiqua in Roman Forum, "Grotta dei cervi" Porto Badisco - Lecce, South Italy). The presented 3D models will be used by cultural heritage conservation authorities for restoration purpose and will available on the Internet for remote inspection.
Wang, Jianwei; Zhang, Yong
2016-01-01
When coming to identify new 2D materials, our intuition would suggest us to look from layered instead of 3D materials. However, since graphite can be hypothetically derived from diamond by stretching it along its [111] axis, many 3D materials can also potentially be explored as new candidates for 2D materials. Using a density functional theory, we perform a systematic study over the common Group IV, III–V, and II–VI semiconductors along different deformation paths to reveal new structures that are topologically connected to but distinctly different from the 3D parent structure. Specifically, we explore two major phase transition paths, originating respectively from wurtzite and NiAs structure, by applying compressive and tensile strain along the symmetry axis, and calculating the total energy changes to search for potential metastable states, as well as phonon spectra to examine the structural stability. Each path is found to further split into two branches under tensile strain–low buckled and high buckled structures, which respectively lead to a low and high buckled monolayer structure. Most promising new layered or planar structures identified include BeO, GaN, and ZnO on the tensile strain side, Ge, Si, and GaP on the compressive strain side. PMID:27090430
Adiabatic photo-steering theory in topological insulators
Inoue, Jun-ichi
2014-01-01
Feasible external control of material properties is a crucial issue in condensed matter physics. A new approach to achieving this aim, named adiabatic photo-steering, is reviewed. The core principle of this scheme is that several material constants are effectively turned into externally tunable variables by irradiation of monochromatic laser light. Two-dimensional topological insulators are selected as the optimal systems that exhibit a prominent change in their properties following the application of this method. Two specific examples of photo-steered quantum phenomena, which reflect topological aspects of the electronic systems at hand, are presented. One is the integer quantum Hall effect described by the Haldane model, and the other is the quantum spin Hall effect described by the Kane–Mele model. The topological quantities associated with these phenomena are the conventional Chern number and spin Chern number, respectively. A recent interesting idea, time-reversal symmetry breaking via a temporary periodic external stimulation, is also discussed. PMID:27877726
Topological Anderson insulators in systems without time-reversal symmetry
NASA Astrophysics Data System (ADS)
Su, Ying; Avishai, Y.; Wang, X. R.
2016-06-01
Occurrence of the topological Anderson insulator (TAI) in a HgTe quantum well suggests that when time-reversal symmetry (TRS) is maintained, the pertinent topological phase transition, marked by re-entrant 2 e2/h quantized conductance contributed by helical edge states, is driven by disorder. Here we show that when TRS is broken, the physics of the TAI becomes even richer. The pattern of longitudinal conductance and nonequilibrium local current distribution displays novel TAI phases characterized by nonzero Chern numbers, indicating the occurrence of multiple chiral edge modes. Tuning either disorder or Fermi energy (in both topologically trivial and nontrivial phases), drives transitions between these distinct TAI phases, characterized by jumps of the quantized conductance from 0 to e2/h and from e2/h to 2 e2/h . An effective medium theory based on the Born approximation yields an accurate description of different TAI phases in parameter space.
Widespread spin polarization effects in photoemission from topological insulators
Jozwiak, C.; Chen, Y. L.; Fedorov, A. V.; Analytis, J. G.; Rotundu, C. R.; Schmid, A. K.; Denlinger, J. D.; Chuang, Y.-D.; Lee, D.-H.; Fisher, I. R.; Birgeneau, R. J.; Shen, Z.-X.; Hussain, Z.; Lanzara, A.
2011-06-22
High-resolution spin- and angle-resolved photoemission spectroscopy (spin-ARPES) was performed on the three-dimensional topological insulator Bi{sub 2}Se{sub 3} using a recently developed high-efficiency spectrometer. The topological surface state's helical spin structure is observed, in agreement with theoretical prediction. Spin textures of both chiralities, at energies above and below the Dirac point, are observed, and the spin structure is found to persist at room temperature. The measurements reveal additional unexpected spin polarization effects, which also originate from the spin-orbit interaction, but are well differentiated from topological physics by contrasting momentum and photon energy and polarization dependencies. These observations demonstrate significant deviations of photoelectron and quasiparticle spin polarizations. Our findings illustrate the inherent complexity of spin-resolved ARPES and demonstrate key considerations for interpreting experimental results.
Virus Enrichment for Single Virus Infection by Using 3D Insulator Based Dielectrophoresis
Masuda, Taisuke; Maruyama, Hisataka; Honda, Ayae; Arai, Fumihito
2014-01-01
We developed an active virus filter (AVF) that enables virus enrichment for single virus infection, by using insulator-based dielectrophoresis (iDEP). A 3D-constricted flow channel design enabled the production of an iDEP force in the microfluidic chip. iDEP using a chip with multiple active virus filters (AVFs) was more accurate and faster than using a chip with a single AVF, and improved the efficiency of virus trapping. We utilized maskless photolithography to achieve the precise 3D gray-scale exposure required for fabrication of constricted flow channel. Influenza virus (A PR/8) was enriched by a negative DEP force when sinusoidal wave was applied to the electrodes within an amplitude range of 20 Vp-p and a frequency of 10 MHz. AVF-mediated virus enrichment can be repeated simply by turning the current ON or OFF. Furthermore, the negative AVF can inhibit virus adhesion onto the glass substrate. We then trapped and transported one of the enriched viruses by using optical tweezers. This microfluidic chip facilitated the effective transport of a single virus from AVFs towards the cell-containing chamber without crossing an electrode. We successfully transported the virus to the cell chamber (v = 10 µm/s) and brought it infected with a selected single H292 cell. PMID:24918921
Kondo-like zero-bias conductance anomaly in a three-dimensional topological insulator nanowire
Cho, Sungjae; Zhong, Ruidan; Schneeloch, John A.; Gu, Genda; Mason, Nadya
2016-01-01
Zero-bias anomalies in topological nanowires have recently captured significant attention, as they are possible signatures of Majorana modes. Yet there are many other possible origins of zero-bias peaks in nanowires—for example, weak localization, Andreev bound states, or the Kondo effect. Here, we discuss observations of differential-conductance peaks at zero-bias voltage in non-superconducting electronic transport through a 3D topological insulator (Bi1.33Sb0.67)Se3 nanowire. The zero-bias conductance peaks show logarithmic temperature dependence and often linear splitting with magnetic fields, both of which are signatures of the Kondo effect in quantum dots. We characterize the zero-bias peaks and discuss their origin. PMID:26911258
Kondo-like zero-bias conductance anomaly in a three-dimensional topological insulator nanowire
Cho, Sungjae; Zhong, Ruidan; Schneeloch, John A.; Gu, Genda; Mason, Nadya
2016-02-25
Zero-bias anomalies in topological nanowires have recently captured significant attention, as they are possible signatures of Majorana modes. Yet there are many other possible origins of zero-bias peaks in nanowires—for example, weak localization, Andreev bound states, or the Kondo effect. Here, we discuss observations of differential-conductance peaks at zero-bias voltage in non-superconducting electronic transport through a 3D topological insulator (Bi_{1.33}Sb_{0.67})Se_{3} nanowire. The zero-bias conductance peaks show logarithmic temperature dependence and often linear splitting with magnetic fields, both of which are signatures of the Kondo effect in quantum dots. As a result, we characterize the zero-bias peaks and discuss their origin.
Massive Dirac Fermion Observed in Lanthanide-Doped Topological Insulator Thin Films.
Harrison, S E; Collins-McIntyre, L J; Schönherr, P; Vailionis, A; Srot, V; van Aken, P A; Kellock, A J; Pushp, A; Parkin, S S P; Harris, J S; Zhou, B; Chen, Y L; Hesjedal, T
2015-10-27
The breaking of time reversal symmetry (TRS) in three-dimensional (3D) topological insulators (TIs), and thus the opening of a 'Dirac-mass gap' in the linearly dispersed Dirac surface state, is a prerequisite for unlocking exotic physical states. Introducing ferromagnetic long-range order by transition metal doping has been shown to break TRS. Here, we present the study of lanthanide (Ln) doped Bi2Te3, where the magnetic doping with high-moment lanthanides promises large energy gaps. Using molecular beam epitaxy, single-crystalline, rhombohedral thin films with Ln concentrations of up to ~35%, substituting on Bi sites, were achieved for Dy, Gd, and Ho doping. Angle-resolved photoemission spectroscopy shows the characteristic Dirac cone for Gd and Ho doping. In contrast, for Dy doping above a critical doping concentration, a gap opening is observed via the decreased spectral intensity at the Dirac point, indicating a topological quantum phase transition persisting up to room-temperature.
Kondo-like zero-bias conductance anomaly in a three-dimensional topological insulator nanowire
Cho, Sungjae; Zhong, Ruidan; Schneeloch, John A.; ...
2016-02-25
Zero-bias anomalies in topological nanowires have recently captured significant attention, as they are possible signatures of Majorana modes. Yet there are many other possible origins of zero-bias peaks in nanowires—for example, weak localization, Andreev bound states, or the Kondo effect. Here, we discuss observations of differential-conductance peaks at zero-bias voltage in non-superconducting electronic transport through a 3D topological insulator (Bi1.33Sb0.67)Se3 nanowire. The zero-bias conductance peaks show logarithmic temperature dependence and often linear splitting with magnetic fields, both of which are signatures of the Kondo effect in quantum dots. As a result, we characterize the zero-bias peaks and discuss their origin.
NASA Astrophysics Data System (ADS)
Adroguer, P.; Liu, Weizhe E.; Culcer, D.; Hankiewicz, E. M.
2015-12-01
The Hikami-Larkin-Nagaoka (HLN) formula [S. Hikami, A. I. Larkin, and Y. Nagaoka, Prog. Theor. Phys. 63, 707 (1980), 10.1143/PTP.63.707] describes the quantum corrections to the magnetoconductivity of a quasi-2D electron gas (quasi-2DEG) with parabolic dispersion. It predicts a crossover from weak localization to antilocalization as a function of the strength of scattering off spin-orbit impurities. Here, we derive the conductivity correction for massless Dirac fermions in 3D topological insulators (TIs) in the presence of spin-orbit impurities. We show that this correction is always positive and therefore we predict weak antilocalization for every value of the spin-orbit disorder. Furthermore, the correction to the diffusion constant is surprisingly linear in the strength of the impurity spin-orbit. Our results call for a reinterpretation of experimental fits for the magnetoconductivity of 3D TIs, which have so far used the standard HLN formula.
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.
Nonlinear conduction via solitons in a topological mechanical insulator
Chen, Bryan Gin-ge; Upadhyaya, Nitin; Vitelli, Vincenzo
2014-01-01
Networks of rigid bars connected by joints, termed linkages, provide a minimal framework to design robotic arms and mechanical metamaterials built of folding components. Here, we investigate a chain-like linkage that, according to linear elasticity, behaves like a topological mechanical insulator whose zero-energy modes are localized at the edge. Simple experiments we performed using prototypes of the chain vividly illustrate how the soft motion, initially localized at the edge, can in fact propagate unobstructed all of the way to the opposite end. Using real prototypes, simulations, and analytical models, we demonstrate that the chain is a mechanical conductor, whose carriers are nonlinear solitary waves, not captured within linear elasticity. Indeed, the linkage prototype can be regarded as the simplest example of a topological metamaterial whose protected mechanical excitations are solitons, moving domain walls between distinct topological mechanical phases. More practically, we have built a topologically protected mechanism that can perform basic tasks such as transporting a mechanical state from one location to another. Our work paves the way toward adopting the principle of topological robustness in the design of robots assembled from activated linkages as well as in the fabrication of complex molecular nanostructures. PMID:25157161
Topological Anderson insulator induced by inter-cell hopping disorder
Lv, Shu-Hui; Song, Juntao Li, Yu-Xian
2013-11-14
We have studied in detail the influence of same-orbit and different-orbit hopping disorders in HgTe/CdTe quantum wells. Intriguingly, similar to the behavior of the on-site Anderson disorder, a phase transition from a topologically trivial phase to a topological phase is induced at a proper strength of the same-orbit hopping disorder. For different-orbit hopping disorder, however, the phase transition does not occur. The results have been analytically verified by using effective medium theory. A consistent conclusion can be obtained by comparing phase diagrams, conductance, and conductance fluctuations. In addition, the influence of Rashba spin-orbit interaction (RSOI) on the system has been studied for different types of disorder, and the RSOI shows different influence on topological phase at different disorders. The topological phase induced by same-orbit hopping disorder is more robust against the RSOI than that induced by on-site Anderson disorder. For different-orbit hopping disorder, no matter whether the RSOI is included or not, the phase transition does not occur. The results indicate, whether or not the topological Anderson insulator can be observed depends on a competition between the different types of the disorder as well as the strength of the RSOI in a system.
β - Ag2Te: A topological insulator with strong anisotropy
NASA Astrophysics Data System (ADS)
Wang, Lan; Sulaev, Azat; Ren, Peng; Xia, Bin; Lin, Qinghua; Yu, Ting; Qiu, Caiyu; Zhang, Shuang-Yuan; Han, Ming-Yong; Li, Zhipeng; Zhu, Wei Guang; Wu, Qingyu; Feng, Yuan Ping; Shen, Lei; Shen, Shun-Qing
2013-03-01
We present evidence of topological surface states in β-Ag2Te through first-principles calculations, periodic quantum interference effect and ambipolar electric field effect in single crystalline nanoribbon. Our first-principles calculations show that β-Ag2Te is a topological insulator with a gapless Dirac cone with strong anisotropy. To experimentally probe the topological surface state, we synthesized high quality β-Ag2Te nanoribbons and performed electron transport measurements. The coexistence of pronounced Aharonov-Bohm oscillations and weak Altshuler-Aronov-Spivak oscillations clearly demonstrates coherent electron transport around the perimeter of β-Ag2Te nanoribbon and therefore the existence of topological surface states, which is further supported by the ambipolar electric field effect for devices fabricated by β-Ag2Te nanoribbons. The experimentally confirmed topological surface states and the theoretically predicted isotropic Dirac cone of β-Ag2Te suggest that the material may be a promising material for fundamental study and future spintronic devices. RCA-08/018 (Singapore), MOE2010-T2-2-059 (Singapore), HKU705150P (Hong Kong), NTU-SUG M4080513
Neupane, Madhab; Xu, Su-Yang; Sankar, R.; Gibson, Q.; Wang, Y. J.; Belopolski, I.; Alidoust, N.; Bian, G.; Shibayev, P. P.; Sanchez, D. S.; Ohtsubo, Y.; Taleb-Ibrahimi, A.; Tsai, W. -F.; Lin, H.; Durakiewicz, Tomasz; Cava, R. J.; Bansil, A.; Chou, F. C.; Hasan, M. Z.
2015-08-20
Here we report the evolution of the surface electronic structure and surface material properties of a topological crystalline insulator (TCI), Pb_{1${-}$x}SnxSe, as a function of various material parameters including composition x, temperature T , and crystal structure. Our spectroscopic data demonstrate the electronic ground-state condition for the saddle point singularity, the tunability of surface chemical potential, and the surface states’ response to circularly polarized light. Our results show that each material parameter can tune the system between the trivial and topological phase in a distinct way, unlike that seen in Bi_{2}Se_{3} and related compounds, leading to a rich topological phase diagram. Our systematic studies of the TCI Pb_{1${-}$x}SnxSe are a valuable materials guide to realize new topological phenomena.
Neupane, Madhab; Xu, Su-Yang; Sankar, R.; ...
2015-08-20
Here we report the evolution of the surface electronic structure and surface material properties of a topological crystalline insulator (TCI), Pb1more » $${-}$$xSnxSe, as a function of various material parameters including composition x, temperature T , and crystal structure. Our spectroscopic data demonstrate the electronic ground-state condition for the saddle point singularity, the tunability of surface chemical potential, and the surface states’ response to circularly polarized light. Our results show that each material parameter can tune the system between the trivial and topological phase in a distinct way, unlike that seen in Bi2Se3 and related compounds, leading to a rich topological phase diagram. Our systematic studies of the TCI Pb1$${-}$$xSnxSe are a valuable materials guide to realize new topological phenomena.« less
Surface-Dominated Transport on a Bulk Topological Insulator
NASA Astrophysics Data System (ADS)
Hofmann, Philip; Barreto, Lucas; Kühnemund, Lisa; Edler, Frederik; Tegenkamp, Christoph; Mi, Jianli; Bremholm, Martin; Brummerstedt Iversen, Bo; Frydendahl, Christian; Bianchi, Marco
2014-03-01
Topological insulators are guaranteed to support metallic surface states on an insulating bulk, and one should thus expect that the electronic transport in these materials is dominated by the surfaces states. Alas, due to the high remaining bulk conductivity, surface contributions to transport have so-far only been singled out indirectly via quantum oscillation, or for devices based on gated and doped topological insulator thin films, a situation in which the surface carrier mobility could be limited by defect and interface scattering. Here we present a direct measurement of surface-dominated conduction on an atomically clean surface of Bi2Te2Se. Using nano-scale four point setups with variable contact distance, we show that the transport at 30 K is two-dimensional rather than three-dimensional and by combining these measurements with angle-resolved photoemission results from the same crystals, we find a surface state mobility of 390(30) cm2V-1s-1 at 30 K at a carrier concentration of 8.71(7) ×1012 cm-2.
NASA Astrophysics Data System (ADS)
Marchewka, Michał
2016-10-01
In this paper the results of the numerical calculation obtained for the three-dimensional (3D) strained Hg1-xCdx Te layers for the x-Cd composition from 0.1 to 0.155 and a different mismatch of the lattice constant are presented. For the investigated region of the Cd composition (x value) the negative energy gap (Eg =Γ8 -Γ6) in the Hg1-xCdx Te is smaller than in the case of pure HgTe which, as it turns out, has a significant influence on the topological surface states (TSS) and the position of the Dirac point. The numerical calculation based on the finite difference method applied for the 8×8 kp model with the in-plane tensile strain for (001) growth oriented structure shows that the Dirac cone inside the induced insulating band gap for non zero of the Cd composition and a bigger strain caused by the bigger lattice mismatch (than for the 3D HgTe TI) can be obtained. It was also shown how different x-Cd compounds move the Dirac cone from the valence band into the band gap. The presented results show that 75 nm wide 3D Hg1-xCdx Te structures with x ≈ 0.155 and 1.6% lattice mismatch make the system a true topological insulator with the dispersion of the topological surface states similar to those ones obtained for the strained CdTe/HgTe QW.
Classification and characterization of topological insulators and superconductors
NASA Astrophysics Data System (ADS)
Mong, Roger
Topological insulators (TIs) are a new class of materials which, until recently, have been overlooked despite decades of study in band insulators. Like semiconductors and ordinary insulators, TIs have a bulk gap, but feature robust surfaces excitations which are protected from disorder and interactions which do not close the bulk gap. TIs are distinguished from ordinary insulators not by the symmetries they possess (or break), but by topological invariants characterizing their bulk band structures. These two pictures, the existence of gapless surface modes, and the nontrivial topology of the bulk states, yield two contrasting approaches to the study of TIs. At the heart of the subject, they are connected by the bulk-boundary correspondence, relating bulk and surface degrees of freedom. In this work, we study both aspects of topological insulators, at the same time providing an illumination to their mysterious connection. First, we present a systematic approach to the classification of bulk states of systems with inversion-like symmetries, deriving a complete set of topological invariants for such ensembles. We find that the topological invariants in all dimensions may be computed algebraically via exact sequences. In particular, systems with spatial inversion symmetries in one-, two-, and three-dimensions can be classified by, respectively, 2, 5, and 11 integer invariants. The values of these integers are related to physical observables such as polarization, Hall conductivity, and magnetoelectric coupling. We also find that, for systems with “antiferromagnetic symmetry,” there is a
Universal anyons at the irradiated surface of topological insulator
Wang, Rui; Chen, Wei; Wang, Baigeng; Xing, D. Y.
2016-01-01
Anyons have recently received great attention due to their promising application in topological quantum computation. The best validated system that enjoys the anyonic excitations are the Laughlin states. The quasi-particles in Laughlin states are neither fermions nor bosons but possess the discrete statistical angle θ = π/m, with m being an integer. Here we report a possible realization of the universal Abelian anyons, whose statistical angle can be tuned continuously by external parameters and can take any arbitrary values interpolating θ = 0 and θ = π. The proposed setup is the surface state of a three dimensional topological insulator driven by an amplitude-modulated circularly-polarized light. It is found that the external field leads to a particular Floquet phase, which is a two-spatial-dimensional analogy of the Weyl semimetal phase in the Floquet first Brillouin zone. The chiral anomaly of this phase results in a U(1) Chern-Simons gauge theory with a tunable Floquet Chern number. Owing to this underlying gauge field theory, the irradiated surface of topological insulator constitutes a promising platform for the observation of the universal anyons. PMID:26830323
Quasiparticle dynamics in reshaped helical Dirac cone of topological insulators.
Miao, Lin; Wang, Z F; Ming, Wenmei; Yao, Meng-Yu; Wang, Meixiao; Yang, Fang; Song, Y R; Zhu, Fengfeng; Fedorov, Alexei V; Sun, Z; Gao, C L; Liu, Canhua; Xue, Qi-Kun; Liu, Chao-Xing; Liu, Feng; Qian, Dong; Jia, Jin-Feng
2013-02-19
Topological insulators and graphene present two unique classes of materials, which are characterized by spin-polarized (helical) and nonpolarized Dirac cone band structures, respectively. The importance of many-body interactions that renormalize the linear bands near Dirac point in graphene has been well recognized and attracted much recent attention. However, renormalization of the helical Dirac point has not been observed in topological insulators. Here, we report the experimental observation of the renormalized quasiparticle spectrum with a skewed Dirac cone in a single Bi bilayer grown on Bi(2)Te(3) substrate from angle-resolved photoemission spectroscopy. First-principles band calculations indicate that the quasiparticle spectra are likely associated with the hybridization between the extrinsic substrate-induced Dirac states of Bi bilayer and the intrinsic surface Dirac states of Bi(2)Te(3) film at close energy proximity. Without such hybridization, only single-particle Dirac spectra are observed in a single Bi bilayer grown on Bi(2)Se(3), where the extrinsic Dirac states Bi bilayer and the intrinsic Dirac states of Bi(2)Se(3) are well separated in energy. The possible origins of many-body interactions are discussed. Our findings provide a means to manipulate topological surface states.
Quasiparticle dynamics in reshaped helical Dirac cone of topological insulators
Miao, Lin; Wang, Z. F.; Ming, Wenmei; Yao, Meng-Yu; Wang, Meixiao; Yang, Fang; Song, Y. R.; Zhu, Fengfeng; Fedorov, Alexei V.; Sun, Z.; Gao, C. L.; Liu, Canhua; Xue, Qi-Kun; Liu, Chao-Xing; Liu, Feng; Qian, Dong; Jia, Jin-Feng
2013-01-01
Topological insulators and graphene present two unique classes of materials, which are characterized by spin-polarized (helical) and nonpolarized Dirac cone band structures, respectively. The importance of many-body interactions that renormalize the linear bands near Dirac point in graphene has been well recognized and attracted much recent attention. However, renormalization of the helical Dirac point has not been observed in topological insulators. Here, we report the experimental observation of the renormalized quasiparticle spectrum with a skewed Dirac cone in a single Bi bilayer grown on Bi2Te3 substrate from angle-resolved photoemission spectroscopy. First-principles band calculations indicate that the quasiparticle spectra are likely associated with the hybridization between the extrinsic substrate-induced Dirac states of Bi bilayer and the intrinsic surface Dirac states of Bi2Te3 film at close energy proximity. Without such hybridization, only single-particle Dirac spectra are observed in a single Bi bilayer grown on Bi2Se3, where the extrinsic Dirac states Bi bilayer and the intrinsic Dirac states of Bi2Se3 are well separated in energy. The possible origins of many-body interactions are discussed. Our findings provide a means to manipulate topological surface states. PMID:23382185
Majorana Fermions at the End of Topological Insulator Nanoribbon
NASA Astrophysics Data System (ADS)
Liu, Xiongjun; Chen, Yuqin
Majorana zero modes can exist at the end of a 1D p-wave SC. 1D semiconductor nanowire approximated a s-wave superconductor is a famous one of those proposals. In which, strong Zeeman field is required to have a large topological region, but unfortunately suppresses superconducting pairing and makes the system more sensitive to disorder. Here we propose a Nanoribbon system made of 2D topological insulator where finite size effect due to the narrow width between two edges plays an important role. A ferromagnetic insulator and an s-wave superconductor are attached at each edge, respectively. We introduce a low energy effective model to investigate the superconducting phase diagram. And, the disorder effect is studied analytically by using the self-consistent Born approximation(SCBA). Furthermore, realistic numerically calculation is carried out with a tight-binding model. We demonstrate that, strong Zeeman field generates a large topological region, and at the same time enhances superconducting pairing and makes the system more immune to disorder.
Fully analytical integration over the 3D volume bounded by the β sphere in topological atoms.
Popelier, Paul L A
2011-11-17
Atomic properties of a topological atom are obtained by 3D integration over the volume of its atomic basin. Algorithms that compute atomic properties typically integrate over two subspaces: the volume bounded by the so-called β sphere, which is centered at the nucleus and completely contained within the atomic basin, and the volume of the remaining part of the basin. Here we show how the usual quadrature over the β sphere volume can be replaced by a fully analytical 3D integration leading to the atomic charge (monopole moment) for s, p, and d functions. Spherical tensor multipole moments have also been implemented and tested up to hexadecupole for s functions only, and up to quadrupole for s and p functions. The new algorithm is illustrated by operating on capped glycine (HF/6-31G, 35 molecular orbitals (MOs), 322 Gaussian primitives, 19 nuclei), the protein crambin (HF/3-21G, 1260 MOs, 5922 primitives and 642 nuclei), and tin (Z = 50) in Sn(2)(CH(3))(2) (B3LYP/cc-pVTZ and LANL2DZ, 59 MOs, 1352 primitives).
CTCF-Mediated Human 3D Genome Architecture Reveals Chromatin Topology for Transcription
Tang, Zhonghui; Luo, Oscar Junhong; Li, Xingwang; Zheng, Meizhen; Zhu, Jacqueline Jufen; Szalaj, Przemyslaw; Trzaskoma, Pawel; Magalska, Adriana; Wlodarczyk, Jakub; Ruszczycki, Blazej; Michalski, Paul; Piecuch, Emaly; Wang, Ping; Wang, Danjuan; Tian, Simon Zhongyuan; Penrad-Mobayed, May; Sachs, Laurent M.; Ruan, Xiaoan; Wei, Chia-Lin; Liu, Edison T.; Wilczynski, Grzegorz M.; Plewczynski, Dariusz; Li, Guoliang; Ruan, Yijun
2015-01-01
Summary Spatial genome organization and its effect on transcription remains a fundamental question. We applied an advanced ChIA-PET strategy to comprehensively map higher-order chromosome folding and specific chromatin interactions mediated by CTCF and RNAPII with haplotype specificity and nucleotide resolution in different human cell lineages. We find that CTCF/cohesin-mediated interaction anchors serve as structural foci for spatial organization of constitutive genes concordant with CTCF-motif orientation, whereas RNAPII interacts within these structures by selectively drawing cell-type-specific genes towards CTCF-foci for coordinated transcription. Furthermore, we show that haplotype-variants and allelic-interactions have differential effects on chromosome configuration influencing gene expression and may provide mechanistic insights into functions associated with disease susceptibility. 3D-genome simulation suggests a model of chromatin folding around chromosomal axes, where CTCF is involved in defining the interface between condensed and open compartments for structural regulation. Our 3D-genome strategy thus provides unique insights in the topological mechanism of human variations and diseases. PMID:26686651
NASA Astrophysics Data System (ADS)
Ludwig, Andreas W. W.
2016-12-01
After briefly recalling the quantum entanglement-based view of topological phases of matter in order to outline the general context, we give an overview of different approaches to the classification problem of topological insulators and superconductors of non-interacting fermions. In particular, we review in some detail general symmetry aspects of the ‘ten-fold way’ which forms the foundation of the classification, and put different approaches to the classification in relationship with each other. We end by briefly mentioning some of the results obtained on the effect of interactions, mainly in three spatial dimensions.
Correlation effects on a topological insulator at finite temperatures
NASA Astrophysics Data System (ADS)
Yoshida, Tsuneya; Fujimoto, Satoshi; Kawakami, Norio
2012-03-01
We analyze the effects of the local Coulomb interaction on a topological band insulator (TBI) by applying the dynamical mean-field theory to a generalized Bernevig-Hughes-Zhang model having electron correlations. It is elucidated how the correlation effects modify electronic properties in the TBI phase at finite temperatures. In particular, the band inversion character of the TBI inevitably leads to the large reduction of the spectral gap via the renormalization effect, which results in the strong temperature dependence of the spin Hall conductivity. We clarify that a quantum phase transition from the TBI to a trivial Mott insulator, if it is nonmagnetic, is of first order with a hysteresis. This is confirmed via the interaction dependence of the double occupancy and the spectral function. A magnetic instability is also addressed. All these results imply that the spectral gap does not close at the transition.
Interacting topological insulator and emergent grand unified theory
NASA Astrophysics Data System (ADS)
You, Yi-Zhuang; Xu, Cenke
2015-03-01
Motivated by the Pati-Salam grand unified theory [J. C. Pati and A. Salam, Phys. Rev. D 10, 275 (1974), 10.1103/PhysRevD.10.275], we study (4 +1 )d topological insulators with SU (4 ) ×SU (2) 1×SU (2) 2 symmetry, whose (3 +1 )d boundary has 16 flavors of left-chiral fermions, which form representations (4 ,2 ,1 ) and (4 ¯,1 ,2 ) . The key result we obtain is that, without any interaction, this topological insulator has a Z classification, namely, any quadratic fermion mass operator at the (3 +1 )d boundary is prohibited by the symmetries listed above; while under interaction, this system becomes trivial, namely, its (3 +1 )d boundary can be gapped out by a properly designed short-range interaction without generating nonzero vacuum expectation value of any fermion bilinear mass, or in other words, its (3 +1 )d boundary can be driven into a "strongly-coupled symmetric gapped (SCSG) phase." Based on this observation, we propose that after coupling the system to a dynamical SU (4 ) ×SU (2) 1×SU (2) 2 lattice gauge field, the Pati-Salam GUT can be fully regularized as the boundary states of a (4 +1 )d topological insulator with a thin fourth spatial dimension, the thin fourth dimension makes the entire system generically a (3 +1 )d system. The mirror sector on the opposite boundary will not interfere with the desired GUT, because the mirror sector is driven to the SCSG phase by a carefully designed interaction and is hence decoupled from the GUT.
The Mott-Hubbard Insulator: localization and topological quantum order
NASA Astrophysics Data System (ADS)
Martin, Richard M.
2010-03-01
An insulating state of condensed matter is characterized by localization of the center of mass of the electrons. This criterion can be addressed in terms of the ground state on a torus with boundary conditions ψK(x1+L,x2, ) = exp( i K L) ψK(x1,x2, ). As shown by Kohn[1], in an insulator the energy is insensitive to K as L ->∞, whereas in an ideal metal it increases as K^2. In addition, Souza, et al. derived expressions for the localization length in terms of the wavefunction as a function of K. The present work generalizes the arguments to provide a fundamental distinction between ``band'' and ``Mott-Hubbard'' insulators. The criteria involve only counting of electrons and experimentally measurable quantities independent of models, and they lead to the requirement that a Mott-Hubbard insulator with no broken local symmetry must have topological quantum order.[4pt] [1] W. Kohn, Phys. Rev. 133, A171 (1964)[0pt] [2] I. Souza, et al., Phys. Rev. B 62, 1666 (2000).
High-Dimensional Topological Insulators with Quaternionic Analytic Landau Levels
NASA Astrophysics Data System (ADS)
Li, Yi; Wu, Congjun
2013-05-01
We study the three-dimensional topological insulators in the continuum by coupling spin-1/2 fermions to the Aharonov-Casher SU(2) gauge field. They exhibit flat Landau levels in which orbital angular momentum and spin are coupled with a fixed helicity. The three-dimensional lowest Landau level wave functions exhibit the quaternionic analyticity as a generalization of the complex analyticity of the two-dimensional case. Each Landau level contributes one branch of gapless helical Dirac modes to the surface spectra, whose topological properties belong to the Z2 class. The flat Landau levels can be generalized to an arbitrary dimension. Interaction effects and experimental realizations are also studied.
Mapping the unconventional orbital texture in topological crystalline insulators
NASA Astrophysics Data System (ADS)
Zeljkovic, Ilija; Okada, Yoshinori; Huang, Cheng-Yi; Sankar, R.; Walkup, Daniel; Zhou, Wenwen; Serbyn, Maksym; Chou, Fangcheng; Tsai, Wei-Feng; Lin, Hsin; Bansil, Arun; Fu, Liang; Hasan, M. Zahid; Madhavan, Vidya
2014-03-01
The newly discovered topological crystalline insulators (TCIs) harbor a complex band structure involving multiple Dirac cones. These materials are potentially highly tunable by external electric field, temperature or strain and could find future applications in field-effect transistors, photodetectors, and nano-mechanical systems. Theoretically, it has been predicted that different Dirac cones, offset in energy and momentum-space, might harbor vastly different orbital character, a unique property which if experimentally realized, would present an ideal platform for accomplishing new spintronic devices. In this study, we unveil the orbital texture in a prototypical TCI Pb1-xSnxSe by using Fourier-transform (FT) scanning tunneling spectroscopy (STS) to measure the interference patterns produced by the scattering of surface state electrons. We discover that the intensity and energy dependences of FTs show distinct characteristics, which can directly be attributed to orbital effects. Our experiments also reveal the complex band topology involving two Lifshitz transitions.
Tuning surface Dirac valleys by strain in topological crystalline insulators
NASA Astrophysics Data System (ADS)
Zhao, Lu; Wang, Jianfeng; Gu, Bing-Lin; Duan, Wenhui
2015-05-01
A topological crystalline insulator has an even number of Dirac cones (i.e., multiple valleys) in its surface band structure, thus potentially leading to valleytronic applications such as graphene. Using the density-functional-theory method, we systematically investigate the strain-induced evolution of topological surface states on the SnTe(111) surface. Our results show that compressive strain can shift the Dirac cones at the Γ ¯ and M ¯ valleys to different extents (even oppositely) in energy, while the tensile strain can induce different band gaps at the valleys due to the enhanced penetration depths of surface states. Exploiting a strain-induced nanostructure with well-defined edges on the (111) surface, we demonstrate strong valley-selective filtering for massless Dirac fermions by dynamically applying local external pressure. Our findings may pave the way for strain-engineered valley-resolved manipulation of Dirac fermions with high tunability and scalability.
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
Photon-modulated impurity scattering on a topological insulator surface
NASA Astrophysics Data System (ADS)
Deng, Ming-Xun; Deng, W. Y.; Shao, D. X.; Wang, Rui-Qiang; Shen, R.; Sheng, L.; Xing, D. Y.
2017-03-01
We consider the Dirac electron scattering off a pointlike impurity absorbed on the surface of a topological insulator, which is irradiated by a beam of circularly polarized light. It is found that the Dirac electron backscattering is allowed even for a nonmagnetic impurity due to the reshuffled spectrum caused by the light, and so exhibits interesting spin texture and Friedel oscillation in the real space. Furthermore, in the charge density of states, the interplay of the light irradiation and impurity scattering can lead to an in-gap bound state around the Dirac point, heavily modulating the Dirac dispersion. We discuss the different scenarios for resonant and off-resonant lights in detail. The impurity scattering feature is sensitive to the parameters of the polarized light, which suggests a possibility to optically manipulate the topological surface states.
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.
Dirac-fermion-induced parity mixing in superconducting topological insulators
NASA Astrophysics Data System (ADS)
Mizushima, Takeshi; Yamakage, Ai; Sato, Masatoshi; Tanaka, Yukio
2014-11-01
We self-consistently study surface states of superconducting topological insulators. We clarify that, if a topologically trivial bulk s -wave pairing symmetry is realized, parity mixing of the pair potential near the surface is anomalously enhanced by surface Dirac fermions, opening an additional surface gap larger than the bulk one. In contrast to classical s -wave superconductors, the resulting surface density of state hosts an extra coherent peak at the induced gap besides a conventional peak at the bulk gap. We also find that no such extra peak appears for odd-parity superconductors with a cylindrical Fermi surface. Our calculation suggests that the simple U-shaped scanning tunneling microscope spectrum in CuxBi2Se3 does not originate from s -wave superconductivity, but can be explained by odd-parity superconductivity with a cylindrical Fermi surface.
Proposed two-dimensional topological insulator in SiTe
NASA Astrophysics Data System (ADS)
Ma, Yandong; Kou, Liangzhi; Dai, Ying; Heine, Thomas
2016-11-01
The two-dimensional (2D) crystal SiTe is identified to be a 2D topological insulator (TI) with bulk band gap of 220 meV by means of first-principles calculations. The synthesis of 2D SiTe has been reported earlier [Phys. Status Solidi RRL 8, 302 (2014), 10.1002/pssr.201409013] as part of a three-dimensional superlattice. The freestanding monolayer is thermally and dynamically stable and only weakly bound within the layered superlattice, offering the possibility of mechanical exfoliation. Our discovery of a topological signature with large band gap raises the expectation that the most apparent showstopper in experimental 2D TI research, the lack of stable materials exposing a quantum spin Hall effect at room temperature, can be overcome. This offers many laboratories an opportunity to participate in investigating exciting new phenomena in condensed matter physics, such as new quasiparticles and dissipationless spin transport.
Exotic quantum phase transitions of strongly interacting topological insulators
NASA Astrophysics Data System (ADS)
Slagle, Kevin; You, Yi-Zhuang; Xu, Cenke
2015-03-01
Using determinant quantum Monte Carlo simulations, we demonstrate that an extended Hubbard model on a bilayer honeycomb lattice has two novel quantum phase transitions. The first is a quantum phase transition between the weakly interacting gapless Dirac fermion phase and a strongly interacting fully gapped and symmetric trivial phase, which cannot be described by the standard Gross-Neveu model. The second is a quantum critical point between a quantum spin Hall insulator with spin Sz conservation and the previously mentioned strongly interacting fully gapped phase. At the latter quantum critical point the single-particle excitations remain gapped, while spin and charge gaps both close. We argue that the first quantum phase transition is related to the Z16 classification of the topological superconductor 3He-B phase with interactions, while the second quantum phase transition is a topological phase transition described by a bosonic O (4 ) nonlinear sigma model field theory with a Θ term.
Inverse spin galvanic effect in topological-insulator based heterostructures
NASA Astrophysics Data System (ADS)
Rodriguez-Vega, Martin; Schwiete, Georg; Sinova, Jairo; Rossi, Enrico
2015-03-01
We study the inverse spin galvanic effect in heterostructures formed by a layer of a three dimensional strong topological insulator (TI) and a magnetic material. We consider different configurations for the heterostructure and for the contacts. We carefully treat the effect on the TI bands of the proximity of a magnetic material and take into account both intra-band and inter-band contributions to the current-induced spin polarization of the TI surface states. Finally, we discuss the relevance of our results for recent experiments. Work supported by ONR-N00014-13-1-0321, ACS-PRF # 53581-DNI5, and the Jeffress Memorial Trust.
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.
Plasmonics in Dirac systems: from graphene to topological insulators.
Stauber, Tobias
2014-03-26
Recent developments in the emerging field of plasmonics in graphene and other Dirac systems are reviewed and a comprehensive introduction to the standard models and techniques is given. In particular, we discuss intrinsic plasmon excitation of single and bilayer graphene via hydrodynamic equations and the random phase approximation, but also comment on double and multilayer structures. Additionally, we address Dirac systems in the retardation limit and also with large spin–orbit coupling including topological insulators. Finally, we summarize basic properties of the charge, current and photon linear response functions in an appendix.
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.
Enhanced thermoelectric performance and anomalous seebeck effects in topological insulators.
Xu, Yong; Gan, Zhongxue; Zhang, Shou-Cheng
2014-06-06
Improving the thermoelectric figure of merit zT is one of the greatest challenges in material science. The recent discovery of topological insulators (TIs) offers new promise in this prospect. In this work, we demonstrate theoretically that zT is strongly size dependent in TIs, and the size parameter can be tuned to enhance zT to be significantly greater than 1. Furthermore, we show that the lifetime of the edge states in TIs is strongly energy dependent, leading to large and anomalous Seebeck effects with an opposite sign to the Hall effect. These striking properties make TIs a promising material for thermoelectric science and technology.
Electrified magnetic catalysis in three-dimensional topological insulators
NASA Astrophysics Data System (ADS)
Gorbar, E. V.; Miransky, V. A.; Shovkovy, I. A.; Sukhachov, P. O.
2016-09-01
The gap equations for the surface quasiparticle propagators in a slab of three-dimensional topological insulator in external electric and magnetic fields perpendicular to the slab surfaces are analyzed and solved. A different type of magnetic catalysis is revealed with the dynamical generation of both Haldane and Dirac gaps. Its characteristic feature manifests itself in the crucial role that the electric field plays in dynamical symmetry breaking and the generation of a Dirac gap in the slab. It is argued that, for a sufficiently large external electric field, the ground state of the system is a phase with a homogeneous surface charge density.
Helical Quantum Edge Gears in 2D Topological Insulators.
Chou, Yang-Zhi; Levchenko, Alex; Foster, Matthew S
2015-10-30
We show that two-terminal transport can measure the Luttinger liquid (LL) parameter K, in helical LLs at the edges of two-dimensional topological insulators (TIs) with Rashba spin-orbit coupling. We consider a Coulomb drag geometry with two coplanar TIs and short-ranged spin-flip interedge scattering. Current injected into one edge loop induces circulation in the second, which floats without leads. In the low-temperature (T→0) perfect drag regime, the conductance is (e^{2}/h)(2K+1)/(K+1). At higher T, we predict a conductivity ~T^{-4K+3}. The conductivity for a single edge is also computed.
Mirror-symmetry protected non-TRIM surface state in the weak topological insulator Bi2TeI
NASA Astrophysics Data System (ADS)
Rusinov, I. P.; Menshchikova, T. V.; Isaeva, A.; Eremeev, S. V.; Koroteev, Yu. M.; Vergniory, M. G.; Echenique, P. M.; Chulkov, E. V.
2016-02-01
Strong topological insulators (TIs) support topological surfaces states on any crystal surface. In contrast, a weak, time-reversal-symmetry-driven TI with at least one non-zero v1, v2, v3 ℤ2 index should host spin-locked topological surface states on the surfaces that are not parallel to the crystal plane with Miller indices (v1 v2 v3). On the other hand, mirror symmetry can protect an even number of topological states on the surfaces that are perpendicular to a mirror plane. Various symmetries in a bulk material with a band inversion can independently preordain distinct crystal planes for realization of topological states. Here we demonstrate the first instance of coexistence of both phenomena in the weak 3D TI Bi2TeI which (v1 v2 v3) surface hosts a gapless spin-split surface state protected by the crystal mirror-symmetry. The observed topological state has an even number of crossing points in the directions of the 2D Brillouin zone due to a non-TRIM bulk-band inversion. Our findings shed light on hitherto uncharted features of the electronic structure of weak topological insulators and open up new vistas for applications of these materials in spintronics.
Mirror-symmetry protected non-TRIM surface state in the weak topological insulator Bi2TeI
Rusinov, I. P.; Menshchikova, T. V.; Isaeva, A.; Eremeev, S. V.; Koroteev, Yu. M.; Vergniory, M. G.; Echenique, P. M.; Chulkov, E. V.
2016-01-01
Strong topological insulators (TIs) support topological surfaces states on any crystal surface. In contrast, a weak, time-reversal-symmetry-driven TI with at least one non-zero v1, v2, v3 ℤ2 index should host spin-locked topological surface states on the surfaces that are not parallel to the crystal plane with Miller indices (v1 v2 v3). On the other hand, mirror symmetry can protect an even number of topological states on the surfaces that are perpendicular to a mirror plane. Various symmetries in a bulk material with a band inversion can independently preordain distinct crystal planes for realization of topological states. Here we demonstrate the first instance of coexistence of both phenomena in the weak 3D TI Bi2TeI which (v1 v2 v3) surface hosts a gapless spin-split surface state protected by the crystal mirror-symmetry. The observed topological state has an even number of crossing points in the directions of the 2D Brillouin zone due to a non-TRIM bulk-band inversion. Our findings shed light on hitherto uncharted features of the electronic structure of weak topological insulators and open up new vistas for applications of these materials in spintronics. PMID:26864814
NASA Astrophysics Data System (ADS)
Metlitski, Max A.; Kane, C. L.; Fisher, Matthew P. A.
2015-09-01
A three-dimensional electron topological insulator (ETI) is a phase of matter protected by particle-number conservation and time-reversal symmetry. It was previously believed that the surface of an ETI must be gapless unless one of these symmetries is broken. A well-known symmetry-preserving, gapless surface termination of an ETI supports an odd number of Dirac cones. In this paper, we deduce a symmetry-respecting, gapped surface termination of an ETI, which carries an intrinsic two-dimensional (2d) topological order, Moore-Read×U (1) -2 . The Moore-Read sector supports non-Abelian charge 1 /4 anyons, while the Abelian, U (1) -2 , (antisemion) sector is electrically neutral. Time-reversal symmetry is implemented in this surface phase in a highly nontrivial way. Moreover, it is impossible to realize this phase strictly in 2d, simultaneously preserving its implementation of both the particle-number and time-reversal symmetries. A one-dimensional (1d) edge on the ETI surface between the topologically ordered phase and the topologically trivial time-reversal-broken phase with a Hall conductivity σx y=1 /2 carries a right-moving neutral Majorana mode, a right-moving bosonic charge mode, and a left-moving bosonic neutral mode. The topologically ordered phase is separated from the surface superconductor by a direct second-order phase transition in the X Y* universality class, which is driven by the condensation of a charge 1 /2 boson, when approached from the topologically ordered side, and proliferation of a flux 4 π (2 h c /e ) vortex, when approached from the superconducting side. In addition, we prove that time-reversal invariant (interacting) electron insulators with no intrinsic topological order and electromagnetic response characterized by a θ angle, θ =π , do not exist if the electrons transform as Kramers singlets under time reversal.
Flux-Fusion Anomaly Test and Bosonic Topological Crystalline Insulators
NASA Astrophysics Data System (ADS)
Hermele, Michael; Chen, Xie
2016-10-01
We introduce a method, dubbed the flux-fusion anomaly test, to detect certain anomalous symmetry fractionalization patterns in two-dimensional symmetry-enriched topological (SET) phases. We focus on bosonic systems with Z2 topological order and a symmetry group of the form G =U (1 )⋊G' , where G' is an arbitrary group that may include spatial symmetries and/or time reversal. The anomalous fractionalization patterns we identify cannot occur in strictly d =2 systems but can occur at surfaces of d =3 symmetry-protected topological (SPT) phases. This observation leads to examples of d =3 bosonic topological crystalline insulators (TCIs) that, to our knowledge, have not previously been identified. In some cases, these d =3 bosonic TCIs can have an anomalous superfluid at the surface, which is characterized by nontrivial projective transformations of the superfluid vortices under symmetry. The basic idea of our anomaly test is to introduce fluxes of the U(1) symmetry and to show that some fractionalization patterns cannot be extended to a consistent action of G' symmetry on the fluxes. For some anomalies, this can be described in terms of dimensional reduction to d =1 SPT phases. We apply our method to several different symmetry groups with nontrivial anomalies, including G =U (1 )×Z2T and G =U (1 )×Z2P , where Z2T and Z2P are time-reversal and d =2 reflection symmetry, respectively.
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.
Numerical Study of a Bosonic Topological Insulator in three dimensions
NASA Astrophysics Data System (ADS)
Geraedts, Scott; Motrunich, Olexei
2014-03-01
We construct a model which realizes a (3+1)-dimensional symmetry-protected topological phase of bosons with U(1) charge conservation and time reversal symmetry, envisioned by A. Vishwanath and T. Senthil [PRX 4 011016]. Our model works by introducing an additional O(3) degree of freedom, and binding its hedgehogs to a species of charged bosons; the continuous symmetry is thus enlarged to SO(3) × U(1) . We study the model using Monte Carlo and determine its bulk phase diagram; the phase where the bound states of hedgehogs and charges condense is the topological phase. We also study surface phase diagram on a (2+1)-dimensional boundary between the topological and trivial insulators. The theory for the surface is the same as for a (2+1)D hedgehog-suppressed non-linear sigma model, which confirms the proposed so-called NCCP1 field theory. We apply a Zeeman field to the surface, which breaks time reversal on the surface only, and observe a surface Hall conductivity which is half of a quantized value allowed for bosons in strictly (2+1)D, thus establishing topological nature of the (3+1)D bulk phase. Support from NSF Grant DMR-1206096; Caltech Institute of Quantum Imformation and Matter, and an NSERC PGS fellowship.
NASA Astrophysics Data System (ADS)
Men'shov, V. N.; Tugushev, V. V.; Chulkov, E. V.
2016-10-01
We theoretically study how magnetic modulation can be used to manipulate the transport properties of heterostructures formed by a thin film of a three-dimensional topological insulator sandwiched between slabs of a normal insulator. Employing the k • p scheme, in the framework of a continual approach, we argue that electron states of the system are spin-polarized when ultrathin magnetic insertions are incorporated into the film. We demonstrate that (i) the spin-polarization magnitude depends strongly on the magnetic insertion position in the film and (ii) there is the optimal insertion position to realize quantum anomalous Hall effect, which is a function of the material parameters, the film thickness and the topological insulator/normal insulator interface potential. For the heterostructure with a pair of symmetrically placed magnetic insertions, we calculate a phase diagram that shows a series of transitions between distinct quantum regimes of transverse conductivity. We provide consistent interpretation of recent experimental findings in the context of our results.
Interacting topological phases in thin films of topological mirror Kondo insulators
NASA Astrophysics Data System (ADS)
Zhang, Rui-Xing; Xu, Cenke; Liu, Chao-Xing
2016-12-01
We study interaction effects on thin films of topological mirror Kondo insulators (TMKIs), where the strong interaction is expected to play an important role. Our study has led to the following results: (i) We identify a rich phase diagram of noninteracting TMKIs with different mirror Chern numbers in the monolayer and bilayer thin films; (ii) we obtain the phase diagram with interaction and identify the regimes of interaction parameters to mimic bosonic symmetry-protected topological phases with either gapless bosonic modes or spontaneous mirror symmetry breaking at the boundary; and (iii) for the spontaneous mirror symmetry-breaking boundary, we also study various domain-wall defects between different mirror symmetry-breaking order parameters at the boundary. Our results reveal that the thin-film TMKI serves as an intriguing platform for experimental studies of interacting topological phases.
Superparamagnetism-induced mesoscopic electron focusing in topological insulators
NASA Astrophysics Data System (ADS)
Sessi, P.; Rüßmann, P.; Bathon, T.; Barla, A.; Kokh, K. A.; Tereshchenko, O. E.; Fauth, K.; Mahatha, S. K.; Valbuena, M. A.; Godey, S.; Glott, F.; Mugarza, A.; Gargiani, P.; Valvidares, M.; Long, N. H.; Carbone, C.; Mavropoulos, P.; Blügel, S.; Bode, M.
2016-08-01
Recently it has been shown that surface magnetic doping of topological insulators induces backscattering of Dirac states which are usually protected by time-reversal symmetry [Sessi et al., Nat. Commun. 5, 5349 (2014), 10.1038/ncomms6349]. Here we report on quasiparticle interference measurements where, by improved Fermi level tuning, strongly focused interference patterns on surface Mn-doped Bi2Te3 could be directly observed by means of scanning tunneling microscopy at 4 K. Ab initio and model calculations reveal that their mesoscopic coherence relies on two prerequisites: (i) a hexagonal Fermi surface with large parallel segments (nesting) and (ii) magnetic dopants which couple to a high-spin state. Indeed, x-ray magnetic circular dichroism shows superparamagnetism even at very dilute Mn concentrations. Our findings provide evidence of strongly anisotropic Dirac-fermion-mediated interactions and demonstrate how spin information can be transmitted over long distances, allowing the design of experiments and devices based on coherent quantum effects in topological insulators.
The half-filled Landau level and topological insulator surfaces
NASA Astrophysics Data System (ADS)
Senthil, T.
The metallic state of the half-filled Landau level - described originally in pioneering work by Halperin , Lee, and Read as a liquid of composite fermions - was proposed recently by Son to be described by a particle-hole symmetric effective field theory distinct from that in the prior literature. This talk will develop a simple picture of the particle-hole symmetric composite fermion through a modification of older pictures as electrically neutral ``dipolar'' particles. This picture, and the proposed particle-hole symmetric theory, will be further substantiated through a recently developed deep connection between the half-filled Landau level and correlated surface states of certain three dimensional topological insulators. The phenomenology of composite fermi liquids (with or without particle-hole symmetry) will be revisited. It will be shown that their heat/electrical transport dramatically violates the conventional Wiedemann-Franz law but satisfies a modified one. References: 1. Chong Wang and T. Senthil, ``Half-filled Landau Level, Topological Insulator Surfaces, and Three Dimensional Quantum Spin Liquids,'' cond-mat arXiv:1507.08290 (2015).
Structure and transport of topological insulators on epitaxial graphene
NASA Astrophysics Data System (ADS)
Kally, James; Reifsnyder Hickey, Danielle; Lin, Yu-Chuan; Richardella, Anthony; Lee, Joon Sue; Robinson, Joshua; Mkhoyan, K. Andre; Samarth, Nitin
Recent advancements in spintronics have shown that a class of materials, topological insulators (TI), can be used as a spin-current generator or detector. Topological insulators have protected surface states with the electron's spin locked to its momentum. To access these surface states, (Bi, Sb)2Te3 can be grown by molecular beam epitaxy to have the Fermi energy near the Dirac point so that transport occurs only through the spin-dependent surface states. Graphene is another 2D material of great interest for spintronics because of its very long spin diffusion length. This is an ideal material to act as a spin channel in devices. The van der Waals nature of the growth exhibited by 2D materials such as (Bi, Sb)2Te3 and graphene allows heterostructures to be formed despite the large lattice mismatch. We explore the structure and transport of (Bi, Sb)2Te3 grown on epitaxial graphene on 6H-SiC substrates for spintronic applications. This work was supported in part by C-SPIN and LEAST, two of the six centers of STARnet, a Semiconductor Research Corporation program, sponsored by MARCO and DARPA.
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.
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.
Enhanced photogalvanic current in topological insulators via Fermi energy tuning
NASA Astrophysics Data System (ADS)
Okada, Ken N.; Ogawa, Naoki; Yoshimi, Ryutaro; Tsukazaki, Atsushi; Takahashi, Kei S.; Kawasaki, Masashi; Tokura, Yoshinori
2016-02-01
We achieve the enhancement of the circular photogalvanic effect arising from the photoinjection of spins in topological insulator thin films by tuning the Fermi level (EF) . A series of (Bi1-xSbx) 2Te3 thin films were tailored so that the Fermi energy ranges above 0.34 eV to below 0.29 eV of the Dirac point, i.e., from the bulk conduction band bottom to the valence band top through the bulk in-gap surface-Dirac cone. The circular photogalvanic current, indicating a flow of spin-polarized surface-Dirac electrons, shows a pronounced peak when the EF is set near the Dirac point and is also correlated with the carrier mobility. Our observation reveals that there are substantial scatterings between the surface-Dirac and bulk state electrons in the generation process of spin-polarized photocurrent, which can be avoided by designing the electronic structure in topological insulators.
Magnetoelectric Effect in Topological Insulator Films Beyond Linear Response Regime
NASA Astrophysics Data System (ADS)
Tretiakov, Oleg; Baasanjav, Dashdeleg; Nomura, Kentaro
2014-03-01
We study the response of topological insulator films to strong magnetic and electric fields beyond the linear response theory. As a model, we use three-dimensional lattice Wilson-Dirac Hamiltonian where we simultaneously introduce both magnetic field as Aharonov Bohm phase and electric field as potential energy depending on lattice coordinate. We compute the energy spectrum by numerically diagonalizing this Hamiltonian for electrons and obtain the quantized magnetoelectric polarizability. In addition, we find that the magnetoelectric effect vanishes as width of the film decreases, due to the hybridization of surface wavefunctions. Furthermore, by applying a gate voltage between the surfaces, we observe multiple quantized plateaus of θ-term. We explain that the multiple quantization rule of θ is mainly determined by the physics of Landau level structures on the top and bottom surfaces of topological insulator, whereas the small deviations from the exact quantization are coming from the asymmetry of the surface wavefunctions in the bulk. We also show that the magnetoelectric effect persists even for strong bulk interactions with magnetic field or magnetic impurities. We acknowledge support by the Grants-in-Aid for Scientific Research (No. 24740211, No. 25800184, and No. 25247056) from the MEXT, Japan.
Two-dimensional topological insulators with large bulk energy gap
NASA Astrophysics Data System (ADS)
Yang, Z. Q.; Jia, Jin-Feng; Qian, Dong
2016-11-01
Two-dimensional (2D) topological insulators (TIs, or quantum spin Hall insulators) are special insulators that possess bulk 2D electronic energy gap and time-reversal symmetry protected one-dimensional (1D) edge state. Carriers in the edge state have the property of spin-momentum locking, enabling dissipation-free conduction along the 1D edge. The existence of 2D TIs was confirmed by experiments in semiconductor quantum wells. However, the 2D bulk gaps in those quantum wells are extremely small, greatly limiting potential application in future electronics and spintronics. Despite this limitation, 2D TIs with a large bulk gap attracted plenty of interest. In this paper, recent progress in searching for TIs with a large bulk gap is reviewed briefly. We start by introducing some theoretical predictions of these new materials and then discuss some recent important achievements in crystal growth and characterization. Project supported by the National Natural Science Foundation of China (Grant Nos. U1632272, 11574201, and 11521404). D. Q. acknowledges support from the Changjiang Scholars Program, China and the Program for Professor of Special Appointment (Eastern Scholar), China.
In-plane topological p-n junction in the three-dimensional topological insulator Bi2-xSbxTe3-ySey
NASA Astrophysics Data System (ADS)
Tu, Ngoc Han; Tanabe, Yoichi; Satake, Yosuke; Huynh, Khuong Kim; Tanigaki, Katsumi
2016-12-01
A topological p-n junction (TPNJ) is an important concept to control spin and charge transport on a surface of three-dimensional topological insulators (3D-TIs). Here we report successful fabrication of such TPNJ on a surface of 3D-TI Bi2-xSbxTe3-ySey thin films and experimental observation of the electrical transport. By tuning the chemical potential of n-type topological Dirac surface of Bi2-xSbxTe3-ySey on its top half by using tetrafluoro-7,7,8,8-tetracyanoquinodimethane as an organic acceptor molecule, a half surface can be converted to p-type with leaving the other half side as the opposite n-type, and consequently TPNJ can be created. By sweeping the back-gate voltage in the field effect transistor structure, the TPNJ was controlled both on the bottom and the top surfaces. A dramatic change in electrical transport observed at the TPNJ on 3D-TI thin films promises novel spin and charge transport of 3D-TIs for future spintronics.
In-plane topological p-n junction in the three-dimensional topological insulator Bi2−xSbxTe3−ySey
Tu, Ngoc Han; Tanabe, Yoichi; Satake, Yosuke; Huynh, Khuong Kim; Tanigaki, Katsumi
2016-01-01
A topological p-n junction (TPNJ) is an important concept to control spin and charge transport on a surface of three-dimensional topological insulators (3D-TIs). Here we report successful fabrication of such TPNJ on a surface of 3D-TI Bi2−xSbxTe3−ySey thin films and experimental observation of the electrical transport. By tuning the chemical potential of n-type topological Dirac surface of Bi2−xSbxTe3−ySey on its top half by using tetrafluoro-7,7,8,8-tetracyanoquinodimethane as an organic acceptor molecule, a half surface can be converted to p-type with leaving the other half side as the opposite n-type, and consequently TPNJ can be created. By sweeping the back-gate voltage in the field effect transistor structure, the TPNJ was controlled both on the bottom and the top surfaces. A dramatic change in electrical transport observed at the TPNJ on 3D-TI thin films promises novel spin and charge transport of 3D-TIs for future spintronics. PMID:27934857
Transport Experiments of Topological Insulators and Dirac Semimetals
NASA Astrophysics Data System (ADS)
Xiong, Jun
The progress in understanding the Berry phase of Bloch electrons in crystals has triggered tremendous interest in discovering novel topological phases of solids. The integration of the Berry curvature in the Brillouin zone can categorize solids into phases such as topological insulators (TI), Dirac semimetals (DSM) and Weyl semimetals (WSM). These new phases have unconventional electronic states at the boundaries, such as the spin polarized electrons on the surface of a three-dimensional TI. Under proper engineering, such edge states can carry a dissipationless current, leading to a great application potential in low-power devices and topological quantum computers. Besides TI, the newly discovered Dirac and Weyl semimetals represent another example in which electrons have a linear energy-momentum dispersion. The paired Weyl nodes have opposite chiralities, and can be regarded as positive and negative monopoles of the Berry flux. Under the time-reversal, inversion and certain crystal symmetries, as in the cases of Cd3As2 and Na3Bi, the Weyl nodes with different chiralities can coexist at the same point in the Brillouin zone and the crystal becomes a Dirac semimetal. Such semimetals provide platforms for some phenomena in high energy physics, such as the chiral anomaly effect. The above predictions lie at the heart of our experimental study of topological materials. We synthesized a topological insulator, Bi2Te2 Se, with a suppressed bulk carrier density. Analysis of the prominent Shubnikov-de Haas oscillations in Bi2Te2Se demonstrates clear evidence for the Dirac surface electrons and their pi Berry phase. We also leveraged the ionic liquid gating technique to bring the chemical potential 50% closer to the Dirac point. Additionally, we studied two types of Na3Bi, a DSM. The first type with a high chemical potential exhibits a large and linear magnetoresistance (MR), implying a transport lifetime steeply tuned by the magnetic field. In the second type of Na3Bi with a
Superradiant Topological Peierls Insulator inside an Optical Cavity
NASA Astrophysics Data System (ADS)
Mivehvar, Farokh; Ritsch, Helmut; Piazza, Francesco
2017-02-01
We consider a spinless ultracold Fermi gas tightly trapped along the axis of an optical resonator and transversely illuminated by a laser closely tuned to a resonator mode. At a certain threshold pump intensity, the homogeneous gas density breaks a Z2 symmetry towards a spatially periodic order, which collectively scatters pump photons into the cavity. We show that this known self-ordering transition also occurs for low field seeking fermionic particles when the laser light is blue detuned to an atomic transition. The emergent superradiant optical lattice in this case is homopolar and possesses two distinct dimerizations. Depending on the spontaneously chosen dimerization, the resulting Bloch bands can have a nontrivial topological structure characterized by a nonvanishing Zak phase. In the case where the Fermi momentum is close to half of the cavity-mode wave number, a Peierls-like instability here creates a topological insulator with a gap at the Fermi surface, which hosts a pair of edge states. The topological features of the system can be nondestructively observed via the cavity output: the Zak phase of the bulk coincides with the relative phase between laser and cavity field, while the fingerprint of edge states can be observed as additional broadening in a well-defined frequency window of the cavity spectrum.
Superradiant Topological Peierls Insulator inside an Optical Cavity.
Mivehvar, Farokh; Ritsch, Helmut; Piazza, Francesco
2017-02-17
We consider a spinless ultracold Fermi gas tightly trapped along the axis of an optical resonator and transversely illuminated by a laser closely tuned to a resonator mode. At a certain threshold pump intensity, the homogeneous gas density breaks a Z_{2} symmetry towards a spatially periodic order, which collectively scatters pump photons into the cavity. We show that this known self-ordering transition also occurs for low field seeking fermionic particles when the laser light is blue detuned to an atomic transition. The emergent superradiant optical lattice in this case is homopolar and possesses two distinct dimerizations. Depending on the spontaneously chosen dimerization, the resulting Bloch bands can have a nontrivial topological structure characterized by a nonvanishing Zak phase. In the case where the Fermi momentum is close to half of the cavity-mode wave number, a Peierls-like instability here creates a topological insulator with a gap at the Fermi surface, which hosts a pair of edge states. The topological features of the system can be nondestructively observed via the cavity output: the Zak phase of the bulk coincides with the relative phase between laser and cavity field, while the fingerprint of edge states can be observed as additional broadening in a well-defined frequency window of the cavity spectrum.
Three-dimensional all-dielectric photonic topological insulator
NASA Astrophysics Data System (ADS)
Slobozhanyuk, Alexey; Mousavi, S. Hossein; Ni, Xiang; Smirnova, Daria; Kivshar, Yuri S.; Khanikaev, Alexander B.
2016-12-01
The discovery of two-dimensional topological photonic systems has transformed our views on the propagation and scattering of electromagnetic waves, and the quest for similar states in three dimensions is open. Here, we theoretically demonstrate that it is possible to design symmetry-protected three-dimensional topological states in an all-dielectric platform, with the electromagnetic duality between electric and magnetic fields being ensured by the structure design. Magneto-electrical coupling plays the role of a synthetic gauge field that determines a topological transition to an 'insulating' regime with a complete three-dimensional photonic bandgap. We reveal the emergence of surface states with conical Dirac dispersion and spin-locking, and we numerically confirm robust propagation of the surface states along two-dimensional domain walls with first-principles studies. The proposed system represents a table-top platform capable of emulating the relativistic dynamics of massive Dirac fermions and the surface states can be interpreted as Jackiw-Rebbi states bound to the interface separating domains with particles of opposite masses.
Stable topological insulators achieved using high energy electron beams
Zhao, Lukas; Konczykowski, Marcin; Deng, Haiming; Korzhovska, Inna; Begliarbekov, Milan; Chen, Zhiyi; Papalazarou, Evangelos; Marsi, Marino; Perfetti, Luca; Hruban, Andrzej; Wołoś, Agnieszka; Krusin-Elbaum, Lia
2016-01-01
Topological insulators are potentially transformative quantum solids with metallic surface states which have Dirac band structure and are immune to disorder. Ubiquitous charged bulk defects, however, pull the Fermi energy into the bulk bands, denying access to surface charge transport. Here we demonstrate that irradiation with swift (∼2.5 MeV energy) electron beams allows to compensate these defects, bring the Fermi level back into the bulk gap and reach the charge neutrality point (CNP). Controlling the beam fluence, we tune bulk conductivity from p- (hole-like) to n-type (electron-like), crossing the Dirac point and back, while preserving the Dirac energy dispersion. The CNP conductance has a two-dimensional character on the order of ten conductance quanta and reveals, both in Bi2Te3 and Bi2Se3, the presence of only two quantum channels corresponding to two topological surfaces. The intrinsic quantum transport of the topological states is accessible disregarding the bulk size. PMID:26961901
Non-commutative geometry in higher dimensional quantum hall effect as A-class topological insulator
NASA Astrophysics Data System (ADS)
Hasebe, K.
2014-09-01
We clarify relations between the higher dimensional quantum Hall effect and A-class topological insulator. In particular, we elucidate physical implications of the higher dimensional non-commutative geometry in the context of A-class topological insulator. This presentation is based on arXiv:1403.5066.
Ramsubhag, Ron R; Massaro, Chelsea L; Dadich, Christina M; Janeczek, Andrew J; Hoang, Tung T; Mazzio, Elizabeth A; Eyunni, Suresh; Soliman, Karam F A; Dudley, Gregory B
2016-08-16
3,3-Dimethylcyclopentanes (neopentylenes) are ubiquitous in Nature but largely absent from synthetic pharmaceutical libraries. Neopentylenes define a hydrophobic and rigid 3-D topology with distinct molecular pharmacology, as exemplified here with two neopentylene-fused analogues of the synthetic anti-inflammatory drug, ibuprofen.
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.
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
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.
Magnetic Topological Insulators and Quantum Anomalous Hall Effect
NASA Astrophysics Data System (ADS)
Kou, Xufeng
The engineering of topological surface states is a key to realize applicable devices based on topological insulators (TIs). Among various proposals, introducing magnetic impurities into TIs has been proven to be an effective way to open a surface gap and integrate additional ferromagnetism with the original topological order. In this Dissertation, we study both the intrinsic electrical and magnetic properties of the magnetic TI thin films grown by molecular beam epitaxy. By doping transition element Cr into the host tetradymite-type V-VI semiconductors, we achieve robust ferromagnetic order with a strong perpendicular magnetic anisotropy. With additional top-gating capability, we realize the electric-field-controlled ferromagnetism in the magnetic TI systems, and demonstrate such magneto-electric effects can be effectively manipulated, depending on the interplays between the band topology, magnetic exchange coupling, and structural engineering. Most significantly, we report the observation of quantum anomalous Hall effect (QAHE) in the Cr-doped (BiSb)2Te3 samples where dissipationless chiral edge conduction is realized in the macroscopic millimeter-size devices without the presence of any external magnetic field, and the stability of the quantized Hall conductance of e2/h is well-maintained as the film thickness varies across the 2D hybridization limit. With additional quantum confinement, we discover the metal-to-insulator switching between two opposite QAHE states, and reveal the universal QAHE phase diagram in the thin magnetic TI samples. In addition to the uniform magnetic TIs, we further investigate the TI/Cr-doped TI bilayer structures prepared by the modulation-doped growth method. By controlling the magnetic interaction profile, we observe the Dirac hole-mediated ferromagnetism and develop an effective way to manipulate its strength. Besides, the giant spin-orbit torque in such magnetic TI-based heterostructures enables us to demonstrate the current
Quantum phenomena in transport measurements of topological insulator nanostructures (Review Article)
NASA Astrophysics Data System (ADS)
Fang, Lei; Kwok, Wai-Kwong
2014-04-01
We review the recent experimental advances on quantum phenomena in transport measurements of topological insulators with emphasis on quantum oscillation, weak antilocalization and Aharonov-Bohm effect and Altshuler-Aronov-Spivak effect. Following a brief introduction on the topic, we discuss the identification of the topological surface state based on quantum phenomena. Research prospect of topological insulators is described at the end of this article.
Tunnel magnetoresistance scan of a pristine three-dimensional topological insulator
NASA Astrophysics Data System (ADS)
Roy, Sthitadhi; Soori, Abhiram; Das, Sourin
2015-01-01
Though the Fermi surface of surface states of a 3D topological insulator (TI) has zero magnetization, an arbitrary segment of the full Fermi surface has a unique magnetic moment consistent with the type of spin-momentum locking in hand. We propose a three-terminal set up, which directly couples to the magnetization of a chosen segment of a Fermi surface hence leading to a finite tunnel magnetoresistance (TMR) response of the nonmagnetic TI surface states, when coupled to spin polarized STM probe. This multiterminal TMR not only provides a unique signature of spin-momentum locking for a pristine TI but also provides a direct measure of momentum resolved out of plane polarization of hexagonally warped Fermi surfaces relevant for Bi2Te3 , which could be as comprehensive as spin-resolved ARPES. Implication of this unconventional TMR is also discussed in the broader context of 2D spin-orbit (SO) materials.
Experimental and density functional study of Mn doped Bi2Te3 topological insulator
NASA Astrophysics Data System (ADS)
Ghasemi, A.; Kepaptsoglou, D.; Figueroa, A. I.; Naydenov, G. A.; Hasnip, P. J.; Probert, M. I. J.; Ramasse, Q.; van der Laan, G.; Hesjedal, T.; Lazarov, V. K.
2016-12-01
We present a nanoscale structural and density functional study of the Mn doped 3D topological insulator Bi2Te3. X-ray absorption near edge structure shows that Mn has valency of nominally 2+. Extended x-ray absorption fine structure spectroscopy in combination with electron energy loss spectroscopy (EELS) shows that Mn is a substitutional dopant of Bi and Te and also resides in the van der Waals gap between the quintuple layers of Bi2Te3. Combination of aberration-corrected scanning transmission electron microscopy and EELS shows that Mn substitution of Te occurs in film regions with increased Mn concentration. First-principles calculations show that the Mn dopants favor octahedral sites and are ferromagnetically coupled.
Group IV semiconductor Ge integration with topological insulator Sb2Te3 for spintronic application
NASA Astrophysics Data System (ADS)
Zheng, Beining; Sun, Yu; Wu, Jie; Han, Mei; Wu, Xiaofeng; Huang, Keke; Feng, Shouhua
2017-03-01
Sb2Te3/Ge heterojunctions were grown on deoxidized GaAs (0 0 1) substrates by molecular beam epitaxy to explore a new type of spin torque device. Despite the large lattice mismatch between Ge and Sb2Te3, the films display highly uniform fabrication and good crystallinity, which have been confirmed by structural characterization. The band structures of Sb2Te3/Ge heterojunctions were investigated by x-ray photoemission spectroscopy and ultraviolet photoemission spectroscopy. Small chemical shift of Sb 3d5/2 indicates that TI conducting surface is not destroyed, and Ge valence band bending contributes to Fermi level depinning. The band offset of Sb2Te3/Ge heterojunctions is different from common gate dielectric/Ge heterojunctions. The integral quality of the heterostructure reveals the potential of combining topological insulators with semiconductors for the advancement of spintronic devices.
70-fs mode-locked erbium-doped fiber laser with topological insulator
Liu, Wenjun; Pang, Lihui; Han, Hainian; Tian, Wenlong; Chen, Hao; Lei, Ming; Yan, Peiguang; Wei, Zhiyi
2016-01-01
Femtosecond optical pulses have applications in optical communication, astronomical frequency combs, and laser spectroscopy. Here, a hybrid mode-locked erbium-doped fiber (EDF) laser with topological insulator (TI) is proposed, for the first time to our best knowledge. The pulsed laser deposition (PLD) method is employed to fabricate the fiber-taper TI saturable absorber (TISA). By virtue of the fiber-taper TISA, the hybrid EDF laser is passively mode-locked using the nonlinear polarization evolution (NPE), and emits 70 fs pulses at 1542 nm, whose 3 dB spectral width is 63 nm with a repetition rate and transfer efficiency of 95.4 MHz and 14.12%, respectively. Our experiments indicate that the proposed hybrid mode-locked EDF lasers have better performance to achieve shorter pulses with higher power and lower mode-locking threshold in the future. PMID:26813439
Two-Dimensional Dirac Fermions in a Topological Insulator: Transport in the Quantum Limit
Analytis, J.G.; McDonald, R.D.; Riggs, S.C.; Chu, J.-H.; Boebinger, G.S.; Fisher, I.R.; /SIMES, Stanford /SLAC /Stanford U., Geballe Lab /Stanford U., Appl. Phys. Dept.
2011-08-12
Pulsed magnetic fields of up to 55T are used to investigate the transport properties of the topological insulator Bi{sub 2}Se{sub 3} in the extreme quantum limit. For samples with a bulk carrier density of n = 2.9 x 10{sup 16} cm{sup -3}, the lowest Landau level of the bulk 3D Fermi surface is reached by a field of 4T. For fields well beyond this limit, Shubnikov-de Haas oscillations arising from quantization of the 2D surface state are observed, with the {nu} = 1 Landau level attained by a field of {approx} 35T. These measurements reveal the presence of additional oscillations which occur at fields corresponding to simple rational fractions of the integer Landau indices.
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
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.
From Graphene and Topological Insulators to Weyl Semimetals
NASA Astrophysics Data System (ADS)
Hills, R. D. Y.; Brada, M.; Liu, Y.; Pierpoint, M.; Sobnack, M. B.; Wu, W. M.; Kusmartsev, F. V.
Here we present a short introduction into physics of Dirac materials. In particular we review main physical properties of various two-dimensional crystals such as graphene, silicene, germanene and others. We comment on the origin of their buckled two-dimensional shape, and address the issues created by Mermin-Wagner theorem prohibiting the existence of strictly two-dimensional, flat crystals. Then we describe main ideas which were leading to the discovery of two and three-dimensional topological insulators and Weyl fermions. We describe some of their outstanding electronic properties which have been originating due to the existence of the Dirac gapless spectrum. We also compare simplest devices made of Dirac materials. Analogies and differences between Dirac materials and optics are also discussed.
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.
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.
Linear magnetoresistance in a topological insulator Ru2Sn3
NASA Astrophysics Data System (ADS)
Shiomi, Y.; Saitoh, E.
2017-03-01
We have studied magnetotransport properties of a topological insulator material Ru2Sn3. Bulk single crystals of Ru2Sn3 were grown by a Bi flux method. The resistivity is semiconducting at high temperatures above 160 K, while it becomes metallic below 160 K. Nonlinear field dependence of Hall resistivity in the metallic region shows conduction of multiple carriers at low temperatures. In the high-temperature semiconducting region, magnetoresistance exhibits a conventional quadratic magnetic-field dependence. In the low-temperature metallic region, however, high-field magnetoresistance is clearly linear with magnetic fields, signaling a linear dispersion in the low-temperature electronic structure. Small changes in the magnetoresistance magnitude with respect to the magnetic field angle indicate that bulk electron carriers are responsible mainly for the observed linear magnetoresistance.
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
Statistical translation invariance protects a topological insulator from interactions
NASA Astrophysics Data System (ADS)
Milsted, A.; Seabra, L.; Fulga, I. C.; Beenakker, C. W. J.; Cobanera, E.
2015-08-01
We investigate the effect of interactions on the stability of a disordered, two-dimensional topological insulator realized as an array of nanowires or chains of magnetic atoms on a superconducting substrate. The Majorana zero-energy modes present at the ends of the wires overlap, forming a dispersive edge mode with thermal conductance determined by the central charge c of the low-energy effective field theory of the edge. We show numerically that, in the presence of disorder, the c =1 /2 Majorana edge mode remains delocalized up to extremely strong attractive interactions, while repulsive interactions drive a transition to a c =3 /2 edge phase localized by disorder. The absence of localization for strong attractive interactions is explained by a self-duality symmetry of the statistical ensemble of disorder configurations and of the edge interactions, originating from translation invariance on the length scale of the underlying mesoscopic array.
Absorption of surface acoustic waves by topological insulator thin films
Li, L. L.; Xu, W.
2014-08-11
We present a theoretical study on the absorption of the surface acoustic waves (SAWs) by Dirac electrons in topological insulator (TI) thin films (TITFs). We find that due to momentum and energy conservation laws, the absorption of the SAWs in TITFs can only be achieved via intra-band electronic transitions. The strong absorption can be observed up to sub-terahertz frequencies. With increasing temperature, the absorption intensity increases significantly and the cut-off frequency is blue-shifted. More interestingly, we find that the absorption of the SAWs by the TITFs can be markedly enhanced by the tunable subgap in the Dirac energy spectrum of the TI surface states. Such a subgap is absent in conventional two-dimensional electron gases (2DEGs) and in the gapless Dirac 2DEG such as graphene. This study is pertinent to the exploration of the acoustic properties of TIs and to potential application of TIs as tunable SAW devices working at hypersonic frequencies.
Absorption of surface acoustic waves by topological insulator thin films
NASA Astrophysics Data System (ADS)
Li, L. L.; Xu, W.
2014-08-01
We present a theoretical study on the absorption of the surface acoustic waves (SAWs) by Dirac electrons in topological insulator (TI) thin films (TITFs). We find that due to momentum and energy conservation laws, the absorption of the SAWs in TITFs can only be achieved via intra-band electronic transitions. The strong absorption can be observed up to sub-terahertz frequencies. With increasing temperature, the absorption intensity increases significantly and the cut-off frequency is blue-shifted. More interestingly, we find that the absorption of the SAWs by the TITFs can be markedly enhanced by the tunable subgap in the Dirac energy spectrum of the TI surface states. Such a subgap is absent in conventional two-dimensional electron gases (2DEGs) and in the gapless Dirac 2DEG such as graphene. This study is pertinent to the exploration of the acoustic properties of TIs and to potential application of TIs as tunable SAW devices working at hypersonic frequencies.
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.
Electrostatic coupling between two surfaces of a topological insulator nanodevice.
Fatemi, Valla; Hunt, Benjamin; Steinberg, Hadar; Eltinge, Stephen L; Mahmood, Fahad; Butch, Nicholas P; Watanabe, Kenji; Taniguchi, Takashi; Gedik, Nuh; Ashoori, Raymond C; Jarillo-Herrero, Pablo
2014-11-14
We report on electronic transport measurements of dual-gated nanodevices of the low-carrier density topological insulator (TI) Bi_{1.5}Sb_{0.5}Te_{1.7}Se_{1.3}. In all devices, the upper and lower surface states are independently tunable to the Dirac point by the top and bottom gate electrodes. In thin devices, electric fields are found to penetrate through the bulk, indicating finite capacitive coupling between the surface states. A charging model allows us to use the penetrating electric field as a measurement of the intersurface capacitance C_{TI} and the surface state energy-density relationship μ(n), which is found to be consistent with independent angle-resolved photoemission spectroscopy measurements. At high magnetic fields, increased field penetration through the surface states is observed, strongly suggestive of the opening of a surface state band gap due to broken time-reversal symmetry.
Cyclotron resonance in topological insulators: non-relativistic effects
NASA Astrophysics Data System (ADS)
Tabert, C. J.; Carbotte, J. P.
2015-09-01
The low-energy Hamiltonian used to describe the dynamics of the helical Dirac fermions on the surface of a topological insulator contains a subdominant non-relativistic (Schrödinger) contribution. This term can have an important effect on some properties while having no effect on others. The Hall plateaus retain the same relativistic quantization as the pure Dirac case. The height of the universal interband background conductivity is unaltered, but its onset is changed. However, the non-relativistic term leads directly to particle-hole asymmetry. It also splits the interband magneto-optical lines into doublets. Here, we find that, while the shape of the semiclassical cyclotron resonance line is unaltered, the cyclotron frequency and its optical spectral weight are changed. There are significant differences in both of these quantities for a fixed value of chemical potential or fixed doping away from charge neutrality depending on whether the Fermi energy lies in the valence or conduction band.
Collective modes at a surface of a topological insulator
NASA Astrophysics Data System (ADS)
Wu, Jhih-Sheng; Fogler, M. M.; Basov, D. N.
2015-03-01
We investigate hybrid plasmon-phonon modes of a polar topological insulator that originate from interaction among the quasiparticles of surface and bulk states, and also optical phonons. As an example, we study electron-doped Bi2Se3. We analyze the dispersion of the collective modes of this compound for (i) a bulk sample with a depletion layer created by acceptor adsorbates on the surface and (ii) a thin film. In the first case, we show that a depletion layer gives rise to two energy-momentum regions, where the surface states dominate the collective modes over the bulk carriers. In a thin film, the phonons are more prominent than the bulk carriers. The anisotropy of the phonon response makes the thin film behaves as a waveguide. We discuss how these various collective modes can be detected by scanning near-field optical microscopy. Supported by ONR and UCOP.
Weyl semimetal emerging from LaBiTe3-class topological insulators
NASA Astrophysics Data System (ADS)
Liu, Jianpeng; Vanderbilt, David
2014-03-01
We study the topological-to-normal transition in LaBiTe3 and LuBiTe3 by tuning the strength of the spin-orbit coupling (SOC). For centrosymmetric 3D topological insulators (TIs), the strong Z2 index can be changed only by an accidental band touching at an odd number of time-reversal invariant momenta in the Brillouin zone (BZ), achieved at some critical value of an external parameter λ. These band-touching points (BTPs) are ``Dirac-like,'' carrying zero chiral charge. For general noncentrosymmetric TIs, however, one expects to see a stable Weyl semimetal phase over some finite interval of λ. As λ is varied, one expects first the appearance of 2(2 n + 1) Dirac-like BTPs in the BZ, which then split into pairs of Weyl points carrying opposite chiral charges. These BTPs then migrate in the BZ and finally annihilate after exchanging partners, leaving behind an inverted strong Z2 index. Based on first-principles calculations, we predict that this phenomenon can be realized as the SOC is tuned in LaBiTe3 and LuBiTe3. We also construct a low-energy effective model to describe the topological phases in these materials. Preliminary results suggest that other interesting phases could be observed when a Zeeman field is applied.
Wojek, B M; Berntsen, M H; Jonsson, V; Szczerbakow, A; Dziawa, P; Kowalski, B J; Story, T; Tjernberg, O
2015-10-13
Since the advent of topological insulators hosting Dirac surface states, efforts have been made to gap these states in a controllable way. A new route to accomplish this was opened up by the discovery of topological crystalline insulators where the topological states are protected by crystal symmetries and thus prone to gap formation by structural changes of the lattice. Here we show a temperature-driven gap opening in Dirac surface states within the topological crystalline insulator phase in (Pb,Sn)Se. By using angle-resolved photoelectron spectroscopy, the gap formation and mass acquisition is studied as a function of composition and temperature. The resulting observations lead to the addition of a temperature- and composition-dependent boundary between massless and massive Dirac states in the topological phase diagram for (Pb,Sn)Se (001). Overall, our results experimentally establish the possibility to tune between massless and massive topological states on the surface of a topological system.
Thermoelectric properties of topological insulator BaSn2
NASA Astrophysics Data System (ADS)
Guo, San-Dong; Qiu, Liang
2017-01-01
Recently, \\text{BaS}{{\\text{n}}2} has been predicted to be a strong topological insulator by the first-principle calculations. It is well known that topological insulators have a close connection to thermoelectric materials, such as the \\text{B}{{\\text{i}}2}\\text{T}{{\\text{e}}3} family. In this work, we investigate thermoelectric properties of \\text{BaS}{{\\text{n}}2} by the first-principles calculations combined with the Boltzmann transport theory. The electronic part is carried out by a modified Becke and Johnson (mBJ) exchange potential, including spin-orbit coupling (SOC), while the phonon part is performed using a generalized gradient approximation (GGA). It was found that the electronic transport coefficients between the in-plane and cross-plane directions showed strong anisotropy, while lattice-lattice thermal conductivities demonstrated almost complete isotropy. Calculated results revealed a very low lattice thermal conductivity for \\text{BaS}{{\\text{n}}2} , and the corresponding average lattice thermal conductivity at room temperature is 1.69 \\text{W}~{{\\text{m}}-1}~{{\\text{K}}-1} , which is comparable or lower than those of lead chalcogenides and bismuth-tellurium systems as classic thermoelectric materials. Due to the complicated scattering mechanism, calculating the scattering time τ is challenging. By using an empirical τ ={{10}-14} s, the n-type figure of merit ZT is greater than 0.40 in wide temperature ranges. Experimentally, it is possible to attain better thermoelectric performance by strain or tuning size parameters. This work indicates that \\text{BaS}{{\\text{n}}2} may be a potential thermoelectric material, which can stimulate further theoretical and experimental work.
Room Temperature Electrical Detection of Spin Polarized Currents in Topological Insulators.
Dankert, André; Geurs, Johannes; Kamalakar, M Venkata; Charpentier, Sophie; Dash, Saroj P
2015-12-09
Topological insulators (TIs) are a new class of quantum materials that exhibit a current-induced spin polarization due to spin-momentum locking of massless Dirac Fermions in their surface states. This helical spin polarization in three-dimensional (3D) TIs has been observed using photoemission spectroscopy up to room temperatures. Recently, spin polarized surface currents in 3D TIs were detected electrically by potentiometric measurements using ferromagnetic detector contacts. However, these electric measurements are so far limited to cryogenic temperatures. Here we report the room temperature electrical detection of the spin polarization on the surface of Bi2Se3 by employing spin sensitive ferromagnetic tunnel contacts. The current-induced spin polarization on the Bi2Se3 surface is probed by measuring the magnetoresistance while switching the magnetization direction of the ferromagnetic detector. A spin resistance of up to 70 mΩ is measured at room temperature, which increases linearly with current bias, reverses sign with current direction, and decreases with higher TI thickness. The magnitude of the spin signal, its sign, and control experiments, using different measurement geometries and interface conditions, rule out other known physical effects. These findings provide further information about the electrical detection of current-induced spin polarizations in 3D TIs at ambient temperatures and could lead to innovative spin-based technologies.
Noblet, Vincent; Heinrich, Christian; Heitz, Fabrice; Armspach, Jean-Paul
2005-05-01
This paper deals with topology preservation in three-dimensional (3-D) deformable image registration. This work is a nontrivial extension of, which addresses the case of two-dimensional (2-D) topology preserving mappings. In both cases, the deformation map is modeled as a hierarchical displacement field, decomposed on a multiresolution B-spline basis. Topology preservation is enforced by controlling the Jacobian of the transformation. Finding the optimal displacement parameters amounts to solving a constrained optimization problem: The residual energy between the target image and the deformed source image is minimized under constraints on the Jacobian. Unlike the 2-D case, in which simple linear constraints are derived, the 3-D B-spline-based deformable mapping yields a difficult (until now, unsolved) optimization problem. In this paper, we tackle the problem by resorting to interval analysis optimization techniques. Care is taken to keep the computational burden as low as possible. Results on multipatient 3-D MRI registration illustrate the ability of the method to preserve topology on the continuous image domain.
NASA Astrophysics Data System (ADS)
Chen, M. N.; Su, W.; Deng, M. X.; Ruan, Jiawei; Luo, W.; Shao, D. X.; Sheng, L.; Xing, D. Y.
2016-11-01
A great deal of attention has been paid to the topological phases engineered by photonics over the past few years. Here, we propose a topological quantum phase transition to a quantum anomalous Hall (QAH) phase induced by off-resonant circularly polarized light in a two-dimensional system that is initially in a quantum spin Hall phase or a trivial insulator phase. This provides an alternative method to realize the QAH effect, other than magnetic doping. The circularly polarized light effectively creates a Zeeman exchange field and a renormalized Dirac mass, which are tunable by varying the intensity of the light and drive the quantum phase transition. Both the transverse and longitudinal Hall conductivities are studied, and the former is consistent with the topological phase transition when the Fermi level lies in the band gap. A highly controllable spin-polarized longitudinal electrical current can be generated when the Fermi level is in the conduction band, which may be useful for designing topological spintronics.
Nonlinear dynamics induced anomalous Hall effect in topological insulators.
Wang, Guanglei; Xu, Hongya; Lai, Ying-Cheng
2016-01-28
We uncover an alternative mechanism for anomalous Hall effect. In particular, we investigate the magnetisation dynamics of an insulating ferromagnet (FM) deposited on the surface of a three-dimensional topological insulator (TI), subject to an external voltage. The spin-polarised current on the TI surface induces a spin-transfer torque on the magnetisation of the top FM while its dynamics can change the transmission probability of the surface electrons through the exchange coupling and hence the current. We find a host of nonlinear dynamical behaviors including multistability, chaos, and phase synchronisation. Strikingly, a dynamics mediated Hall-like current can arise, which exhibits a nontrivial dependence on the channel conductance. We develop a physical understanding of the mechanism that leads to the anomalous Hall effect. The nonlinear dynamical origin of the effect stipulates that a rich variety of final states exist, implying that the associated Hall current can be controlled to yield desirable behaviors. The phenomenon can find applications in Dirac-material based spintronics.
Nonlinear dynamics induced anomalous Hall effect in topological insulators
NASA Astrophysics Data System (ADS)
Wang, Guanglei; Xu, Hongya; Lai, Ying-Cheng
2016-01-01
We uncover an alternative mechanism for anomalous Hall effect. In particular, we investigate the magnetisation dynamics of an insulating ferromagnet (FM) deposited on the surface of a three-dimensional topological insulator (TI), subject to an external voltage. The spin-polarised current on the TI surface induces a spin-transfer torque on the magnetisation of the top FM while its dynamics can change the transmission probability of the surface electrons through the exchange coupling and hence the current. We find a host of nonlinear dynamical behaviors including multistability, chaos, and phase synchronisation. Strikingly, a dynamics mediated Hall-like current can arise, which exhibits a nontrivial dependence on the channel conductance. We develop a physical understanding of the mechanism that leads to the anomalous Hall effect. The nonlinear dynamical origin of the effect stipulates that a rich variety of final states exist, implying that the associated Hall current can be controlled to yield desirable behaviors. The phenomenon can find applications in Dirac-material based spintronics.
Chiral exciton in the topological insulator Bi2Se3
NASA Astrophysics Data System (ADS)
Kung, Hsiang-Hsi; Salehi, Maryam; Wang, Xueyun; Koirala, Nikesh; Brahlek, Matthew; Lee, Alexander; Cheong, Sang-Wook; Oh, Seongshik; Blumberg, Girsh
Materials with novel band structures can host ``chiral excitons'', where the exciton emission preserves the helicity of the excitation photon, as recently demonstrated in transition metal dichalcogenide monolayers. Here, we report the observation of a highly polarized photoluminescence peak, which is due to chiral exciton emission in the topological insulator Bi2Se3. Surprisingly, the energy of the emission is centered at 2.26 eV, much higher than the 0.3 eV bulk band gap of Bi2Se3. The excitation profile shows maximum polarization around 2.60 eV excitation, suggesting the chiral exciton is due to interband transition between the topological surface states and a bulk band. We demonstrate that the polarization of the exciton emission is insensitive to temperature and Bi2Se3 film thickness, providing a convenient and robust platform for optoelectronic applications. Gb, HHK and AL acknowledge support from NSF Award DMR-1104884. MS, NK, MB and SO are funded by Gordon and Betty Moore Foundation's EPiQS initiative (GBMF4418) and NSF(DMR-1308142). XYW and SWC acknowledge support from NSF Award DMREF-1233349.
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.
Encapsulated silicene: A robust large-gap topological insulator
Kou, Liangzhi; Ma, Yandong; Yan, Binghai; ...
2015-08-20
The quantum spin Hall (QSH) effect predicted in silicene has raised exciting prospects of new device applications compatible with current microelectronic technology. Efforts to explore this novel phenomenon, however, have been impeded by fundamental challenges imposed by silicene’s small topologically nontrivial band gap and fragile electronic properties susceptible to environmental degradation effects. Here we propose a strategy to circumvent these challenges by encapsulating silicene between transition-metal dichalcogenides (TMDCs) layers. First-principles calculations show that such encapsulated silicene exhibit a two-orders-of-magnitude enhancement in its nontrivial band gap, which is driven by the strong spin–orbit coupling effect in TMDCs via the proximity effect.more » Moreover, the cladding TMDCs layers also shield silicene from environmental gases that are detrimental to the QSH state in free-standing silicene. In conclusion, the encapsulated silicene represents a novel two-dimensional topological insulator with a robust nontrivial band gap suitable for room-temperature applications, which has significant implications for innovative QSH device design and fabrication.« less
Encapsulated silicene: A robust large-gap topological insulator
Kou, Liangzhi; Ma, Yandong; Yan, Binghai; Tan, Xin; Chen, Changfeng; Smith, Sean C.
2015-08-20
The quantum spin Hall (QSH) effect predicted in silicene has raised exciting prospects of new device applications compatible with current microelectronic technology. Efforts to explore this novel phenomenon, however, have been impeded by fundamental challenges imposed by silicene’s small topologically nontrivial band gap and fragile electronic properties susceptible to environmental degradation effects. Here we propose a strategy to circumvent these challenges by encapsulating silicene between transition-metal dichalcogenides (TMDCs) layers. First-principles calculations show that such encapsulated silicene exhibit a two-orders-of-magnitude enhancement in its nontrivial band gap, which is driven by the strong spin–orbit coupling effect in TMDCs via the proximity effect. Moreover, the cladding TMDCs layers also shield silicene from environmental gases that are detrimental to the QSH state in free-standing silicene. In conclusion, the encapsulated silicene represents a novel two-dimensional topological insulator with a robust nontrivial band gap suitable for room-temperature applications, which has significant implications for innovative QSH device design and fabrication.
Nonadiabatic bulk-surface oscillations in driven topological insulators
NASA Astrophysics Data System (ADS)
Kolodrubetz, Michael; Fregoso, Benjamin M.; Moore, Joel E.
2016-11-01
Recent theoretical and experimental work has suggested the tantalizing possibility of opening a topological gap upon driving the surface states of a three-dimensional strong topological insulator (TI) with circularly polarized light. With this motivation, we study the response of TIs to a driving field that couples to states near the surface. We unexpectedly find coherent oscillations between the surface and the bulk and trace their appearance to unavoidable resonances caused by photon absorption from the drive. We show how these resonant oscillations may be captured by the Demkov-Osherov model of multilevel Landau-Zener physics, leading to nontrivial consequences such as the loss of adiabaticity upon slow ramping of the amplitude. We numerically demonstrate that these oscillations are observable in the time-dependent Wigner distribution, which is directly measurable in time-resolved angle-resolved photoemission spectroscopy (ARPES) experiments. Our results apply to any system with surface states in the presence of a gapped bulk, and thus suggest experimental signatures of a generic surface-bulk coupling mechanism that is fundamental for proposals to engineer nontrivial states by periodic driving.
(1 + 3)-D topological superconductors: Screening and confinement in the presence of external fields
NASA Astrophysics Data System (ADS)
Gaete, Patricio; Helayël-Neto, José A.
2016-12-01
Adopting the gauge-invariant and path-dependent variables formalism, we compute the interaction energy for a topological field theory describing (1 + 3)-D topological superconductors in the presence of external fields. As a result, in the case of a constant electric-field strength expectation value, we show that the interaction energy describes a purely screening phase, encoded in a Yukawa potential. On the other hand, in the case of a constant magnetic-field strength and for a very small Josephson coupling constant, the particle-antiparticle binding potential displays a linear term leading to the confinement of static charge probes along with a screening contribution.
Strain in a silicon-on-insulator nanostructure revealed by 3D x-ray Bragg ptychography
Chamard, V.; Allain, M.; Godard, P.; Talneau, A.; Patriarche, G.; Burghammer, M.
2015-01-01
Progresses in the design of well-defined electronic band structure and dedicated functionalities rely on the high control of complex architectural device nano-scaled structures. This includes the challenging accurate description of strain fields in crystalline structures, which requires non invasive and three-dimensional (3D) imaging methods. Here, we demonstrate in details how x-ray Bragg ptychography can be used to quantify in 3D a displacement field in a lithographically patterned silicon-on-insulator structure. The image of the crystalline properties, which results from the phase retrieval of a coherent intensity data set, is obtained from a well-controlled optimized process, for which all steps are detailed. These results confirm the promising perspectives of 3D Bragg ptychography for the investigation of complex nano-structured crystals in material science. PMID:25984829
Topology-based Simplification for Feature Extraction from 3D Scalar Fields
Gyulassy, A; Natarajan, V; Pascucci, V; Bremer, P; Hamann, B
2005-10-13
This paper describes a topological approach for simplifying continuous functions defined on volumetric domains. We present a combinatorial algorithm that simplifies the Morse-Smale complex by repeated application of two atomic operations that removes pairs of critical points. The Morse-Smale complex is a topological data structure that provides a compact representation of gradient flows between critical points of a function. Critical points paired by the Morse-Smale complex identify topological features and their importance. The simplification procedure leaves important critical points untouched, and is therefore useful for extracting desirable features. We also present a visualization of the simplified topology.
Observation of a topological 3D Dirac semimetal phase in high-mobility Cd3As2
NASA Astrophysics Data System (ADS)
Neupane, M.; Xu, S.-Y.; Sankar, R.; Alidoust, N.; Bian, G.; Liu, Chang; Belopolski, I.; Chang, T.-R.; Jeng, H.-T.; Lin, H.; Bansil, A.; Chou, Fangcheng; Hasan, M. Z.
2014-03-01
Experimental identification of three-dimensional (3D) Dirac semimetals in solid state systems is critical for realizing exotic topological phenomena and quantum transport. Using high-resolution angle-resolved photoemission spectroscopy, we performed systematic electronic structure studies on well-known compound Cd3As2. For the first time, we observe a highly linear bulk Dirac cone located at the Brillouin zone center projected onto the (001) surface, which is consistent with a 3D Dirac semimetal phase in Cd3As2. Remarkably, an unusually high Dirac Fermion velocity is seen in samples where the mobility far exceeds 20,000 cm2/V.s suggesting that Cd3As2 can be a promising candidate as a hypercone analog of graphene in many device-applications, which can also incorporate topological quantum phenomena in a large gap setting. This work is primarily supported by U.S. DOE and Princeton University.
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.
Magnetic transitions in the topological magnon insulator Cu(1,3-bdc)
NASA Astrophysics Data System (ADS)
Chisnell, R.; Helton, J. S.; Freedman, D. E.; Singh, D. K.; Demmel, F.; Stock, C.; Nocera, D. G.; Lee, Y. S.
2016-06-01
Topological magnon insulators are a new class of magnetic materials that possess topologically nontrivial magnon bands. As a result, magnons in these materials display properties analogous to those of electrons in topological insulators. Here we present magnetization, specific heat, and neutron scattering measurements of the ferromagnetic kagome magnet Cu(1,3-bdc). Our measurements provide a detailed description of the magnetic structure and interactions in this material and confirm that it is an ideal prototype for topological magnon physics in a system with a simple spin Hamiltonian.
Okada, Yoshinori; Dhital, Chetan; Zhou, Wenwen; Huemiller, Erik D; Lin, Hsin; Basak, S; Bansil, A; Huang, Y-B; Ding, H; Wang, Z; Wilson, Stephen D; Madhavan, V
2011-05-20
We study interference patterns of a magnetically doped topological insulator Bi(2-x)Fe(x)Te(3+d) by using Fourier transform scanning tunneling spectroscopy and observe several new scattering channels. A comparison with angle-resolved photoemission spectroscopy allows us to unambiguously ascertain the momentum-space origin of distinct dispersing channels along high-symmetry directions and identify those originating from time-reversal symmetry breaking. Our analysis also reveals that the surface state survives far above the energy where angle-resolved photoemission spectroscopy finds the onset of continuum bulk bands.
Self-organized pseudo-graphene on grain boundaries in topological band insulators
NASA Astrophysics Data System (ADS)
Slager, Robert-Jan; Juričić, Vladimir; Lahtinen, Ville; Zaanen, Jan
2016-06-01
Semimetals are characterized by nodal band structures that give rise to exotic electronic properties. The stability of Dirac semimetals, such as graphene in two spatial dimensions, requires the presence of lattice symmetries, while akin to the surface states of topological band insulators, Weyl semimetals in three spatial dimensions are protected by band topology. Here we show that in the bulk of topological band insulators, self-organized topologically protected semimetals can emerge along a grain boundary, a ubiquitous extended lattice defect in any crystalline material. In addition to experimentally accessible electronic transport measurements, these states exhibit a valley anomaly in two dimensions influencing edge spin transport, whereas in three dimensions they appear as graphenelike states that may exhibit an odd-integer quantum Hall effect. The general mechanism underlying these semimetals—the hybridization of spinon modes bound to the grain boundary—suggests that topological semimetals can emerge in any topological material where lattice dislocations bind localized topological modes.
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.
Two-dimensional oxide topological insulator with iron-pnictide superconductor LiFeAs structure
NASA Astrophysics Data System (ADS)
Xu, Qiunan; Song, Zhida; Nie, Simin; Weng, Hongming; Fang, Zhong; Dai, Xi
2015-11-01
By using first-principles calculations, we propose that ZrSiO can be looked at as a three-dimensional (3D) oxide weak topological insulator (TI) and its single layer is a long-sought-after 2D oxide TI with a band gap up to 30 meV. Calculated phonon spectrum of the single layer ZrSiO indicates it is dynamically stable and the experimental achievements in growing oxides with atomic precision ensure that it can be readily synthesized. This will lead to novel devices based on TIs, the so-called "topotronic" devices, operating under room temperature and stable when exposed in the air. Thus a new field of "topotronics" will arise. Another intriguing thing is this oxide 2D TI has the similar crystal structure as the well-known iron-pnictide superconductor LiFeAs. This brings great promise in realizing the combination of superconductor and TI, paving the way to various extraordinary quantum phenomena, such as topological superconductor and Majorana modes. We further find that there are many other isostructural compounds hosting the similar electronic structure and forming a W H M family with W being Zr, Hf, or La, H being group IV or group V element, and M being group VI one.
NASA Astrophysics Data System (ADS)
Liu, Y.; Long, Y. J.; Zhao, L. X.; Nie, S. M.; Zhang, S. J.; Weng, Y. X.; Jin, M. L.; Li, W. M.; Liu, Q. Q.; Long, Y. W.; Yu, R. C.; Gu, C. Z.; Sun, F.; Yang, W. G.; Mao, H. K.; Feng, X. L.; Li, Q.; Zheng, W. T.; Weng, H. M.; Dai, X.; Fang, Z.; Chen, G. F.; Jin, C. Q.
2017-03-01
Recently, theoretical studies show that layered HfTe5 is at the boundary of weak & strong topological insulator (TI) and might crossover to a Dirac semimetal state by changing lattice parameters. The topological properties of 3D stacked HfTe5 are expected hence to be sensitive to pressures tuning. Here, we report pressure induced phase evolution in both electronic & crystal structures for HfTe5 with a culmination of pressure induced superconductivity. Our experiments indicated that the temperature for anomaly resistance peak (Tp) due to Lifshitz transition decreases first before climbs up to a maximum with pressure while the Tp minimum corresponds to the transition from a weak TI to strong TI. The HfTe5 crystal becomes superconductive above ~5.5 GPa where the Tp reaches maximum. The highest superconducting transition temperature (Tc) around 5 K was achieved at 20 GPa. Crystal structure studies indicate that HfTe5 transforms from a Cmcm phase across a monoclinic C2/m phase then to a P-1 phase with increasing pressure. Based on transport, structure studies a comprehensive phase diagram of HfTe5 is constructed as function of pressure. The work provides valuable experimental insights into the evolution on how to proceed from a weak TI precursor across a strong TI to superconductors.
Massive Dirac Fermion Observed in Lanthanide-Doped Topological Insulator Thin Films
Harrison, S. E.; Collins-McIntyre, L. J.; Schönherr, P.; Vailionis, A.; Srot, V.; van Aken, P. A.; Kellock, A. J.; Pushp, A.; Parkin, S. S. P.; Harris, J. S.; Zhou, B.; Chen, Y. L.; Hesjedal, T.
2015-01-01
The breaking of time reversal symmetry (TRS) in three-dimensional (3D) topological insulators (TIs), and thus the opening of a ‘Dirac-mass gap’ in the linearly dispersed Dirac surface state, is a prerequisite for unlocking exotic physical states. Introducing ferromagnetic long-range order by transition metal doping has been shown to break TRS. Here, we present the study of lanthanide (Ln) doped Bi2Te3, where the magnetic doping with high-moment lanthanides promises large energy gaps. Using molecular beam epitaxy, single-crystalline, rhombohedral thin films with Ln concentrations of up to ~35%, substituting on Bi sites, were achieved for Dy, Gd, and Ho doping. Angle-resolved photoemission spectroscopy shows the characteristic Dirac cone for Gd and Ho doping. In contrast, for Dy doping above a critical doping concentration, a gap opening is observed via the decreased spectral intensity at the Dirac point, indicating a topological quantum phase transition persisting up to room-temperature. PMID:26503435
Liu, Y.; Long, Y. J.; Zhao, L. X.; Nie, S. M.; Zhang, S. J.; Weng, Y. X.; Jin, M. L.; Li, W. M.; Liu, Q. Q.; Long, Y. W.; Yu, R. C.; Gu, C. Z.; Sun, F.; Yang, W. G.; Mao, H. K.; Feng, X. L.; Li, Q.; Zheng, W. T.; Weng, H. M.; Dai, X.; Fang, Z.; Chen, G. F.; Jin, C. Q.
2017-01-01
Recently, theoretical studies show that layered HfTe5 is at the boundary of weak & strong topological insulator (TI) and might crossover to a Dirac semimetal state by changing lattice parameters. The topological properties of 3D stacked HfTe5 are expected hence to be sensitive to pressures tuning. Here, we report pressure induced phase evolution in both electronic & crystal structures for HfTe5 with a culmination of pressure induced superconductivity. Our experiments indicated that the temperature for anomaly resistance peak (Tp) due to Lifshitz transition decreases first before climbs up to a maximum with pressure while the Tp minimum corresponds to the transition from a weak TI to strong TI. The HfTe5 crystal becomes superconductive above ~5.5 GPa where the Tp reaches maximum. The highest superconducting transition temperature (Tc) around 5 K was achieved at 20 GPa. Crystal structure studies indicate that HfTe5 transforms from a Cmcm phase across a monoclinic C2/m phase then to a P-1 phase with increasing pressure. Based on transport, structure studies a comprehensive phase diagram of HfTe5 is constructed as function of pressure. The work provides valuable experimental insights into the evolution on how to proceed from a weak TI precursor across a strong TI to superconductors. PMID:28300156
NASA Astrophysics Data System (ADS)
Geraedts, Scott D.; Motrunich, Olexei I.
2014-10-01
We study a topological phase of interacting bosons in (3 +1 ) dimensions that is protected by charge conservation and time-reversal symmetry. We present an explicit lattice model that realizes this phase and that can be studied in sign-free Monte Carlo simulations. The idea behind our model is to bind bosons to topological defects called hedgehogs. We determine the phase diagram of the model and identify a phase where such bound states are proliferated. In this phase, we observe a Witten effect in the bulk whereby an external monopole binds half of the elementary boson charge, which confirms that it is a bosonic topological insulator. We also study the boundary between the topological insulator and a trivial insulator. We find a surface phase diagram that includes exotic superfluids, a topologically ordered phase, and a phase with a Hall effect quantized to one-half of the value possible in a purely two-dimensional system. We also present models that realize symmetry-enriched topologically ordered phases by binding multiple hedgehogs to each boson; these phases show charge fractionalization and intrinsic topological order as well as a fractional Witten effect.
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.
Electronic properties of SnTe-class topological crystalline insulator materials
NASA Astrophysics Data System (ADS)
Wang, Jianfeng; Wang, Na; Huang, Huaqing; Duan, Wenhui
2016-11-01
The rise of topological insulators in recent years has broken new ground both in the conceptual cognition of condensed matter physics and the promising revolution of the electronic devices. It also stimulates the explorations of more topological states of matter. Topological crystalline insulator is a new topological phase, which combines the electronic topology and crystal symmetry together. In this article, we review the recent progress in the studies of SnTe-class topological crystalline insulator materials. Starting from the topological identifications in the aspects of the bulk topology, surface states calculations, and experimental observations, we present the electronic properties of topological crystalline insulators under various perturbations, including native defect, chemical doping, strain, and thickness-dependent confinement effects, and then discuss their unique quantum transport properties, such as valley-selective filtering and helicity-resolved functionalities for Dirac fermions. The rich properties and high tunability make SnTe-class materials promising candidates for novel quantum devices. Project supported by the Ministry of Science and Technology of China (Grant No. 2016YFA0301000) and the National Natural Science Foundation of China (Grant No. 11334006).
X-boson cumulant approach to the topological Kondo insulators
NASA Astrophysics Data System (ADS)
Ramos, E.; Franco, R.; Silva-Valencia, J.; Foglio, M. E.; Figueira, M. S.
2014-12-01
In this work we present a generalization of our previous work of the X-boson approach to the periodic Anderson model (PAM), adequate to study a novel class of intermetallic 4f and 5f orbitals materials: the topological Kondo insulators, whose paradigmatic material is the compound SmB6. For simplicity, we consider a version of the PAM on a 2D square lattice, adequate to describe Ce-based compounds in two dimensions. The starting point of the model is the 4f - Ce ions orbitals, with J = 5/2 multiplet, in the presence of spin-orbit coupling. Our technique works well for all of the parameters of the model and avoids the unwanted phase transitions of the slave boson mean field theory. We present a critical comparison of our results with those of the usual slave boson method, that has been intensively used to describe this class of materials. We also obtain a new valence first order transition which we attribute to the vec k dependence of the hybridization.
Tunneling spectroscopy of a magnetic adatoms on topological insulator surfaces
NASA Astrophysics Data System (ADS)
Misiorny, M.; Bjerngaard, M.; Paaske, J.
In this communication, we address the question of how the presence of a magnetic impurity on a topological insulator (TI) surface manifests in the inelastic electron tunneling spectroscopy (IETS) when such a system is probed by a STM. For this purpose, we consider a single magnetic adatom with arbitrary spin, whose dynamics is governed by the local magnetic anisotropy. The spin is exchange-coupled to two-dimensional helical surface electrons, corresponding to the surface of a three-dimensional TI like Bi2Se3, with its characteristic hexagonally warped Dirac cone band structure. Employing an effective exchange-tunneling model, we calculate the non-linear differential conductance from a spin-polarized STM tip to the helical substrate, valid in the perturbative regime of weak exchange-tunneling and including the nonequilibrium pumping of the adatom spin states. The interplay between the magnetic anisotropy and the spin-momentum locked surface electrons is shown to give a number of specific imprints in the IETS, which could be investigated by spin-resolved scanning tunneling spectroscopy. M. Misiorny, M. Bjerngaard and J. Paaske, manuscript in preparation Work supported by the Polish Ministry of Science and Education as `Iuventus Plus' project (IP2014 030973) in years 2015-2016.
Strain induced novel quantum magnetotransport properties of topological insulators
NASA Astrophysics Data System (ADS)
Ma, Ning; Zhang, Shengli; Liu, Daqing
2016-12-01
Recent theoretical and experimental researches have revealed that the strained bulk HgTe can be regarded as a three-dimensional topological insulator (TI). Motivated by this, we explore the strain effects on the transport properties of the HgTe surface states, which are modulated by a weak 1D in-plane electrostatic periodic potential in the presence of a perpendicular magnetic field. We analytically derive the zero frequency (dc) diffusion conductivity for the case of quasielastic scattering in the Kubo formalism, and find that, in strong magnetic field regime, the Shubnikov-de Haas oscillations are superimposed on top of the Weiss oscillations due to the electric modulation for null and finite strain. Furthermore, the strain is shown to remove the degeneracy in inversion symmetric Dirac cones on the top and bottom surfaces. This accordingly gives rise to the splitting and mixture of Landau levels, and the asymmetric spectrum of the dc conductivity. These phenomena, not known in a conventional 2D electron gas and even in a strainless TI and graphene, are a consequence of the anomalous spectrum of surface states in a fully stained TI. These results should be valuable for electronic and spintronic applications of TIs, and thus we fully expect to see them in the further experiment.
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.
Phase coherence and Andreev reflection in topological insulator devices
Finck, A. D. K.; Kurter, C.; Hor, Y. S.; ...
2014-11-04
Topological insulators (TIs) have attracted immense interest because they host helical surface states. Protected by time-reversal symmetry, they are robust to nonmagnetic disorder. When superconductivity is induced in these helical states, they are predicted to emulate p-wave pairing symmetry, with Majorana states bound to vortices. Majorana bound states possess non-Abelian exchange statistics that can be probed through interferometry. Here, we take a significant step towards Majorana interferometry by observing pronounced Fabry-Pérot oscillations in a TI sandwiched between a superconducting and a normal lead. For energies below the superconducting gap, we observe a doubling in the frequency of the oscillations, arisingmore » from an additional phase from Andreev reflection. When a magnetic field is applied perpendicular to the TI surface, a number of very sharp and gate-tunable conductance peaks appear at or near zero energy, which has consequences for interpreting spectroscopic probes of Majorana fermions. Our results show that TIs are a promising platform for exploring phase-coherent transport in a solid-state system.« less
Phase coherence and Andreev reflection in topological insulator devices
Finck, A. D. K.; Kurter, C.; Hor, Y. S.; Van Harlingen, D. J.
2014-11-04
Topological insulators (TIs) have attracted immense interest because they host helical surface states. Protected by time-reversal symmetry, they are robust to nonmagnetic disorder. When superconductivity is induced in these helical states, they are predicted to emulate p-wave pairing symmetry, with Majorana states bound to vortices. Majorana bound states possess non-Abelian exchange statistics that can be probed through interferometry. Here, we take a significant step towards Majorana interferometry by observing pronounced Fabry-Pérot oscillations in a TI sandwiched between a superconducting and a normal lead. For energies below the superconducting gap, we observe a doubling in the frequency of the oscillations, arising from an additional phase from Andreev reflection. When a magnetic field is applied perpendicular to the TI surface, a number of very sharp and gate-tunable conductance peaks appear at or near zero energy, which has consequences for interpreting spectroscopic probes of Majorana fermions. Our results show that TIs are a promising platform for exploring phase-coherent transport in a solid-state system.
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
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
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.
Transport on the surface of a topological insulator
Vargiamidis, V.; Vasilopoulos, P.
2014-08-14
We study theoretically dc and ac transport on the surface of a three-dimensional topological insulator when its time-reversal symmetry is broken. Starting with a Kubo formula, we derive an explicit expression for the dc Hall conductivity, valid for finite temperatures. At zero temperature this expression gives the dc half-quantum Hall conductivity, provided the Fermi level lies in the gap. Corrections when the Fermi level is outside the gap and scattering by impurities are quantified. The longitudinal conductivity is also examined. At finite frequencies, we find a modified Drude term in σ{sub xx}(ω) and logarithmic, frequency-dependent corrections in σ{sub yx}(ω). The ac Hall conductivity exhibits a robust logarithmic singularity for excitation energies equal to the gapwidth. For these energies, we also find that the power spectrum, which is pertinent to optical experiments, exhibits drastic increase. The Hall conductivity remains almost unaffected for temperatures up to approximately 300 K.
Transport in selectively magnetically doped topological insulator wires
NASA Astrophysics Data System (ADS)
Acero, Sergio; Brey, Luis; Herrera, William J.; Yeyati, Alfredo Levy
2015-12-01
We study the electronic and transport properties of a topological insulator nanowire including selective magnetic doping of its surfaces. We use a model which is appropriate to describe materials like Bi2Se3 within a k .p approximation and consider nanowires with a rectangular geometry. Within this model the magnetic doping at the (111) surfaces induces a Zeeman field which opens a gap at the Dirac cones corresponding to the surface states. For obtaining the transport properties in a two terminal configuration we use a recursive Green's function method based on a tight-binding model which is obtained by discretizing the original continuous model. For the case of uniform magnetization of two opposite nanowire (111) surfaces we show that the conductance can switch from a quantized value of e2/h (when the magnetizations are equal) to a very small value (when they are opposite). We also analyze the case of nonuniform magnetizations in which the Zeeman field on the two opposite surfaces change sign at the middle of the wire. For this case we find that conduction by resonant tunneling through a chiral state bound at the middle of the wire is possible. The resonant level position can be tuned by imposing an Aharonov-Bohm flux through the nanowire cross section.
Tunneling in graphene-topological insulator hybrid devices
NASA Astrophysics Data System (ADS)
Steinberg, H.; Orona, L. A.; Fatemi, V.; Sanchez-Yamagishi, J. D.; Watanabe, K.; Taniguchi, T.; Jarillo-Herrero, P.
2015-12-01
Hybrid graphene-topological insulator (TI) devices were fabricated using a mechanical transfer method and studied via electronic transport. Devices consisting of bilayer graphene (BLG) under the TI Bi2Se3 exhibit differential conductance characteristics which appear to be dominated by tunneling, roughly reproducing the Bi2Se3 density of states. Similar results were obtained for BLG on top of Bi2Se3 , with tenfold greater conductance consistent with a larger contact area due to better surface conformity. The devices further show evidence of inelastic phonon-assisted tunneling processes involving both Bi2Se3 and graphene phonons. These processes favor phonons which compensate for momentum mismatch between the TI Γ and graphene K ,K' points. Finally, the utility of these tunnel junctions is demonstrated on a density-tunable BLG device, where the charge neutrality point is traced along the energy-density trajectory. This trajectory is used as a measure of the ground-state density of states.
Acquisition of He3 Cryostat Insert for Experiments on Topological Insulators
2016-02-03
P.O. Box 12211 Research Triangle Park, NC 27709-2211 Cryogenic equipment with 9 Tesla magnet, low temperature experiments REPORT DOCUMENTATION PAGE...facilitated by the cryogenic system are i) In-situ low - temperature tuning of the chemical potential in the topological insulator using ionic liquid gating...situ low - temperature tuning of the chemical potential in the topological insulator using ionic liquid gating. The intense electric field applied
Chemically gated electronic structure of a superconducting doped topological insulator system
NASA Astrophysics Data System (ADS)
Wray, L. A.; Xu, S.; Neupane, M.; Fedorov, A. V.; Hor, Y. S.; Cava, R. J.; Hasan, M. Z.
2013-07-01
Angle resolved photoemission spectroscopy is used to observe changes in the electronic structure of bulk-doped topological insulator CuxBi2Se3 as additional copper atoms are deposited onto the cleaved crystal surface. Carrier density and surface-normal electrical field strength near the crystal surface are estimated to consider the effect of chemical surface gating on atypical superconducting properties associated with topological insulator order, such as the dynamics of theoretically predicted Majorana Fermion vortices.
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)
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.
NASA Astrophysics Data System (ADS)
Ye, Zhen-Yu; Deng, Hui-Xiong; Wu, Hui-Zhen; Li, Shu-Shen; Wei, Su-Huai; Luo, Jun-Wei
2015-11-01
Group-IV tellurides have exhibited exotic band structures. Specifically, despite the fact that Sn sits between Ge and Pb in the same column of the periodic table, cubic SnTe is a topological crystalline insulator with band inversion, but both isovalent GeTe and PbTe are trivial semiconductors with normal band order. By performing first-principles band structure calculations, we unravel the origin of this abnormal behaviour by using symmetry analysis and the atomic orbital energy levels and atomic sizes of these elements. In group-IV tellurides, the s lone pair band of the group-IV element is allowed by symmetry to couple with the anion valence p band at the L-point, and such s-p coupling leads to the occurrence of bandgap at the L-point. We find that such s-p coupling is so strong in SnTe that it inverts the band order near the bandgap; however, it is not strong enough in both GeTe and PbTe, so they remain normal semiconductors. The reason for this is the incomplete screening of the core of the relatively tight-binding Ge 4s orbital by its 3d orbitals and the large atomic size and strong relativistic effect in Pb, respectively. Interestingly, we also find that the rhombohedral distortion removes the inversion symmetry and the reduced s-p coupling transforms the α-SnTe back to a normal semiconductor. Our study demonstrates that, in addition to spin-orbital coupling, strain and interface dipole fields, inter-orbital coupling is another effective way to engineer the topological insulators.
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.
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.
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.
Quantum transport in 3D Weyl semimetals: Is there a metal-insulator transition?
NASA Astrophysics Data System (ADS)
Ziegler, Klaus
2016-12-01
We calculate the transport properties of three-dimensional Weyl fermions in a disordered environment. The resulting conductivity depends only on the Fermi energy and the scattering rate. First we study the conductivity at the spectral node for a fixed scattering rate and obtain a continuous transition from an insulator at weak disorder to a metal at stronger disorder. Within the self-consistent Born approximation the scattering rate depends on the Fermi energy. Then it is crucial that the limits of the conductivity for a vanishing Fermi energy and a vanishing scattering rate do not commute. As a result, there is also metallic behavior in the phase with vanishing scattering rate and only a quantum critical point remains as an insulating phase. The latter turns out to be a critical fixed point in terms of a renormalization-group flow.
Interacting weak topological insulators and their transition to Dirac semimetal phases
NASA Astrophysics Data System (ADS)
Li, Gang; Hanke, Werner; Sangiovanni, Giorgio; Trauzettel, Björn
2015-12-01
Topological insulators in the presence of a strong Coulomb interaction constitute novel phases of matter. Transitions between these phases can be driven by single-particle or many-body effects. On the basis of ab initio calculations, we identify a concrete material, i.e., Ca2PtO4 , that turns out to be a hole-doped weak topological insulator. Interestingly, the Pt d orbitals in this material are relevant for the band inversion that gives rise to the topological phase. Therefore, Coulomb interactions should be of importance in Ca2PtO4 . To study the influence of interactions on the weak topological insulating phase, we look at a toy model corresponding to a layer-stacked three-dimensional version of the Bernevig-Hughes-Zhang model with local interactions. For a low to intermediate interaction strength, we discover novel interaction-driven topological phase transitions between the weak topological insulator and two Dirac semimetal phases. The latter correspond to gapless topological phases. For strong interactions, the system eventually becomes a Mott insulator.
NASA Astrophysics Data System (ADS)
Isobe, Hiroki; Fu, Liang
2015-03-01
The effects of electron-electron interaction in edge states of mirror-symmetry protected topological crystalline insulators (TCI's) are discussed. The analysis is performed by using bosonized Hamiltonian following the Tomonaga-Luttinger liquid theory. When two pairs of helical edge states exist, electron-electron interaction could gap out one edge mode, which is a possible realization of interacting symmetry-protected topological (SPT) phases. This type of SPT phase is closely related to a Luther-Emery liquid in spinful 1D system. We also propose a method of detecting the SPT phases by STM. The other focus of the study is the classification of SPT phases in mirror-symmetry protected TCI's. By adopting the Chern-Simons theory, we find that electron-electron interaction reduces the classification from Z to Z4. It means that the edge states can be gapped out when four pairs of edge states exist. In other cases, the edge modes cannot be fully gapped. Each of these states corresponds to a different SPT phase depending on the relevant interaction process.
NASA Astrophysics Data System (ADS)
Grusdt, Fabian; Abanin, Dmitry; Demler, Eugene
2013-05-01
Recently experiments with ultracold atoms started to explore topological phases in 1D optical lattices. While transport measurements are challenging in these systems, ways to directly measure topological quantum numbers using a combination of Bloch oscillations and Ramsey interferometry have been explored (Atala et al., arXiv:1212.0572). In this talk I will present ways to measure the Z2 topological quantum numbers of two and three dimensional time-reversal invariant (TR) topological insulators. In this case non-Abelian Bloch oscillations can be combined with Ramsey interferometry to map out the topological properties of a given band-structure. Our method is very general and works even in the presence of accidental degeneracies. The applicability of the scheme is discussed for different theoretically proposed implementations of TR topological insulators using ultracold atoms. F. G. is grateful to Harvard University for hospitality and acknowledges financial support from Graduate School Materials Science in Mainz (MAINZ).
NASA Astrophysics Data System (ADS)
Lapa, Matthew F.; Jian, Chao-Ming; Ye, Peng; Hughes, Taylor L.
2017-01-01
We calculate the topological part of the electromagnetic response of bosonic integer quantum Hall (BIQH) phases in odd (space-time) dimensions, and bosonic topological insulator (BTI) and bosonic chiral semimetal (BCSM) phases in even dimensions. To do this, we use the nonlinear sigma model (NLSM) description of bosonic symmetry-protected topological (SPT) phases, and the method of gauged Wess-Zumino (WZ) actions. We find the surprising result that for BIQH states in dimension 2 m -1 (m =1 ,2 ,⋯ ), the bulk response to an electromagnetic field Aμ is characterized by a Chern-Simons term for Aμ with a level quantized in integer multiples of m ! (factorial). We also show that BTI states (which have an extra Z2 symmetry) can exhibit a Z2-breaking quantum Hall effect on their boundaries, with this boundary quantum Hall effect described by a Chern-Simons term at level m/! 2 . We show that the factor of m ! can be understood by requiring gauge invariance of the exponential of the Chern-Simons term on a general Euclidean manifold, and we also use this argument to characterize the electromagnetic and gravitational responses of fermionic SPT phases with U(1 ) symmetry in all odd dimensions. We then use our gauged boundary actions for the BIQH and BTI states to (i) construct a bosonic analog of a chiral semimetal (BCSM) in even dimensions, (ii) show that the boundary of the BTI state exhibits a bosonic analog of the parity anomaly of Dirac fermions in odd dimensions, and (iii) study anomaly inflow at domain walls on the boundary of BTI states. In a series of Appendixes we derive important formulas and additional results. In particular, in Appendix A we use the connection between equivariant cohomology and gauged WZ actions to give a mathematical interpretation of the actions for the BIQH and BTI boundaries constructed in this paper.
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.
NASA Astrophysics Data System (ADS)
Xypakis, Emmanouil; Bardarson, Jens H.
2017-01-01
Clean topological insulators exposed to a magnetic field develop Landau levels accompanied by a nonzero Hall conductivity for the infinite slab geometry. In this work we consider the case of disordered topological insulator nanowires and find, in contrast, that a zero Hall plateau emerges within a broad energy window close to the Dirac point. We numerically calculate the conductance and its distribution for a statistical ensemble of disordered nanowires, and use the conductance fluctuations to study the dependence of the insulating phase on system parameters, such as the nanowire length, disorder strength, and the magnetic field.
Tuning thermoelectricity in a Bi2Se3 topological insulator via varied film thickness
Guo, Minghua; Wang, Zhenyu; Xu, Yong; ...
2016-01-12
We report thermoelectric transport studies on Bi2Se3 topological insulator thin films with varied thickness grown by molecular beam epitaxy. We find that the Seebeck coefficient and thermoelectric power factor decrease systematically with the reduction of film thickness. These experimental observations can be explained quantitatively by theoretical calculations based on realistic electronic band structure of the Bi2Se3 thin films. Lastly, this work illustrates the crucial role played by the topological surface states on the thermoelectric transport of topological insulators, and sheds new light on further improvement of their thermoelectric performance.
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.
Topological Bose-Mott insulators in a one-dimensional optical superlattice.
Zhu, Shi-Liang; Wang, Z-D; Chan, Y-H; Duan, L-M
2013-02-15
We study topological properties of the Bose-Hubbard model with repulsive interactions in a one-dimensional optical superlattice. We find that the Mott insulator states of the single-component (two-component) Bose-Hubbard model under fractional fillings are topological insulators characterized by a nonzero charge (or spin) Chern number with nontrivial edge states. For ultracold atomic experiments, we show that the topological Chern number can be detected through measuring the density profiles of the bosonic atoms in a harmonic trap.
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.
Dimensional crossover and cold-atom realization of topological Mott insulators
Scheurer, Mathias S.; Rachel, Stephan; Orth, Peter P.
2015-01-01
Interacting cold-atomic gases in optical lattices offer an experimental approach to outstanding problems of many body physics. One important example is the interplay of interaction and topology which promises to generate a variety of exotic phases such as the fractionalized Chern insulator or the topological Mott insulator. Both theoretically understanding these states of matter and finding suitable systems that host them have proven to be challenging problems. Here we propose a cold-atom setup where Hubbard on-site interactions give rise to spin liquid-like phases: weak and strong topological Mott insulators. They represent the celebrated paradigm of an interacting and topological quantum state with fractionalized spinon excitations that inherit the topology of the non-interacting system. Our proposal shall help to pave the way for a controlled experimental investigation of this exotic state of matter in optical lattices. Furthermore, it allows for the investigation of a dimensional crossover from a two-dimensional quantum spin Hall insulating phase to a three-dimensional strong topological insulator by tuning the hopping between the layers. PMID:25669431
Dimensional crossover and cold-atom realization of topological Mott insulators.
Scheurer, Mathias S; Rachel, Stephan; Orth, Peter P
2015-02-11
Interacting cold-atomic gases in optical lattices offer an experimental approach to outstanding problems of many body physics. One important example is the interplay of interaction and topology which promises to generate a variety of exotic phases such as the fractionalized Chern insulator or the topological Mott insulator. Both theoretically understanding these states of matter and finding suitable systems that host them have proven to be challenging problems. Here we propose a cold-atom setup where Hubbard on-site interactions give rise to spin liquid-like phases: weak and strong topological Mott insulators. They represent the celebrated paradigm of an interacting and topological quantum state with fractionalized spinon excitations that inherit the topology of the non-interacting system. Our proposal shall help to pave the way for a controlled experimental investigation of this exotic state of matter in optical lattices. Furthermore, it allows for the investigation of a dimensional crossover from a two-dimensional quantum spin Hall insulating phase to a three-dimensional strong topological insulator by tuning the hopping between the layers.
Quantum transport and two-parameter scaling at the surface of a weak topological insulator.
Mong, Roger S K; Bardarson, Jens H; Moore, Joel E
2012-02-17
Weak topological insulators have an even number of Dirac cones in their surface spectrum and are thought to be unstable to disorder, which leads to an insulating surface. Here we argue that the presence of disorder alone will not localize the surface states; rather, the presence of a time-reversal symmetric mass term is required for localization. Through numerical simulations, we show that in the absence of the mass term the surface always flow to a stable metallic phase and the conductivity obeys a one-parameter scaling relation, just as in the case of a strong topological insulator surface. With the inclusion of the mass, the transport properties of the surface of a weak topological insulator follow a two-parameter scaling form.
Ultrafast helicity control of surface currents in topological insulators with near-unity fidelity.
Kastl, Christoph; Karnetzky, Christoph; Karl, Helmut; Holleitner, Alexander W
2015-03-26
In recent years, a class of solid-state materials, called three-dimensional topological insulators, has emerged. In the bulk, a topological insulator behaves like an ordinary insulator with a band gap. At the surface, conducting gapless states exist showing remarkable properties such as helical Dirac dispersion and suppression of backscattering of spin-polarized charge carriers. The characterization and control of the surface states via transport experiments is often hindered by residual bulk contributions. Here we show that surface currents in Bi2Se3 can be controlled by circularly polarized light on a picosecond timescale with a fidelity near unity even at room temperature. We reveal the temporal separation of such ultrafast helicity-dependent surface currents from photo-induced thermoelectric and drift currents in the bulk. Our results uncover the functionality of ultrafast optoelectronic devices based on surface currents in topological insulators.
Massive Dirac Fermion on the Surface of a Magnetically Doped Topological Insulator
Chen, Y.L.; Chu, J.-H.; Analytis, J.G.; Liu, Z.K.; Igarashi, K.; Kuo, H.-H.; Qi, X.L.; Mo, S.K.; Moore, R.G.; Lu, D.H.; Hashimoto, M.; Sasagawa, T.; Zhang, S.C.; Fisher, I.R.; Hussain, Z.; Shen, Z.-X.; /SLAC
2011-05-20
Topological insulators are characterized by a massless Dirac surface state and a bulk energy gap. An insulating massive Dirac fermion state is predicted to occur if the breaking of the time reversal symmetry opens an energy gap at the Dirac point, provided that the Fermi-energy resides inside both the surface and bulk gaps. By introducing magnetic dopants into the three dimensional topological insulator Bi{sub 2}Se{sub 3} to break the time reversal symmetry, we observed the formation of a massive Dirac fermion on the surface; simultaneous magnetic and charge doping furthermore positioned the Fermi-energy inside the Dirac gap. The insulating massive Dirac Fermion state thus obtained may provide a tool for studying a range of topological phenomena relevant to both condensed matter and particle physics.
Ultrafast helicity control of surface currents in topological insulators with near-unity fidelity
Kastl, Christoph; Karnetzky, Christoph; Karl, Helmut; Holleitner, Alexander W.
2015-01-01
In recent years, a class of solid-state materials, called three-dimensional topological insulators, has emerged. In the bulk, a topological insulator behaves like an ordinary insulator with a band gap. At the surface, conducting gapless states exist showing remarkable properties such as helical Dirac dispersion and suppression of backscattering of spin-polarized charge carriers. The characterization and control of the surface states via transport experiments is often hindered by residual bulk contributions. Here we show that surface currents in Bi2Se3 can be controlled by circularly polarized light on a picosecond timescale with a fidelity near unity even at room temperature. We reveal the temporal separation of such ultrafast helicity-dependent surface currents from photo-induced thermoelectric and drift currents in the bulk. Our results uncover the functionality of ultrafast optoelectronic devices based on surface currents in topological insulators. PMID:25808213
NASA Astrophysics Data System (ADS)
Poon, Kelvin; Hamarneh, Ghassan; Abugharbieh, Rafeef
2007-03-01
Segmentation of 3D data is one of the most challenging tasks in medical image analysis. While reliable automatic methods are typically preferred, their success is often hindered by poor image quality and significant variations in anatomy. Recent years have thus seen an increasing interest in the development of semi-automated segmentation methods that combine computational tools with intuitive, minimal user interaction. In an earlier work, we introduced a highly-automated technique for medical image segmentation, where a 3D extension of the traditional 2D Livewire was proposed. In this paper, we present an enhanced and more powerful 3D Livewire-based segmentation approach with new features designed to primarily enable the handling of complex object topologies that are common in biological structures. The point ordering algorithm we proposed earlier, which automatically pairs up seedpoints in 3D, is improved in this work such that multiple sets of points are allowed to simultaneously exist. Point sets can now be automatically merged and split to accommodate for the presence of concavities, protrusions, and non-spherical topologies. The robustness of the method is further improved by extending the 'turtle algorithm', presented earlier, by using a turtle-path pruning step. Tests on both synthetic and real medical images demonstrate the efficiency, reproducibility, accuracy, and robustness of the proposed approach. Among the examples illustrated is the segmentation of the left and right ventricles from a T1-weighted MRI scan, where an average task time reduction of 84.7% was achieved when compared to a user performing 2D Livewire segmentation on every slice.
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
Lee, Yea-Lee; Park, Hee Chul; Ihm, Jisoon; Son, Young-Woo
2015-09-15
Because topological surface states of a single-crystal topological insulator can exist on all surfaces with different crystal orientations enclosing the crystal, mutual interactions among those states contiguous to each other through edges can lead to unique phenomena inconceivable in normal insulators. Here we show, based on a first-principles approach, that the difference in the work function between adjacent surfaces with different crystal-face orientations generates a built-in electric field around facet edges of a prototypical topological insulator such as Bi2Se3. Owing to the topological magnetoelectric coupling for a given broken time-reversal symmetry in the crystal, the electric field, in turn, forces effective magnetic dipoles to accumulate along the edges, realizing the facet-edge magnetic ordering. We demonstrate that the predicted magnetic ordering is in fact a manifestation of the axion electrodynamics in real solids.
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.
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.
Spin generation via bulk spin current in three-dimensional topological insulators
Peng, Xingyue; Yang, Yiming; Singh, Rajiv R.P.; Savrasov, Sergey Y.; Yu, Dong
2016-01-01
To date, spin generation in three-dimensional topological insulators is primarily modelled as a single-surface phenomenon, attributed to the momentum-spin locking on each individual surface. In this article, we propose a mechanism of spin generation where the role of the insulating yet topologically non-trivial bulk becomes explicit: an external electric field creates a transverse pure spin current through the bulk of a three-dimensional topological insulator, which transports spins between the top and bottom surfaces. Under sufficiently high surface disorder, the spin relaxation time can be extended via the Dyakonov–Perel mechanism. Consequently, both the spin generation efficiency and surface conductivity are largely enhanced. Numerical simulation confirms that this spin generation mechanism originates from the unique topological connection of the top and bottom surfaces and is absent in other two-dimensional systems such as graphene, even though they possess a similar Dirac cone-type dispersion. PMID:26932574
Manifestation of axion electrodynamics through magnetic ordering on edges of a topological insulator
Lee, Yea-Lee; Park, Hee Chul; Ihm, Jisoon; Son, Young-Woo
2015-01-01
Because topological surface states of a single-crystal topological insulator can exist on all surfaces with different crystal orientations enclosing the crystal, mutual interactions among those states contiguous to each other through edges can lead to unique phenomena inconceivable in normal insulators. Here we show, based on a first-principles approach, that the difference in the work function between adjacent surfaces with different crystal-face orientations generates a built-in electric field around facet edges of a prototypical topological insulator such as Bi2Se3. Owing to the topological magnetoelectric coupling for a given broken time-reversal symmetry in the crystal, the electric field, in turn, forces effective magnetic dipoles to accumulate along the edges, realizing the facet-edge magnetic ordering. We demonstrate that the predicted magnetic ordering is in fact a manifestation of the axion electrodynamics in real solids. PMID:26324891
Underlapped FinFET on insulator: Quasi3D analytical model
NASA Astrophysics Data System (ADS)
Kumari, Vandana; Sharmetha, K.; Saxena, Manoj; Gupta, Mridula
2017-03-01
The work presented in this paper analyse the influence of gate underlap region (present either near the source end or near the drain end) on the performance of FinFET using an efficient quasi 3D analytical model carried out by using separation of variable technique. Various parameters analysed in this work are: surface potential, electric field, threshold voltage (Vth), Subthreshold slope (SS), Drain Induced Barrier Lowering (DIBL) and sub-threshold drain current for different channel and underlap length. Analytical results obtained from the developed model are validated by 3-D ATLAS device simulation software results. Analog and RF performance metrics are also extracted for different lengths of underlap region and compared with the conventional FinFET through extensive device simulation. The influence of the back gate voltage on the electrostatics of the underlap FinFET is also investigated. The single stage common source amplifier using conventional and underlap FinFET has also been analysed. Apart from this, switching speed of the device is also investigated by comparing Ion/Ioff ratio and delay for different underlap and channel length.
Jamali, Mahdi; Lee, Joon Sue; Jeong, Jong Seok; Mahfouzi, Farzad; Lv, Yang; Zhao, Zhengyang; Nikolić, Branislav K; Mkhoyan, K Andre; Samarth, Nitin; Wang, Jian-Ping
2015-10-14
Three-dimensional (3D) topological insulators are known for their strong spin-orbit coupling (SOC) and the existence of spin-textured surface states that might be potentially exploited for "topological spintronics." Here, we use spin pumping and the inverse spin Hall effect to demonstrate successful spin injection at room temperature from a metallic ferromagnet (CoFeB) into the prototypical 3D topological insulator Bi2Se3. The spin pumping process, driven by the magnetization dynamics of the metallic ferromagnet, introduces a spin current into the topological insulator layer, resulting in a broadening of the ferromagnetic resonance (FMR) line width. Theoretical modeling of spin pumping through the surface of Bi2Se3, as well as of the measured angular dependence of spin-charge conversion signal, suggests that pumped spin current is first greatly enhanced by the surface SOC and then converted into a dc-voltage signal primarily by the inverse spin Hall effect due to SOC of the bulk of Bi2Se3. We find that the FMR line width broadens significantly (more than a factor of 5) and we deduce a spin Hall angle as large as 0.43 in the Bi2Se3 layer.
Observation of unconventional quantum spin textures in topological insulators.
Hsieh, D; Xia, Y; Wray, L; Qian, D; Pal, A; Dil, J H; Osterwalder, J; Meier, F; Bihlmayer, G; Kane, C L; Hor, Y S; Cava, R J; Hasan, M Z
2009-02-13
A topologically ordered material is characterized by a rare quantum organization of electrons that evades the conventional spontaneously broken symmetry-based classification of condensed matter. Exotic spin-transport phenomena, such as the dissipationless quantum spin Hall effect, have been speculated to originate from a topological order whose identification requires a spin-sensitive measurement, which does not exist to this date in any system. Using Mott polarimetry, we probed the spin degrees of freedom and demonstrated that topological quantum numbers are completely determined from spin texture-imaging measurements. Applying this method to Sb and Bi(1-x)Sb(x), we identified the origin of its topological order and unusual chiral properties. These results taken together constitute the first observation of surface electrons collectively carrying a topological quantum Berry's phase and definite spin chirality, which are the key electronic properties component for realizing topological quantum computing bits with intrinsic spin Hall-like topological phenomena.
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.
Z2 topology in nonsymmorphic crystalline insulators: Möbius twist in surface states
NASA Astrophysics Data System (ADS)
Shiozaki, Ken; Sato, Masatoshi; Gomi, Kiyonori
2015-04-01
It has been known that an antiunitary symmetry such as time-reversal or charge conjugation is needed to realize Z2 topological phases in noninteracting systems. Topological insulators and superconducting nanowires are representative examples of such Z2 topological matters. Here we report the Z2 topological phase protected by only unitary symmetries. We show that the presence of a nonsymmorphic space group symmetry opens a possibility to realize Z2 topological phases without assuming any antiunitary symmetry. The Z2 topological phases are constructed in various dimensions, which are closely related to each other by Hamiltonian mapping. In two and three dimensions, the Z2 phases have a surface consistent with the nonsymmorphic space group symmetry, and thus they support topological gapless surface states. Remarkably, the surface states have a unique energy dispersion with the Möbius twist, which identifies the Z2 phases experimentally. We also provide the relevant structure in the K theory.
Peyrin, Francoise; Attali, Dominique; Chappard, Christine; Benhamou, Claude Laurent
2010-08-15
Purpose: Trabecular bone microarchitecture is made of a complex network of plate and rod structures evolving with age and disease. The purpose of this article is to propose a new 3D local analysis method for the quantitative assessment of parameters related to the geometry of trabecular bone microarchitecture. Methods: The method is based on the topologic classification of the medial axis of the 3D image into branches, rods, and plates. Thanks to the reversibility of the medial axis, the classification is next extended to the whole 3D image. Finally, the percentages of rods and plates as well as their mean thicknesses are calculated. The method was applied both to simulated test images and 3D micro-CT images of human trabecular bone. Results: The classification of simulated phantoms made of plates and rods shows that the maximum error in the quantitative percentages of plate and rods is less than 6% and smaller than with the structure model index (SMI). Micro-CT images of human femoral bone taken in osteoporosis and early or advanced osteoarthritis were analyzed. Despite the large physiological variability, the present method avoids the underestimation of rods observed with other local methods. The relative percentages of rods and plates were not significantly different between osteoarthritis and osteoporotic groups, whereas their absolute percentages were in relation to an increase of rod and plate thicknesses in advanced osteoarthritis with also higher relative and absolute number of nodes. Conclusions: The proposed method is model-independent, robust to surface irregularities, and enables geometrical characterization of not only skeletal structures but entire 3D images. Its application provided more accurate results than the standard SMI on simple simulated phantoms, but the discrepancy observed on the advanced osteoarthritis group raises questions that will require further investigations. The systematic use of such a local method in the characterization of
NASA Astrophysics Data System (ADS)
Yoshiura, Shintaro; Shimabukuro, Hayato; Takahashi, Keitaro; Matsubara, Takahiko
2017-02-01
The brightness temperature of the redshifted 21-cm line brings rich information about the intergalactic medium (IGM) from the cosmic dawn and epoch of reionization (EoR). While the power spectrum is a useful tool to investigate the 21-cm signal statistically, the 21-cm brightness temperature field is highly non-Gaussian and the power spectrum is inadequate to characterize the non-Gaussianity. Minkowski functionals (MFs) are promising tools to extract non-Gaussian features of the 21-cm signal and give topological information, such as morphology of ionized bubbles. In this work, we study the 21-cm line signal in detail with MFs. To promote understanding of basic features of the 21-cm signal, we calculate the MFs of not only the hydrogen neutral fraction but also the matter density and spin temperature, which contribute to brightness-temperature fluctuations. We find that the structure of the brightness temperature depends mainly on the ionized fraction and the spin temperature at late and early stages of the EoR, respectively. Further, we investigate the redshift evolution of MFs at 7 < z < 20. We find that, after the onset of reionization, MFs mainly reflect the ionized bubble property. In addition, MFs are sensitive to model parameters related to the topology of ionized bubbles and we consider the possibility of constraining the parameters using future 21-cm signal observations.
NASA Astrophysics Data System (ADS)
Olbrich, P.; Golub, L. E.; Herrmann, T.; Danilov, S. N.; Plank, H.; Bel'kov, V. V.; Mussler, G.; Weyrich, Ch.; Schneider, C. M.; Kampmeier, J.; Grützmacher, D.; Plucinski, L.; Eschbach, M.; Ganichev, S. D.
2014-08-01
We report on the observation of photogalvanic effects in epitaxially grown Sb2Te3 and Bi2Te3 three-dimensional (3D) topological insulators (TI). We show that asymmetric scattering of Dirac fermions driven back and forth by the terahertz electric field results in a dc electric current. Because of the "symmetry filtration" the dc current is generated by the surface electrons only and provides an optoelectronic access to probe the electron transport in TI, surface domains orientation, and details of electron scattering in 3D TI even at room temperature.
Strong nonlinear terahertz response induced by Dirac surface states in Bi2Se3 topological insulator.
Giorgianni, Flavio; Chiadroni, Enrica; Rovere, Andrea; Cestelli-Guidi, Mariangela; Perucchi, Andrea; Bellaveglia, Marco; Castellano, Michele; Di Giovenale, Domenico; Di Pirro, Giampiero; Ferrario, Massimo; Pompili, Riccardo; Vaccarezza, Cristina; Villa, Fabio; Cianchi, Alessandro; Mostacci, Andrea; Petrarca, Massimo; Brahlek, Matthew; Koirala, Nikesh; Oh, Seongshik; Lupi, Stefano
2016-04-26
Electrons with a linear energy/momentum dispersion are called massless Dirac electrons and represent the low-energy excitations in exotic materials such as graphene and topological insulators. Dirac electrons are characterized by notable properties such as a high mobility, a tunable density and, in topological insulators, a protection against backscattering through the spin-momentum locking mechanism. All those properties make graphene and topological insulators appealing for plasmonics applications. However, Dirac electrons are expected to present also a strong nonlinear optical behaviour. This should mirror in phenomena such as electromagnetic-induced transparency and harmonic generation. Here we demonstrate that in Bi2Se3 topological insulator, an electromagnetic-induced transparency is achieved under the application of a strong terahertz electric field. This effect, concomitantly determined by harmonic generation and charge-mobility reduction, is exclusively related to the presence of Dirac electron at the surface of Bi2Se3, and opens the road towards tunable terahertz nonlinear optical devices based on topological insulator materials.
Strong nonlinear terahertz response induced by Dirac surface states in Bi2Se3 topological insulator
NASA Astrophysics Data System (ADS)
Giorgianni, Flavio; Chiadroni, Enrica; Rovere, Andrea; Cestelli-Guidi, Mariangela; Perucchi, Andrea; Bellaveglia, Marco; Castellano, Michele; di Giovenale, Domenico; di Pirro, Giampiero; Ferrario, Massimo; Pompili, Riccardo; Vaccarezza, Cristina; Villa, Fabio; Cianchi, Alessandro; Mostacci, Andrea; Petrarca, Massimo; Brahlek, Matthew; Koirala, Nikesh; Oh, Seongshik; Lupi, Stefano
2016-04-01
Electrons with a linear energy/momentum dispersion are called massless Dirac electrons and represent the low-energy excitations in exotic materials such as graphene and topological insulators. Dirac electrons are characterized by notable properties such as a high mobility, a tunable density and, in topological insulators, a protection against backscattering through the spin-momentum locking mechanism. All those properties make graphene and topological insulators appealing for plasmonics applications. However, Dirac electrons are expected to present also a strong nonlinear optical behaviour. This should mirror in phenomena such as electromagnetic-induced transparency and harmonic generation. Here we demonstrate that in Bi2Se3 topological insulator, an electromagnetic-induced transparency is achieved under the application of a strong terahertz electric field. This effect, concomitantly determined by harmonic generation and charge-mobility reduction, is exclusively related to the presence of Dirac electron at the surface of Bi2Se3, and opens the road towards tunable terahertz nonlinear optical devices based on topological insulator materials.
NASA Astrophysics Data System (ADS)
Liu, Luqiao
2016-10-01
Spin orbit interactions give rise to interesting physics phenomena in solid state materials such as the spin Hall effect (SHE) and topological insulator surface states. Those effects have been extensively studied using various electrical detection methods. However, to date most experiments focus only on characterizing electrons near the Fermi surface, while spin-orbit interaction is expected to be energy dependent. Here we developed a tunneling spectroscopy technique to measure spin Hall materials and topological insulators under finite bias voltages. By electrically injecting spin polarized electrons into spin Hall metals or topological insulators using tunnel junctions and measuring the induced transverse voltage, we are able to study SHE in typical 5d transition metals and the spin momentum locking in topological insulators. For spin Hall effect metals, the magnitude of the spin Hall angle has been a highly controversial topic in previous studies. Results obtained from various techniques can differ by more than an order of magnitude. Our results from this transport measurement turned out to be consistent with the values obtained from spin Hall torque measurements, which can help to address the long debating issue. Besides the magnitude, the voltage dependent spectra from our experiment also provide useful information in distinguishing between different potential mechanisms. Finally, because of the impedance matching capability of tunnel junctions, the spin polarized tunneling technique can also be used as a powerful tool to measure resistive materials such as the topological insulators. Orders of magnitude improvement in the effective spin Hall angle was demonstrated through our measurement
Strong nonlinear terahertz response induced by Dirac surface states in Bi2Se3 topological insulator
Giorgianni, Flavio; Chiadroni, Enrica; Rovere, Andrea; Cestelli-Guidi, Mariangela; Perucchi, Andrea; Bellaveglia, Marco; Castellano, Michele; Di Giovenale, Domenico; Di Pirro, Giampiero; Ferrario, Massimo; Pompili, Riccardo; Vaccarezza, Cristina; Villa, Fabio; Cianchi, Alessandro; Mostacci, Andrea; Petrarca, Massimo; Brahlek, Matthew; Koirala, Nikesh; Oh, Seongshik; Lupi, Stefano
2016-01-01
Electrons with a linear energy/momentum dispersion are called massless Dirac electrons and represent the low-energy excitations in exotic materials such as graphene and topological insulators. Dirac electrons are characterized by notable properties such as a high mobility, a tunable density and, in topological insulators, a protection against backscattering through the spin–momentum locking mechanism. All those properties make graphene and topological insulators appealing for plasmonics applications. However, Dirac electrons are expected to present also a strong nonlinear optical behaviour. This should mirror in phenomena such as electromagnetic-induced transparency and harmonic generation. Here we demonstrate that in Bi2Se3 topological insulator, an electromagnetic-induced transparency is achieved under the application of a strong terahertz electric field. This effect, concomitantly determined by harmonic generation and charge-mobility reduction, is exclusively related to the presence of Dirac electron at the surface of Bi2Se3, and opens the road towards tunable terahertz nonlinear optical devices based on topological insulator materials. PMID:27113395
Is the black phase of SmS a topological Kondo insulator?
NASA Astrophysics Data System (ADS)
Bauer, Eric; Ghimire, N. J.; Ronning, F.; Batista, C.; Byler, D.; Thompson, J. D.; Rahmanisisan, A.; Fisk, Z.
2015-03-01
SmS is a prototypical Kondo insulator where electronic correlations drive a system insulating that would otherwise be metallic. Whether or not such a system is also a topological insulator that hosts a protected metallic surface state, depends on the parity of the wavefunction of the occupied states. However, unlike weakly correlated materials, it is unclear whether state-of-the-art electronic structure calculations accurately predict the parity of the occupied wavefunctions of correlated insulators. Nevertheless, Dzero and collaborators suggest that Kondo insulators such as SmB6 can be topological. Like SmB6, Cubic SmS is a non-magnetic semiconductor with an insulating behavior at ambient pressure and low temperatures driven by hybridization with the Sm f-electrons. At 6 kbar, SmS undergoes a phase transition into a valance fluctuating phase accompanied by a visible color change from black to gold. It then undergoes a second phase transition at about 20 kbar to an antiferromagnetic order at low temperatures. We will discuss whether electronic structure calculations indicate a topological state of SmS at P =0. We will also discuss whether or not the magnetic, thermal and transport properties of the black phase of SmS are consistent with the existence of a topological protected surface state.
Kelch, Inken D.; Bogle, Gib; Sands, Gregory B.; Phillips, Anthony R. J.; LeGrice, Ian J.; Rod Dunbar, P.
2015-01-01
Understanding of the microvasculature has previously been limited by the lack of methods capable of capturing and modelling complete vascular networks. We used novel imaging and computational techniques to establish the topology of the entire blood vessel network of a murine lymph node, combining 63706 confocal images at 2 μm pixel resolution to cover a volume of 3.88 mm3. Detailed measurements including the distribution of vessel diameters, branch counts, and identification of voids were subsequently re-visualised in 3D revealing regional specialisation within the network. By focussing on critical immune microenvironments we quantified differences in their vascular topology. We further developed a morphology-based approach to identify High Endothelial Venules, key sites for lymphocyte extravasation. These data represent a comprehensive and continuous blood vessel network of an entire organ and provide benchmark measurements that will inform modelling of blood vessel networks as well as enable comparison of vascular topology in different organs. PMID:26567707
Effects of 2D and 3D Error Fields on the SAS Divertor Magnetic Topology
NASA Astrophysics Data System (ADS)
Trevisan, G. L.; Lao, L. L.; Strait, E. J.; Guo, H. Y.; Wu, W.; Evans, T. E.
2016-10-01
The successful design of plasma-facing components in fusion experiments is of paramount importance in both the operation of future reactors and in the modification of operating machines. Indeed, the Small Angle Slot (SAS) divertor concept, proposed for application on the DIII-D experiment, combines a small incident angle at the plasma strike point with a progressively opening slot, so as to better control heat flux and erosion in high-performance tokamak plasmas. Uncertainty quantification of the error fields expected around the striking point provides additional useful information in both the design and the modeling phases of the new divertor, in part due to the particular geometric requirement of the striking flux surfaces. The presented work involves both 2D and 3D magnetic error field analysis on the SAS strike point carried out using the EFIT code for 2D equilibrium reconstruction, V3POST for vacuum 3D computations and the OMFIT integrated modeling framework for data analysis. An uncertainty in the magnetic probes' signals is found to propagate non-linearly as an uncertainty in the striking point and angle, which can be quantified through statistical analysis to yield robust estimates. Work supported by contracts DE-FG02-95ER54309 and DE-FC02-04ER54698.
The d-p band-inversion topological insulator in bismuth-based skutterudites
Yang, Ming; Liu, Wu-Ming
2014-01-01
Skutterudites, a class of materials with cage-like crystal structure which have received considerable research interest in recent years, are the breeding ground of several unusual phenomena such as heavy fermion superconductivity, exciton-mediated superconducting state and Weyl fermions. Here, we predict a new topological insulator in bismuth-based skutterudites, in which the bands involved in the topological band-inversion process are d- and p-orbitals, which is distinctive with usual topological insulators, for instance in Bi2Se3 and BiTeI the bands involved in the topological band-inversion process are only p-orbitals. Due to the present of large d-electronic states, the electronic interaction in this topological insulator is much stronger than that in other conventional topological insulators. The stability of the new material is verified by binding energy calculation, phonon modes analysis, and the finite temperature molecular dynamics simulations. This new material can provide nearly zero-resistivity signal current for devices and is expected to be applied in spintronics devices. PMID:24875345
The d-p band-inversion topological insulator in bismuth-based skutterudites.
Yang, Ming; Liu, Wu-Ming
2014-05-30
Skutterudites, a class of materials with cage-like crystal structure which have received considerable research interest in recent years, are the breeding ground of several unusual phenomena such as heavy fermion superconductivity, exciton-mediated superconducting state and Weyl fermions. Here, we predict a new topological insulator in bismuth-based skutterudites, in which the bands involved in the topological band-inversion process are d- and p-orbitals, which is distinctive with usual topological insulators, for instance in Bi2Se3 and BiTeI the bands involved in the topological band-inversion process are only p-orbitals. Due to the present of large d-electronic states, the electronic interaction in this topological insulator is much stronger than that in other conventional topological insulators. The stability of the new material is verified by binding energy calculation, phonon modes analysis, and the finite temperature molecular dynamics simulations. This new material can provide nearly zero-resistivity signal current for devices and is expected to be applied in spintronics devices.
Topological magnon insulators: Chern numbers and surface magnons
NASA Astrophysics Data System (ADS)
Mook, Alexander; Henk, Jürgen; Mertig, Ingrid
2016-10-01
Within the framework of a quantum-mechanical Heisenberg model, we address the topology of the magnon band structures of ferromagnetic pyrochlores, in particular Lu2V2O7. The bridge from Chern numbers of bulk magnons to essential properties of topological surface magnons is constructed by the bulk-boundary correspondence. The topological properties of the system which originate from the Dzyaloshinskii-Moriya interaction and show up as nonzero Berry curvatures suggest a method to clearly distinguish topological surface magnons from other, trivial magnons. The method is illustrated with regard to experiments.
NASA Astrophysics Data System (ADS)
Wang, Yaoping; Chui, Cheekong K.; Cai, Yiyu; Mak, KoonHou
1998-06-01
This study presents an approach to build a 3D vascular system of coronary for the development of a virtual cardiology simulator. The 3D model of the coronary arterial tree is reconstructed from the geometric information segmented from the Visible Human data set for physical analysis of catheterization. The process of segmentation is guided by a 3D topologic hierarchy structure of coronary vessels which is obtained from a mechanical model by using Coordinate Measuring Machine (CMM) probing. This mechanical professional model includes all major coronary arterials ranging from right coronary artery to atrioventricular branch and from left main trunk to left anterior descending branch. All those branches are considered as the main operating sites for cardiology catheterization. Along with the primary arterial vasculature and accompanying secondary and tertiary networks obtained from a previous work, a more complete vascular structure can then be built for the simulation of catheterization. A novel method has been developed for real time Finite Element Analysis of catheter navigation based on this featured vasculature of vessels.
Effect of Band-Alignment Operation on Carrier Transport in Bi2Se3 Topological Insulator
Gupta, Gaurav; Jalil, Mansoor Bin Abdul; Liang, Gengchiau
2014-01-01
Band-alignment induced current modulation in Bi2Se3 three-dimensional topological insulator slab has been investigated by quantum transport simulations for three different device designs, one for purely lateral transport and other two with vertical transport. Non-Equilibrium Green Function formalism has been deployed to understand the transport mechanism in band-alignment devices to appraise the possibility of a 3D-TI based resonant device. A resonance condition is observed when the Dirac-points (bands) are aligned. This results in the maximum current at resonance for the design with only lateral transport. However, current ratio between resonant and non-resonant condition is found to be relatively small and strong temperature dependence is also noticed. The other two designs with vertical transport have degraded transfer characteristics, although from state-of-art literature they are expected to manifest nearly an ideal resonance peak. The physical insights for these observations have been posited along with the suggestions for attaining close to an ideal operation for the first design, which we also suggest for the pursuit in the future for spintronic oscillators and analog multipliers based on band-alignment induced resonance. PMID:25164148
Switching Current Distributions in Superconductor-Topological Insulator-Superconductor Junctions
NASA Astrophysics Data System (ADS)
Murphy, Andrew; Zhang, Can; Huemiller, Erik; Oh, Seongshik; Eckstein, James; van Harlingen, Dale; Bezryadin, Alexey
It has been proposed that localized Majorana fermion (MF) modes can exist in lateral Josephson junctions with a 3D-topological insulator barrier at locations at which the phase difference across the junction is an odd multiple of π. These states enter the junctions bound to the nodes of the Josephson vortices as a perpendicular magnetic field is increased. Each mode contributes a local 4 π-periodic sin(φ/2)-component to the junction's current-phase relation, adding to the usual sin(φ) dependence. The sign of this new term encodes the parity of the Majorana pair. As a way to detect these states and measure their parity, we study the distribution of switching currents in Nb-Bi2Se3-Nb junctions fabricated on thin Bi2Se3 films in which the superconductivity is induced by a pair of closely spaced Nb electrodes. We expect that such measurements will be sensitive to the parity of the MFs, yielding a splitting of the distribution. Preliminary measurements of the critical current distributions show the onset of unusual features when the magnetic field is increased which we are analyzing to determine if they may arise from Majorana fermions in the junctions.
NASA Astrophysics Data System (ADS)
Fang, Chen; Hsieh, Timothy; Fu, Liang
2015-03-01
We theoretically predict two new classes of 3D topological crystalline insulators (TCI) that have protected, robust surface states. In first class, the surface states are protected by a single glide mirror symmetry. On a symmetry-preserving surface, a single Dirac point can appear at any position along either one of the two mirror symmetric lines inside the surface Brillouin zone (SBZ). In the second class, the surface Dirac point is protected by a combination of twofold rotation and time-reversal symmetry, and appears on the crystal surface perpendicular to the rotation axis. Its position in the SBZ is completely free to move by symmetry-preserving perturbations. In each class, we prove the existence of a Z2 bulk invariant and find its explicit analytic expression. These new classes of TCI do not presume the presence or the absence of spin-orbital coupling. DOE Office of Basic Energy Sciences, Division of Materials Sciences and Engineering under Award DE-SC0010526 (LF) and the STC Center for Integrated Quantum Materials, NSF Grant No. DMR-1231319 (CF).
Moiré superlattices at the topological insulator Bi2Te3
Schouteden, Koen; Li, Zhe; Chen, Taishi; Song, Fengqi; Partoens, Bart; Van Haesendonck, Chris; Park, Kyungwha
2016-01-01
We report on the observation of complex superlattices at the surface of the topological insulator Bi2Te3. Scanning tunneling microscopy reveals the existence of two different periodic structures in addition to the Bi2Te3 atomic lattice, which is found to strongly affect the local electronic structure. These three different periodicities are interpreted to result from a single small in-plane rotation of the topmost quintuple layer only. Density functional theory calculations support the observed increase in the DOS near the Fermi level, and exclude the possibility that strain is at the origin of the observed Moiré pattern. Exploration of Moiré superlattices formed by the quintuple layers of topological insulators holds great potential for further tuning of the properties of topological insulators. PMID:26854069
Criticality of the metal-topological insulator transition driven by disorder
NASA Astrophysics Data System (ADS)
Yamakage, Ai; Nomura, Kentaro; Imura, Ken-Ichiro; Kuramoto, Yoshio
2013-05-01
Employing scaling analysis of the localization length, we deduce the critical exponent of the metal-topological insulator transitions induced by disorder. The obtained exponent ν˜2.7 shows no conspicuous deviation from the value established for metal-ordinary insulator transitions in systems of the symplectic class. We investigate the topological phase diagram upon carrier doping to reveal the nature of the so-called topological Anderson insulator (TAI) region. The critical exponent of the metal-TAI transition is also first estimated, shown to be undistinguishable from the above value within the numerical error. By symmetry considerations we determine the explicit form of Rashba spin-orbit coupling in systems of C4v point group symmetry.
Topological insulator and particle pumping in a one-dimensional shaken optical lattice
NASA Astrophysics Data System (ADS)
Mei, Feng; You, Jia-Bin; Zhang, Dan-Wei; Yang, X. C.; Fazio, R.; Zhu, Shi-Liang; Kwek, L. C.
2014-12-01
We propose a simple method to simulate and detect topological insulators with cold atoms trapped in a one-dimensional bichromatic optical lattice subjected to a time-periodic modulation. The tight-binding form of this shaken system is equivalent to the periodically driven Aubry-Andre model. We demonstrate that this model can be mapped into a two-dimensional Chern insulator model, whose energy spectrum hosts a topological phase within an experimentally accessible parameter regime. By tuning the laser phase adiabatically, such one-dimensional system constitutes a natural platform to realize topological particle pumping. We show that the Chern number characterizing the topological features of this system can be measured by detecting the density shift after one cycle of pumping.
Defect Physics and Intrinsic p-TYPE Conductivity in Topological Insulator AuTlS2
NASA Astrophysics Data System (ADS)
Zhang, Jian-Min; Feng, Wangxiang; Yang, Pei; Shi, Lijie; Zhang, Ying
2014-01-01
Using first-principles calculations, we systematically investigate the defect physics in topological insulator AuTlS2. An optimal growth condition is explicitly proposed to guide for the experimental synthesis. The stabilities of various native point defects under different growth conditions and different carrier environments are studied in detail. We show that the p-type conductivity is strongly preferred in AuTlS2, and the band gap can be engineered by the control of intrinsic defects. Our results demonstrate that AuTlS2 is an ideal p-type topological insulator which can be easily integrated with traditional semiconductor.
Spin-orbit torque in two-dimensional antiferromagnetic topological insulators
NASA Astrophysics Data System (ADS)
Ghosh, S.; Manchon, A.
2017-01-01
We investigate spin transport in two-dimensional ferromagnetic (FTI) and antiferromagnetic (AFTI) topological insulators. In the presence of an in-plane magnetization AFTI supports zero energy modes, which enables topologically protected edge conduction at low energy. We address the nature of current-driven spin torque in these structures and study the impact of spin-independent disorder. Interestingly, upon strong disorder the spin torque develops an antidamping component (i.e., even upon magnetization reversal) along the edges, which could enable current-driven manipulation of the antiferromagnetic order parameter. This antidamping torque decreases when increasing the system size and when the system enters the trivial insulator regime.
Surface waves at the interface between a dielectric and a topological insulator
NASA Astrophysics Data System (ADS)
Maimistov, A. I.; Lyashko, E. I.
2016-10-01
Surface waves that propagate along the interface between an isotropic linear or nonlinear (of the Kerr type) dielectric and a topological insulator have been studied theoretically. A dispersion relation for surface waves, which are represented by superpositions of TE and TM waves, has been obtained. This hybridization occurs because, upon passage through the interface, the polarization of a surface wave changes, which is caused by an induced surface current (which is transverse to the electric field vector of the wave). The surface current of this kind is characteristic of topological insulators. Expressions for the energy flux transferred by a surface wave have been given.
Geometrically induced reversion of Hall current in a topological insulator cavity
NASA Astrophysics Data System (ADS)
Campos, W. H.; Moura-Melo, W. A.; Fonseca, J. M.
2017-02-01
An electric charge near the surface of a topological insulator induces an image magnetic monopole. Here, we show that if the topological insulator surface has a negative curvature, namely in the case of a semispherical cavity, the induced Hall current reverses its rotation as the electric charge crosses the semisphere geometric focus. Such a reversion is shown to be equivalent of inverting the charge of the image magnetic monopole. We also discuss upon the case of a semicylindrical cavity, where Hall current reversion appears to be feasible of probing in realistic experiments.
Emergent Momentum-Space Skyrmion Texture on the Surface of Topological Insulators
Mohanta, Narayan; Kampf, Arno P.; Kopp, Thilo
2017-01-01
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 e2/2h. 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. PMID:28378779
Spacetime as a topological insulator: mechanism for the origin of the fermion generations.
Kaplan, David B; Sun, Sichun
2012-05-04
We suggest a mechanism whereby the three generations of quarks and leptons correspond to surface modes in a five-dimensional theory. These modes arise from a nonlinear fermion dispersion relation in the extra dimension, much in the same manner as fermion surface modes in a topological insulator or lattice implementation of domain wall fermions. We also show that the topological properties can persist in a deconstructed version of the model in four dimensions.
NASA Astrophysics Data System (ADS)
Nagai, Yuki; Ota, Yukihiro
2016-10-01
We study the temperature dependence of nuclear magnetic relaxation (NMR) rates to detect unconventional superconductivity in doped topological insulators, such as M (=Cu ,Nb,Sr) xBi2Se3 and Sn1 -xInxTe . The Hebel-Slichter coherence effect below a critical temperature Tc depends on the superconducting states predicted by a minimal model of doped topological insulators. In a nodal anisotropic topological state similar to the ABM phase in 3He, the NMR rate has a conventional s -wave-like coherence peak below Tc. In contrast, in a fully-gapped isotropic topological superconducting state, this rate below Tc exhibits an antipeak profile. Moreover, in a twofold in-plane anisotropic topological superconducting state, there is no coherence effect, which is similar to that in a chiral p -wave state. We also claim in a model of CuxBi2Se3 that a signal of the fully-gapped odd-parity state is attainable from the change of the antipeak behavior depending on doping level. Thus, we reveal that the NMR rates shed light on unconventional superconductivity in doped topological insulators.
Das, Tanmoy; Balatsky, A. V.
2013-01-01
Topological insulators represent a new class of quantum phase defined by invariant symmetries and spin-orbit coupling that guarantees metallic Dirac excitations at its surface. The discoveries of these states have sparked the hope of realizing non-trivial excitations and novel effects such as a magnetoelectric effect and topological Majorana excitations. Here we develop a theoretical formalism to show that a three-dimensional topological insulator can be designed artificially via stacking bilayers of two-dimensional Fermi gases with opposite Rashba-type spin-orbit coupling on adjacent layers, and with interlayer quantum tunneling. We demonstrate that in the stack of bilayers grown along a (001)-direction, a non-trivial topological phase transition occurs above a critical number of Rashba bilayers. In the topological phase, we find the formation of a single spin-polarized Dirac cone at the -point. This approach offers an accessible way to design artificial topological insulators in a set up that takes full advantage of the atomic layer deposition approach. This design principle is tunable and also allows us to bypass limitations imposed by bulk crystal geometry. PMID:23739724
NASA Astrophysics Data System (ADS)
Yadav, Anil K.; Majhi, Kunjalata; Banerjee, Abhishek; Devi, Poonam; Ganesan, R.; Mishra, P.; Lohani, H.; Sekhar, B. R.; Kumar, P. S. Anil
2016-05-01
Recently discovered, Topological Insulators (TIs) have garnered enormous amount of attention owing to its unique surface properties which has potential applications in the field of spintronics and other modern technologies. For all this, it should require a very good quality samples. There are a number of techniques suggested by people for the growth of good quality TIs. Here, we are reporting the growth of high quality single crystals of Bi2Se3 (a TI) by slow cooling solid-state reaction method. X-ray diffraction measurements performed on a cleaved flake of single crystal Bi2Se3 showed up with proper orientations of the crystal planes. High energy X-ray diffraction has been performed to confirm the stoichiometry of the compound and also recorded Laue patterns prove the single crystalline nature of Bi2Se3. Moreover, angle resolved photo-emission spectroscopy (ARPES) carried out on a flat crystal flake shows distinct Dirac dispersion of surface bands at the gamma point clarifying it as a 3D topological insulator.
Knutsen, Ashleen R; Borkowski, Sean L; Ebramzadeh, Edward; Flanagan, Colleen L; Hollister, Scott J; Sangiorgio, Sophia N
2015-09-01
Recently, as an alternative to metal spinal fusion cages, 3D printed bioresorbable materials have been explored; however, the static and fatigue properties of these novel cages are not well known. Unfortunately, current ASTM testing standards used to determine these properties were designed prior to the advent of bioresorbable materials for cages. Therefore, the applicability of these standards for bioresorbable materials is unknown. In this study, an image-based topology and a conventional 3D printed bioresorbable poly(ε)-caprolactone (PCL) cervical cage design were tested in compression, compression-shear, and torsion, to establish their static and fatigue properties. Difficulties were in fact identified in establishing failure criteria and in particular determining compressive failure load. Given these limitations, under static loads, both designs withstood loads of over 650 N in compression, 395 N in compression-shear, and 0.25 Nm in torsion, prior to yielding. Under dynamic testing, both designs withstood 5 million (5M) cycles of compression at 125% of their respective yield forces. Geometry significantly affected both the static and fatigue properties of the cages. The measured compressive yield loads fall within the reported physiological ranges; consequently, these PCL bioresorbable cages would likely require supplemental fixation. Most importantly, supplemental testing methods may be necessary beyond the current ASTM standards, to provide more accurate and reliable results, ultimately improving preclinical evaluation of these devices.
Knutsen, Ashleen R.; Borkowski, Sean L.; Ebramzadeh, Edward; Flanagan, Colleen L.; Hollister, Scott J.; Sangiorgio, Sophia N.
2015-01-01
Recently, as an alternative to metal spinal fusion cages, 3D printed bioresorbable materials have been explored; however, the static and fatigue properties of these novel cages are not well known. Unfortunately, current ASTM testing standards used to determine these properties were designed prior to the advent of bioresorbable materials for cages. Therefore, the applicability of these standards for bioresorbable materials is unknown. In this study, an image-based topology and a conventional 3D printed bioresorbable poly(ε)-caprolactone (PCL) cervical cage design were tested in compression, compression-shear, and torsion, to establish their static and fatigue properties. Difficulties were in fact identified in establishing failure criteria and in particular determining compressive failure load. Given these limitations, under static loads, both designs withstood loads of over 650N in compression, 395N in compression-shear, and 0.25Nm in torsion, prior to yielding. Under dynamic testing, both designs withstood 5 million (5M) cycles of compression at 125% of their respective yield forces. Geometry significantly affected both the static and fatigue properties of the cages. The measured compressive yield loads fall within the reported physiological ranges; consequently, these PCL bioresorbable cages would likely require supplemental fixation. Most importantly, supplemental testing methods may be necessary beyond the current ASTM standards, to provide more accurate and reliable results, ultimately improving preclinical evaluation of these devices. PMID:26072198
A tunable topological insulator in the spin helical Dirac transport regime.
Hsieh, D; Xia, Y; Qian, D; Wray, L; Dil, J H; Meier, F; Osterwalder, J; Patthey, L; Checkelsky, J G; Ong, N P; Fedorov, A V; Lin, H; Bansil, A; Grauer, D; Hor, Y S; Cava, R J; Hasan, M Z
2009-08-27
Helical Dirac fermions-charge carriers that behave as massless relativistic particles with an intrinsic angular momentum (spin) locked to its translational momentum-are proposed to be the key to realizing fundamentally new phenomena in condensed matter physics. Prominent examples include the anomalous quantization of magneto-electric coupling, half-fermion states that are their own antiparticle, and charge fractionalization in a Bose-Einstein condensate, all of which are not possible with conventional Dirac fermions of the graphene variety. Helical Dirac fermions have so far remained elusive owing to the lack of necessary spin-sensitive measurements and because such fermions are forbidden to exist in conventional materials harbouring relativistic electrons, such as graphene or bismuth. It has recently been proposed that helical Dirac fermions may exist at the edges of certain types of topologically ordered insulators-materials with a bulk insulating gap of spin-orbit origin and surface states protected against scattering by time-reversal symmetry-and that their peculiar properties may be accessed provided the insulator is tuned into the so-called topological transport regime. However, helical Dirac fermions have not been observed in existing topological insulators. Here we report the realization and characterization of a tunable topological insulator in a bismuth-based class of material by combining spin-imaging and momentum-resolved spectroscopies, bulk charge compensation, Hall transport measurements and surface quantum control. Our results reveal a spin-momentum locked Dirac cone carrying a non-trivial Berry's phase that is nearly 100 per cent spin-polarized, which exhibits a tunable topological fermion density in the vicinity of the Kramers point and can be driven to the long-sought topological spin transport regime. The observed topological nodal state is shown to be protected even up to 300 K. Our demonstration of room-temperature topological order and non
Electronic transport in nano-devices based on graphene and topological insulators
NASA Astrophysics Data System (ADS)
Jauregui, Luis A.
In this thesis we experimentally focused on quantum transport in graphene and topological insulators and we worked on four main projects. 1) The extraordinary properties and potential applications of graphene have motivated the development of large-scale, synthetic graphene, such as chemical vapor deposition (CVD) on Cu. In this research, we study graphene grains (either isolated grains or a small number of merged grains) formed during the early stage of ambient CVD growth on Cu foils. We show that grain boundaries give a significant Raman 'D' peak, impede electrical transport, and induce prominent weak localization indicative of intervalley scattering in graphene. In order for graphene to realize its promise in \\carbon-based" electronics, it is necessary to achieve a better control over the nucleation of individual graphene grains and to avoid the grain boundaries in fabricated graphene devices. 2) Topological Insulators are materials where the bandstructure has a bulk bandgap and conductive surface. The topological surface state is characterized by a linear dispersion, similar to graphene, but it is topologically protected, suppressing backscattering. One of the main sources of scattering in ultrathin nanowire devices is backscattering, and this could be suppressed in topological insulator nanowires (TINWs). In this study we have measured ambipolar field effect and metal-insulator transitions on topological insulator nanowire field effect devices. We also have measured the Aharonov-Bohn (A-B) and Shubnikov de Haas (SdH) oscillations showing electrons on the surface of TINWs behave like massless Dirac particles. 3) We measured the gate voltage dependent A-B oscillation phase and we infer the surface of TINWs is quantized forming subbands with a gap at the Dirac point when 0 or even multiples of half magnetic flux quantum (Phi_0) is threaded through the TINWs (axial magnetic field). However, it undergoes through topological transitions when odd multiples of phi0
Lattice-matched heterojunctions between topological and normal insulators: A first-principles study
NASA Astrophysics Data System (ADS)
Lee, Hyungjun; Yazyev, Oleg V.
2017-02-01
Gapless boundary modes at the interface between topologically distinct regions are one of the most salient manifestations of topology in physics. Metallic boundary states of time-reversal-invariant topological insulators (TIs), a realization of topological order in condensed matter, have been of much interest not only due to such a fundamental nature, but also due to their practical significance. These boundary states are immune to backscattering and localization owing to their topological origin, thereby opening up the possibility to tailor them for potential uses in spintronics and quantum computing. The heterojunction between a TI and a normal insulator (NI) is a representative playground for exploring such a topologically protected metallic boundary state and expected to constitute a building block for future electronic and spintronic solid-state devices based on TIs. Here, we report a first-principles study of two experimentally realized lattice-matched heterojunctions between TIs and NIs, Bi2Se3 (0001)/InP(111) and Bi2Te3 (0001)/BaF2(111). We evaluate the band offsets at these interfaces from many-body perturbation theory within the G W approximation as well as density-functional theory. Furthermore, we investigate the topological interface states, demonstrating that at these lattice-matched heterointerfaces, they are strictly localized and their helical spin textures are as well preserved as those at the vacuum-facing surfaces. These results taken together may help in designing devices relying on spin-helical metallic boundary states of TIs.
Si, Weidong E-mail: qiangli@bnl.gov; Zhang, Cheng; Wu, Lijun; Ozaki, Toshinori; Gu, Genda; Li, Qiang E-mail: qiangli@bnl.gov
2015-08-31
Recent discovery of the topological crystalline insulator SnTe has triggered a search for topological superconductors, which have potential application to topological quantum computing. The present work reports on the superconducting properties of indium doped SnTe thin films. The (100) and (111) oriented thin films were epitaxially grown by pulsed-laser deposition on (100) and (111) BaF{sub 2} crystalline substrates, respectively. The onset superconducting transition temperatures are about 3.8 K for (100) and 3.6 K for (111) orientations, slightly lower than that of the bulk. Magneto-resistive measurements indicate that these thin films may have upper critical fields higher than that of the bulk. With large surface-to-bulk ratio, superconducting indium doped SnTe thin films provide a rich platform for the study of topological superconductivity and potential device applications based on topological superconductors.
Si, W.; Zhang, C.; Wu, L.; ...
2015-09-01
Recent discovery of the topological crystalline insulator SnTe has triggered a search for topological superconductors, which have potential application to topological quantum computing. The present work reports on the superconducting properties of indium doped SnTe thin films. The (100) and (111) oriented thin films were epitaxially grown by pulsed-laser deposition on (100) and (111) BaF2 crystalline substrates respectively. The onset superconducting transition temperatures are about 3.8 K for (100) and 3.6 K for (111) orientations, slightly lower than that of the bulk. Magneto-resistive measurements indicate that these thin films may have upper critical fields higher than that of the bulk.more » With large surface-to-bulk ratio, superconducting indium doped SnTe thin films provide a rich platform for the study of topological superconductivity and potential device applications based on topological superconductors.« less
Si, W.; Zhang, C.; Wu, L.; Ozaki, T.; Gu, G.; Li, Q.
2015-09-01
Recent discovery of the topological crystalline insulator SnTe has triggered a search for topological superconductors, which have potential application to topological quantum computing. The present work reports on the superconducting properties of indium doped SnTe thin films. The (100) and (111) oriented thin films were epitaxially grown by pulsed-laser deposition on (100) and (111) BaF2 crystalline substrates respectively. The onset superconducting transition temperatures are about 3.8 K for (100) and 3.6 K for (111) orientations, slightly lower than that of the bulk. Magneto-resistive measurements indicate that these thin films may have upper critical fields higher than that of the bulk. With large surface-to-bulk ratio, superconducting indium doped SnTe thin films provide a rich platform for the study of topological superconductivity and potential device applications based on topological superconductors.
Electrical Probing of Inherent Spin Polarization in a Topological Insulator with Electrical Gating
NASA Astrophysics Data System (ADS)
Lee, Joon Sue; Richardella, Anthony; Samarth, Nitin
2015-03-01
The hallmark of a time-reversal symmetry protected three-dimensional topological insulator is the helically spin-textured surface state. Although electrical detection of spin polarization in topological insulators has been demonstrated very recently, there have not been any electrical measurements to demonstrate the entire mapping of the spin polarization throughout the surface state. We report the electrical probing of the spin-polarized surface state using a magnetic tunnel junction as a spin detector while the chemical potential of a topological insulator (Bi,Sb)2Te3 is tuned by back gating. Hysteretic spin signals were observed as the magnetization of the detector ferromagnet (permalloy) switches with in-plane magnetic field. Changing the direction of bias current through the topological insulator channel flips the direction of the spin polarization, resulting in the reverse of sign of the detected spin signals. We demonstrate the control of the Fermi energy, which has importance not only in further understanding of the spin-momentum locking in the surface state but also in possible electrical tuning of the spin polarization for potential spin-based devices. Supported by C-SPIN & DARPA/SRC.
Wang, Yu-Qi; Wu, Xu; Wang, Ye-Liang; Shao, Yan; Lei, Tao; Wang, Jia-Ou; Zhu, Shi-Yu; Guo, Haiming; Zhao, Ling-Xiao; Chen, Gen-Fu; Nie, Simin; Weng, Hong-Ming; Ibrahim, Kurash; Dai, Xi; Fang, Zhong; Gao, Hong-Jun
2016-07-01
2D materials with heterolayered structures beyond graphene are explored. A theoretically predicted superconductor-topological insulator-normal metal heterolayered structure is realized experimentally. The generated hybrid structure HfTe3 /HfTe5 /Hf has potential applications in both quantum-spin Hall effect-based and Majorana-based devices.
Higher dimensional quantum Hall effect as A-class topological insulator
NASA Astrophysics Data System (ADS)
Hasebe, Kazuki
2014-09-01
We perform a detail study of higher dimensional quantum Hall effects and A-class topological insulators with emphasis on their relations to non-commutative geometry. There are two different formulations of non-commutative geometry for higher dimensional fuzzy spheres: the ordinary commutator formulation and quantum Nambu bracket formulation. Corresponding to these formulations, we introduce two kinds of monopole gauge fields: non-abelian gauge field and antisymmetric tensor gauge field, which respectively realize the non-commutative geometry of fuzzy sphere in the lowest Landau level. We establish connection between the two types of monopole gauge fields through Chern-Simons term, and derive explicit form of tensor monopole gauge fields with higher string-like singularity. The connection between two types of monopole is applied to generalize the concept of flux attachment in quantum Hall effect to A-class topological insulator. We propose tensor type Chern-Simons theory as the effective field theory for membranes in A-class topological insulators. Membranes turn out to be fractionally charged objects and the phase entanglement mediated by tensor gauge field transforms the membrane statistics to be anyonic. The index theorem supports the dimensional hierarchy of A-class topological insulator. Analogies to D-brane physics of string theory are discussed too.
Hydrodynamic theory of surface excitations of three-dimensional topological insulators
NASA Astrophysics Data System (ADS)
Vildanov, N. M.
2011-03-01
Edge excitations of a fractional quantum Hall system can be derived as surface excitations of an incompressible quantum droplet using one-dimensional chiral bosonization. Here we show that an analogous approach can be developed to characterize surface states of three-dimensional time-reversal invariant topological insulators. The key ingredient of our theory is the Luther’s multidimensional bosonization construction.
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.
Topological Insulators and Nematic Phases from Spontaneous Symmetry Breaking in
Sun, K.
2010-05-26
We investigate the stability of a quadratic band-crossing point (QBCP) in 2D fermionic systems. At the non-interacting level, we show that a QBCP exists and is topologically stable for a Berry flux {-+}2{pi}, if the point symmetry group has either fourfold or sixfold rotational symmetries. This putative topologically stable free-fermion QBCP is marginally unstable to arbitrarily weak shortrange repulsive interactions. We consider both spinless and spin-1/2 fermions. Four possible ordered states result: a quantum anomalous Hall phase, a quantum spin Hall phase, a nematic phase, and a nematic-spin-nematic phase.
BaSn2: A wide-gap strong topological insulator
NASA Astrophysics Data System (ADS)
Young, Steve M.; Manni, S.; Shao, Junping; Canfield, Paul C.; Kolmogorov, Aleksey N.
2017-02-01
BaSn2 has been shown to form as layers of buckled stanene intercalated by barium ions. However, despite an apparently straightforward synthesis and significant interest in stanene as a topological material, BaSn2 has been left largely unexplored, and has only recently been recognized as a potential topological insulator. Belonging to neither the lead nor bismuth chalcogenide families, it would represent a unique manifestation of the topological insulating phase. Here we present a detailed investigation of BaSn2, using both ab initio and experimental methods. First-principles calculations demonstrate that this overlooked material is indeed a strong, wide-gap topological insulator with a bulk band gap of 200 meV. We characterize the surface state dependence on termination chemistry, providing guidance for experimental efforts to measure and manipulate its topological properties. Additionally, through ab initio modeling and synthesis experiments, we explore the stability and accessibility of this phase, revealing a complicated phase diagram that indicates a challenging path to obtaining single crystals.
Disorder-induced structural transitions in topological insulating Ge-Sb-Te compounds
Kim, Jeongwoo; Jhi, Seung-Hoon
2015-05-21
The mechanism for the fast switching between amorphous, metastable, and crystalline structures in chalcogenide phase-change materials has been a long-standing puzzle. Based on first-principles calculations, we study the atomic and electronic properties of metastable Ge{sub 2}Sb{sub 2}Te{sub 5} and investigate the atomic disorder to understand the transition between crystalline hexagonal and cubic structures. In addition, we study the topological insulating property embedded in these compounds and its evolution upon structural changes and atomic disorder. We also discuss the role of the surface-like states arising from the topological insulating property in the metal-insulator transition observed in the hexagonal structure.
Ultrafast electron dynamics at the Dirac node of the topological insulator Sb2Te3.
Zhu, Siyuan; Ishida, Yukiaki; Kuroda, Kenta; Sumida, Kazuki; Ye, Mao; Wang, Jiajia; Pan, Hong; Taniguchi, Masaki; Qiao, Shan; Shin, Shik; Kimura, Akio
2015-08-21
Topological insulators (TIs) are a new quantum state of matter. Their surfaces and interfaces act as a topological boundary to generate massless Dirac fermions with spin-helical textures. Investigation of fermion dynamics near the Dirac point (DP) is crucial for the future development of spintronic devices incorporating topological insulators. However, research so far has been unsatisfactory because of a substantial overlap with the bulk valence band and a lack of a completely unoccupied DP. Here, we explore the surface Dirac fermion dynamics in the TI Sb2Te3 by time- and angle-resolved photoemission spectroscopy (TrARPES). Sb2Te3 has an in-gap DP located completely above the Fermi energy (EF). The excited electrons in the upper Dirac cone stay longer than those below the DP to form an inverted population. This was attributed to a reduced density of states (DOS) near the DP.
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.
Ultrafast electron dynamics at the Dirac node of the topological insulator Sb2Te3
Zhu, Siyuan; Ishida, Yukiaki; Kuroda, Kenta; Sumida, Kazuki; Ye, Mao; Wang, Jiajia; Pan, Hong; Taniguchi, Masaki; Qiao, Shan; Shin, Shik; Kimura, Akio
2015-01-01
Topological insulators (TIs) are a new quantum state of matter. Their surfaces and interfaces act as a topological boundary to generate massless Dirac fermions with spin-helical textures. Investigation of fermion dynamics near the Dirac point (DP) is crucial for the future development of spintronic devices incorporating topological insulators. However, research so far has been unsatisfactory because of a substantial overlap with the bulk valence band and a lack of a completely unoccupied DP. Here, we explore the surface Dirac fermion dynamics in the TI Sb2Te3 by time- and angle-resolved photoemission spectroscopy (TrARPES). Sb2Te3 has an in-gap DP located completely above the Fermi energy (EF). The excited electrons in the upper Dirac cone stay longer than those below the DP to form an inverted population. This was attributed to a reduced density of states (DOS) near the DP. PMID:26294343
Zhou, Tong; Zhang, Jiayong; Zhao, Bao; Zhang, Huisheng; Yang, Zhongqin
2015-08-12
Electronic and topological behaviors of Sb(111) monolayers decorated with H and certain magnetic atoms are investigated by using ab initio methods. The drastic exchange field induced by the magnetic atoms, together with strong spin-orbit coupling (SOC) of Sb atoms, generates one new category of valley polarized topological insulators, called quantum spin-quantum anomalous Hall (QSQAH) insulators in the monolayer, with a band gap up to 53 meV. The strong SOC is closely related to Sb px and py orbitals, instead of pz orbitals in usual two-dimensional (2D) materials. Topological transitions from quantum anomalous Hall states to QSQAH states and then to time-reversal-symmetry-broken quantum spin Hall states are achieved by tuning the SOC strength. The behind mechanism is revealed. Our work is helpful for future valleytronic and spintronic applications in 2D materials.
Dual nature of magnetic dopants and competing trends in topological insulators.
Sessi, Paolo; Biswas, Rudro R; Bathon, Thomas; Storz, Oliver; Wilfert, Stefan; Barla, Alessandro; Kokh, Konstantin A; Tereshchenko, Oleg E; Fauth, Kai; Bode, Matthias; Balatsky, Alexander V
2016-06-27
Topological insulators interacting with magnetic impurities have been reported to host several unconventional effects. These phenomena are described within the framework of gapping Dirac quasiparticles due to broken time-reversal symmetry. However, the overwhelming majority of studies demonstrate the presence of a finite density of states near the Dirac point even once topological insulators become magnetic. Here, we map the response of topological states to magnetic impurities at the atomic scale. We demonstrate that magnetic order and gapless states can coexist. We show how this is the result of the delicate balance between two opposite trends, that is, gap opening and emergence of a Dirac node impurity band, both induced by the magnetic dopants. Our results evidence a more intricate and rich scenario with respect to the once generally assumed, showing how different electronic and magnetic states may be generated and controlled in this fascinating class of materials.
Dual nature of magnetic dopants and competing trends in topological insulators
Sessi, Paolo; Biswas, Rudro R.; Bathon, Thomas; Storz, Oliver; Wilfert, Stefan; Barla, Alessandro; Kokh, Konstantin A.; Tereshchenko, Oleg E.; Fauth, Kai; Bode, Matthias; Balatsky, Alexander V.
2016-01-01
Topological insulators interacting with magnetic impurities have been reported to host several unconventional effects. These phenomena are described within the framework of gapping Dirac quasiparticles due to broken time-reversal symmetry. However, the overwhelming majority of studies demonstrate the presence of a finite density of states near the Dirac point even once topological insulators become magnetic. Here, we map the response of topological states to magnetic impurities at the atomic scale. We demonstrate that magnetic order and gapless states can coexist. We show how this is the result of the delicate balance between two opposite trends, that is, gap opening and emergence of a Dirac node impurity band, both induced by the magnetic dopants. Our results evidence a more intricate and rich scenario with respect to the once generally assumed, showing how different electronic and magnetic states may be generated and controlled in this fascinating class of materials. PMID:27345240
Quantum Hall Effect on Surface States of Topological Insulator (Bi1-xSbx)2Te3 Thin Films
NASA Astrophysics Data System (ADS)
Yoshimi, Ryutaro; Tsukazaki, Atsushi; Kozuka, Yusuke; Falson, Joseph; Takahashi, Kei; Checkelsky, Joseph; Nagaosa, Naoto; Kawasaki, Masashi; Tokura, Yoshinori
2015-03-01
The three-dimensional (3D) topological insulator (TI) is a novel state of matter as characterized by two-dimensional metallic Dirac states on its surface. Quantum transport in Dirac systems has been attracting much attention for the half-integer quantum Hall effect (QHE), as typically observed in graphene. Unlike the case of graphene, the Dirac states of TIs have no degeneracy including spin degree of freedom. Instead, both top and bottom surfaces host Dirac states with opposite spin-momentum locked modes. Such a helicity degree of freedom in real space is expected to yield intriguing quantum phenomena in 3D TIs. Bi-based chalcogenides such as Bi2Se3,Bi2Te3,Sb2Te3 and their compounds are candidates where the Dirac features can be detected via quantum transport phenomena in thin films form. Here, we report the realization of the QHE on the surface Dirac states in (Bi1-xSbx)2Te3 films (x = 0.84 and 0.88). With electrostatic gate-tuning of Fermi level under magnetic fields, QH states with filling factor ν = +/- 1 are resolved with quantized Hall resistance of Ryx = +/- h /e2 and zero Rxx, owing to the chiral edge modes at top/bottom surface Dirac states. The significant technical advance in 3D TI films may pave a way toward TI-based electronics.
NASA Astrophysics Data System (ADS)
Suzuki, Y.; Geiger, J.
2016-06-01
The impact of the 3D equilibrium response on the plasma edge topology is studied. In Wendelstein 7-X, the island divertor concept is used to assess scenarios for quasi-steady-state operation. However, the boundary islands necessary for the island divertor are strongly susceptible to plasma beta and toroidal current density effects because of the low magnetic shear. The edge magnetic topology for quasi-steady-state operation scenarios is calculated with the HINT-code to study the accompanying changes of the magnetic field structures. Two magnetic configurations have been selected, which had been investigated in self consistent neoclassical transport simulations for low bootstrap current but which use the alternative natural island chains to the standard iota value of 1 (ι b = 5/5, periodicity), namely, at high-iota (ι b = 5/4) and at low-iota (ι b = 5/6). For the high-iota configuration, the boundary islands are robust but the stochasticity around them is enhanced with beta. The addition of toroidal current densities enhances the stochasticity further. The increased stochasticity changes the footprints on in-vessel components with a direct impact on the corresponding heat loads. In the low-iota configuration the boundary islands used for the island divertor are dislocated radially due to the low shear and even show healing effects, i.e. the island width vanishes. In the latter case the plasma changes from divertor to limiter operation. To realize the predicted high-performance quasi-steady-state operation of the transport simulations, further adjustments of the magnetic configuration may be necessary to achieve a proper divertor compatibility of the scenarios.
Sun, Yayong; Zhao, Siwei; Ma, Haoran; Han, Yi; Liu, Kang; Wang, Lei
2016-06-15
Two novel three-dimensional (3D) pillar-layered metal-organic frameworks (MOFs), namely [Zn{sub 2}(μ{sub 2}-OH)(boaba)(1,4-bmimb)]{sub n} (1) and {[Zn_5K_2(μ_2-H_2O)_2(boaba)_4(1,2-bmimb)_2(H_2O)_2]·H_2O}{sub n} (2), were prepared by hydrothermal reactions (H{sub 3}boaba=3,5-bis-oxyacetate-benzoic acid; 1,4-bmimb=1,4-bis((2-methyl-1H-imidazol-1-yl)methyl)benzene; 1,2-bmimb =1,2-bis((2-methyl-1H-imidazol-1-yl)methyl)benzene). Notably, 1 exhibits a (3,5)-connected binodal (6{sup 3})(6{sup 9}·8)-gra net with binuclear [Zn{sub 2}(μ{sub 2}-OH)(COO)]{sup 2+} clusters, while 2 shows a novel (4,4,5,9)-connected 4-nodal net constructed from the unique Zn(II)-K(I) heterometal rod-like substructures. The results indicate that the disposition of the 2-methylimidazolyl groups of bis(imidazole) ligands have a significant effect on structural diversity. Moreover, the photoluminescence properties of 1 and 2 have been investigated. - Graphical abstract: Two novel 3D pillar-layered metal-organic coordination networks with aromatic multicarboxylate anion and bis(imidazole) ligands have been synthesized and structurally characterized. Display Omitted - Highlights: • It is rarely reported that metal-organic frameworks prepared with 3,5-bis-oxyacetate-benzoic acid. • Two metal-organic frameworks based on positional isomeric ligands were synthesized and structurally characterized. • Compond 1 displays unique (3,5)-connected binodal gra topology. • Compound 2 exhibits (4,4,5,9)-connected 4-nodal topology based on the Zn(II)-K(I) heterometal rod-like substructures. • The photoluminescence properties of compound 1 and 2 have been investigated.
Aharonov-Bohm oscillations in a quasi-ballistic three-dimensional topological insulator nanowire
NASA Astrophysics Data System (ADS)
Cho, Sungjae; Dellabetta, Brian; Zhong, Ruidan; Schneeloch, John; Liu, Tiansheng; Gu, Genda; Gilbert, Matthew J.; Mason, Nadya
2015-07-01
Aharonov-Bohm oscillations effectively demonstrate coherent, ballistic transport in mesoscopic rings and tubes. In three-dimensional topological insulator nanowires, they can be used to not only characterize surface states but also to test predictions of unique topological behaviour. Here we report measurements of Aharonov-Bohm oscillations in (Bi1.33Sb0.67)Se3 that demonstrate salient features of topological nanowires. By fabricating quasi-ballistic three-dimensional topological insulator nanowire devices that are gate-tunable through the Dirac point, we are able to observe alternations of conductance maxima and minima with gate voltage. Near the Dirac point, we observe conductance minima for zero magnetic flux through the nanowire and corresponding maxima (having magnitudes of almost a conductance quantum) at magnetic flux equal to half a flux quantum; this is consistent with the presence of a low-energy topological mode. The observation of this mode is a necessary step towards utilizing topological properties at the nanoscale in post-CMOS applications.
Aharonov-Bohm oscillations in a quasi-ballistic three-dimensional topological insulator nanowire.
Cho, Sungjae; Dellabetta, Brian; Zhong, Ruidan; Schneeloch, John; Liu, Tiansheng; Gu, Genda; Gilbert, Matthew J; Mason, Nadya
2015-07-09
Aharonov-Bohm oscillations effectively demonstrate coherent, ballistic transport in mesoscopic rings and tubes. In three-dimensional topological insulator nanowires, they can be used to not only characterize surface states but also to test predictions of unique topological behaviour. Here we report measurements of Aharonov-Bohm oscillations in (Bi1.33Sb0.67)Se3 that demonstrate salient features of topological nanowires. By fabricating quasi-ballistic three-dimensional topological insulator nanowire devices that are gate-tunable through the Dirac point, we are able to observe alternations of conductance maxima and minima with gate voltage. Near the Dirac point, we observe conductance minima for zero magnetic flux through the nanowire and corresponding maxima (having magnitudes of almost a conductance quantum) at magnetic flux equal to half a flux quantum; this is consistent with the presence of a low-energy topological mode. The observation of this mode is a necessary step towards utilizing topological properties at the nanoscale in post-CMOS applications.
Disorder Effects on Electron Transport in Nanocrystal Assemblies and Topological Insulators
NASA Astrophysics Data System (ADS)
Chen, Tianran
The continuing development of new energy technologies for electronic devices and medical applications necessitates the search for advanced nanomaterials. Among the more promising candidates are two novel materials: nanocrystal (NC) assemblies and three-dimensional (3D) topological insulators (TIs). The former have great promise for optoelectronic and photovoltaic devices, while the latter can be applied in spintronics and quantum computing. Thus far, however, the development of NC- and TI-based devices have been slowed by a lack of a solid theoretical understanding of many of their electronic properties, in particular, the influence of the presence of disorder on charge transport. In this thesis we propose to help address this need by performing a detailed, theoretical analysis of the disorder effects on electronic transport properties of NC arrays and TIs. NC assemblies can be made from different materials. Specifically, we consider three types of systems: semiconductor NCs, metallic NCs and superconducting grains. As-grown semiconductor NCs are insulators, and in order for them to be useful in photovoltaic devices, their electrical conductivity must be tuned by doping. Recent experiments have shown that the resistivity of a dense crystalline array of semiconductor NCs depends in a sensitive way on the level of doping as well as on the NC size and spacing. We show that in sufficiently small NCs, the fluctuations in donor number from one NC to another provide disorder that helps to determine the conduction mechanism in the array. Using this model, we explain how the different regimes of resistivity observed in experiment arise based on the interplay between the charging spectrum of NCs, the long-ranged Coulomb interactions between charged NCs, and the discrete quantum energy levels of confined electrons. We supplement our theory with a computer simulation, which we use to calculate the single particle density of states (DOS) and the resistivity. Compared to
NASA Astrophysics Data System (ADS)
Kulatov, E. T.; Men'shov, V. N.; Tugushev, V. V.; Uspenskii, Yu. A.
2016-09-01
This letter presents the first-principles study of a three-dimensional (3D) topological insulator (TI) Bi2Se3 doped with magnetic atoms of 3d-transition metal (TM), Cr or Fe. Calculations are performed within the density functional theory (DFT) and a modified potential of Becke and Johnson (MBJ). Using the MBJ scheme, we show that both systems are semiconductors with a direct gap of 0.21 eV (Bi2Se3:Cr) and 0.27 eV (Bi2Se3:Fe), respectively. The magnetic structures of studied materials are, however, different: Bi2Se3:Cr exhibits a clear ferromagnetic (FM) order, while Bi2Se3:Fe has an antiferromagnetic (AFM) order. In view of the great scientific and practical interest to the 3D FMTI systems, the optical and magneto-optical spectra of Bi2Se3:Cr are calculated.
Excitons in topological Kondo insulators: Theory of thermodynamic and transport anomalies in SmB6
NASA Astrophysics Data System (ADS)
Knolle, Johannes; Cooper, Nigel R.
2017-03-01
Kondo insulating materials lie outside the usual dichotomy of weakly versus correlated—band versus Mott—insulators. They are metallic at high temperatures but resemble band insulators at low temperatures because of the opening of an interaction-induced band gap. The first discovered Kondo insulator (KI) SmB6 has been predicted to form a topological KI (TKI). However, since its discovery thermodynamic and transport anomalies have been observed that have defied a theoretical explanation. Enigmatic signatures of collective modes inside the charge gap are seen in specific heat, thermal transport, and quantum oscillation experiments in strong magnetic fields. Here, we show that TKIs are susceptible to the formation of excitons and magnetoexcitons. These charge neutral composite particles can account for long-standing anomalies in SmB6 .
Knolle, Johannes; Cooper, Nigel R
2017-03-03
Kondo insulating materials lie outside the usual dichotomy of weakly versus correlated-band versus Mott-insulators. They are metallic at high temperatures but resemble band insulators at low temperatures because of the opening of an interaction-induced band gap. The first discovered Kondo insulator (KI) SmB_{6} has been predicted to form a topological KI (TKI). However, since its discovery thermodynamic and transport anomalies have been observed that have defied a theoretical explanation. Enigmatic signatures of collective modes inside the charge gap are seen in specific heat, thermal transport, and quantum oscillation experiments in strong magnetic fields. Here, we show that TKIs are susceptible to the formation of excitons and magnetoexcitons. These charge neutral composite particles can account for long-standing anomalies in SmB_{6}.
Tunable Dirac cone in the topological insulator Bi(2-x)Sb(x)Te(3-y)Se(y).
Arakane, T; Sato, T; Souma, S; Kosaka, K; Nakayama, K; Komatsu, M; Takahashi, T; Ren, Zhi; Segawa, Kouji; Ando, Yoichi
2012-01-24
The three-dimensional topological insulator is a quantum state of matter characterized by an insulating bulk state and gapless Dirac cone surface states. Device applications of topological insulators require a highly insulating bulk and tunable Dirac carriers, which has so far been difficult to achieve. Here we demonstrate that Bi(2-x)Sb(x)Te(3-y)Se(y) is a system that simultaneously satisfies both of these requirements. For a series of compositions presenting bulk-insulating transport behaviour, angle-resolved photoemission spectroscopy reveals that the chemical potential is always located in the bulk band gap, whereas the Dirac cone dispersion changes systematically so that the Dirac point moves up in energy with increasing x, leading to a sign change of the Dirac carriers at x~0.9. Such a tunable Dirac cone opens a promising pathway to the development of novel devices based on topological insulators.
Ultrafast photocurrents at the surface of the three-dimensional topological insulator Bi2Se3
NASA Astrophysics Data System (ADS)
Braun, Lukas; Mussler, Gregor; Hruban, Andrzej; Konczykowski, Marcin; Schumann, Thomas; Wolf, Martin; Münzenberg, Markus; Perfetti, Luca; Kampfrath, Tobias
2016-10-01
Three-dimensional topological insulators are fascinating materials with insulating bulk yet metallic surfaces that host highly mobile charge carriers with locked spin and momentum. Remarkably, surface currents with tunable direction and magnitude can be launched with tailored light beams. To better understand the underlying mechanisms, the current dynamics need to be resolved on the timescale of elementary scattering events (~10 fs). Here, we excite and measure photocurrents in the model topological insulator Bi2Se3 with a time resolution of 20 fs by sampling the concomitantly emitted broadband terahertz (THz) electromagnetic field from 0.3 to 40 THz. Strikingly, the surface current response is dominated by an ultrafast charge transfer along the Se-Bi bonds. In contrast, photon-helicity-dependent photocurrents are found to be orders of magnitude smaller than expected from generation scenarios based on asymmetric depopulation of the Dirac cone. Our findings are of direct relevance for broadband optoelectronic devices based on topological-insulator surface currents.
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.
Ultrafast photocurrents at the surface of the three-dimensional topological insulator Bi2Se3
Braun, Lukas; Mussler, Gregor; Hruban, Andrzej; Konczykowski, Marcin; Schumann, Thomas; Wolf, Martin; Münzenberg, Markus; Perfetti, Luca; Kampfrath, Tobias
2016-01-01
Three-dimensional topological insulators are fascinating materials with insulating bulk yet metallic surfaces that host highly mobile charge carriers with locked spin and momentum. Remarkably, surface currents with tunable direction and magnitude can be launched with tailored light beams. To better understand the underlying mechanisms, the current dynamics need to be resolved on the timescale of elementary scattering events (∼10 fs). Here, we excite and measure photocurrents in the model topological insulator Bi2Se3 with a time resolution of 20 fs by sampling the concomitantly emitted broadband terahertz (THz) electromagnetic field from 0.3 to 40 THz. Strikingly, the surface current response is dominated by an ultrafast charge transfer along the Se–Bi bonds. In contrast, photon-helicity-dependent photocurrents are found to be orders of magnitude smaller than expected from generation scenarios based on asymmetric depopulation of the Dirac cone. Our findings are of direct relevance for broadband optoelectronic devices based on topological-insulator surface currents. PMID:27796297
Gehring, Pascal; Vaklinova, Kristina; Hoyer, Alexander; Benia, Hadj M; Skakalova, Viera; Argentero, Giacomo; Eder, Franz; Meyer, Jannik C; Burghard, Marko; Kern, Klaus
2015-06-29
Three-dimensional topological insulators comprise topologically protected surface states displaying massless, Dirac-like linear dispersion with spin-momentum locking. Electrical conduction through such surface states has been documented to manifest itself in a two-dimensional character of the angle-dependent magnetotransport behavior. Here, we explore the size-dependent electronic properties of nanostructures made of the lead-containing mineral Aleksite, a naturally occurring topological insulator. Compared to its sister compound Kawazulite, a member of the well-studied Tetradymite crystal class, the crystal structure of Aleksite is distinguished by its lack of any counterpart within the group of synthetic topological insulators. Low temperature Hall measurements on thin Aleksite nanosheets reveal a significant carrier mobility on the order of 1000 cm(2)/(Vs), and a high carrier density of n = 3.9 × 10(25) m(-3). Importantly, for Aleksite nanoribbons with a width below 150 nm, a 1D weak antilocalization effect along with 1D universal conductance fluctuations emerges, which transforms into 2D behavior for larger ribbon widths.
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.
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.
NASA Astrophysics Data System (ADS)
Gehring, Pascal; Vaklinova, Kristina; Hoyer, Alexander; Benia, Hadj M.; Skakalova, Viera; Argentero, Giacomo; Eder, Franz; Meyer, Jannik C.; Burghard, Marko; Kern, Klaus
2015-06-01
Three-dimensional topological insulators comprise topologically protected surface states displaying massless, Dirac-like linear dispersion with spin-momentum locking. Electrical conduction through such surface states has been documented to manifest itself in a two-dimensional character of the angle-dependent magnetotransport behavior. Here, we explore the size-dependent electronic properties of nanostructures made of the lead-containing mineral Aleksite, a naturally occurring topological insulator. Compared to its sister compound Kawazulite, a member of the well-studied Tetradymite crystal class, the crystal structure of Aleksite is distinguished by its lack of any counterpart within the group of synthetic topological insulators. Low temperature Hall measurements on thin Aleksite nanosheets reveal a significant carrier mobility on the order of 1000 cm2/(Vs), and a high carrier density of n = 3.9 × 1025 m-3. Importantly, for Aleksite nanoribbons with a width below 150 nm, a 1D weak antilocalization effect along with 1D universal conductance fluctuations emerges, which transforms into 2D behavior for larger ribbon widths
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.
NASA Astrophysics Data System (ADS)
Shvets, Gennady B.
2016-09-01
Electromagnetic (EM) waves propagating through an inhomogeneous medium inevitably scatter whenever the medium's electromagnetic properties change on the scale of a single wavelength. This fundamental phenomenon constrains how optical structures are designed and interfaced with each other. Recent theoretical work indicates that electromagnetic structures collectively known as photonic topological insulators (PTIs) can be employed to overcome this fundamental limitation, thereby paving the way for ultra-compact photonic structures that no longer have to be wavelength-scale smooth. I will review some of the recent developments in the field of topological photonics and discuss several novel directions. Those include the first experimental realization of the topologically robust (i.e. reflections- and interference-free) delay line that enables nearly-arbitrary engineering of the optical phase along the light's path. Novel concepts such as all-dielectric PTIs that emulate the valley degree of freedom, as well as topologically protected high-Q cavities will also be discussed.
Enhancement of the Bulk Photovoltaic Effect in Topological Insulators
NASA Astrophysics Data System (ADS)
Tan, Liang Z.; Rappe, Andrew M.
2016-06-01
We investigate the shift current bulk photovoltaic response of materials close to a band inversion topological phase transition. We find that the bulk photocurrent reverses direction across the band inversion transition, and that its magnitude is enhanced in the vicinity of the phase transition. These results are demonstrated with first principles density functional theory calculations of BiTeI and CsPbI3 under hydrostatic pressure, and explained with an analytical model, suggesting that this phenomenon remains robust across disparate material systems.
Enhancement of the Bulk Photovoltaic Effect in Topological Insulators.
Tan, Liang Z; Rappe, Andrew M
2016-06-10
We investigate the shift current bulk photovoltaic response of materials close to a band inversion topological phase transition. We find that the bulk photocurrent reverses direction across the band inversion transition, and that its magnitude is enhanced in the vicinity of the phase transition. These results are demonstrated with first principles density functional theory calculations of BiTeI and CsPbI_{3} under hydrostatic pressure, and explained with an analytical model, suggesting that this phenomenon remains robust across disparate material systems.
Thermoelectric characterization of individual bismuth selenide topological insulator nanoribbons
NASA Astrophysics Data System (ADS)
Tang, Hao; Wang, Xiaomeng; Xiong, Yucheng; Zhao, Yang; Zhang, Yin; Zhang, Yan; Yang, Juekuan; Xu, Dongyan
2015-04-01
Bismuth selenide (Bi2Se3) nanoribbons have attracted tremendous research interest recently to study the properties of topologically protected surface states that enable new opportunities to enhance the thermoelectric performance. However, the thermoelectric characterization of individual Bi2Se3 nanoribbons is rare due to the technological challenges in the measurements. One challenge is to ensure good contacts between the nanoribbon and electrodes in order to determine the thermal and electrical properties accurately. In this work, we report the thermoelectric characterization of individual Bi2Se3 nanoribbons via a suspended microdevice method. Through careful measurements, we have demonstrated that contact thermal resistance is negligible after the electron-beam-induced deposition (EBID) of platinum/carbon (Pt/C) composites at the contacts between the nanoribbon and electrodes. It is shown that the thermal conductivity of the Bi2Se3 nanoribbons is less than 50% of the bulk value over the whole measurement temperature range, which can be attributed to enhanced phonon boundary scattering. Our results indicate that intrinsic Bi2Se3 nanoribbons prepared in this work are highly doped n-type semiconductors, and therefore the Fermi level should be in the conduction band and no topological transport behavior can be observed in the intrinsic system.
Search for new topological insulators: ternary Li2AgSb-class semiconductors and related compounds
NASA Astrophysics Data System (ADS)
Lin, Hsin; Das, Tanmoy; Wang, Y. J.; Wray, L. A.; Xu, S.-Y.; Hasan, M. Z.; Bansil, Arun
2012-02-01
Topological insulators host a rare quantum phase of electrons which is characterized by a topological invariant number of bulk states of combined spin-orbit and time-reversal symmetry origin. Despite recent progress the available classes of topological insulators are still quite limited for use in device applications and experimental exploration of exotic topological phenomena. For this reason, the search for new materials with greater structural flexibility and tunability in various local order broken symmetry phases is continuing worldwide with great intensity. Here we discuss our effort based on first-principles calculations to show that the adiabatic continuation method can provide a very powerful tool for predicting non-trivial topological phases with the example of ternary intermetallic series, Li2M'X (M'=Cu, Ag, Au, and Cd, X=Sb, Bi, and Sn) as well as other compounds with zinc-blende type sublattice. [1-3] Work supported by the Office of Basic Energy Sciences, US DOE.[1] H. Lin, et al. Nature Materials 9, 546 (2010). [2] Y. J. Wang, et al. New J. Phys. 13, 085017 (2011). [3] H. Lin, et al., arXiv:1007.5111.