Topological Anderson Insulator in cation-disordered Cu2ZnSnS4
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Abstract: Using ab initio calculations supported by experimental transport measurements, we present the first credible candidate for the realization of a disorder-induced Topological Anderson Insulator in a real material system. High energy reactive ball-milling produces a polymorph of Cu2ZnSnS4 with high cation disorder, which shows an inverted ordering of bands at the Brillouin zone center, in contrast to its ordered phase. Adiabatic continuity arguments establish that this disordered Cu2ZnSnS4 can be connected to the closely related Cu2ZnSnSe4, previously predicted to be a 3D topological insulator. Band structure calculations with a slab geometry reveal the presence of robust surface states, while impedance spectroscopy coupled with resistivity measurements point to the surface-dominated transport which such states would imply; thus making a strong case in favor of a novel topological phase. As such, this study opens up a window to understanding and potentially exploiting topological behavior in a rich class of easily-synthesized multinary, disordered compounds.Keywords:
Topological insulator
Brillouin zone
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The bandstructures of [110] and [001] Bi2Te3 nanowires are solved with the atomistic 20 band tight binding functionality of NEMO5. The theoretical results reveal: The popular assumption that all topological insulator 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.
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Bi2Te3 is a topological insulator with time reversal symmetry possessing a single Dirac cone at a given surface. The surface states of topological insulators play a critical role in exotic physical phenomena and their applications. We investigate the surface states of thin films of Bi2Te3(111) using density‐functional theory including spin‐orbit coupling. Considering one to six quintuple layers (QLs) of Bi2Te3 films, we identify the surface states from calculated band structures using the decay length of the surface states and electron density plots. We show that the films of 1 and 2 QLs are too thin to hold the surface states protected topologically, and that for thicker films bands identified as surface states at Γ̄ lose their surface‐state features away from Γ̄. This method can be applied to other topological insulators.
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Quantitative analysis of the weak antilocalization (WAL) effect of topological surface states in topological insulators is of tremendous importance. The major obstacle to achieve accurate results is how to eliminate the contribution of the anisotropic magnetoconductance of bulk states when the Fermi level lies in bulk bands. Here, we demonstrate that we can analyze quantitatively and accurately the WAL effect of topological surface states in topological insulator, BiSbTeSe2 (BSTS), by measuring the anisotropic magnetoconductance. The anomalous conductance peaks induced by the WAL effect of topological surface states of BSTS together with the anisotropic magnetoconductance of bulk states have been observed. By subtracting the anisotropic magnetoconductance of bulk states, we are able to analyze the WAL effect of topological surface states using the Hikami–Larkin–Nagaoka expression. Our findings offer an alternative strategy for the quantitative exploration of the WAL effect of topological surface states in topological insulators.
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Topological insulator
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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. In ambient conditions, the detection of topological surface states in topological insulators is impaired by the presence of bulk and quantum well states. Here, Park et al. demonstrate how such topological states in epitaxial ultrathin Bi2Se3may be isolated using terahertz time-domain spectroscopy.
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It is well known that the three-dimensional (3D) electronic topological insulator (TI) with charge-conservation and time-reversal symmetry cannot have a trivial insulating surface that preserves symmetry. It is often implicitly assumed that if the TI surface preserves both symmetries then it must be gapless. Here we show that it is possible for the TI surface to be both gapped and symmetry preserving, at the expense of having surface-topological order. In contrast to analogous bosonic topological insulators, this symmetric surface topological order is intrinsically non-Abelian. We show that the surface-topological order provides a complete nonperturbative definition of the electron TI that transcends a free-particle band-structure picture, and could provide a useful perspective for studying strongly correlated topological Mott insulators.
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It is well known that the three-dimensional (3D) electronic topological insulator (TI) with charge-conservation and time-reversal symmetry cannot have a trivial insulating surface that preserves symmetry. It is often implicitly assumed that if the TI surface preserves both symmetries then it must be gapless. Here we show that it is possible for the TI surface to be both gapped and symmetry preserving, at the expense of having surface-topological order. In contrast to analogous bosonic topological insulators, this symmetric surface topological order is intrinsically non-Abelian. We show that the surface-topological order provides a complete nonperturbative definition of the electron TI that transcends a free-particle band-structure picture, and could provide a useful perspective for studying strongly correlated topological Mott insulators.
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We have presented an introductory study on surface states of topological insulator, Bi2Se3 based on the first principle
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Nontrivial surface states in topological materials have emerged as exciting targets for surface chemistry research. In particular, topological insulators have been used as electrodes in electrocatalytic reactions. Herein, we investigate the robustness of the topological surface states and band topology under electrochemical conditions, specifically in the presence of an electric double layer. First-principles band structure calculations are performed on the electrified (111) surfaces of Bi2Te3, Bi2Se3, and Sb2Te3 using an implicit electrolyte model. Our results demonstrate the adiabatic evolution of the surface states upon surface charging. Under oxidizing potentials, the surface states are shifted upward in energy, preserving the Dirac point on the surface and the band inversion in the bulk. Conversely, under reduced potentials, hybridization is observed between the surface and bulk states, suggesting a likely breakdown of topological protection. The position of the Fermi level, which dictates the working states in catalytic reactions, should ideally be confined within the bulk bandgap. This requirement defines a potential window for the effective application of topological electrocatalysis.
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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, Bi${}_{2}$Se${}_{3}$, 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.
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