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Register a new userLarge Gap Topological Insulator Bi2Te3 with a Single Dirac Cone on the Surface
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We investigate the surface state of Bi$_2$Te$_3$ using angle resolved photoemission spectroscopy (ARPES) and transport measurements. By scanning over the entire Brillouin zone (BZ), we demonstrate that the surface state consists of a single non-degenerate Dirac cone centered at the $Gamma$ point. Furthermore, with appropriate hole (Sn) doping to counteract intrinsic n-type doping from vacancy and anti-site defects, the Fermi level can be tuned to intersect only the surface states, indicating a full energy gap for the bulk states, consistent with a carrier sign change near this doping in transport properties. Our experimental results establish for the first time that Bi$_2$Te$_3$ is a three dimensional topological insulator with a single Dirac cone on the surface, as predicted by a recent theory.
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Angle resolved photoemission spectroscopy (ARPES) studies were performed on two compounds (TlBiTe$_2$ and TlBiSe$_2$) from a recently proposed three dimensional topological insulator family in Thallium-based III-V-VI$_2$ ternary chalcogenides. For both materials, we show that the electronic band structures are in broad agreement with the $ab$ $initio$ calculations; by surveying over the entire surface Brillouin zone (BZ), we demonstrate that there is a single Dirac cone reside at the center of BZ, indicating its topological non-triviality. For TlBiSe$_2$, the observed Dirac point resides at the top of the bulk valance band, making it a large gap ($geq$200$meV$) topological insulator; while for TlBiTe$_2$, we found there exist a negative indirect gap between the bulk conduction band at $M$ point and the bulk valance band near $Gamma$, making it a semi-metal at proper doping. Interestingly, the unique band structures of TlBiTe$_2$ we observed further suggest TlBiTe$_2$ may be a candidate for topological superconductors.
We discuss the ultrafast evolution of the surface electronic structure of the topological insulator Bi$_2$Te$_3$ following a femtosecond laser excitation. Using time and angle resolved photoelectron spectroscopy, we provide a direct real-time visualisation of the transient carrier population of both the surface states and the bulk conduction band. We find that the thermalization of the surface states is initially determined by interband scattering from the bulk conduction band, lasting for about 0.5 ps; subsequently, few ps are necessary for the Dirac cone non-equilibrium electrons to recover a Fermi-Dirac distribution, while their relaxation extends over more than 10 ps. The surface sensitivity of our measurements makes it possible to estimate the range of the bulk-surface interband scattering channel, indicating that the process is effective over a distance of 5 nm or less. This establishes a correlation between the nanoscale thickness of the bulk charge reservoir and the evolution of the ultrafast carrier dynamics in the surface Dirac cone.
Two-dimensional (2D) topological insulators (TIs) with a large bulk band-gap are promising for experimental studies of the quantum spin Hall effect and for spintronic device applications. Despite considerable theoretical efforts in predicting large-gap 2D TI candidates, only few of them have been experimentally verified. Here, by combining scanning tunneling microscopy/spectroscopy and angle-resolved photoemission spectroscopy, we reveal that the top monolayer of ZrTe5 crystals hosts a large band gap of ~100 meV on the surface and a finite constant density-of-states within the gap at the step edge. Our first-principles calculations confirm the topologically nontrivial nature of the edge states. These results demonstrate that the top monolayer of ZrTe5 crystals is a large-gap 2D TI suitable for topotronic applications at high temperature.
The unoccupied states in topological insulators Bi_2Se_3, PbSb_2Te_4, and Pb_2Bi_2Te_2S_3 are studied by the density functional theory methods. It is shown that a surface state with linear dispersion emerges in the inverted conduction band energy gap at the center of the surface Brillouin zone on the (0001) surface of these insulators. The alternative expression of Z_2 invariant allowed us to show that a necessary condition for the existence of the second Gamma Dirac cone is the presence of local gaps at the time reversal invariant momentum points of the bulk spectrum and change of parity in one of these points.
Magnetic topological quantum materials (TQMs) provide a fertile ground for the emergence of fascinating topological magneto-electric effects. Recently, the discovery of intrinsic antiferromagnetic (AFM) topological insulator MnBi2Te4 that could realize quantized anomalous Hall effect and axion insulator phase ignited intensive study on this family of TQM compounds. Here, we investigated the AFM compound MnBi4Te7 where Bi2Te3 and MnBi2Te4 layers alternate to form a superlattice. Using spatial- and angle-resolved photoemission spectroscopy, we identified ubiquitous (albeit termination dependent) topological electronic structures from both Bi2Te3 and MnBi2Te4 terminations. Unexpectedly, while the bulk bands show strong temperature dependence correlated with the AFM transition, the topological surface states show little temperature dependence and remain gapless across the AFM transition. The detailed electronic structure of MnBi4Te7 and its temperature evolution, together with the results of its sister compound MnBi2Te4, will not only help understand the exotic properties of this family of magnetic TQMs, but also guide the design for possible applications.
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