Results 1 - 10 of 547
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[en] We have extended the search for topological insulators to the ternary tetradymite-like compounds M2X2Y (M=Bi or Sb; X and Y=S, Se or Te), which are variations of the well-known binary compounds Bi2Se3 and Bi2Te3. Our first-principles computations suggest that five existing compounds are strong topological insulators with a single Dirac cone on the surface. In particular, stoichiometric Bi2Se2S, Sb2Te2Se and Sb2Te2S are predicted to have an isolated Dirac cone on their naturally cleaved surface. This finding paves the way for the realization of the topological transport regime.
[en] We investigate the intrinsic quasi-particle (QP) dynamics in the topological metallic (TM) states of Bi2Se3, Bi2-xSnxTe3 (x=0.0067) and Sb. Surprisingly, the intrinsic QP dynamics are mostly determined by the scattering within the TM states and are unaffected by the bulk electronic states. We observe the binding energy-independent imaginary part of the self-energy Im Σ for the QPs in the TM states. We attribute the binding energy-independent Im Σ to momentum mixing due to flakes or warped surfaces of the cleaved samples. This makes the intrinsic scattering rate of the QPs extremely small in the TM states. We discuss a few possible contributions to the smallness of the Im Σ for QPs. Our new observations on the extremely small intrinsic QP scattering rates may open up the possibility of room-temperature quantum devices.
[en] Thin films of Sb-doped Bi2Se3 were grown in different deposition pressure and substrate temperature conditions in a pulsed laser deposition system. It was observed that with increase in the Ar pressure during deposition, the charge carrier concentration decreases. A two order change in carrier density was observed between films grown in 10-5 mbar vacuum and 10-1 mbar Ar. Raman studies on the low pressure-deposited films revealed an additional peak at around 92 cm-1, corresponding to A1g mode of Bi suggesting that Bi has segregated in these films. It was found that the segregation was due to a significant Se deficiency in films grown at low pressures. Raman studies on films grown in low temperature conditions revealed an additional broad mode at 250 cm-1, corresponding to Sb, indicating segregation of Sb in low temperature growth. We have achieved growth of stoichiometric Sb-doped Bi2Se3 films in 5x10-1 mbar Ar environment and 350°C substrate temperature. (author)
[en] The three-dimensional topological insulator differs from other topological insulators in the family in that the effective Hamiltonian of its surface states on a flat semi-infinite slab requires the addition of a cubic momentum hexagonal warping term in order to reproduce the experimentally measured constant energy contours. In this work, we derive the appropriate effective Hamiltonian for the surface states of a cylinder incorporating the corresponding hexagonal warping terms in a cylindrical geometry. We show that at the energy range where the surface states dominate, the effective Hamiltonian adequately reproduces the dispersion relation obtained from a full four-band Hamiltonian which describes both the bulk and surface states. As an example application of our effective Hamiltonian, we study the transmission between two collinear cylinders magnetized in different directions perpendicular to their axes. We show that the hexagonal warping term results in a transmission profile between the cylinders which may be of utility in a multiple state magnetic memory bit. (paper)
[en] Within the Boltzmann equation formalism we evaluate the transport relaxation time of Dirac surface states (SSs) in the typical topological insulator(TI) Bi2Se3 due to the phonon scattering. We find that although the back-scattering of the SSs in TIs is strictly forbidden, the in-plane scattering between SSs in 3-dimensional TIs is allowed, maximum around the right-angle scattering. Thus the topological property of the SSs only reduces the scattering rate to its one half approximately. Besides, the larger LA deformation potential and lower sound velocity of Bi2Se3 enhance the scattering rate significantly. Compared with the Dirac electrons in graphene, we find the scattering rate of SSs in Bi2Se3 are two orders of magnitudes larger, which agree with the recent transport experiments
[en] Topological insulators are a quantum state of matter that has recently created a great interest among the scientific community, with being one of the most extensively studied materials. Here, we demonstrate that polycrystalline nanostructured samples of preserve the existence of topological surface states, where electrons cannot be localised. The nanosheet crystals were synthesised by a microwave-assisted method and their structure, composition and morphology thoroughly characterised. The transport properties of a textured polycrystalline sample with strong preferred orientation along the c-axis were measured, showing the presence of the weak anti-localisation effect and Shubnikov-de Haas oscillations. These features are robust against the presence of non-magnetic impurities and structural defects.
[en] We report a systematic study of bulk and surface chemical doping effects on single Dirac cone topological insulator Bi2Se3 and Bi2Te3. By bulk doping, we were able to achieve full range control of charge carrier types and concentration, with the exact Fermi energy measured by angle-resolved photoemission spectroscopy (ARPES). Due to the unusual robustness of the topological surface state, we further realized the bi-polar control of the surface carriers by gaseous or alkaline surface doping without affecting the topological nature of these materials. The doping progress monitored by in situ ARPES study clearly demonstrated the switching between different carrier types through the Dirac point. The ability to control the carrier types and the concentration of topological insulators will greatly facilitate future applications. (invited paper)
[en] Pressure-induced electrical transport properties of Bi2Se3, including Hall coefficient, carrier concentration, mobility, and electrical resistivity, have been investigated under pressure up to 29.8 GPa by in situ Hall-effect measurements. The results indicate that the structural and electronic phase transitions of Bi2Se3 induce discontinuous changes in these electrical parameters. The significant anomaly in Hall coefficient at 5 GPa reveals an electronic topological transition deriving from the topological change of the band extremum (Van Hove singularity). Additionally, electrical resistivity measurements under variable temperatures show that the insulating state of Bi2Se3 becomes increasingly stable with an increase of pressure below 9.7 GPa. But above 9.7 GPa, Bi2Se3 enters into a fully metallic state. As the metallization occurs, the topological property of Bi2Se3 disappears
[en] Topological insulators are very interesting from a fundamental point of view, and their unique properties may be useful for electronic and spintronic device applications. From the point of view of applications it is important to understand the decay behavior of carriers injected in the band gap of the topological insulator, which is determined by its complex band structure (CBS). Using first-principles calculations, we investigate the dispersion and symmetry of the complex bands of Bi2Se3 family of three-dimensional topological insulators. We compare the CBS of a band insulator and a topological insulator and follow the CBS evolution in both when the spin–orbit interaction is turned on. We find significant differences in the CBS linked to the topological band structure. In particular, our results demonstrate that the evanescent states in Bi2Se3 are non-trivially complex, i.e. contain both the real and imaginary contributions. This explains quantitatively the oscillatory behavior of the band gap obtained from Bi2Se3 (0 0 0 1) slab calculations. (paper)
[en] Recently, topological insulator materials have been theoretically predicted and experimentally observed in both 2D and 3D systems. We first review the basic models and physical properties of topological insulators, using HgTe and Bi2Se3 as prime examples. We then give a comprehensive survey of topological insulators which have been predicted so far, and discuss the current experimental status. (review article)