32 research outputs found
Pressure effects on crystal and electronic structure of bismuth tellurohalides
We study the possibility of pressure-induced transitions from a normal
semiconductor to a topological insulator (TI) in bismuth tellurohalides using
density functional theory and tight-binding method. In BiTeI this transition is
realized through the formation of an intermediate phase, a Weyl semimetal, that
leads to modification of surface state dispersions. In the topologically
trivial phase, the surface states exhibit a Bychkov-Rashba type dispersion. The
Weyl semimetal phase exists in a narrow pressure interval of 0.2 GPa. After the
Weyl semimetal--TI transition occurs, the surface electronic structure is
characterized by gapless states with linear dispersion. The peculiarities of
the surface states modification under pressure depend on the band-bending
effect. We have also calculated the frequencies of Raman active modes for BiTeI
in the proposed high-pressure crystal phases in order to compare them with
available experimental data. Unlike BiTeI, in BiTeBr and BiTeCl the topological
phase transition does not occur. In BiTeBr, the crystal structure changes with
pressure but the phase remains a trivial one. However, the transition appears
to be possible if the low-pressure crystal structure is retained. In BiTeCl
under pressure, the topological phase does not appear up to 18 GPa due to a
relatively large band gap width in this compound
Pressure effects on crystal and electronic structure of bismuth tellurohalides
Westudy the possibility of pressure-induced transitions from a normal semiconductor to a topological insulator (TI) in bismuth tellurohalides using density functional theory and tight-binding method. In BiTeI this transition is realized through the formation of an intermediate phase, a Weyl semimetal, that leads to modification of surface state dispersions. In the topologically trivial phase, the
surface states exhibit a Bychkov–Rashba type dispersion. The Weyl semimetal phase exists in a narrow pressure interval of 0.2 GPa. After the Weyl semimetal–TI transition occurs, the surface electronic structure is characterized by gapless states with linear dispersion. The peculiarities of the surface states modification under pressure depend on the band-bending effect.Wehave also calculated the frequencies of Raman active modes for BiTeI in the proposed high-pressure crystal phases in order to compare them with available experimental data. Unlike BiTeI, in BiTeBr and BiTeCl the topological phase transition does not occur. In BiTeBr, the crystal structure changes with pressure but the phase remains a trivial one. However, the transition appears to be possible if the low-pressure crystal structure is retained. In BiTeCl under pressure, the topological phase does not appear up to 18 GPa due to a relatively large band gap width in this compound
Experimental verification of PbBiTe as a 3D topological insulator
The first experimental evidence is presented of the topological insulator
state in PbBiTe. A single surface Dirac cone is observed by
angle-resolved photoemission spectroscopy (ARPES) with synchrotron radiation.
Topological invariants are calculated from the {\it ab initio}
band structure to be 1; (111). The observed two-dimensional iso-energy contours
in the bulk energy gap are found to be the largest among the known
three-dimensional topological insulators. This opens a pathway to achieving a
sufficiently large spin current density in future spintronic devices.Comment: 5 pages, 5 figures, accepted for publication in Phys. Rev. Let
Emergent quantum confinement at topological insulator surfaces
Bismuth-chalchogenides are model examples of three-dimensional topological
insulators. Their ideal bulk-truncated surface hosts a single spin-helical
surface state, which is the simplest possible surface electronic structure
allowed by their non-trivial topology. They are therefore widely
regarded ideal templates to realize the predicted exotic phenomena and
applications of this topological surface state. However, real surfaces of such
compounds, even if kept in ultra-high vacuum, rapidly develop a much more
complex electronic structure whose origin and properties have proved
controversial. Here, we demonstrate that a conceptually simple model,
implementing a semiconductor-like band bending in a parameter-free
tight-binding supercell calculation, can quantitatively explain the entire
measured hierarchy of electronic states. In combination with circular dichroism
in angle-resolved photoemission (ARPES) experiments, we further uncover a rich
three-dimensional spin texture of this surface electronic system, resulting
from the non-trivial topology of the bulk band structure. Moreover, our study
reveals how the full surface-bulk connectivity in topological insulators is
modified by quantum confinement.Comment: 9 pages, including supplementary information, 4+4 figures. A high
resolution version is available at
http://www.st-andrews.ac.uk/~pdk6/pub_files/TI_quant_conf_high_res.pd
New interpretation of the origin of 2DEG states at the surface of layered topological insulators
On the basis of relativistic ab-initio calculations we show that the driving
mechanism of simultaneous emergence of parabolic and M-shaped 2D electron gas
(2DEG) bands at the surface of layered topological insulators as well as
Rashba-splitting of the former states is an expansion of van der Waals (vdW)
spacings caused by intercalation of metal atoms or residual gases. The
expansion of vdW spacings and emergence of the 2DEG states localized in the
(sub)surface region are also accompanied by a relocation of the topological
surface state to the lower quintuple layers, that can explain the absence of
interband scattering found experimentally.Comment: 5 pages, 4 figure
MONOPHENOL TS-13 improves SURVIVAL in MICE INFECTed with VIRULENT <i>M</i><i>ycobacterium tuberculosis</i>
The aim of the study was to determine the dose of a virulent strain of Mycobacterium tuberculosis H37Rv that is optimal for modeling experimental tuberculosis granulomatosis in mice and to investigate the effect of the original inductor of the Keap1/Nrf2/ARE system TS-13 (sodium 3- (3'- tert -butyl-4'-hydroxyphenyl) propylthiosulfonate) on animal survival and the dynamics of granuloma formation. Material and methods. Generalized tuberculosis granulomatosis was modeled by a single injection into the tail vein of male BALB/c mice of the 2-month-old M. tuberculosis strain H37Rv at doses of 106, 107 and 108 microbial bodies. Another group of animals on the day of infection with M. tuberculosis (107 microbial bodies) began to receive TS-13 with drinking water (100 mg/kg body weight). Survival was fixed daily; after 5 weeks, mice were euthanized and liver samples were taken for histological examination. Results and discussion. The dose of 107 microbial bodies was found to be the most adequate when modeling in BALB/c mice the tuberculosis granulomatosis caused by the intravenous injection of virulent M. tuberculosis strain H37Rv. At the 36th day after the injection of 107 microbial bodies, mortality was significantly lower in the group of mice receiving the inducer of the signal system Keap1/Nrf2/ARE monophenol TS-13 with drinking water (44 and 15% mice survived, respectively). At the same time, these two groups did not differ in the number and diameter of liver granulomas. The results show a high prospect of studying the role of oxidative stress and the redox-sensitive signal system Keap1/Nrf2/ARE in tuberculosis granulomatosis