994 research outputs found
Semiconductor-metal transition in semiconducting bilayer sheets of transition metal dichalcogenides
Using first-principles calculations we show that the band gap of bilayer
sheets of semiconducting transition metal dichalcogenides (TMDs) can be reduced
smoothly by applying vertical compressive pressure. These materials undergo a
universal reversible semiconductor to metal (S-M) transition at a critical
pressure. S-M transition is attributed to lifting the degeneracy of the bands
at fermi level caused by inter-layer interactions via charge transfer from
metal to chalcogens. The S-M transition can be reproduced even after
incorporating the band gap corrections using hybrid functionals and GW method.
The ability to tune the band gap of TMDs in a controlled fashion over a wide
range of energy, opens-up possibility for its usage in a range of applications.Comment: Accepted in Phys. Rev.
Quantum Kerr tunneling vacua on a -brane: An emergent Kerr black hole in five dimensions
We revisit a non-perturbative space-time curvature theory, underlying a two
form U(1) gauge dynamics, on a D4-brane. In particular, two different gauge
choices for a two form are explored underlying the dynamics of a geometric
torsion in a second order formalism. We obtain two non-extremal quantum Kerr
geometries in five dimensions on a pair of -brane in a type IIA
superstring theory. The quantum vacua are described by a vanishing torsion in a
gauge choice, underlying a geometric realization, on a non-BPS brane. It is
argued that the quantum Kerr vacua undergo tunneling and lead to a five
dimensional Kerr black hole in Einstein vacuum. A low energy limit in the
quantum Kerr vacua further re-assures an emergent Kerr black hole.Comment: 21 pages, 8 figure
Emergent Schwarzschild and Reissner-Nordstrom black holes in 4D: An effective curvature sourced by a B2-field on a D4-brane
We obtain a Schwarzschild and a Reissner-Nordstrom emergent black holes, by
exploring the torsion dynamics in a generalized curvature formulation,
underlying an effective D4-brane on S1. It is shown that a constant effective
metric, sourced by a background fluctuation in B2-potential, on a D3-brane
receives a dynamical quantum correction in presence of an electric charge.Comment: 7-pages, minor corrections, references added, to appear in Physical
Review
Quintessence and effective AdS brane geometries
A geometric torsion dynamics leading to an effective curvature in a second
order formalism on a D4-brane is revisited with a renewed interest. We obtain
two effective brane geometries on a vacuum created pair of -brane. One of them is shown to describe an AdS Schwarzschild spinning
black hole and the other is shown to describe a spinning black hole bound
state. It is argued that a D-instanton in a vacuum created anti D3-brane within
a pair may describe a quintessence. It may seen to incorporate a varying vacuum
energy density in a brane universe. We consider the effective curvature scalar
on to analyze torsion-less geometries on a vacuum created pair
of -brane. The emergent brane is shown to describe a
Schwarzschild and a Reissner-Nordstrom (RN) geometries in presence of extra
dimension(s).Comment: 20 pages, expanded discussion and added referenc
Emergent gravity/Non-linear U(1) gauge theory correspondence
Kaluza-Klein gravity is revisted, with renewed interest, in a type IIB string
theory on . The irreducible curvature tensors are worked out in
the, T-dual, emergent gravity in 4D to yield a non-linear U(1) gauge theory.
Interestingly, the T-duality may be seen to describe an open/closed string
duality at a self-dual string coupling. The obtained deformation in
black hole is analyzed to introduce the notion of temperature in the emergent
gravity underlying the recent idea of entropic force.Comment: 6 page
Pressure-Induced Topological Phase Transitions in CdGeSb and CdSnSb
Topological quantum phase transitions (TQPTs) in a material induced by
external perturbations are often characterized by band touching points in the
Brillouin zone. The low-energy excitations near the degenerate band touching
points host different types of fermions while preserving the topological
protection of surface states. An interplay of different tunable topological
phases offers an insight into the evolution of the topological character. In
this paper, we study the occurrence of TQPTs as a function of hydrostatic
pressure in CdGeSb and CdSnSb chalcopyrites, using the first-principles
calculations. At ambient pressure, both materials are topological insulators
having a finite band gap with inverted order of Sb- and Sb-,
orbitals of valence bands at the point. On the application of
hydrostatic pressure the band gap reduces, and at the critical point of the
phase transition, these materials turn into Dirac semimetals. On further
increasing the pressure beyond the critical point, the band inversion is
reverted making them trivial insulators. The pressure-induced change in band
topology from non-trivial to trivial phase is also captured by L\"{u}ttinger
model Hamiltonian calculations. Our model demonstrates the critical role played
by a pressure-induced anisotropy in frontier bands in driving the phase
transitions. These theoretical findings of peculiar coexistence of multiple
topological phases in the same material provide a realistic and promising
platform for the experimental realization of the TQPT.Comment: 7 pages, 7 figure
Strain-induced electronic phase transition and strong enhancement of thermopower of TiS2
Using first principles density functional theory calculations, we show a
semimetal to semiconducting electronic phase transition for bulk TiS 2 by
applying uniform biaxial tensile strain. This electronic phase transition is
triggered by charge transfer from Ti to S, which eventually reduces the overlap
between Ti-(d) and S-(p) orbitals. The electronic transport calculations show a
large anisotropy in electrical conductivity and thermopower, which is due to
the difference in the effective masses along the in-plane and out of plane
directions. Strain induced opening of band gap together with changes in
dispersion of bands lead to three-fold enhancement in thermopower for both p-
and n-type TiS2 . We further demonstrate that the uniform tensile strain, which
enhances the thermoelectric performance, can be achieved by doping TiS2 with
larger iso-electronic elements such as Zr or Hf at Ti sites.Comment: 8 pages, 6 figure
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