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 (DDˉ)4(D{\bar D})_4-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 $(D{\bar D})_4$-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 $AdS_4$ brane geometries on a vacuum created pair of $(D{\bar D})_3$-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 $S^1\times S^1$ to analyze torsion-less geometries on a vacuum created pair of $(D{\bar D})_2$-brane. The emergent $AdS_3$ 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 $S^1\times K3$. 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 $AdS_5$ black hole is analyzed to introduce the notion of temperature in the emergent gravity underlying the recent idea of entropic force.Comment: 6 page

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