1,180 research outputs found
Chiral universality class of the normal-superconducting and the exciton condensation transition on the surface of topological insulator
New two dimensional systems like surface of topological insulator and
graphene offer a possibility to experimentally investigate situations
considered "exotic" just a decade ago. One of those is the quantum phase
transition of the "chiral" type in electronic systems with relativistic
spectrum. Phonon mediated ("conventional") pairing in the Dirac semimetal
appearing on the surface of topological insulator leads to transition into a
chiral superconducting state, while exciton condensation in these gapless
systems has been envisioned long time ago in the physics of the narrow band
semiconductors. Starting from the microscopic Dirac Hamiltonian with local
attraction or repulsion, the BCS type gaussian approximation is developed in
the framework of functional integrals. It is shown that due to an
"ultra-relativistic" dispersion relation there is a quantum critical point
governing the zero temperature transition to a superconducting or the exciton
condensed state. The quantum transitions that have critical exponents very
different from the conventional ones. They belong to the chiral universality
class. We discuss the application of these results to recent experiments in
which surface superconductivity was found in topological insulators and
estimate feasibility of the phonon pairing.Comment: 19pages, 4 figures. arXiv admin note: substantial text overlap with
arXiv:1405.594
Superconductivity in 2D electron gas induced by high energy optical phonon mode and large polarization of the STO substrate
Theory of superconductivity generated in one atomic layer thick two
dimensional electron gas by a single flat band of high energy longitudinal
optical phonons is considered. The polar dielectric (STO) exhibits
such an energetic phonon mode and the 2DEG is created both when one unit cell
layer is grown on its surface and on the interface
with another dielectric like (LAO). We obtain a quantitative
description of both systems solving the gap equation for without making
use of approximations like the Kirzhnits Ansatz for arbitrary chemical
potential , electron-phonon coupling and the phonon frequency
, and direct (RPA) electron-electron repulsion strength . The
high temperature superconductivity in 1UC/STO is possible due to a
combination of three factors: high LO phonon frequency, large electron-phonon
coupling and huge dielectric constant of the substrate
suppression the Coulomb repulsion. It is shown that very low density electron
gas in the interfaces is still capable of generating superconductivity of the
order of K in LAO/STO. Superconductivity persists even on the band edge
.Comment: 19 pages, 7 figure
Magnetic impurities make superconductivity in 3D Dirac semi-metal triplet
Conventional electron-phonon coupling induces either odd (triplet) or even
(singlet) pairing states in a time reversal and inversion invariant Dirac semi
- metal. In certain range of the chemical potential and parameters
characterizing the pairing attraction (effective electron-electron coupling
constant and the Debye energy ) the energy of the singlet
although always lower, prevails by a very slim margin over the triplet. This
means that interactions that are small but discriminate between the spin
singlet and the spin triplet determine the nature of the superconducting order
there. It shown that in materials close enough to the Dirac point () magnetic impurities stabilize the odd pairing superconducting
state.Comment: 5 pages, 4 figures.Supplemenatry materials, 5 pages, 2 figures. arXiv
admin note: substantial text overlap with arXiv:1407.077
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