Showing papers by "Sergey Y. Savrasov published in 2013"

Electron-phonon superconductivity near charge-density-wave instability in LaO0.5F0.5BiS2: Density-functional calculations

Abstract: We discuss the electronic structure, lattice dynamics, and electron-phonon interaction of the newly discovered superconductor LaO${}_{0.5}$F${}_{0.5}$BiS${}_{2}$ using density-functional-based calculations. A strong Fermi surface nesting at $\mathbf{k}=(\ensuremath{\pi},\ensuremath{\pi},0)$ suggests a proximity to charge-density-wave instability and leads to imaginary harmonic phonons at this $\mathbf{k}$ point associated with in-plane displacements of S atoms. Total energy analysis resolves only a shallow double-well potential well preventing the appearance of static long-range order. Both harmonic and anharmonic contributions to electron-phonon coupling are evaluated and give a total coupling constant $\ensuremath{\lambda}\ensuremath{\simeq}0.85$, prompting this material to be a conventional superconductor contrary to structurally similar FeAs materials.

Floquet Weyl Semimetal Induced by Off-Resonant Light

Abstract: We propose that a Floquet Weyl semimetal state can be induced in three-dimensional topological insulators, either nonmagnetic or magnetic, by the application of off-resonant light. The virtual photon processes play a critical role in renormalizing the Dirac mass and so resulting in a topological semimetal with vanishing gap at Weyl points. The present mechanism via off-resonant light is quite different from that via on-resonant light, the latter being recently suggested to give rise to a Floquet topological state in ordinary band insulators.

Calculation of Multipolar Exchange Interactions in Spin-Orbital Coupled Systems

Abstract: A new method of computing multipolar exchange interaction in spin-orbit coupled systems is developed using multipolar tensor expansion of the density matrix in local density $\text{approximation}+\mathrm{U}$ electronic structure calculation. Within the mean field approximation, exchange constants can be mapped into a series of total energy calculations by the pair-flip approximation technique. The application to uranium dioxide shows an antiferromagnetic superexchange coupling in dipoles but a ferromagnetic one in quadrupoles which is very different from past studies. Further calculation of the spin-lattice interaction indicates it is of the same order with the superexchange and characterizes the overall behavior of the quadrupolar part as a competition between them.

Scaling Between Periodic Anderson and Kondo Lattice Models

Abstract: Theoretical Physics III, Center for Electronic Correlations and Magnetism,Institute of Physics, University of Augsburg, D-86135 Augsburg, Germany(Dated: December 27, 2012)Continuous–Time Quantum Monte Carlo (CT-QMC) method combined with Dynamical MeanField Theory (DMFT) is used to calculate both Periodic Anderson Model (PAM) and Kondo Lat-tice Model (KLM). Different parameter sets of both models are connected by the Schrieffer–Wolfftransformation. For degeneracy N = 2, a special particle–hole symmetric case of PAM at half fillingwhich always fixes one electron per impurity site is compared with the results of the KLM. Wefind a good mapping between PAM and KLM in the limit of large on–site Hubbard interaction Ufor different properties like self–energy, quasiparticle residue and susceptibility. This allows us toextract quasiparticle mass renormalizations for the f electrons directly from KLM. The method isfurther applied to higher degenerate case and to realsitic heavy fermion system CeRhIn

Unconventional Superconductivity in Doped Topological Insulators

Abstract: Turning topological insulator into a superconductor may lead to some promising applications but requires a creation of an exotic state of fermion pairs. Here we use a first principles linear response calculation to show that in doped Bi2Se3 an unconventional odd-parity state can be favored via a conventional phonon-mediated mechanism, as driven by an unusual, almost singular behavior of the electron-phonon interaction at long wavelengths. This may provide a new platform for our understanding superconductivity phenomena in doped band insulators.

Floquet Weyl semimetal induced by topological phase transitions

Abstract: We investigate the effect of a time--periodical electromagnetic field on magnetic 3D topological insulators by using Floquet theory. It is found that the external field can change the topology of the undriven system by renormalizing its Dirac mass, leading to a new Weyl semimetal phase. A diamond lattice model is further studied to confirm the above results, where topological phase transitions between quantum anomalous Hall and Weyl semimetal phases take place due to the periodical perturbation.