51 research outputs found
Efficient computation of the second-Born self-energy using tensor-contraction operations
In the nonequilibrium Green's function approach, the approximation of the
correlation self-energy at the second-Born level is of particular interest,
since it allows for a maximal speed-up in computational scaling when used
together with the Generalized Kadanoff-Baym Ansatz for the Green's function.
The present day numerical time-propagation algorithms for the Green's function
are able to tackle first principles simulations of atoms and molecules, but
they are limited to relatively small systems due to unfavourable scaling of
self-energy diagrams with respect to the basis size. We propose an efficient
computation of the self-energy diagrams by using tensor-contraction operations
to transform the internal summations into functions of external low-level
linear algebra libraries. We discuss the achieved computational speed-up in
transient electron dynamics in selected molecular systems.Comment: 9 pages, 4 figures, 1 tabl
Spin transport in Heisenberg antiferromagnets
We analyze spin transport in insulating antiferromagnets described by the XXZ
Heisenberg model in two and three dimensions. Spin currents can be generated by
a magnetic-field gradient or, in systems with spin-orbit coupling,
perpendicular to a time-dependent electric field. The Kubo formula for the
longitudinal spin conductivity is derived analogously to the Kubo formula for
the optical conductivity of electronic systems. The spin conductivity is
calculated within interacting spin-wave theory. In the Ising regime, the XXZ
magnet is a spin insulator. For the isotropic Heisenberg model, the
dimensionality of the system plays a crucial role: In d=3 the regular part of
the spin conductivity vanishes linearly in the zero frequency limit, whereas in
d=2 it approaches a finite zero frequency value.Comment: 9 pages, 5 figure
Ultrafast modification of Hubbard in a strongly correlated material: ab initio high-harmonic generation in NiO
Engineering effective electronic parameters is a major focus in condensed
matter physics. Their dynamical modulation opens the possibility of creating
and controlling physical properties in systems driven out of equilibrium. In
this work, we demonstrate that the Hubbard , the on-site Coulomb repulsion
in strongly correlated materials, can be modified on femtosecond time scales by
a strong nonresonant laser excitation in the prototypical charge transfer
insulator NiO. Using our recently developed time-dependent density functional
theory plus self-consistent (TDDFT+U) method, we demonstrate the importance
of a dynamically modulated in the description of the high-harmonic
generation of NiO. Our study opens the door to novel ways of modifying
effective interactions in strongly correlated materials via laser driving,
which may lead to new control paradigms for field-induced phase transitions and
perhaps laser-induced Mott insulation in charge-transfer materials
Superconductivity and Pairing Fluctuations in the Half-Filled Two-Dimensional Hubbard Model
The two-dimensional Hubbard model exhibits superconductivity with d-wave
symmetry even at half-filling in the presence of next-nearest neighbor hopping.
Using plaquette cluster dynamical mean-field theory with a continuous-time
quantum Monte Carlo impurity solver, we reveal the non-Fermi liquid character
of the metallic phase in proximity to the superconducting state. Specifically,
the low-frequency scattering rate for momenta near (\pi, 0) varies
non-monotonously at low temperatures, and the dc conductivity is T-linear at
elevated temperatures with an upturn upon cooling. Evidence is provided that
pairing fluctuations dominate the normal-conducting state even considerably
above the superconducting transition temperature.Comment: 4.3 pages, 4 figure
Correlations in a band insulator
We study a model of a covalent band insulator with on-site Coulomb repulsion
at half-filling using dynamical mean-field theory. Upon increasing the
interaction strength the system undergoes a discontinuous transition from a
correlated band insulator to a Mott insulator with hysteretic behavior at low
temperatures. Increasing the temperature in the band insulator close to the
insulator-insulator transition we find a crossover to a Mott insulator at
elevated temperatures. Remarkably, correlations decrease the energy gap in the
correlated band insulator. The gap renormalization can be traced to the
low-frequency behavior of the self-energy, analogously to the quasiparticle
renormalization in a Fermi liquid. While the uncorrelated band insulator is
characterized by a single gap for both charge and spin excitations, the spin
gap is smaller than the charge gap in the correlated system.Comment: 7 pages, 7 figure
Cavity Quantum-Electrodynamical Chern Insulator: Route Towards Light-Induced Quantized Anomalous Hall Effect in Graphene
We show that an energy gap is induced in graphene by light-matter coupling to
a circularly polarized photon mode in a cavity. Using many-body perturbation
theory we compute the electronic spectra which exhibit photon-dressed sidebands
akin to Floquet sidebands for laser-driven materials. In contrast with Floquet
topological insulators, in which a strictly quantized Hall response is induced
by light only for off-resonant driving in the high-frequency limit, the
photon-dressed Dirac fermions in the cavity show a quantized Hall response
characterized by an integer Chern number. Specifically for graphene we predict
that a Hall conductance of can be induced in the low-temperature
limit.Comment: 8 pages, 4 figures, incl. Supplementary Materia
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