40 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
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
All-optical nonequilibrium pathway to stabilizing magnetic Weyl semimetals in pyrochlore iridates
Nonequilibrium many-body dynamics is becoming one of the central topics of
modern condensed matter physics. Floquet topological states were suggested to
emerge in photodressed band structures in the presence of periodic laser
driving. Here we propose a viable nonequilibrium route without requiring
coherent Floquet states to reach the elusive magnetic Weyl semimetallic phase
in pyrochlore iridates by ultrafast modification of the effective
electron-electron interaction with short laser pulses. Combining \textit{ab
initio} calculations for a time-dependent self-consistent reduced Hubbard
controlled by laser intensity and nonequilibrium magnetism simulations for
quantum quenches, we find dynamically modified magnetic order giving rise to
transiently emerging Weyl cones that are probed by time- and angle-resolved
photoemission spectroscopy. Our work offers a unique and realistic pathway for
nonequilibrium materials engineering beyond Floquet physics to create and
sustain Weyl semimetals. This may lead to ultrafast, tens-of-femtoseconds
switching protocols for light-engineered Berry curvature in combination with
ultrafast magnetism.Comment: 27 pages including methods and supplementary information, 4 figures,
4 supplementary figure
Comparing the generalized Kadanoff-Baym ansatz with the full Kadanoff-Baym equations for an excitonic insulator out of equilibrium
We investigate out-of-equilibrium dynamics in an excitonic insulator (EI)
with a finite momentum pairing perturbed by a laser-pulse excitation and a
sudden coupling to fermionic baths. The transient dynamics of the excitonic
order parameter is resolved using the full nonequilibrium Green's function
approach and the generalized Kadanoff-Baym ansatz (GKBA) within the second-Born
approximation. The comparison between the two approaches after a laser pulse
excitation shows a good agreement in the weak and the intermediate photo-doping
regime. In contrast, the laser-pulse dynamics resolved by the GKBA does not
show a complete melting of the excitonic order after a strong excitation.
Instead we observe persistent oscillations of the excitonic order parameter
with a predominant frequency given by the renormalized equilibrium bandgap.
This anomalous behavior can be overcome within the GKBA formalism by coupling
to an external bath, which leads to a transition of the EI system towards the
normal state. We analyze the long-time evolution of the system and distinguish
decay timescales related to dephasing and thermalization.Comment: 13 pages, 12 figure
Quantum Electrodynamical Bloch Theory with Homogeneous Magnetic Fields
We propose a solution to the problem of Bloch electrons in a homogeneous
magnetic field by including the quantum fluctuations of the photon field. A
generalized quantum electrodynamical (QED) Bloch theory from first principles
is presented. In the limit of vanishing quantum fluctuations we recover the
standard results of solid-state physics, for instance, the fractal spectrum of
the Hofstadter butterfly. As a further application we show how the well known
Landau physics is modified by the photon field and that Landau polaritons
emerge. This shows that our QED-Bloch theory does not only allow to capture the
physics of solid-state systems in homogeneous magnetic fields, but also novel
features that appear at the interface of condensed matter physics and quantum
optics