165 research outputs found
Relationship between Population Dynamics and the Self-Energy in Driven Non-Equilibrium Systems
We compare the decay rates of excited populations directly calculated within
a Keldysh formalism to the equation of motion of the population itself for a
Hubbard-Holstein model in two dimensions. While it is true that these two
approaches must give the same answer, it is common to make a number of
simplifying assumptions within the differential equation for the populations
that allows one to interpret the decay in terms of hot electrons interacting
with a phonon bath. Here we show how care must be taken to ensure an accurate
treatment of the equation of motion for the populations due to the fact that
there are identities that require cancellations of terms that naively look like
they contribute to the decay rates. In particular, the average time dependence
of the Green's functions and self-energies plays a pivotal role in determining
these decay rates.Comment: Submitted to Entrop
Quantum Eigenvector Continuation for Chemistry Applications
A typical task for classical and quantum computing in chemistry is finding a
potential energy surface (PES) along a reaction coordinate, which involves
solving the quantum chemistry problem for many points along the reaction path.
Developing algorithms to accomplish this task on quantum computers has been an
active area of development, yet finding all the relevant eigenstates along the
reaction coordinate remains a difficult problem, and determining PESs is thus a
costly proposal. In this paper, we demonstrate the use of a eigenvector
continuation -- a subspace expansion that uses a few eigenstates as a basis --
as a tool for rapidly exploring potential energy surfaces. We apply this to
determining the binding PES or torsion PES for several molecules of varying
complexity. In all cases, we show that the PES can be captured using relatively
few basis states; suggesting that a significant amount of (quantum)
computational effort can be saved by making use of already calculated ground
states in this manner.Comment: 13 pages, 8 figures, 3 pages of appendi
Creating stable Floquet-Weyl semimetals by laser-driving of 3D Dirac materials
Tuning and stabilising topological states, such as Weyl semimetals, Dirac
semimetals, or topological insulators, is emerging as one of the major topics
in materials science. Periodic driving of many-body systems offers a platform
to design Floquet states of matter with tunable electronic properties on
ultrafast time scales. Here we show by first principles calculations how
femtosecond laser pulses with circularly polarised light can be used to switch
between Weyl semimetal, Dirac semimetal, and topological insulator states in a
prototypical 3D Dirac material, NaBi. Our findings are general and apply to
any 3D Dirac semimetal. We discuss the concept of time-dependent bands and
steering of Floquet-Weyl points (Floquet-WPs), and demonstrate how light can
enhance topological protection against lattice perturbations. Our work has
potential practical implications for the ultrafast switching of materials
properties, like optical band gaps or anomalous magnetoresistance. Moreover, we
introduce Floquet time-dependent density functional theory (Floquet-TDDFT) as a
general and robust first principles method for predictive Floquet engineering
of topological states of matter.Comment: 21 pages, 4 figure
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
On the Positive Definiteness of Response Functions in the Time Domain
Response functions of quantum systems, such as electron Green's functions,
magnetic, or charge susceptibilities, describe the response of a system to an
external perturbation. They are the central objects of interest in field
theories and quantum computing and measured directly in experiment. Response
functions are intrinsically causal. In equilibrium and steady-state systems,
they correspond to a positive spectral function in the frequency domain. This
article shows that response functions define an inner product on a Hilbert
space and thereby induce a positive definite function. The properties of this
function can be used to reduce noise in measured data and, in equilibrium and
steady state, to construct positive definite extensions for data known on
finite time intervals, which are then guaranteed to correspond to positive
spectra
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