166 research outputs found
Coupled forward-backward trajectory approach for non-equilibrium electron-ion dynamics
We introduce a simple ansatz for the wavefunction of a many-body system based
on coupled forward and backward-propagating semiclassical trajectories. This
method is primarily aimed at, but not limited to, treating nonequilibrium
dynamics in electron-phonon systems. The time-evolution of the system is
obtained from the Euler-Lagrange variational principle, and we show that this
ansatz yields Ehrenfest mean field theory in the limit that the forward and
backward trajectories are orthogonal, and in the limit that they coalesce. We
investigate accuracy and performance of this method by simulating electronic
relaxation in the spin-boson model and the Holstein model. Although this method
involves only pairs of semiclassical trajectories, it shows a substantial
improvement over mean field theory, capturing quantum coherence of nuclear
dynamics as well as electron-nuclear correlations. This improvement is
particularly evident in nonadiabatic systems, where the accuracy of this
coupled trajectory method extends well beyond the perturbative electron-phonon
coupling regime. This approach thus provides an attractive route forward to the
ab-initio description of relaxation processes, such as thermalization, in
condensed phase systems
Photovoltaic Effect from the Viewpoint of Time-reversal Symmetry
We theoretically investigate field-induced charge-transport processes from
the viewpoint of time-reversal symmetry. We analytically demonstrate that
breaking of the time-reversal symmetry is a necessary condition to induce
charge-transport and direct-current by external fields. This finding provides
microscopic insights into photovoltaic effects and optical-control of currents
Frequency-resolved microscopic current density analysis of linear and nonlinear optical phenomena in solids
We perform a frequency-resolved analysis of electron dynamics in solids to
obtain microscopic insight into linear and nonlinear optical phenomena. For the
analysis, we first compute the electron dynamics under optical electric fields
and evaluate the microscopic current density as a function of time and space.
Subsequently, we perform the Fourier transformation on the microscopic current
density and obtain the corresponding quantity in the frequency domain. The
frequency-resolved microscopic current density provides insight into the
microscopic electron dynamics in real-space at the frequency of linear and
nonlinear optical responses. We apply frequency-resolved microscopic current
density analysis to light-induced electron dynamics in aluminum, silicon, and
diamond based on the first-principles electron dynamics simulation according to
the time-dependent density functional theory. Consequently, the nature of
delocalized electrons in metals and bound electrons in semiconductors and
insulators is suitably captured by the developed scheme
Nonlinear polarization evolution using time-dependent density functional theory
We propose a theoretical and computational approach to investigate temporal
behavior of a nonlinear polarization in perturbative regime induced by an
intense and ultrashort pulsed electric field. First-principles time-dependent
density functional theory is employed to describe the electron dynamics.
Temporal evolution of third-order nonlinear polarization is extracted from a
few calculations of electron dynamics induced by pulsed electric fields with
the same time profile but different amplitudes. We discuss characteristic
features of the nonlinear polarization evolution as well as an extraction of
nonlinear susceptibilities and time delays by fitting the polarization. We also
carry out a decomposition of temporal and spatial changes of the electron
density in power series with respect to the field amplitude. It helps to get
insight into the origin of the nonlinear polarization in atomic scale.Comment: 11 pages, 9 figure
Nonlinear electric conductivity and THz-induced charge transport in graphene
Based on the quantum master equation approach, the nonlinear electric
conductivity of graphene is investigated under static electric fields for
various chemical potential shifts. The simulation results show that, as the
field strength increases, the effective conductivity is firstly suppressed,
reflecting the depletion of effective carriers due to the large displacement in
the Brillouin zone caused by the strong field. Then, as the field strength
exceeds ~MV/m, the effective conductivity increases, overcoming the carrier
depletion via the Landau--Zener tunneling process. Based on the nonlinear
behavior of the conductivity, the charge transport induced by few-cycle THz
pulses is studied to elucidate the ultrafast control of electric current in
matter
Limitations of mean-field approximations in describing shift-current and injection-current in materials
We theoretically investigate bulk photovoltaic effects, with a specific focus
on shift-current and injection-current. Initially, we perform a numerical
analysis of the direct current (dc) induced by a laser pulse with a
one-dimensional model, utilizing mean-field theories such as time-dependent
Hartree--Fock and time-dependent Hartree methods. Our numerical results,
obtained with mean-field theories, reveal that the dc component of the current
exists even after irradiation with linearly polarized light as a second-order
nonlinear effect, indicating the generation of injection current. Conversely,
when we employ the independent particle approximation, no injection current is
generated by linearly polarized light. To develop the microscopic understanding
of injection current within the mean-field approximation, we further analyze
the dc component of the current with the perturbation theory, employing the
mean-field approximations, the independent-particle approximation, and the
exact solution of the many-body Schr\"odinger equation. The perturbation
analysis clarifies that the injection current induced by linearly polarized
light under the mean-field approximations is an artifact caused by population
imbalance, created through quantum interference from unphysical self-excitation
pathways. Therefore, investigation of many-body effects on the bulk
photovoltaic effects have to be carefully conducted in mean-field schemes due
to potential contamination by unphysical dc current. Additionally, we perform
the first-principles electron dynamics calculation for BaTiO based on the
time-dependent density functional theory, and we confirm that the above
findings from the one-dimensional model calculation and the perturbation
analysis apply to realistic systems
Floquet engineering non-equilibrium steady states: on the optimization of system properties with gradient-based methods
Non-equilibrium steady states are created when a periodically driven quantum
system is also incoherently interacting with an environment -- as it is the
case in most realistic situations. The notion of Floquet engineering refers to
the manipulation of the properties of systems under periodic perturbations.
Although it more frequently refers to the coherent states of isolated systems
(or to the transient phase for states that are weakly coupled to the
environment), it may sometimes be of more interest to consider the final steady
states that are reached after decoherence and dissipation take place. In this
work, we propose a computational method to find the multicolor periodic
perturbations that lead to the final steady states that are optimal with
respect to a given predefined metric, such as for example the maximization of
the temporal average value of some observable. We exemplify the concept using a
simple model for the nitrogen-vacancy center in diamond: the goal in this case
is to find the driving periodic magnetic field that maximizes a time-averaged
spin component. We show that, for example, this technique permits to prepare
states whose spin values are forbidden in thermal equilibrium at any
temperature.Comment: 14 pages, 3 figures. Added a few more paragraphs commenting on the
relationship of this work with other methods, computational issues and other
minor detail
Terahertz-induced high-order harmonic generation and nonlinear charge transport in graphene
We theoretically study the THz-induced high-order harmonic generation (HHG)
and nonlinear electric transport in graphene based on the quantum master
equation with the relaxation time approximation. To obtain microscopic insight
into the phenomena, we compare the results of the fully dynamical calculations
with those under a quasi-static approximation, where the electronic system is
approximated as a nonequilibrium steady state. As a result, we find that the
THz-induced electron dynamics in graphene can be accurately modeled with the
nonequilibrium steady-state at each instance. The population distribution
analysis further clarifies that the THz-induced HHG in graphene originates from
the reduction of effective conductivity due to a large displacement of
electrons in the Brillouin zone. By comparing the present nonequilibrium
picture with a thermodynamic picture, we explore the role of the nonequilibrium
nature of electron dynamics on the extremely nonlinear optical and transport
phenomena in graphene
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