53 research outputs found
Effective bias and potentials in steady-state quantum transport: A NEGF reverse-engineering study
Using non-equilibrium Green's functions combined with many-body perturbation
theory, we have calculated steady-state densities and currents through short
interacting chains subject to a finite electric bias. By using a steady-state
reverse-engineering procedure, the effective potential and bias which reproduce
such densities and currents in a non-interacting system have been determined.
The role of the effective bias is characterised with the aid of the so-called
exchange-correlation bias, recently introduced in a steady-state
density-functional-theory formulation for partitioned systems. We find that the
effective bias (or, equivalently, the exchange-correlation bias) depends
strongly on the interaction strength and the length of the central (chain)
region. Moreover, it is rather sensitive to the level of many-body
approximation used. Our study shows the importance of the
effective/exchange-correlation bias out of equilibrium, thereby offering hints
on how to improve the description of density-functional-theory based approaches
to quantum transport
Transport of Correlated Electrons through Disordered Chains: A Perspective on Entanglement, Conductance, and Disorder Averaging
We investigate electron transport in disordered Hubbard chains contacted to
macroscopic leads, via the non-equilibrium Green's functions technique. We
observe a cross-over of currents and conductances at finite bias which depends
on the relative strength of disorder and interactions. The finite-size scaling
of the conductance is highly dependent on the interaction strength, and
exponential attenuation is not always seen. We provide a proof that the
Coherent Potential Approximation, a widely used method for treating disorder
averages, fulfils particle conservation at finite bias with or without electron
correlations. Finally, our results hint that the observed trends in conductance
due to interactions and disorder also appear as signatures in the single-site
entanglement entropy.Comment: 5 pages, 4 figure
Time-resolved spectroscopy at surfaces and adsorbate dynamics: insights from a model-system approach
We introduce a model description of femtosecond laser induced desorption at
surfaces. The substrate part of the system is taken into account as a (possibly
semi-infinite) linear chain. Here, being especially interested in the early
stages of dissociation, we consider a finite-size implementation of the model
(i.e., a finite substrate), for which an exact numerical solution is possible.
By time-evolving the many-body wave function, and also using results from a
time-dependent density functional theory description for electron-nuclear
systems, we analyze the competition between several surface-response mechanisms
and electronic correlations in the transient and longer time dynamics under the
influence of dipole-coupled fields. Our model allows us to explore how coherent
multiple-pulse protocols can impact desorption in a variety of prototypical
experiments.Comment: replaces a shorter versio
Time Dependent Density Functional Theory meets Dynamical Mean Field Theory: Real-Time Dynamics for the 3D Hubbard model
We introduce a new class of exchange-correlation potentials for a static and
time-dependent Density Functional Theory of strongly correlated systems in 3D.
The potentials are obtained via Dynamical Mean Field Theory and, for strong
enough interactions, exhibit a discontinuity at half filling density, a
signature of the Mott transition. For time-dependent perturbations, the
dynamics is described in the adiabatic local density approximation. Results
from the new scheme compare very favorably to exact ones in clusters. As an
application, we study Bloch oscillations in the 3D Hubbard model.Comment: 4 pages, 3 figure
Nonadiabatic Van der Pol oscillations in molecular transport
The force exerted by the electrons on the nuclei of a current-carrying
molecular junction can be manipulated to engineer nanoscale mechanical systems.
In the adiabatic regime a peculiarity of these forces is negative friction,
responsible for Van der Pol oscillations of the nuclear coordinates. In this
work we study the robustness of the Van der Pol oscillations against
high-frequency bias and gate voltage. For this purpose we go beyond the
adiabatic approximation and perform full Ehrenfest dynamics simulations. The
numerical scheme implements a mixed quantum-classical algorithm for open
systems and is capable to deal with arbitrary time-dependent driving fields. We
find that the Van der Pol oscillations are extremely stable. The nonadiabatic
electron dynamics distorts the trajectory in the momentum-coordinate phase
space but preserves the limit cycles in an average sense. We further show that
high-frequency fields change both the oscillation amplitudes and the average
nuclear positions. By switching the fields off at different times one obtains
cycles of different amplitudes which attain the limit cycle only after
considerably long times.Comment: 12 pages, 7 figure
Molecular junctions and molecular motors: Including Coulomb repulsion in electronic friction using nonequilibrium Green's functions
We present a theory of molecular motors based on the Ehrenfest dynamics for
the nuclear coordinates and the adiabatic limit of the Kadanoff-Baym equations
for the current-induced forces. Electron-electron interactions can be
systematically included through many-body perturbation theory, making the
nonequilibrium Green's functions formulation suitable for first-principles
treatments of realistic junctions. The method is benchmarked against
simulations via real-time Kadanoff-Baym equations, finding an excellent
agreement. Results on a paradigmatic model of molecular motor show that
correlations can change dramatically the physical scenario by, e.g. introducing
a sizable damping in the self-sustained van der Pol oscillations.Comment: 7 pages , 3 figs + Suppl. Informatio
Lanczos-adapted time evolution for open boundary quantum transport
We increase the efficiency of a recently proposed time integration scheme for
time dependent quantum transport by using the Lanczos method for time
evolution. We illustrate our modified scheme in terms of a simple one
dimensional model. Our results show that the Lanczos-adapted scheme gives a
large increase in numerical efficiency, and is an advantageous route for
numerical time integration in ab-initio treatment of open boundary quantum
transport phenomena.Comment: 11 pages, 1 figur
Classical Nuclear Motion in Quantum Transport
An ab initio quantum-classical mixed scheme for the time evolution of
electrode-device-electrode systems is introduced to study nuclear dynamics in
quantum transport. Two model systems are discussed to illustrate the method.
Our results provide the first example of current-induced molecular desorption
as obtained from a full time-dependent approach and suggest the use of ac
biases as a way to tailor electromigration. They also show the importance of
non-adiabatic effects for ultrafast phenomena in nanodevices.Comment: 5 pages, 3 figure
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