58,630 research outputs found
Green function techniques in the treatment of quantum transport at the molecular scale
The theoretical investigation of charge (and spin) transport at nanometer
length scales requires the use of advanced and powerful techniques able to deal
with the dynamical properties of the relevant physical systems, to explicitly
include out-of-equilibrium situations typical for electrical/heat transport as
well as to take into account interaction effects in a systematic way.
Equilibrium Green function techniques and their extension to non-equilibrium
situations via the Keldysh formalism build one of the pillars of current
state-of-the-art approaches to quantum transport which have been implemented in
both model Hamiltonian formulations and first-principle methodologies. We offer
a tutorial overview of the applications of Green functions to deal with some
fundamental aspects of charge transport at the nanoscale, mainly focusing on
applications to model Hamiltonian formulations.Comment: Tutorial review, LaTeX, 129 pages, 41 figures, 300 references,
submitted to Springer series "Lecture Notes in Physics
Spectral moment sum rules for strongly correlated electrons in time-dependent electric fields
We derive exact operator average expressions for the first two spectral
moments of nonequilibrium Green's functions for the Falicov-Kimball model and
the Hubbard model in the presence of a spatially uniform, time-dependent
electric field. The moments are similar to the well-known moments in
equilibrium, but we extend those results to systems in arbitrary time-dependent
electric fields. Moment sum rules can be employed to estimate the accuracy of
numerical calculations; we compare our theoretical results to numerical
calculations for the nonequilibrium dynamical mean-field theory solution of the
Falicov-Kimball model at half-filling.Comment: (16 pages, submitted to Phys. Rev. B
Electron transport through strongly interacting quantum dot coupled to normal metal and superconductor
We study the electron transport through the quantum dot coupled to the normal
metal and BCS-like superconductor (N - QD - S) in the presence of the Kondo
effect and Andreev scattering. The system is described by the single impurity
Anderson model in the limit of strong on-dot interaction. We use recently
proposed equation of motion technique for Keldysh nonequilibrium Green's
function together with the modified slave boson approach to study the electron
transport. We derive formula for the current which contains various tunneling
processes and apply it to study the transport through the system. We find that
the Andreev conductance is strongly suppressed and there is no zero-bias
(Kondo) anomaly in the differential conductance. We discuss effects of the
particle-hole asymmetry in the electrodes as well as the asymmetry in the
couplings.Comment: Supercond. Sci. Technol. - accepted for publicatio
Time-Dependent Partition-Free Approach in Resonant Tunneling Systems
An extended Keldysh formalism, well suited to properly take into account the
initial correlations, is used in order to deal with the time-dependent current
response of a resonant tunneling system. We use a \textit{partition-free}
approach by Cini in which the whole system is in equilibrium before an external
bias is switched on. No fictitious partitions are used. Besides the
steady-state responses one can also calculate physical dynamical responses. In
the noninteracting case we clarify under what circumstances a steady-state
current develops and compare our result with the one obtained in the
partitioned scheme. We prove a Theorem of asymptotic Equivalence between the
two schemes for arbitrary time-dependent disturbances. We also show that the
steady-state current is independent of the history of the external perturbation
(Memory Loss Theorem). In the so called wide-band limit an analytic result for
the time-dependent current is obtained. In the interacting case we propose an
exact non-equilibrium Green function approach based on Time Dependent Density
Functional Theory. The equations are no more difficult than an ordinary Mean
Field treatment. We show how the scattering-state scheme by Lang follows from
our formulation. An exact formula for the steady-state current of an arbitrary
interacting resonant tunneling system is obtained. As an example the
time-dependent current response is calculated in the Random Phase
Approximation.Comment: final version, 18 pages, 9 figure
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