579 research outputs found
Kondo effect in interacting nanoscopic systems: Keldysh field integral theory
Kondo physics in nonequilibrium interacting nanoscale devices is an
attractive fundamental many-particle phenomenon with a rich potential for
applications. Due to enormous complexity its clear and flexible theory is still
highly desirable. We develop a physically transparent analytical theory capable
to correctly describe the Kondo effect in strongly interacting systems at
temperatures close to and above the Kondo temperature. We derive a
nonequilibrium Keldysh field theory valid for a system with any finite
electron-electron interaction which is much stronger than the coupling of the
system to contacts. Finite electron-electron interactions are treated involving
as many slave-boson degrees of freedom as one needs for a concrete many-body
system. In a small vicinity of the zero slave-bosonic field configuration weak
slave-bosonic oscillations, induced by the dot-contacts tunneling, are
described by an effective Keldysh action quadratic in the slave-bosonic fields.
For clarity the theory is presented for the single impurity Anderson model but
the construction of the Keldysh field integral is universal and applicable to
systems with more complex many-body spectra.Comment: 5 pages, 2 figure
Keldysh effective action theory for universal physics in spin-1/2 Kondo dots
We present a theory for the Kondo spin-1/2 effect in strongly correlated
quantum dots. The theory is applicable at any temperature and voltage. It is
based on a quadratic Keldysh effective action parameterized by a universal
function. We provide a general analytical form for the tunneling density of
states through this universal function for which we propose a simple
microscopic model. We apply our theory to the highly asymmetric Anderson model
with and describe its strong coupling limit, weak coupling limit and
crossover region within a single analytical expression. We compare our results
with numerical renormalization group in equilibrium and with a real-time
renormalization group out of equilibrium and show that the universal shapes of
the linear and differential conductance obtained in our theory and in these
theories are very close to each other in a wide range of temperatures and
voltages. In particular, as in the real-time renormalization group, we predict
that at the Kondo voltage the differential conductance is equal to 2/3 of its
maximum.Comment: 5 pages, 2 figures + supp.ma
Performance analysis of an interacting quantum dot thermoelectric system
We analyze the nanocaloritronic performance of an interacting quantum dot
that is subject to an applied bias and an applied temperature gradient. It is
now well known that, in the absence of phonon contribution, a weakly coupled
non-interacting quantum dot can operate at thermoelectric efficiencies
approaching the Carnot limit. However, it has also been recently pointed out
that such peak efficiencies can only be achieved when operated in the
reversible limit, with a vanishing current and hence a vanishing power output.
In this paper, we point out three fundamental results affecting the
thermoelectric performance due to the inclusion of Coulomb interactions: a) The
reversible operating point carries zero efficiency, b) operation at finite
power output is possible even at peak efficiencies approaching the Carnot
value, and c) the evaluated trends of the the maximum efficiency deviate
considerably from the conventional {\it{figure of merit}} based result.
Finally, we also analyze our system for thermoelectric operation at maximum
power output.Comment: 10 pages, 6 figures, Resubmission- to be published in Phys. Rev.
Driven Tunneling Dynamics: Bloch-Redfield Theory versus Path Integral Approach
In the regime of weak bath coupling and low temperature we demonstrate
numerically for the spin-boson dynamics the equivalence between two widely used
but seemingly different roads of approximation, namely the path integral
approach and the Bloch-Redfield theory. The excellent agreement between these
two methods is corroborated by a novel efficient analytical high-frequency
approach: it well approximates the decay of quantum coherence via a series of
damped coherent oscillations. Moreover, a suitably tuned control field can
selectively enhance or suppress quantum coherence.Comment: 4 pages including 3 figures, submitted for publicatio
Slave-boson Keldysh field theory for the Kondo effect in quantum dots
We present a {\it nonequilibrium nonperturbative} field theory for the Kondo
effect in strongly interacting quantum dots at finite temperatures. Unifying
the slave-boson representation with the Keldysh field integral an effective
Keldysh action is derived and explored in the vicinity of the zero
slave-bosonic field configuration. The theory properly reflects the essential
features of the Kondo physics and at the same time significantly simplifies a
field-theoretic treatment of the phenomenon, avoiding complicated saddle point
analysis or 1/N expansions, used so far. Importantly, our theory admits a {\it
closed analytical} solution which explains the mechanism of the Kondo effect in
terms of an interplay between the real and imaginary parts of the slave-bosonic
self-energy. It thus provides a convenient nonperturbative building block,
playing the role of a "free propagator", for more advanced theories. We finally
demonstrate that already this simplest possible field theory is able to
correctly reproduce experimental data on the Kondo peak observed in the
differential conductance, correctly predicts the Kondo temperature and, within
its applicability range, has the same universal temperature dependence of the
conductance as the one obtained in numerical renormalization group
calculations.Comment: published versio
Driving-Induced Symmetry Breaking in the Spin-Boson System
A symmetric dissipative two-state system is asymptotically completely
delocalized independent of the initial state. We show that driving-induced
localization at long times can take place when both the bias and tunneling
coupling energy are harmonically modulated. Dynamical symmetry breaking on
average occurs when the driving frequencies are odd multiples of some reference
frequency. This effect is universal, as it is independent of the dissipative
mechanism. Possible candidates for an experimental observation are flux
tunneling in the variable barrier rf SQUID and magnetization tunneling in
magnetic molecular clusters.Comment: 4 pages, 4 figures, to be published in PR
Sub-gap spectroscopy of thermally excited quasiparticles in a Nb contacted carbon nanotube quantum dot
We present electronic transport measurements of a single wall carbon nanotube
quantum dot coupled to Nb superconducting contacts. For temperatures comparable
to the superconducting gap peculiar transport features are observed inside the
Coulomb blockade and superconducting energy gap regions. The observed
temperature dependence can be explained in terms of sequential tunneling
processes involving thermally excited quasiparticles. In particular, these new
channels give rise to two unusual conductance peaks at zero bias in the
vicinity of the charge degeneracy point and allow to determine the degeneracy
of the ground states involved in transport. The measurements are in good
agreement with model calculations.Comment: 5 pages, 4 figure
Thermally induced subgap features in the cotunneling spectroscopy of a carbon nanotube
We report on nonlinear cotunneling spectroscopy of a carbon nanotube quantum
dot coupled to Nb superconducting contacts. Our measurements show rich subgap
features in the stability diagram which become more pronounced as the
temperature is increased. Applying a transport theory based on the
Liouville-von Neumann equation for the density matrix, we show that the
transport properties can be attributed to processes involving sequential as
well as elastic and inelastic cotunneling of quasiparticles thermally excited
across the gap. In particular, we predict thermal replicas of the elastic and
inelastic cotunneling peaks, in agreement with our experimental results.Comment: 21 pages, 9 figures, submitted to New Journal of Physic
Inelastic cotunneling in quantum dots and molecules with weakly broken degeneracies
We calculate the nonlinear cotunneling conductance through interacting
quantum dot systems in the deep Coulomb blockade regime using a rate equation
approach based on the T-matrix formalism, which shows in the concerned regions
very good agreement with a generalized master equation approach. Our focus is
on inelastic cotunneling in systems with weakly broken degeneracies, such as
complex quantum dots or molecules. We find for these systems a characteristic
gate dependence of the non-equilibrium cotunneling conductance. While on one
side of a Coulomb diamond the conductance decreases after the inelastic
cotunneling threshold towards its saturation value, on the other side it
increases monotonously even after the threshold. We show that this behavior
originates from an asymmetric gate voltage dependence of the effective
cotunneling amplitudes.Comment: 12 pages, 12 figures; revised published versio
Iterative algorithm versus analytic solutions of the parametrically driven dissipative quantum harmonic oscillator
We consider the Brownian motion of a quantum mechanical particle in a
one-dimensional parabolic potential with periodically modulated curvature under
the influence of a thermal heat bath. Analytic expressions for the
time-dependent position and momentum variances are compared with results of an
iterative algorithm, the so-called quasiadiabatic propagator path integral
algorithm (QUAPI). We obtain good agreement over an extended range of
parameters for this spatially continuous quantum system. These findings
indicate the reliability of the algorithm also in cases for which analytic
results may not be available a priori.Comment: 15 pages including 11 figures, one reference added, minor typos
correcte
- âŚ