804 research outputs found

### Low energy theory of disordered graphene

At low values of external doping graphene displays a wealth of unconventional
transport properties. Perhaps most strikingly, it supports a robust 'metallic'
regime, with universal conductance of the order of the conductance quantum. We
here apply a combination of mean field and bosonization methods to explore the
large scale transport properties of the system. We find that, irrespective of
the doping level, disordered graphene is subject to common mechanisms of
Anderson localization. However, at low doping a number of renormalization
mechanisms conspire to protect the conductivity of the system, to an extend
that strong localization may not be seen even at temperatures much smaller than
those underlying present experimental work.Comment: 4 page

### Strong Anderson localization in cold atom quantum quenches

Signatures of strong Anderson localization in the momentum distribution of a
cold atom cloud after a quantum quench are studied. We consider a quasi
one-dimensional cloud initially prepared in a well defined momentum state, and
expanding for some time in a disorder speckle potential. Anderson localization
leads to a formation of a coherence peak in the \emph{forward} scattering
direction (as opposed to the common weak localization backscattering peak). We
present a microscopic, and fully time resolved description of the phenomenon,
covering the entire diffusion--to--localization crossover. Our results should
be observable by present day technology.Comment: 4 pages, 2 figures, published versio

### Echo spectroscopy of Anderson localization

We propose a conceptually new framework to study the onset of Anderson
localization in disordered systems. The idea is to expose waves propagating in
a random scattering environment to a sequence of short dephasing pulses. The
system responds through coherence peaks forming at specific echo times, each
echo representing a particular process of quantum interference. We suggest a
concrete realization for cold gases, where quantum interferences are observed
in the momentum distribution of matter waves in a laser speckle potential. This
defines a challenging, but arguably realistic framework promising to yield
unprecedented insight into the mechanisms of Anderson localization.Comment: 14 pages, 7 figures; published versio

### 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 $U=\infty$ 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

### Spectral and Transport Properties of d-Wave Superconductors With Strong Impurities

One of the remarkable features of disordered d-wave superconductors is strong
sensitivity of long range properties to the microscopic realization of the
disorder potential. Particularly rich phenomenology is observed for the --
experimentally relevant -- case of dilute distributions of isolated impurity
centers. Building on earlier diagrammatic analyses, the present paper derives
and analyses a low energy effective field theory of this system. Specifically,
the results of previous diagrammatic T-matrix approaches are extended into the
perturbatively inaccessible low energy regimes, and the long range (thermal)
transport behaviour of the system is discussed. It turns out that in the
extreme case of a half-filled tight binding band and infinitely strong
impurities (impurities at the unitary limit), the system is in a delocalized
phase.Comment: 14 pages, two figures include

### 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

### Crossover of magnetoconductance autocorrelation for a ballistic chaotic quantum dot

The autocorrelation function C_{\varphi,\eps}(\Delta\varphi,\,\Delta \eps)=
\langle \delta g(\varphi,\,\eps)\, \delta
g(\varphi+\Delta\varphi,\,\eps+\Delta \eps)\rangle ($\varphi$ and \eps are
rescaled magnetic flux and energy) for the magnetoconductance of a ballistic
chaotic quantum dot is calculated in the framework of the supersymmetric
non-linear $\sigma$-model. The Hamiltonian of the quantum dot is modelled by a
Gaussian random matrix. The particular form of the symmetry breaking matrix is
found to be relevant for the autocorrelation function but not for the average
conductance. Our results are valid for the complete crossover from orthogonal
to unitary symmetry and their relation with semiclassical theory and an
$S$-matrix Brownian motion ensemble is discussed.Comment: 7 pages, no figures, accepted for publication in Europhysics Letter

### Finite temperature damping of collective modes of a BCS-BEC crossover superfluid

A new mechanism is proposed to explain the puzzling damping of collective
excitations, which was recently observed in the experiments of strongly
interacting Fermi gases below the superfluid critical temperature on the
fermionic (BCS) side of Feshbach resonance. Sound velocity, superfluid density
and damping rate are calculated with effective field theory. We find that a
dominant damping process is due to the interaction between superfluid phonons
and thermally excited fermionic quasiparticles, in contrast to the previously
proposed pair-breaking mechanism. Results from our effective model are compared
quantitatively with recent experimental findings, showing a good agreement.Comment: final version, 9 pages, 4 figure

### Transient fluctuation relations for time-dependent particle transport

We consider particle transport under the influence of time-varying driving forces, where fluctuation relations connect the statistics of pairs of time reversed evolutions of physical observables. In many "mesoscopic" transport processes, the effective many-particle dynamics is dominantly classical, while the microscopic rates governing particle motion are of quantum-mechanical origin. We here employ the stochastic path integral approach as an optimal tool to probe the fluctuation statistics in such applications. Describing the classical limit of the Keldysh quantum nonequilibrium field theory, the stochastic path integral encapsulates the quantum origin of microscopic particle exchange rates. Dynamically, it is equivalent to a transport master equation which is a formalism general enough to describe many applications of practical interest. We apply the stochastic path integral to derive general functional fluctuation relations for current flow induced by time-varying forces. We show that the successive measurement processes implied by this setup do not put the derivation of quantum fluctuation relations in jeopardy. While in many cases the fluctuation relation for a full time-dependent current profile may contain excessive information, we formulate a number of reduced relations, and demonstrate their application to mesoscopic transport. Examples include the distribution of transmitted charge, where we show that the derivation of a fluctuation relation requires the combined monitoring of the statistics of charge and work

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