306 research outputs found
Electronic dephasing in wires due to metallic gates
The dephasing effect of metallic gates on electrons moving in one
quasi--one--dimensional diffusive wires is analyzed. The incomplete screening
in this geometry implies that the effect of the gate can be described, at high
energies or temperatures, as an electric field fluctuating in time. The
resulting system can be considered a realization of the Caldeira-Leggett model
of an environment coupled to many particles. Within the range of temperatures
where this approximation is valid, a simple estimation of the inverse dephasing
time gives .Comment: 6 page
Detecting charge noise with a Josephson junction: A problem of thermal escape in presence of non-Gaussian fluctuations
Motivated by several experimental activities to detect charge noise produced
by a mesoscopic conductor with a Josephson junction as on-chip detector, the
switching rate out of its zero-voltage state is studied. This process is
related to the fundamental problem of thermal escape in presence of
non-Gaussian fluctuations. In the relevant case of weak higher than second
order cumulants, an effective Fokker-Planck equation is derived, which is then
used to obtain an explicit expression for the escape rate. Specific results for
the rate asymmetry due to the third moment of current noise allow to analyse
experimental data and to optimize detection circuits.Comment: 4 pages, 1 figure; minor typos corrected, some revisions in the tex
Pseudo-diffusive magnetotransport in graphene
Transport properties through wide and short ballistic graphene junctions are
studied in the presence of arbitrary dopings and magnetic fields. No dependence
on the magnetic field is observed at the Dirac point for any current cumulant,
just as in a classical diffusive system, both in normal-graphene-normal and
normal-graphene-superconductor junctions. This pseudo-diffusive regime is
however extremely fragile respect to doping at finite fields. We identify the
crossovers to a field-suppressed and a normal ballistic transport regime in the
magnetic field - doping parameter space, and provide a physical interpretation
of the phase diagram. Remarkably, pseudo-diffusive transport is recovered away
from the Dirac point in resonance with Landau levels at high magnetic fields.Comment: 4+ pages, 2 figures. Minor corrections. Published version
Weak values of electron spin in a double quantum dot
We propose a protocol for a controlled experiment to measure a weak value of
the electron's spin in a solid state device. The weak value is obtained by a
two step procedure -- weak measurement followed by a strong one
(post-selection), where the outcome of the first measurement is kept provided a
second post-selected outcome occurs. The set-up consists of a double quantum
dot and a weakly coupled quantum point contact to be used as a detector.
Anomalously large values of the spin of a two electron system are predicted, as
well as negative values of the total spin. We also show how to incorporate the
adverse effect of decoherence into this procedure.Comment: 4+ pages, 3 figures, final published versio
Frequency Dependence of Magnetopolarizability of Mesoscopic Grains
We calculate average magnetopolarizability of an isolated metallic sample at
frequency comparable to the mean level spacing . The frequency
dependence of the magnetopolarizability is described by a universal function of
.Comment: 3 pages, 1 figur
Effect of Interactions on the Admittance of Ballistic Wires
A self-consistent theory of the admittance of a perfect ballistic, locally
charge neutral wire is proposed. Compared to a non-interacting theory,
screening effects drastically change the frequency behavior of the conductance.
In the single-channel case the frequency dependence of the admittance is
monotonic, while for two or more channels collective interchannel excitations
lead to resonant structures in the admittance. The imaginary part of the
admittance is typically positive, but can become negative near resonances.Comment: Presentation considerably modified; the results are unchanged. 4
pages, 2 figures .eps-format include
Spin-Dependent Hubbard Model and a Quantum Phase Transition in Cold Atoms
We describe an experimental protocol for introducing spin-dependent lattice
structure in a cold atomic fermi gas using lasers. It can be used to realize
Hubbard models whose hopping parameters depend on spin and whose interaction
strength can be controlled with an external magnetic field. We suggest that
exotic superfluidities will arise in this framework. An especially interesting
possibility is a class of states that support coexisting superfluid and normal
components, even at zero temperature. The quantity of normal component varies
with external parameters. We discuss some aspects of the quantum phase
transition that arises at the point where it vanishes.Comment: 9 pages, 7 figures; added/corrected references in [11] and [44
Cascade Boltzmann - Langevin approach to higher-order current correlations in diffusive metal contacts
The Boltzmann - Langevin approach is extended to calculations of third and
fourth cumulants of current in diffusive-metal contacts. These cumulants result
from indirect correlations between current fluctuations, which may be
considered as "noise of noise". The calculated third cumulant coincides exactly
with its quantum-mechanical value. The fourth cumulant tends to its
quantum-mechanical value at high voltages and to a positive value
at V=0 changing its sign at .Comment: 6 pages, 2 eps figures, typo corrected, minor change
Magneto-polarisability of mesoscopic systems
In order to understand how screening is modified by electronic interferences
in a mesoscopic isolated system, we have computed both analytically and
numerically the average thermodynamic and time dependent polarisabilities of
two dimensional mesoscopic samples in the presence of an Aharonov-Bohm flux.
Two geometries have been considered: rings and squares. Mesoscopic correction
to screening are taken into account in a self consistent way, using the
response function formalism. The role of the statistical ensemble (canonical
and grand canonical), disorder and frequency have been investigated. We have
also computed first order corrections to the polarisability due to
electron-electron interactions. Our main results concern the diffusive regime.
In the canonical ensemble, there is no flux dependence polarisability when the
frequency is smaller than the level spacing. On the other hand, in the grand
canonical ensemble for frequencies larger than the mean broadening of the
energy levels (but still small compared to the level spacing), the
polarisability oscillates with flux, with the periodicity . The order of
magnitude of the effect is given by , where is the Thomas Fermi screening length, the
width of the rings or the size of the squares and their average
dimensionless conductance. This magnetopolarisability of Aharonov-Bohm rings
has been recently measured experimentally \cite{PRL_deblock00} and is in good
agreement with our grand canonical result.Comment: 12 pages, 10 figures, revte
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