546 research outputs found
Shot noise in diffusive ferromagnetic metals
We show that shot noise in a diffusive ferromagnetic wire connected by tunnel
contacts to two ferromagnetic electrodes can probe the intrinsic density of
states and the extrinsic impurity scattering spin-polarization contributions in
the polarization of the wire conductivity. The effect is more pronounced when
the electrodes are perfectly polarized in opposite directions. While in this
case the shot noise has a weak dependence on the impurity scattering
polarization, it is strongly affected by the polarization of the density of
states. For a finite spin-flip scattering rate the shot noise increases well
above the normal state value and can reach the full Poissonian value when the
density of states tends to be perfectly polarized. For the parallel
configuration we find that the shot noise depends on the relative sign of the
intrinsic and the extrinsic polarizations.Comment: 4 pages, 3 figure
Doubled Full Shot Noise in Quantum Coherent Superconductor - Semiconductor Junctions
We performed low temperature shot noise measurements in Superconductor (TiN)
- strongly disordered normal metal (heavily doped Si) weakly transparent
junctions. We show that the conductance has a maximum due to coherent multiple
reflections at low energy and that shot noise is then twice the Poisson noise
(S=4eI). The shot noise changes to the normal value (S=2eI) due to a large
quasiparticle contribution.Comment: published in Physical Review Letter
Shot noise in the chaotic-to-regular crossover regime
We investigate the shot noise for phase-coherent quantum transport in the
chaotic-to-regular crossover regime. Employing the Modular Recursive Green's
Function Method for both ballistic and disordered two-dimensional cavities we
find the Fano factor and the transmission eigenvalue distribution for regular
systems to be surprisingly similar to those for chaotic systems. We argue that
in the case of regular dynamics in the cavity, diffraction at the lead openings
is the dominant source of shot noise. We also explore the onset of the
crossover from quantum to classical transport and develop a quasi-classical
transport model for shot noise suppression which agrees with the numerical
quantum data.Comment: 4 pages, 3 figures, submitted to Phys.Rev.Let
Enhanced Shot Noise in Tunneling through a Stack of Coupled Quantum Dots
We have investigated the noise properties of the tunneling current through
vertically coupled self-assembled InAs quantum dots. We observe
super-Poissonian shot noise at low temperatures. For increased temperature this
effect is suppressed. The super-Poissonian noise is explained by capacitive
coupling between different stacks of quantum dots
Full counting statistics of chiral Luttinger liquids with impurities
We study the statistics of charge transfer through an impurity in a chiral
Luttinger liquid (realized experimentally as a quantum point contact in a
fractional quantum Hall edge state device). Taking advantage of the
integrability we present a procedure for obtaining the cumulant generating
function of the probability distribution to transfer a fixed amount of charge
through the constriction. Using this approach we analyze in detail the
behaviour of the third cumulant C_3 as a function of applied voltage,
temperature and barrier height. We predict that C_3 can be used to measure the
fractional charge at temperatures, which are several orders of magnitude higher
than those needed to extract the fractional charge from the measurement of the
second cumulant. Moreover, we identify the component of C_3, which carries the
information about the fractional charge.Comment: 5 pages, 2 figures (EPS files
Shot Noise and Full Counting Statistics from Non-equilibrium Plasmons in Luttinger-Liquid Junctions
We consider a quantum wire double junction system with each wire segment
described by a spinless Luttinger model, and study theoretically shot noise in
this system in the sequential tunneling regime. We find that the
non-equilibrium plasmonic excitations in the central wire segment give rise to
qualitatively different behavior compared to the case with equilibrium
plasmons. In particular, shot noise is greatly enhanced by them, and exceeds
the Poisson limit. We show that the enhancement can be explained by the
emergence of several current-carrying processes, and that the effect disappears
if the channels effectively collapse to one due to, {\em e.g.}, fast plasmon
relaxation processes.Comment: 9 pages; IOP Journal style; several changes in the tex
Finite size effects, super-and sub-poissonian noise in a nanotube connected to leads
The injection of electrons in the bulk of carbon nanotube which is connected
to ideal Fermi liquid leads is considered. While the presence of the leads
gives a cancellation of the noise cross-correlations, the auto-correlation
noise has a Fano factor which deviates strongly from the Schottky behavior at
voltages where finite size effects are expected. Indeed, as the voltage is
increased from zero, the noise is first super-poissonian, then sub-poissonian,
and eventually it reaches the Schottky limit. These finite size effects are
also tested using a diagnosis of photo-assisted transport, where a small AC
modulation is superposed to the DC bias voltage between the injection tip and
the nanotube. When finite size effects are at play, we obtain a stepwise
behavior for the noise derivative, as expected for normal metal systems,
whereas in the absence of finite size effects, due to the presence of Coulomb
interactions, a smoothed staircase is observed. The present work shows that it
is possible to explore finite size effects in nanotube transport via a zero
frequency noise measurement
Full counting statistics of spin transfer through the Kondo dot
We calculate the spin current distribution function for a Kondo dot in two
different regimes. In the exactly solvable Toulouse limit the linear response,
zero temperature statistics of the spin transfer is trinomial, such that all
the odd moments vanish and the even moments follow a binomial distribution. On
the contrary, the corresponding spin-resolved distribution turns out to be
binomial. The combined spin and charge statistics is also determined. In
particular, we find that in the case of a finite magnetic field or an
asymmetric junction the spin and charge measurements become statistically
dependent. Furthermore, we analyzed the spin counting statistics of a generic
Kondo dot at and around the strong-coupling fixed point (the unitary limit).
Comparing these results with the Toulouse limit calculation we determine which
features of the latter are generic and which ones are artifacts of the spin
symmetry breaking.Comment: 9 pages, 3 eps figure
Generalizing with perceptrons in case of structured phase- and pattern-spaces
We investigate the influence of different kinds of structure on the learning
behaviour of a perceptron performing a classification task defined by a teacher
rule. The underlying pattern distribution is permitted to have spatial
correlations. The prior distribution for the teacher coupling vectors itself is
assumed to be nonuniform. Thus classification tasks of quite different
difficulty are included. As learning algorithms we discuss Hebbian learning,
Gibbs learning, and Bayesian learning with different priors, using methods from
statistics and the replica formalism. We find that the Hebb rule is quite
sensitive to the structure of the actual learning problem, failing
asymptotically in most cases. Contrarily, the behaviour of the more
sophisticated methods of Gibbs and Bayes learning is influenced by the spatial
correlations only in an intermediate regime of , where
specifies the size of the training set. Concerning the Bayesian case we show,
how enhanced prior knowledge improves the performance.Comment: LaTeX, 32 pages with eps-figs, accepted by J Phys
Using a quantum dot as a high-frequency shot noise detector
We present the experimental realization of a Quantum Dot (QD) operating as a
high-frequency noise detector. Current fluctuations produced in a nearby
Quantum Point Contact (QPC) ionize the QD and induce transport through excited
states. The resulting transient current through the QD represents our detector
signal. We investigate its dependence on the QPC transmission and voltage bias.
We observe and explain a quantum threshold feature and a saturation in the
detector signal. This experimental and theoretical study is relevant in
understanding the backaction of a QPC used as a charge detector.Comment: 4 pages, 4 figures, accepted for publication in Physical Review
Letter
- …