80 research outputs found
Fredholm determinants and the statistics of charge transport
Using operator algebraic methods we show that the moment generating function
of charge transport in a system with infinitely many non-interacting Fermions
is given by a determinant of a certain operator in the one-particle Hilbert
space. The formula is equivalent to a formula of Levitov and Lesovik in the
finite dimensional case and may be viewed as its regularized form in general.
Our result embodies two tenets often realized in mesoscopic physics, namely,
that the transport properties are essentially independent of the length of the
leads and of the depth of the Fermi sea.Comment: 30 pages, 2 figures, reference added, credit amende
Laboratory study of magnetic reconnection in lunar-relevant mini-magnetospheres
Mini-magnetospheres are small ion-scale structures that are well-suited to
studying kinetic-scale physics of collisionless space plasmas. Such ion-scale
magnetospheres can be found on local regions of the Moon, associated with the
lunar crustal magnetic field. In this paper, we report on the laboratory
experimental study of magnetic reconnection in laser-driven, lunar-like
ion-scale magnetospheres on the Large Plasma Device (LAPD) at the University of
California - Los Angeles. In the experiment, a high-repetition rate (1 Hz),
nanosecond laser is used to drive a fast moving, collisionless plasma that
expands into the field generated by a pulsed magnetic dipole embedded into a
background plasma and magnetic field. The high-repetition rate enables the
acquisition of time-resolved volumetric data of the magnetic and electric
fields to characterize magnetic reconnection and calculate the reconnection
rate. We notably observe the formation of Hall fields associated with
reconnection. Particle-in-cell simulations reproducing the experimental results
were performed to study the micro-physics of the interaction. By analyzing the
generalized Ohm's law terms, we find that the electron-only reconnection is
driven by kinetic effects, through the electron pressure anisotropy. These
results are compared to recent satellite measurements that found evidence of
magnetic reconnection near the lunar surface
Dephasing and Measurement Efficiency via a Quantum Dot Detector
We study charge detection and controlled dephasing of a mesoscopic system via
a quantum dot detector (QDD), where the mesoscopic system and the QDD are
capacitively coupled. The QDD is considered to have coherent resonant
tunnelling via a single level. It is found that the dephasing rate is
proportional to the square of the conductance of the QDD for the Breit-Wigner
model, showing that the dephasing is completely different from the shot noise
of the detector. The measurement rate, on the other hand, shows a dip near the
resonance. Our findings are peculiar especially for a symmetric detector in the
following aspect: The dephasing rate is maximum at resonance of the QDD where
the detector conductance is insensitive to the charge state of the mesoscopic
system. As a result, the efficiency of the detector shows a dip and vanishes at
resonance, in contrast to the single-channel symmetric non-resonant detector
that has always a maximum efficiency. We find that this difference originates
from a very general property of the scattering matrix: The abrupt phase change
exists in the scattering amplitudes in the presence of the symmetry, which is
insensitive to the detector current but {\em stores} the information of the
quantum state of the mesoscopic system.Comment: 7 pages, 3 figure
Quantum Disorder and Quantum Chaos in Andreev Billiards
We investigate the crossover from the semiclassical to the quantum
description of electron energy states in a chaotic metal grain connected to a
superconductor. We consider the influence of scattering off point impurities
(quantum disorder) and of quantum diffraction (quantum chaos) on the electron
density of states. We show that both the quantum disorder and the quantum chaos
open a gap near the Fermi energy. The size of the gap is determined by the mean
free time in disordered systems and by the Ehrenfest time in clean chaotic
systems. Particularly, if both times become infinitely large, the density of
states is gapless, and if either of these times becomes shorter than the
electron escape time, the density of states is described by random matrix
theory. Using the Usadel equation, we also study the density of states in a
grain connected to a superconductor by a diffusive contact.Comment: 20 pages, 10 figure
Proximity effects at ferromagnet-superconductor interfaces
We study proximity effects at ferromagnet superconductor interfaces by
self-consistent numerical solution of the Bogoliubov-de Gennes equations for
the continuum, without any approximations. Our procedures allow us to study
systems with long superconducting coherence lengths. We obtain results for the
pair potential, the pair amplitude, and the local density of states. We use
these results to extract the relevant proximity lengths. We find that the
superconducting correlations in the ferromagnet exhibit a damped oscillatory
behavior that is reflected in both the pair amplitude and the local density of
states. The characteristic length scale of these oscillations is approximately
inversely proportional to the exchange field, and is independent of the
superconducting coherence length in the range studied. We find the
superconducting coherence length to be nearly independent of the ferromagnetic
polarization.Comment: 13 Pages total. Compressed .eps figs might display poorly, but will
print fin
Proximity effects and characteristic lengths in ferromagnet-superconductor structures
We present an extensive theoretical investigation of the proximity effects
that occur in Ferromagnet/Superconductor () systems. We use a numerical
method to solve self consistently the Bogoliubov-de Gennes equations in the
continuum. We obtain the pair amplitude and the local density of states (DOS),
and use these results to extract the relevant lengths characterizing the
leakage of superconductivity into the magnet and to study spin splitting into
the superconductor. These phenomena are investigated as a function of
parameters such as temperature, magnet polarization, interfacial scattering,
sample size and Fermi wavevector mismatch, all of which turn out to have
important influence on the results. These comprehensive results should help
characterize and analyze future data and are shown to be in agreement with
existing experiments.Comment: 24 pages, including 26 figure
Measurement of finite-frequency current statistics in a single-electron transistor
Electron transport in nano-scale structures is strongly influenced by the
Coulomb interaction which gives rise to correlations in the stream of charges
and leaves clear fingerprints in the fluctuations of the electrical current. A
complete understanding of the underlying physical processes requires
measurements of the electrical fluctuations on all time and frequency scales,
but experiments have so far been restricted to fixed frequency ranges as
broadband detection of current fluctuations is an inherently difficult
experimental procedure. Here we demonstrate that the electrical fluctuations in
a single electron transistor (SET) can be accurately measured on all relevant
frequencies using a nearby quantum point contact for on-chip real-time
detection of the current pulses in the SET. We have directly measured the
frequency-dependent current statistics and hereby fully characterized the
fundamental tunneling processes in the SET. Our experiment paves the way for
future investigations of interaction and coherence induced correlation effects
in quantum transport.Comment: 7 pages, 3 figures, published in Nature Communications (open access
Charge densities and charge noise in mesoscopic conductors
We introduce a hierarchy of density of states to characterize the charge
distribution in a mesoscopic conductor. At the bottom of this hierarchy are the
partial density of states which represent the contribution to the local density
of states if both the incident and the out-going scattering channel is
prescribed. The partial density of states play a prominent role in measurements
with a scanning tunneling microscope on multiprobe conductors in the presence
of current flow. The partial density of states determine the degree of
dephasing generated by a weakly coupled voltage probe. In addition the partial
density of states determine the frequency-dependent response of mesoscopic
conductors in the presence of slowly oscillating voltages applied to the
contacts of the sample. The partial density of states permit the formulation of
a Friedel sum rule which can be applied locally. We introduce the off-diagonal
elements of the partial density of states matrix to describe charge fluctuation
processes. This generalization leads to a local Wigner-Smith life-time matrix.Comment: 10 pages, 2 figure
Nonlinear complexity analysis of brain fMRI signals in schizophrenia
Peer reviewedPublisher PD
Association between age of cannabis initiation and gray matter covariance networks in recent onset psychosis
Cannabis use during adolescence is associated with an increased risk of developing psychosis. According to a current hypothesis, this results from detrimental effects of early cannabis use on brain maturation during this vulnerable period. However, studies investigating the interaction between early cannabis use and brain structural alterations hitherto reported inconclusive findings. We investigated effects of age of cannabis initiation on psychosis using data from the multicentric Personalized Prognostic Tools for Early Psychosis Management (PRONIA) and the Cannabis Induced Psychosis (CIP) studies, yielding a total sample of 102 clinically-relevant cannabis users with recent onset psychosis. GM covariance underlies shared maturational processes. Therefore, we performed source-based morphometry analysis with spatial constraints on structural brain networks showing significant alterations in schizophrenia in a previous multisite study, thus testing associations of these networks with the age of cannabis initiation and with confounding factors. Earlier cannabis initiation was associated with more severe positive symptoms in our cohort. Greater gray matter volume (GMV) in the previously identified cerebellar schizophrenia-related network had a significant association with early cannabis use, independent of several possibly confounding factors. Moreover, GMV in the cerebellar network was associated with lower volume in another network previously associated with schizophrenia, comprising the insula, superior temporal, and inferior frontal gyrus. These findings are in line with previous investigations in healthy cannabis users, and suggest that early initiation of cannabis perturbs the developmental trajectory of certain structural brain networks in a manner imparting risk for psychosis later in life
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