1,225 research outputs found
Magnetic states in multiply-connected flat nano-elements
Flat magnetic nano-elements are an essential component of current and future
spintronic devices. By shaping an element it is possible to select and
stabilize chosen metastable magnetic states, control its magnetization
dynamics. Here, using a recent significant development in mathematics of
conformal mapping, complex variable based approach to the description of
magnetic states in planar nano-elements is extended to the case when elements
are multiply-connected (that is, contain holes or magnetic anti-dots). We show
that presence of holes implies a certain restriction on the set of magnetic
states of nano-element.Comment: 5 pages, 7 figure
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
The non-existence of stable Schottky forms
Let be the Satake compactification of the moduli space of
principally polarized abelian -folds and the closure of the image of
the moduli space of genus curves in under the Jacobian
morphism. Then lies in the boundary of for any . We
prove that and do not meet transversely in , but
rather that their intersection contains the th order infinitesimal
neighbourhood of in . We deduce that there is no non-trivial
stable Siegel modular form that vanishes on for every . In particular,
given two inequivalent positive even unimodular quadratic forms and ,
there is a curve whose period matrix distinguishes between the theta series of
and .Comment: Corrected version, using Yamada's correct version of Fay's formula
for the period matrix of a certain degenerating family of curves. To appear
in Compositio Mathematic
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
Phase transitions in soft-committee machines
Equilibrium statistical physics is applied to layered neural networks with
differentiable activation functions. A first analysis of off-line learning in
soft-committee machines with a finite number (K) of hidden units learning a
perfectly matching rule is performed. Our results are exact in the limit of
high training temperatures. For K=2 we find a second order phase transition
from unspecialized to specialized student configurations at a critical size P
of the training set, whereas for K > 2 the transition is first order. Monte
Carlo simulations indicate that our results are also valid for moderately low
temperatures qualitatively. The limit K to infinity can be performed
analytically, the transition occurs after presenting on the order of N K
examples. However, an unspecialized metastable state persists up to P= O (N
K^2).Comment: 8 pages, 4 figure
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
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
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
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
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