3,473 research outputs found
Dielectric susceptibility of the Coulomb-glass
We derive a microscopic expression for the dielectric susceptibility
of a Coulomb glass, which corresponds to the definition used in classical
electrodynamics, the derivative of the polarization with respect to the
electric field. The fluctuation-dissipation theorem tells us that is a
function of the thermal fluctuations of the dipole moment of the system. We
calculate numerically for three-dimensional Coulomb glasses as a
function of temperature and frequency
Universal Crossover between Efros-Shklovskii and Mott Variable-Range-Hopping Regimes
A universal scaling function, describing the crossover between the Mott and
the Efros-Shklovskii hopping regimes, is derived, using the percolation picture
of transport in strongly localized systems. This function is agrees very well
with experimental data. Quantitative comparison with experiment allows for the
possible determination of the role played by polarons in the transport.Comment: 7 pages + 1 figure, Revte
Off-equilibrium dynamics of the two-dimensional Coulomb glass
The dynamics of the 2D Coulomb glass model is investigated by kinetic Monte
Carlo simulation. An exponential divergence of the relaxation time signals a
zero-temperature freezing transition. At low temperatures the dynamics of the
system is glassy. The local charge correlations and the response to
perturbations of the local potential show aging. The dynamics of formation of
the Coulomb gap is slow and the density of states at the Fermi level decays in
time as a power law. The relevance of these findings for recent transport
experiments in Anderson-insulating films is pointed out.Comment: 7 pages, 7 figure
Electronic correlation effects and the Coulomb gap at finite temperature
We have investigated the effect of the long-range Coulomb interaction on the
one-particle excitation spectrum of n-type Germanium, using tunneling
spectroscopy on mechanically controllable break junctions. The tunnel
conductance was measured as a function of energy and temperature. At low
temperatures, the spectra reveal a minimum at zero bias voltage due to the
Coulomb gap. In the temperature range above 1 K the Coulomb gap is filled by
thermal excitations. This behavior is reflected in the temperature dependence
of the variable-range hopping resitivity measured on the same samples: Up to a
few degrees Kelvin the Efros-Shkovskii ln law is obeyed,
whereas at higher temperatures deviations from this law are observed,
indicating a cross-over to Mott's ln law. The mechanism of
this cross-over is different from that considered previously in the literature.Comment: 3 pages, 3 figure
An Exact Formula for the Average Run Length to False Alarm of the Generalized Shiryaev-Roberts Procedure for Change-Point Detection under Exponential Observations
We derive analytically an exact closed-form formula for the standard minimax
Average Run Length (ARL) to false alarm delivered by the Generalized
Shiryaev-Roberts (GSR) change-point detection procedure devised to detect a
shift in the baseline mean of a sequence of independent exponentially
distributed observations. Specifically, the formula is found through direct
solution of the respective integral (renewal) equation, and is a general result
in that the GSR procedure's headstart is not restricted to a bounded range, nor
is there a "ceiling" value for the detection threshold. Apart from the
theoretical significance (in change-point detection, exact closed-form
performance formulae are typically either difficult or impossible to get,
especially for the GSR procedure), the obtained formula is also useful to a
practitioner: in cases of practical interest, the formula is a function linear
in both the detection threshold and the headstart, and, therefore, the ARL to
false alarm of the GSR procedure can be easily computed.Comment: 9 pages; Accepted for publication in Proceedings of the 12-th
German-Polish Workshop on Stochastic Models, Statistics and Their
Application
Cross-link governed dynamics of biopolymer networks
Cytoskeletal networks of biopolymers are cross-linked by a variety of
proteins. Experiments have shown that dynamic cross-linking with physiological
linker proteins leads to complex stress relaxation and enables network flow at
long times. We present a model for the mechanical properties of transient
networks. By a combination of simulations and analytical techniques we show
that a single microscopic timescale for cross-linker unbinding leads to a broad
spectrum of macroscopic relaxation times, resulting in a weak power-law
dependence of the shear modulus on frequency. By performing rheological
experiments, we demonstrate that our model quantitatively describes the
frequency behavior of actin network cross-linked with -Actinin- over
four decades in frequency.Comment: 4 page
Coherent State Path Integrals in the Weyl Representation
We construct a representation of the coherent state path integral using the
Weyl symbol of the Hamiltonian operator. This representation is very different
from the usual path integral forms suggested by Klauder and Skagerstan in
\cite{Klau85}, which involve the normal or the antinormal ordering of the
Hamiltonian. These different representations, although equivalent quantum
mechanically, lead to different semiclassical limits. We show that the
semiclassical limit of the coherent state propagator in Weyl representation is
involves classical trajectories that are independent on the coherent states
width. This propagator is also free from the phase corrections found in
\cite{Bar01} for the two Klauder forms and provides an explicit connection
between the Wigner and the Husimi representations of the evolution operator.Comment: 23 page
Semiclassical Propagation of Wavepackets with Real and Complex Trajectories
We consider a semiclassical approximation for the time evolution of an
originally gaussian wave packet in terms of complex trajectories. We also
derive additional approximations replacing the complex trajectories by real
ones. These yield three different semiclassical formulae involving different
real trajectories. One of these formulae is Heller's thawed gaussian
approximation. The other approximations are non-gaussian and may involve
several trajectories determined by mixed initial-final conditions. These
different formulae are tested for the cases of scattering by a hard wall,
scattering by an attractive gaussian potential, and bound motion in a quartic
oscillator. The formula with complex trajectories gives good results in all
cases. The non-gaussian approximations with real trajectories work well in some
cases, whereas the thawed gaussian works only in very simple situations.Comment: revised text, 24 pages, 6 figure
Non-Markovian Configurational Diffusion and Reaction Coordinates for Protein Folding
The non-Markovian nature of polymer motions is accounted for in folding
kinetics, using frequency-dependent friction. Folding, like many other problems
in the physics of disordered systems, involves barrier crossing on a correlated
energy landscape. A variational transition state theory (VTST) that reduces to
the usual Bryngelson-Wolynes Kramers approach when the non-Markovian aspects
are neglected is used to obtain the rate, without making any assumptions
regarding the size of the barrier, or the memory time of the friction. The
transformation to collective variables dependent on the dynamics of the system
allows the theory to address the controversial issue of what are ``good''
reaction coordinates for folding.Comment: 9 pages RevTeX, 3 eps-figures included, submitted to PR
Neuroimmune disorders in COVID-19
Severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), the aetiologic agent of the coronavirus disease 2019 (COVID-19), is now rapidly disseminating throughout the world with 147,443,848 cases reported so far. Around 30–80% of cases (depending on COVID-19 severity) are reported to have neurological manifestations including anosmia, stroke, and encephalopathy. In addition, some patients have recognised autoimmune neurological disorders, including both central (limbic and brainstem encephalitis, acute disseminated encephalomyelitis [ADEM], and myelitis) and peripheral diseases (Guillain–Barré and Miller Fisher syndrome). We systematically describe data from 133 reported series on the Neurology and Neuropsychiatry of COVID-19 blog (https://blogs.bmj.com/jnnp/2020/05/01/the-neurology-and-neuropsychiatry-of-covid-19/) providing a comprehensive overview concerning the diagnosis, and treatment of patients with neurological immune-mediated complications of SARS-CoV-2. In most cases the latency to neurological disorder was highly variable and the immunological or other mechanisms involved were unclear. Despite specific neuronal or ganglioside antibodies only being identified in 10, many had apparent responses to immunotherapies. Although the proportion of patients experiencing immune-mediated neurological disorders is small, the total number is likely to be underestimated. The early recognition and improvement seen with use of immunomodulatory treatment, even in those without identified autoantibodies, makes delayed or missed diagnoses risk the potential for long-term disability, including the emerging challenge of post-acute COVID-19 sequelae (PACS). Finally, potential issues regarding the use of immunotherapies in patients with pre-existent neuro-immunological disorders are also discussed
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