3,359 research outputs found
The Effect of Columnar Disorder on the Superconducting Transition of a Type-II Superconductor in Zero Applied Magnetic Field
We investigate the effect of random columnar disorder on the superconducting
phase transition of a type-II superconductor in zero applied magnetic field
using numerical simulations of three dimensional XY and vortex loop models. We
consider both an unscreened model, in which the bare magnetic penetration
length is approximated as infinite, and a strongly screened model, in which the
magnetic penetration length is of order the vortex core radius. We consider
both equilibrium and dynamic critical exponents. We show that, as in the
disorder free case, the equilibrium transitions of the unscreened and strongly
screened models lie in the same universality class, however scaling is now
anisotropic. We find for the correlation length exponent , and
for the anisotropy exponent . We find different dynamic
critical exponents for the unscreened and strongly screened models.Comment: 30 pages 12 ps figure
Self-Organized Criticality model for Brain Plasticity
Networks of living neurons exhibit an avalanche mode of activity,
experimentally found in organotypic cultures. Here we present a model based on
self-organized criticality and taking into account brain plasticity, which is
able to reproduce the spectrum of electroencephalograms (EEG). The model
consists in an electrical network with threshold firing and activity-dependent
synapse strenghts. The system exhibits an avalanche activity power law
distributed. The analysis of the power spectra of the electrical signal
reproduces very robustly the power law behaviour with the exponent 0.8,
experimentally measured in EEG spectra. The same value of the exponent is found
on small-world lattices and for leaky neurons, indicating that universality
holds for a wide class of brain models.Comment: 4 pages, 3 figure
Non-invasive detection of animal nerve impulses with an atomic magnetometer operating near quantum limited sensitivity
Magnetic fields generated by human and animal organs, such as the heart,
brain and nervous system carry information useful for biological and medical
purposes. These magnetic fields are most commonly detected using
cryogenically-cooled superconducting magnetometers. Here we present the frst
detection of action potentials from an animal nerve using an optical atomic
magnetometer. Using an optimal design we are able to achieve the sensitivity
dominated by the quantum shot noise of light and quantum projection noise of
atomic spins. Such sensitivity allows us to measure the nerve impulse with a
miniature room-temperature sensor which is a critical advantage for biomedical
applications. Positioning the sensor at a distance of a few millimeters from
the nerve, corresponding to the distance between the skin and nerves in
biological studies, we detect the magnetic field generated by an action
potential of a frog sciatic nerve. From the magnetic field measurements we
determine the activity of the nerve and the temporal shape of the nerve
impulse. This work opens new ways towards implementing optical magnetometers as
practical devices for medical diagnostics.Comment: Main text with figures, and methods and supplementary informatio
Force fluctuation in a driven elastic chain
We study the dynamics of an elastic chain driven on a disordered substrate
and analyze numerically the statistics of force fluctuations at the depinning
transition. The probability distribution function of the amplitude of the slip
events for small velocities is a power law with an exponent
depending on the driving velocity. This result is in qualitative agreement with
experimental measurements performed on sliding elastic surfaces with
macroscopic asperities. We explore the properties of the depinning transition
as a function of the driving mode (i.e. constant force or constant velocity)
and compute the force-velocity diagram using finite size scaling methods. The
scaling exponents are in excellent agreement with the values expected in
interface models and, contrary to previous studies, we found no difference in
the exponents for periodic and disordered chains.Comment: 8 page
Monte Carlo calculation of the current-voltage characteristics of a two dimensional lattice Coulomb gas
We have studied the nonlinear current-voltage characteristic of a two
dimensional lattice Coulomb gas by Monte Carlo simulation. We present three
different determinations of the power-law exponent of the nonlinear
current-voltage characteristic, . The determinations rely on
both equilibrium and non-equilibrium simulations. We find good agreement
between the different determinations, and our results also agree closely with
experimental results for Hg-Xe thin film superconductors and for certain single
crystal thin-film high temperature superconductors.Comment: late
Monte Carlo calculation of the linear resistance of a three dimensional lattice Superconductor model in the London limit
We have studied the linear resistance of a three dimensional lattice
Superconductor model in the London limit London lattice model by Monte Carlo
simulation of the vortex loop dynamics. We find excellent finite size scaling
at the phase transition. We determine the dynamical exponent for the
isotropic London lattice model.Comment: 4 pages, RevTeX with 3 postscript figures include
Neural Response After a Single ECT Session During Retrieval of Emotional Self-Referent Words in Depression:A Randomized, Sham-Controlled fMRI Study
Comparison of treatment with insulin degludec and glargine U100 in patients with type 1 diabetes prone to nocturnal severe hypoglycaemia:The HypoDeg randomized, controlled, open-label, crossover trial
AIM: To investigate whether the long‐acting insulin analogue insulin degludec compared with insulin glargine U100 reduces the risk of nocturnal symptomatic hypoglycaemia in patients with type 1 diabetes (T1D). METHODS: Adults with T1D and at least one episode of nocturnal severe hypoglycaemia during the last 2 years were included in a 2‐year prospective, randomized, open, multicentre, crossover trial. A total of 149 patients were randomized 1:1 to basal‐bolus therapy with insulin degludec and insulin aspart or insulin glargine U100 and insulin aspart. Each treatment period lasted 1 year and consisted of 3 months of run‐in or crossover followed by 9 months of maintenance. The primary endpoint was the number of blindly adjudicated nocturnal symptomatic hypoglycaemic episodes. Secondary endpoints included the occurrence of severe hypoglycaemia. We analysed all endpoints by intention‐to‐treat. RESULTS: Treatment with insulin degludec resulted in a 28% (95% CI: 9%‐43%; P = .02) relative rate reduction (RRR) of nocturnal symptomatic hypoglycaemia at level 1 (≤3.9 mmol/L), a 37% (95% CI: 16%‐53%; P = .002) RRR at level 2 (≤3.0 mmol/L), and a 35% (95% CI: 1%‐58%; P = .04) RRR in all‐day severe hypoglycaemia compared with insulin glargine U100. CONCLUSIONS: Patients with T1D prone to nocturnal severe hypoglycaemia have lower rates of nocturnal symptomatic hypoglycaemia and all‐day severe hypoglycaemia with insulin degludec compared with insulin glargine U100
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