3,149 research outputs found
Injection statistics simulator for dynamic analysis of noise in mesoscopic devices
We present a model for electron injection from thermal reservoirs which is
applied to particle simulations of one-dimensional mesoscopic conductors. The
statistics of injected carriers is correctly described from nondegenerate to
completely degenerate conditions. The model is validated by comparing Monte
Carlo simulations with existing analytical results for the case of ballistic
conductors. An excellent agreement is found for average and noise
characteristics, in particular, the fundamental unities of electrical and
thermal conductances are exactly reproduced.Comment: 4 pages, revtex, 4 PS figures, accepted Semicond. Sci. Techno
New models for two real scalar fields and their kinklike solutions
In this work we study the presence of kinks in models described by two real
scalar fields in bi-dimensional space-time. We generate new two-field models,
constructed from distinct but important one-field models, and we solve them
with techniques that we introduce in the current work. We illustrate the
results with several examples of current interest to high energy physics.Comment: 8 pages, 6 figures; To appear in Adv. High Energy Phy
Constructing networks of defects with scalar fields
We propose a new way to build networks of defects. The idea takes advantage
of the deformation procedure recently employed to describe defect structures,
which we use to construct networks, spread from small rudimentary networks that
appear in simple models of scalar fields.Comment: 5 pages, 4 figures, version with new title, motivations and
references, to appear in PL
Closed Geodesics on Godel-type Backgrounds
We consider radial oscillations of supertube probes in the Godel-type
background which is U-dual to the compactified pp-wave obtained from the
Penrose limit of the NS five-brane near horizon geometry. The supertube probe
computation can be carried over directly to a string probe calculation on the
U-dual background. The classical equations of motion are solved explicitly. In
general, the probe is not restricted to travel unidirectionally through any
global time coordinate. In particular, we find geodesics that close.Comment: latex, 15 pages, 1 figure. v3: reference added, clarifications added
and some discussions expande
Lagged and instantaneous dynamical influences related to brain structural connectivity
Contemporary neuroimaging methods can shed light on the basis of human neural
and cognitive specializations, with important implications for neuroscience and
medicine. Different MRI acquisitions provide different brain networks at the
macroscale; whilst diffusion-weighted MRI (dMRI) provides a structural
connectivity (SC) coincident with the bundles of parallel fibers between brain
areas, functional MRI (fMRI) accounts for the variations in the
blood-oxygenation-level-dependent T2* signal, providing functional connectivity
(FC).Understanding the precise relation between FC and SC, that is, between
brain dynamics and structure, is still a challenge for neuroscience. To
investigate this problem, we acquired data at rest and built the corresponding
SC (with matrix elements corresponding to the fiber number between brain areas)
to be compared with FC connectivity matrices obtained by 3 different methods:
directed dependencies by an exploratory version of structural equation modeling
(eSEM), linear correlations (C) and partial correlations (PC). We also
considered the possibility of using lagged correlations in time series; so, we
compared a lagged version of eSEM and Granger causality (GC). Our results were
two-fold: firstly, eSEM performance in correlating with SC was comparable to
those obtained from C and PC, but eSEM (not C nor PC) provides information
about directionality of the functional interactions. Second, interactions on a
time scale much smaller than the sampling time, captured by instantaneous
connectivity methods, are much more related to SC than slow directed influences
captured by the lagged analysis. Indeed the performance in correlating with SC
was much worse for GC and for the lagged version of eSEM. We expect these
results to supply further insights to the interplay between SC and functional
patterns, an important issue in the study of brain physiology and function.Comment: Accepted and published in Frontiers in Psychology in its current
form. 27 pages, 1 table, 5 figures, 2 suppl. figure
Noise suppression due to long-range Coulomb interaction: Crossover between diffusive and ballistic transport regimes
We present a Monte Carlo analysis of shot-noise suppression due to long-range
Coulomb interaction in semiconductor samples under a crossover between
diffusive and ballistic transport regimes. By varying the mean time between
collisions we find that the strong suppression observed under the ballistic
regime persists under quasi-ballistic conditions, before being washed out when
a complete diffusive regime is reached.Comment: RevTex, 3 pages, 4 figures, minor correction
String Theory and Quantum Chromodynamics
I review recent progress on the connection between string theory and quantum
chromodynamics in the context of the gauge/gravity duality. Emphasis is placed
on conciseness and conceptual aspects rather than on technical details. Topics
covered include the large-Nc limit of gauge theories, the gravitational
description of gauge theory thermodynamics and hydrodynamics, and
confinement/deconfinement thermal phase transitions.Comment: 38 pages, 24 figures. Lectures given at the RTN Winter School on
"Strings, Supergravity and Gauge Theories" at CERN on January 15-19, 200
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