10,100 research outputs found
Melting and structure of the vortex solid in strongly anisotropic layered superconductors with random columnar pins
We study the melting transition of the low-temperature vortex solid in
strongly anisotropic layered superconductors with a concentration of random
columnar pinning centers small enough so that the areal density of the pins is
much less than that of the vortex lines. Both the external magnetic field and
the columnar pins are assumed to be oriented perpendicular to the layers Our
method, involving numerical minimization of a model free energy functional,
yields not only the free energy values at the local minima of the functional
but also the detailed density distribution of the system at each minimum: this
allows us to study in detail the structure of the different phases. We find
that at these pin concentrations and low temperatures, the thermodynamically
stable state is a topologically ordered Bragg glass. This nearly crystalline
state melts into an interstitial liquid (a liquid in which a small fraction of
vortex lines remain localized at the pinning centers) in two steps, so that the
Bragg glass and the liquid are separated by a narrow phase that we identify
from analysis of its density structure as a polycrystalline Bose glass. Both
the Bragg glass to Bose glass and the Bose glass to interstitial liquid
transitions are first-order. We also find that a local melting temperature
defined using a criterion based on the degree of localization of the vortex
lines exhibits spatial variations similar to those observed in recent
experiments.Comment: 17 page
Phase effects in neutrino conversions during a supernova shock wave
Neutrinos escaping from a core collapse supernova a few seconds after bounce
pass through the shock wave, where they may encounter one or more resonances
corresponding to . The neutrino mass eigenstates in
matter may stay coherent between these multiple resonances, giving rise to
oscillations in the survival probabilities of neutrino species. We provide an
analytical approximation to these inevitable phase effects, that relates the
density profile of the shock wave to the oscillation pattern. The phase effects
are present only if the multiple resonances encountered by neutrinos are
semi-adiabatic, which typically happens for 10^{-5} \lsim \sin^2 \theta_{13}
\lsim 10^{-3}. The observability of these oscillations is severely limited by
the inability of the detectors to reconstruct the neutrino energy faithfully.
For typical shock wave profiles, the detection of these phase effects seems
rather unlikely. However, if the effects are indeed identified in the \nuebar
spectra, they would establish inverted hierarchy and a nonzero value of
.Comment: 10 pages, 9 eps figures. Major changes made. Final version to be
published in PR
Warped brane-world compactification with Gauss-Bonnet term
In the Randall-Sundrum (RS) brane-world model a singular delta-function
source is matched by the second derivative of the warp factor. So one should
take possible curvature corrections in the effective action of the RS models in
a Gauss-Bonnet (GB) form. We present a linearized treatment of gravity in the
RS brane-world with the Gauss-Bonnet modification to Einstein gravity. We give
explicit expressions for the Neumann propagator in arbitrary D dimensions and
show that a bulk GB term gives, along with a tower of Kaluza-Klein modes in the
bulk, a massless graviton on the brane, as in the standard RS model. Moreover,
a non-trivial GB coupling can allow a new branch of solutions with finite
Planck scale and no naked bulk singularity, which might be useful to avoid some
of the previously known ``no--go theorems'' for RS brane-world
compactifications.Comment: 23 pages, typos in Secs. 5 & 6 corrected, expanded/published version
(IJMPA
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Narrow Identities
A person's social identity has many facets, involving language, personal interests, customs, religion, and ethnicity, among other attributes. Yet, all over the world many people seek to define themselves in exclusive terms. This paper develops a simple model of personal incentives and group interests to offer one possible explanation for the puzzle
Phase diagram of vortex matter in layered superconductors with random point pinning
We study the phase diagram of the superconducting vortex system in layered
high-temperature superconductors in the presence of a magnetic field
perpendicular to the layers and of random atomic scale point pinning centers.
We consider the highly anisotropic limit where the pancake vortices on
different layer are coupled only by their electromagnetic interaction. The free
energy of the vortex system is then represented as a Ramakrishnan-Yussouff free
energy functional of the time averaged vortex density. We numerically minimize
this functional and examine the properties of the resulting phases. We find
that, in the temperature () -- pinning strength () plane at constant
magnetic induction, the equilibrium phase at low and is a Bragg glass.
As one increases or a first order phase transition occurs to another
phase that we characterize as a pinned vortex liquid. The weakly pinned vortex
liquid obtained for high and small smoothly crosses over to the
strongly pinned vortex liquid as is decreased or increased -- we do not
find evidence for the existence, in thermodynamic equilibrium, of a distinct
vortex glass phase in the range of pinning parameters considered here. %cdr We
present results for the density correlation functions, the density and defect
distributions, and the local field distribution accessible via SR
experiments. These results are compared with those of existing theoretical,
numerical and experimental studies.Comment: 15 pages, including figures. Higher resolution files for Figs 3a and
11 available from author
Adiabatic Faraday effect in a two-level Hamiltonian formalism
The helicity of a photon traversing a magnetized plasma can flip when the
B-field along the trajectory slowly reverses. Broderick and Blandford have
recently shown that this intriguing effect can profoundly change the usual
Faraday effect for radio waves. We study this phenomenon in a formalism
analogous to neutrino flavor oscillations: the evolution is governed by a
Schroedinger equation for a two-level system consisting of the two photon
helicities. Our treatment allows for a transparent physical understanding of
this system and its dynamics. In particular, it allows us to investigate the
nature of transitions at intermediate adiabaticities.Comment: 8 pages, 2 eps figures, and a note added. Title changed. Matches
published versio
Comparative study of FeCr2S4 and FeSc2S4: Spinels with orbitally active A site
Using first-principles density functional calculations, we perform a
comparative study of two Fe based spinel compounds, FeCr2S4 and FeSc2S4. Though
both systems contain an orbitally active A site with an Fe2+ ion, their
properties are rather dissimilar. Our study unravels the microscopic origin of
their behavior driven by the differences in hybridization of Fe d states with
Cr/Sc d states and S p states in the two cases. This leads to important
differences in the nature of the magnetic exchanges as well as the nearest
versus next nearest neighbor exchange parameter ratios, resulting into
significant frustration effects in FeSc2S4 which are absent in FeCr2S4.Comment: 5 pages, 4 figures Phys Rev B (rapid commun) to appear (2010
Stability analysis of collective neutrino oscillations in the supernova accretion phase with realistic energy and angle distributions
We revisit our previous results on the matter suppression of self-induced
neutrino flavor conversions during a supernova (SN) accretion phase, performing
a linearized stability analysis of the neutrino equations of motion, in the
presence of realistic SN density profiles. In our previous numerical study, we
used a simplified model based on an isotropic neutrino emission with a single
typical energy. Here, we take into account realistic neutrino energy and angle
distributions. We find that multi-energy effects have a sub-leading impact in
the flavor stability of the SN neutrino fluxes with respect to our previous
single-energy results. Conversely, realistic forward-peaked neutrino angular
distributions would enhance the matter suppression of the self-induced
oscillations with respect to an isotropic neutrino emission. As a result, in
our models for iron-core SNe, collective flavor conversions have a negligible
impact on the characterization of the observable neutrino signal during the
accretion phase. Instead, for a low-mass O-Ne-Mg core SN model, with lower
matter density profile and less forward-peaked angular distributions,
collective conversions are possible also at early times.Comment: v2: 8 pages, 3 eps figures. Revised version. Minor changes.
References updated. Matches the version published on PR
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