63,759 research outputs found
Crystallization of Simple Fluids: Relative Stability of f.c.c. and b.c.c Structures
A free-energy functional for a crystal that contains both the symmetry
conserved and symmetry broken parts of the direct pair correlation function is
developed. The free-energy functional is used to investigate the
crystallization of fluids interacting via the inverse power potential ;
. In agreement with simulation results we find
that for the freezing is into close packed f.c.c structure while for
soft repulsions b.c.c phase is more stable.Comment: 9 pages, 2 figure
Cosmic Evolution in Generalised Brans-Dicke Theory
We have studied the Generalised Brans-Dicke theory and obtained exact
solutions of a(t),phi(t),and omega(t) for different epochs of the cosmic
evolution .We discuss how inflation,decceleration,cosmic acceleration can
result from this solution.The time variation of G(t) is also examined.Comment: 12 pages, no figure
ATTac-2000: An Adaptive Autonomous Bidding Agent
The First Trading Agent Competition (TAC) was held from June 22nd to July
8th, 2000. TAC was designed to create a benchmark problem in the complex domain
of e-marketplaces and to motivate researchers to apply unique approaches to a
common task. This article describes ATTac-2000, the first-place finisher in
TAC. ATTac-2000 uses a principled bidding strategy that includes several
elements of adaptivity. In addition to the success at the competition, isolated
empirical results are presented indicating the robustness and effectiveness of
ATTac-2000's adaptive strategy
Magnetoresistance behavior of a ferromagnetic shape memory alloy: Ni_1.75Mn_1.25Ga
A negative-positive-negative switching behavior of magnetoresistance (MR)
with temperature is observed in a ferromagnetic shape memory alloy
Ni_1.75Mn_1.25Ga. In the austenitic phase between 300 and 120 K, MR is negative
due to s-d scattering. Curiously, below 120K MR is positive, while at still
lower temperatures in the martensitic phase, MR is negative again. The positive
MR cannot be explained by Lorentz contribution and is related to a magnetic
transition. Evidence for this is obtained from ab initio density functional
theory, a decrease in magnetization and resistivity upturn at 120 K. Theory
shows that a ferrimagnetic state with anti-ferromagnetic alignment between the
local magnetic moments of the Mn atoms is the energetically favoured ground
state. In the martensitic phase, there are two competing factors that govern
the MR behavior: a dominant negative trend up to the saturation field due to
the decrease of electron scattering at twin and domain boundaries; and a weaker
positive trend due to the ferrimagnetic nature of the magnetic state. MR
exhibits a hysteresis between heating and cooling that is related to the first
order nature of the martensitic phase transition.Comment: 17 pages, 5 figures. Accepted in Phys. Rev.
Analytic derivation of the map of null rays passing near a naked singularity
Recently the energy emission from a naked singularity forming in spherical
dust collapse has been investigated. This radiation is due to the particle
creation in a curved spacetime. In this discussion, the central role is played
by the mapping formula between the incoming and the outgoing null coordinates.
For the self-similar model, this mapping formula has been derived analytically.
But for the model with density profile, the mapping formula has
been obtained only numerically. In the present paper, we argue that the
singular nature of the mapping is determined by the local geometry around the
point at which the singularity is first formed. If this is the case, it would
be natural to expect that the mapping formula can be derived analytically. In
the present paper, we analytically rederive the same mapping formula for the
model with density profile that has been earlier derived using a
numerical technique.Comment: 4 pages, submitted to Phys. Rev.
Crossover to non-Fermi-liquid spin dynamics in cuprates
The antiferromagnetic spin correlation function , the staggered
spin susceptibility and the energy scale are studied numerically within the t-J model and the Hubbard
model, as relevant to cuprates. It is shown that , related to the
onset of the non-Fermi-liquid spin response at , is very low in
the regime below the 'optimum' hole doping , while it
shows a steep increase in the overdoped regime. A quantitative analysis of NMR
spin-spin relaxation-rate for various cuprates reveals a similar
behavior, indicating on a sharp, but continuous, crossover between a
Fermi-liquid and a non-Fermi-liquid behavior as a function of doping.Comment: 4 pages, 4 figures. Submitted to PR
Optically mediated nonlinear quantum optomechanics
We consider theoretically the optomechanical interaction of several
mechanical modes with a single quantized cavity field mode for linear and
quadratic coupling. We focus specifically on situations where the optical
dissipation is the dominant source of damping, in which case the optical field
can be adiabatically eliminated, resulting in effective multimode interactions
between the mechanical modes. In the case of linear coupling, the coherent
contribution to the interaction can be exploited e.g. in quantum state swapping
protocols, while the incoherent part leads to significant modifications of cold
damping or amplification from the single-mode situation. Quadratic coupling can
result in a wealth of possible effective interactions including the analogs of
second-harmonic generation and four-wave mixing in nonlinear optics, with
specific forms depending sensitively on the sign of the coupling. The
cavity-mediated mechanical interaction of two modes is investigated in two
limiting cases, the resolved sideband and the Doppler regime. As an
illustrative application of the formal analysis we discuss in some detail a
two-mode system where a Bose-Einstein condensate is optomechanically linearly
coupled to the moving end mirror of a Fabry-P\'erot cavity.Comment: 11 pages, 8 figure
SU(2) Chiral Sigma Model Study of Phase Transition in Hybrid Stars
We use a modified SU(2) chiral sigma model to study nuclear matter component
and simple bag model for quark matter constituting a neutron star. We also
study the phase transition of nuclear matter to quark matter with the mixed
phase characterized by two conserved charges in the interior of highly dense
neutron stars. Stable solutions of Tolman-Oppenheimer-Volkoff equations
representing hybrid stars are obtained with a maximum mass of 1.67
and radius around 8.9 km.Comment: 14 pages, 5 figure
Origin of Large Dielectric Constant with Large Remnant Polarization and Evidence of Magnetoelectric Coupling in Multiferroic La modified BiFeO3-PbTiO3 Solid Solution
The presence of superlattice reflections and detailed analyses of the powder
neutron and x-ray diffraction data reveal that La rich
(BF-LF)-(PT) (BF-LF-PT) has ferroelectric
rhombohedral crystal structure with space group \textit{} at ambient
conditions. The temperature dependence of lattice parameters, tilt angle,
calculated polarization , volume, and integrated intensity of
superlattice and magnetic reflections show an anomaly around 170 K. Impedance
spectroscopy, dielectric and ac conductivity measurements were performed in
temperature range to probe the origin of large remnant
polarization and frequency dependent broad transitions with large dielectric
constant near . Results of impedance spectroscopy measurements
clearly show contributions of both grain and grain boundaries throughout the
frequency range ( Hz Hz). It could be concluded
that the grain boundaries are more resistive and capacitive as compared to the
grains, resulting in inhomogeneities in the sample causing broad frequency
dependent dielectric anomalies. Enhancement in dielectric constant and remnant
polarization values are possibly due to space charge polarization caused by
piling of charges at the interface of grains and grain boundaries. The
imaginary parts of dielectric constant () Vs frequency
data were fitted using Maxwell-Wagner model at K) and model
fits very well with the data up to Hz. Magnetodielectric measurements
prove that the sample starts exhibiting magnetoelectric coupling at
K, which is also validated by neutron diffraction data.Comment: 20 pages, 10 figure
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