1,903 research outputs found

### Infinite compressibility states in the Hierarchical Reference Theory of fluids. II. Numerical evidence

Continuing our investigation into the Hierarchical Reference Theory of fluids
for thermodynamic states of infinite isothermal compressibility kappa[T] we now
turn to the available numerical evidence to elucidate the character of the
partial differential equation: Of the three scenarios identified previously,
only the assumption of the equations turning stiff when building up the
divergence of kappa[T] allows for a satisfactory interpretation of the data. In
addition to the asymptotic regime where the arguments of part I
(cond-mat/0308467) directly apply, a similar mechanism is identified that gives
rise to transient stiffness at intermediate cutoff for low enough temperature.
Heuristic arguments point to a connection between the form of the Fourier
transform of the perturbational part of the interaction potential and the
cutoff where finite difference approximations of the differential equation
cease to be applicable, and they highlight the rather special standing of the
hard-core Yukawa potential as regards the severity of the computational
difficulties.Comment: J. Stat. Phys., in press. Minor changes to match published versio

### Implementation of the Hierarchical Reference Theory for simple one-component fluids

Combining renormalization group theoretical ideas with the integral equation
approach to fluid structure and thermodynamics, the Hierarchical Reference
Theory is known to be successful even in the vicinity of the critical point and
for sub-critical temperatures. We here present a software package independent
of earlier programs for the application of this theory to simple fluids
composed of particles interacting via spherically symmetrical pair potentials,
restricting ourselves to hard sphere reference systems. Using the hard-core
Yukawa potential with z=1.8/sigma for illustration, we discuss our
implementation and the results it yields, paying special attention to the core
condition and emphasizing the decoupling assumption's role.Comment: RevTeX, 16 pages, 2 figures. Minor changes, published versio

### Shell Effects and Phase Separation in a Trapped Multi-Component Fermi System

Shell effects in the coordinate space can be seen with degenerate Fermi
vapors in non-uniform trapping potentials. In particular, below the Fermi
temperature, the density profile of a Fermi gas in a confining harmonic
potential is characterized by several local maxima. This effect is enhanced for
"magic numbers" of particles and in quasi-1D (cigar-shaped) configurations. In
the case of a multi-component Fermi vapor, the separation of Fermi components
in different spatial shells (phase-separation) depends on temperature, number
of particles and scattering length. We derive analytical formulas, based on
bifurcation theory, for the critical density of Fermions and the critical
chemical potential, which give rise to the phase-separation.Comment: to be published in the Proceedings of the VIII Meeting on Problems in
Theoretical Nuclear Physics, Cortona, October 18-20, 2000, Ed. G. Pisent, A.
Fabrocini and L. Canton (World Scientific

### Approximating the ground state of fermion system by multiple determinant states: matching pursuit approach

We present a simple and stable numerical method to approximate the ground
state of a quantum many-body system by multiple determinant states. This method
searches these determinant states one by one according to the matching pursuit
algorithm. The first determinant state is identical to that of the Hartree-Fock
theory. Calculations for two-dimensional Hubbard model serve as a
demonstration.Comment: 5 Pages, 1 figur

### Bosonic clouds with attractive interaction beyond the local interaction approximation

We study the properties of a Bose-Einstein condensed cloud of atoms with
negative scattering length confined in a harmonic trap. When a realistic non
local (finite range) effective interaction is taken into account, we find that,
besides the known low density metastable solution, a new branch of Bose
condensate appears at higher density. This state is self-bound but its density
can be quite low if the number $N$ of atoms is not too big. The transition
between the two classes of solutions as a function of $N$ can be either sharp
or smooth according to the ratio between the range of the attractive
interaction and the length of the trap. A tight trap leads to a smooth
transition. In addition to the energy and the shape of the cloud we study also
the dynamics of the system. In particular, we study the frequencies of
collective oscillation of the Bose condensate as a function of the number of
atoms both in the local and in the non local case. Moreover, we consider the
dynamics of the cloud when the external trap is switched off.Comment: Latex, 6 pages, 2 figure, 1 table, presented to the International
Symposium of Quantum Fluids and Solids 98, Amherst (USA), 9-14 June 199

### Effective wave-equations for the dynamics of cigar-shaped and disc-shaped Bose condensates

Starting from the 3D Gross-Pitaevskii equation and using a variational
approach, we derive an effective 1D wave-equation that describes the axial
dynamics of a Bose condensate confined in an external potential with
cylindrical symmetry. The trapping potential is harmonic in the transverse
direction and generic in the axial one. Our equation, that is a time-dependent
non-polynomial nonlinear Schr\"odinger equation (1D NPSE), can be used to model
cigar-shaped condensates, whose dynamics is essentially 1D. We show that 1D
NPSE gives much more accurate results than all other effective equations
recently proposed. By using 1D NPSE we find analytical solutions for bright and
dark solitons, which generalize the ones known in the literature. We deduce
also an effective 2D non-polynomial Schr\"odinger equation (2D NPSE) that
models disc-shaped Bose condensates confined in an external trap that is
harmonic along the axial direction and generic in the transverse direction. In
the limiting cases of weak and strong interaction, our approach gives rise to
Schr\"odinger-like equations with different polynomial nonlinearities.Comment: 7 pages, 5 figures, to be published in Phys. Rev.

### Recent developments of the Hierarchical Reference Theory of Fluids and its relation to the Renormalization Group

The Hierarchical Reference Theory (HRT) of fluids is a general framework for
the description of phase transitions in microscopic models of classical and
quantum statistical physics. The foundations of HRT are briefly reviewed in a
self-consistent formulation which includes both the original sharp cut-off
procedure and the smooth cut-off implementation, which has been recently
investigated. The critical properties of HRT are summarized, together with the
behavior of the theory at first order phase transitions. However, the emphasis
of this presentation is on the close relationship between HRT and non
perturbative renormalization group methods, as well as on recent
generalizations of HRT to microscopic models of interest in soft matter and
quantum many body physics.Comment: 17 pages, 5 figures. Review paper to appear in Molecular Physic

### A search for Galactic transients disguised as gamma-ray bursts

A significant fraction of cosmological gamma-ray bursts (GRBs) are
characterised by a fast rise and exponential decay (FRED) temporal structure.
This is not a distinctive feature of this class, since it is observed in many
Galactic transients and is likely descriptive of a sudden release of energy
followed by a diffusion process. Possible evidence has recently been reported
by Tello et al. (2012) for a Galactic contamination in the sample of FRED GRBs
discovered with Swift. We searched for possible Galactic intruders disguised as
FRED GRBs in the Swift catalogue up to September 2014. We selected 181 FRED
GRBs (2/3 with unknown redshift) and considered different subsamples. We tested
the degree of isotropy through the dipole and the quadrupole moment
distributions, both with reference to the Galaxy and in a
coordinate-system-independent way, as well as with the two-point angular
autocovariance function. In addition, we searched for possible indicators of a
Galactic origin among the spectral and temporal properties of individual GRBs.
We found marginal (~3 sigma) evidence for an excess of FREDs with unknown
redshift towards the Galactic plane compared with what is expected for an
isotropic distribution corrected for the non-uniform sky exposure. However,
when we account for the observational bias against optical follow-up
observations of low-Galactic latitude GRBs, the evidence for anisotropy
decreases to ~2 sigma. In addition, we found no statistical evidence for
different spectral or temporal properties from the bulk of cosmological GRBs.
We found marginal evidence for the presence of a disguised Galactic population
among Swift GRBs with unknown redshift. The estimated fraction is f=(19 +-
11)%, with an upper limit of 34% (90% confidence).Comment: 6 pages, 4 figures, accepted by A&

### Swift reveals the eclipsing nature of the high mass X-ray binary IGR~J16195-4945

IGR J16195-4945 is a hard X-ray source discovered by INTEGRAL during the Core
Program observations performed in 2003. We analyzed the X-ray emission of this
source exploiting the Swift-BAT survey data from December 2004 to March 2015,
and all the available Swift-XRT pointed observations. The source is detected at
a high significance level in the 123-month BAT survey data, with an average
15-150 keV flux of the source of ~1.6 mCrab. The timing analysis on the BAT
data reveals with a significance higher than 6 standard deviations the presence
of a modulated signal with a period of 3.945 d, that we interpret as the
orbital period of the binary system. The folded light curve shows a flat
profile with a narrow full eclipse lasting ~3.5% of the orbital period. We
requested phase-constrained XRT observations to obtain a more detailed
characterization of the eclipse in the soft X-ray range. Adopting resonable
guess values for the mass and radius of the companion star, we derive a
semi-major orbital axis of ~31 R_sun, equivalent to ~1.8 times the radius of
the companion star. From these estimates and from the duration of the eclipse
we derive an orbital inclination between 55 and 60 degrees. The broad band
time-averaged XRT+BAT spectrum is well modeled with a strongly absorbed flat
power law, with absorbing column N_H=7x 10^22 cm^(-2) and photon index
Gamma=0.5, modified by a high energy exponential cutoff at E_cut=14 keV.Comment: 5 pages, 5 figures, 2 tables. Published on MNRA

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