8,452 research outputs found
New constraints on a triaxial model of the Galaxy
We determine the values of parameters of an N-body model for the Galaxy
developed by Fux via comparison with an unbiased, homogeneous sample of OH/IR
stars. Via Monte-Carlo simulation, we find the plausibilities of the
best-fitting models, as well as their errors. The parameters that are
constrained best by these projected data are the total mass of the model and
the viewing angle of the central Bar, although the distribution of the latter
has multiple maxima. The best model has a viewing angle of 44 degrees,
semi-major axis of 2.5 kpc, a bar mass of 1.7E10 solar masses and a tangential
velocity of the local standard of rest of 171 km/s . We argue that the lower
values that are commonly found from stellar data for the viewing angle (around
25 degrees) arise when too few coordinates are available, when the longitude
range is too narrow or when low latitudes are excluded from the fit. The new
constraints on the viewing angle of the galactic Bar from stellar line-of-sight
velocities decrease further the ability of the Bar's distribution to account
for the observed micro-lensing optical depth toward Baade's window : our model
reproduces only half the observed value. The signal of triaxiality diminishes
quickly with increasing latitude, fading within approximately one scaleheight.
This suggests that Baade's window is not a very appropriate region to sample
Bar properties.Comment: 10 pages, 8 figures, TeX, accepted for publication in MNRA
Characteristic energies, transition temperatures, and switching effects in clean SNS graphene nanostructures
We study proximity effects in clean nanoscale superconductor-normal
metal-superconductor (SNS) graphene heterostructures using a
self-consistent numerical solution to the continuum Dirac Bogoliubov-de Gennes
(DBdG) equations. We obtain results for the pair amplitude and the local
density of states (DOS), as a function of doping and of the geometrical
parameters determining the width of the structures. The superconducting
correlations are found to penetrate the normal graphene layers even when there
is extreme mismatch in the normal and superconducting doping levels, where
specular Andreev reflection dominates. The local DOS exhibits peculiar
features, which we discuss, arising from the Dirac cone dispersion relation and
from the interplay between the superconducting and Thouless energy scales. The
corresponding characteristic energies emerge in the form of resonant peaks in
the local DOS, that depend strongly on the doping level, as does the energy
gap, which declines sharply as the relative difference in doping between the S
and N regions is reduced. We also linearize the DBdG equations and develop an
essentially analytical method that determines the critical temperature of
an \sns nanostructure self-consistently. We find that for S regions that occupy
a fraction of the coherence length, can undergo substantial variations as
a function of the relative doping. At finite temperatures and by manipulating
the doping levels, the self consistent pair amplitudes reveal dramatic
transitions between a superconducting and resistive normal state of the
structure. Such behavior suggests the possibility of using the proposed system
as a carbon-based superconducting switch, turning superconductivity on or off
by tuning the relative doping levels.Comment: 13 pages, figures include
Free Energy Landscape Of Simple Liquids Near The Glass Transition
Properties of the free energy landscape in phase space of a dense hard sphere
system characterized by a discretized free energy functional of the
Ramakrishnan-Yussouff form are investigated numerically. A considerable number
of glassy local minima of the free energy are located and the distribution of
an appropriately defined ``overlap'' between minima is calculated. The process
of transition from the basin of attraction of a minimum to that of another one
is studied using a new ``microcanonical'' Monte Carlo procedure, leading to a
determination of the effective height of free energy barriers that separate
different glassy minima. The general appearance of the free energy landscape
resembles that of a putting green: deep minima separated by a fairly flat
structure. The growth of the effective free-energy barriers with increasing
density is consistent with the Vogel-Fulcher law, and this growth is primarily
driven by an entropic mechanism.Comment: 10 pages, 6 postscript figures, uses iopart.cls and iopart10.clo
(included). Invited talk at the ICTP Trieste Conference on "Unifying Concepts
in Glass Physics", September 1999. To be published in J. Phys. Cond. Ma
A companion to a quasar at redshift 4.7
There is a growing consensus that the emergence of quasars at high redshifts
is related to the onset of galaxy formation, suggesting that the detection of
concentrations of gas accompanying such quasars should provide clues about the
early history of galaxies. Quasar companions have been recently identified at
redshifts up to . Here we report observations of Lyman-
emission (a tracer of ionised hydrogen) from the companion to a quasar at
=4.702, corresponding to a time when the Universe was less than ten per cent
of its present age. We argue that most of the emission arises in a gaseous
nebula that has been photoionised by the quasar, but an additional component of
continuum light -perhaps quasar light scattered from dust in the companion
body, or emission from young stars within the nebula- appears necessary to
explain the observations. These observations may be indicative of the first
stages in the assembly of galaxy-sized structures.Comment: 8 pages, 4 figures, plain LaTeX. Accepted for publication in Natur
Nonlinear Hydrodynamics of a Hard Sphere Fluid Near the Glass Transition
We conduct a numerical study of the dynamic behavior of a dense hard sphere
fluid by deriving and integrating a set of Langevin equations. The statics of
the system is described by a free energy functional of the
Ramakrishnan-Yussouff form. We find that the system exhibits glassy behavior as
evidenced through stretched exponential decay and two-stage relaxation of the
density correlation function. The characteristic times grow with increasing
density according to the Vogel-Fulcher law. The wavenumber dependence of the
kinetics is extensively explored. The connection of our results with
experiment, mode coupling theory, and molecular dynamics results is discussed.Comment: 34 Pages, Plain TeX, 12 PostScript Figures (not included, available
on request
Time Scales for transitions between free energy minima of a hard sphere system
Time scales associated with activated transitions between glassy metastable
states of a free energy functional appropriate for a dense hard sphere system
are calculated by using a new Monte Carlo method for the local density
variables. We calculate the time the system,initially placed in a shallow
glassy minimum of the free energy, spends in the neighborhood of this minimum
before making a transition to the basin of attarction of another free energy
minimum. This time scale is found to increase with the average density. We find
a crossover density near which this time scale increases very sharply and
becomes longer than the longest times accessible in our simulation. This scale
shows no evidence of dependence on sample size.Comment: 25 pages, Revtex, 6 postscript figures. Will appear in Phys Rev E,
March 1996 or s
Superconducting gap node spectroscopy using nonlinear electrodynamics
We present a method to determine the nodal structure of the energy gap of
unconventional superconductors such as high materials. We show how
nonlinear electrodynamics phenomena in the Meissner regime, arising from the
presence of lines on the Fermi surface where the superconducting energy gap is
very small or zero, can be used to perform ``node spectroscopy'', that is, as a
sensitive bulk probe to locate the angular position of those lines. In
calculating the nonlinear supercurrent response, we include the effects of
orthorhombic distortion and plane anisotropy. Analytic results presented
demonstrate a systematic way to experimentally distinguish order parameters of
different symmetries, including cases with mixed symmetry (for example,
and ). We consider, as suggested by various experiments, order parameters
with predominantly -wave character, and describe how to determine the
possible presence of other symmetries. The nonlinear magnetic moment displays a
distinct behavior if nodes in the gap are absent but regions with small,
finite, values of the energy gap exist.Comment: 18 pages, Revtex, 9 postscript figures. Submitted to Phys. Rev
The Anomaly in the Candidate Microlensing Event PA-99-N2
The lightcurve of PA-99-N2, one of the recently announced microlensing
candidates towards M31, shows small deviations from the standard Paczynski
form. We explore a number of possible explanations, including correlations with
the seeing, the parallax effect and a binary lens. We find that the
observations are consistent with an unresolved RGB or AGB star in M31 being
microlensed by a binary lens. We find that the best fit binary lens mass ratio
is about one hundredth, which is one of most extreme values found for a binary
lens so far. If both the source and lens lie in the M31 disk, then the standard
M31 model predicts the probable mass range of the system to be 0.02-3.6 solar
masses (95 % confidence limit). In this scenario, the mass of the secondary
component is therefore likely to be below the hydrogen-burning limit. On the
other hand, if a compact halo object in M31 is lensing a disk or spheroid
source, then the total lens mass is likely to lie between 0.09-32 solar masses,
which is consistent with the primary being a stellar remnant and the secondary
a low mass star or brown dwarf. The optical depth (or alternatively the
differential rate) along the line of sight toward the event indicates that a
halo lens is more likely than a stellar lens provided that dark compact objects
comprise no less than 15 per cent (or 5 per cent) of haloes.Comment: Latex, 23 pages, 9 figures, in press at The Astrophysical Journa
Entropic Origin of the Growth of Relaxation Times in Simple Glassy Liquids
Transitions between ``glassy'' local minima of a model free-energy functional
for a dense hard-sphere system are studied numerically using a
``microcanonical'' Monte Carlo method that enables us to obtain the transition
probability as a function of the free energy and the Monte Carlo ``time''. The
growth of the height of the effective free energy barrier with density is found
to be consistent with a Vogel-Fulcher law. The dependence of the transition
probability on time indicates that this growth is primarily due to entropic
effects arising from the difficulty of finding low-free-energy saddle points
connecting glassy minima.Comment: Four pages, plus three postscript figure
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