5,429 research outputs found
Specific-heat evidence for strong electron correlations in the thermoelectric material (Na,Ca)Co_{2}O_{4}
The specific heat of (Na,Ca)Co_{2}O_{4} is measured at low-temperatures to
determine the magnitude of the electronic specific-heat coefficient \gamma, in
an attempt to gain an insight into the origin of the unusually large
thermoelectric power of this compound. It is found that \gamma is as large as
48 mJ/molK^2, which is an order of magnitude larger than \gamma of simple
metals. This indicates that (Na,Ca)Co_{2}O_{4} is a strongly-correlated
electron system, where the strong correlation probably comes from the
low-dimensionality and the frustrated spin structure. We discuss how the large
thermopower and its dependence on Ca doping can be understood with the strong
electron correlations.Comment: 5 pages, 4 figures, accepted for publication in Phys. Rev.
History Memorized and Recalled upon Glass Transition
The memory effect upon glassification is studied in the glass to rubber
transition of vulcanized rubber with the strain as a controlling parameter. A
phenomenological model is proposed taking the history of the temperature and
the strain into account, by which the experimental results are interpreted. The
data and the model demonstrate that the glassy state memorizes the time-course
of strain upon glassification, not as a single parameter but as the history
itself. The data also show that the effect of irreversible deformation in the
glassy state is beyond the scope of the present model.
Authors' remark: The title of the paper in the accepted version is above. The
title appeared in PRL is the one changed by a Senior Assistant Editor after
acceptance of the paper. The recovery of the title was rejected in the
correction process.Comment: 4 pages, 4 figure
High and Low Dimensions in The Black Hole Negative Mode
The negative mode of the Schwarzschild black hole is central to Euclidean
quantum gravity around hot flat space and for the Gregory-Laflamme black string
instability. We analyze the eigenvalue as a function of space-time dimension by
constructing two perturbative expansions: one for large d and the other for
small d-3, and determining as many coefficients as we are able to compute
analytically. Joining the two expansions we obtain an interpolating rational
function accurate to better than 2% through the whole range of dimensions
including d=4.Comment: 17 pages, 4 figures. v2: added reference. v3: published versio
Theoretical Study of One-dimensional Chains of Metal Atoms in Nanotubes
Using first-principles total-energy pseudopotential calculations, we have
studied the properties of chains of potassium and aluminum in nanotubes. For BN
tubes, there is little interaction between the metal chains and the tubes, and
the conductivity of these tubes is through carriers located at the inner part
of the tube. In contrast, for small radius carbon nanotubes, there are two
types of interactions: charge-transfer (dominant for alkali atoms) leading to
strong ionic cohesion, and hybridization (for multivalent metal atoms)
resulting in a smaller cohesion. For Al-atomic chains in carbon tubes, we show
that both effects contribute. New electronic properties related to these
confined atomic chains of metal are analyzed.Comment: 12 pages + 3 figure
Modelling the dynamical evolution of the Bootes dwarf spheroidal galaxy
We investigate a wide range of possible evolutionary histories for the
recently discovered Bootes dwarf spheroidal galaxy, a Milky Way satellite. By
means of N-body simulations we follow the evolution of possible progenitor
galaxies of Bootes for a variety of orbits in the gravitational potential of
the Milky Way. The progenitors considered cover the range from dark-matter-free
star clusters to massive, dark-matter dominated outcomes of cosmological
simulations. For each type of progenitor and orbit we compare the observable
properties of the remnant after 10 Gyr with those of Bootes observed today. Our
study suggests that the progenitor of Bootes must have been, and remains now,
dark matter dominated. In general our models are unable to reproduce the
observed high velocity dispersion in Bootes without dark matter. Our models do
not support time-dependent tidal effects as a mechanism able to inflate
significantly the internal velocity dispersion. As none of our initially
spherical models is able to reproduce the elongation of Bootes, our results
suggest that the progenitor of Bootes may have had some intrinsic flattening.
Although the focus of the present paper is the Bootes dwarf spheroidal, these
models may be of general relevance to understanding the structure, stability
and dark matter content of all dwarf spheroidal galaxies.Comment: 10 pages, 7 figures, accepted by MNRA
The structure of invariant tori in a 3D galactic potential
We study in detail the structure of phase space in the neighborhood of stable
periodic orbits in a rotating 3D potential of galactic type. We have used the
color and rotation method to investigate the properties of the invariant tori
in the 4D spaces of section. We compare our results with those of previous
works and we describe the morphology of the rotational, as well as of the tube
tori in the 4D space. We find sticky chaotic orbits in the immediate
neighborhood of sets of invariant tori surrounding 3D stable periodic orbits.
Particularly useful for galactic dynamics is the behavior of chaotic orbits
trapped for long time between 4D invariant tori. We find that they support
during this time the same structure as the quasi-periodic orbits around the
stable periodic orbits, contributing however to a local increase of the
dispersion of velocities. Finally we find that the tube tori do not appear in
the 3D projections of the spaces of section in the axisymmetric Hamiltonian we
examined.Comment: 26 pages, 34 figures, accepted for publication in the International
Journal of Bifurcation and Chao
Electronic correlations on a metallic nanosphere
We consider the correlation functions in a gas of electrons moving within a
thin layer on the surface of nanosize sphere. A closed form of expressions for
the RKKY indirect exchange, superconducting Cooper loop and `density-density'
correlation function is obtained. The systematic comparison with planar results
is made, the effects of spherical geometry are outlined. The quantum coherence
of electrons leads to the enhancement of all correlations for the
points--antipodes on the sphere. This effect is lost when the radius of the
sphere exceeds the temperature coherence length.Comment: 5 pages, no figures, to appear in PRB (RC
Population synthesis of old neutron stars in the Galaxy
The paucity of old isolated accreting neutron stars in ROSAT observations is
used to derive a lower limit on the mean velocity of neutron stars at birth.
The secular evolution of the population is simulated following the paths of a
statistical sample of stars for different values of the initial kick velocity,
drawn from an isotropic Gaussian distribution with mean velocity . The spin-down, induced by dipole losses and the
interaction with the ambient medium, is tracked together with the dynamical
evolution in the Galactic potential, allowing for the determination of the
fraction of stars which are, at present, in each of the four possible stages:
Ejector, Propeller, Accretor, and Georotator. Taking from the ROSAT All Sky
Survey an upper limit of accreting neutron stars within pc
from the Sun, we infer a lower bound for the mean kick velocity, \ga
200-300 . The same conclusion is reached for both a constant
( G) and a magnetic field decaying exponentially with a
timescale yr. Present results, moreover, constrain the fraction of
low velocity stars, which could have escaped pulsar statistics, to \la 1%.Comment: 8 pages, 4 PostScript figures, to appear in the proceedings of IAU
Symposium 19
A new approach towards volumetric assessment of left ventricular function with MSCT
Cardiovascular CT is considered the diagnostic standard for establishing the presence of a functional and dynamic imaging system. It is difficult, however, to estimate the ventricular motion and volumes that are processed using hundreds and thousands of CT images, in a few moments
The Effect of Satellite Galaxies on Gravitational Lensing Flux Ratios
Gravitational lenses with anomalous flux ratios are often cited as possible
evidence for dark matter satellites predicted by simulations of hierarchical
merging in cold dark matter cosmogonies. We show that the fraction of quads
with anomalous flux ratios depends primarily on the total mass and spatial
extent of the satellites, and the characteristic lengthscale R of their
distribution. If R is 100 kpc, then for a moderately elliptical galaxy with a
line-of-sight velocity dispersion of 250 km/s, a mass of 3 x 10^9 solar masses
in highly-concentrated (Plummer model) satellites is needed for 20% of
quadruplets to show anomalous flux ratios, rising to 1.25 x 10^10 solar masses
for 50%. Several times these masses are required if the satellites have more
extended Hernquist profiles. Compared to a typical elliptical, the flux ratios
of quads formed by typical edge-on disc galaxies with maximum discs are
significantly less susceptible to changes through substructure -- three times
the mass in satellite galaxies is needed to affect 50% of the systems. In many
of the lens systems with anomalous flux ratios, there is evidence for visible
satellites (e.g., B2045+265 or MG0414+0534). We show that optically identified
substructure should not be preponderant among lens systems with anomalies.
There are two possible resolutions of this difficulty. First, in some cases,
visible substructure may be projected within or close to the Einstein radius
and wrongly ascribed as the culprit, whereas dark matter substructure is
causing the flux anomaly. Second, bright satellites, in which baryon cooling
and condensation has taken place, may have higher central densities than dark
satellites, rendering them more efficient at causing flux anomalies.Comment: 10 pages, 14 figures, MNRAS, in pres
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