1,325 research outputs found
Quantum critical origin of the superconducting dome in SrTiO
We investigate the origin of superconductivity in doped SrTiO (STO) using
a combination of density functional and strong coupling theories within the
framework of quantum criticality. Our density functional calculations of the
ferroelectric soft mode frequency as a function of doping reveal a crossover
from quantum paraelectric to ferroelectric behavior at a doping level
coincident with the experimentally observed top of the superconducting dome.
Based on this finding, we explore a model in which the superconductivity in STO
is enabled by its proximity to the ferroelectric quantum critical point and the
soft mode fluctuations provide the pairing interaction on introduction of
carriers. Within our model, the low doping limit of the superconducting dome is
explained by the emergence of the Fermi surface, and the high doping limit by
departure from the quantum critical regime. We predict that the highest
critical temperature will increase and shift to lower carrier doping with
increasing O isotope substitution, a scenario that is experimentally
verifiable.Comment: 4 pages + supplemental, 3 + 2 figure
Effects of Galaxy Formation on Thermodynamics of the Intracluster Medium
We present detailed comparisons of the intracluster medium (ICM) in
cosmological Eulerian cluster simulations with deep Chandra observations of
nearby relaxed clusters. To assess the impact of galaxy formation, we compare
two sets of simulations, one performed in the non-radiative regime and another
with radiative cooling and several physical processes critical to various
aspects of galaxy formation: star formation, metal enrichment and stellar
feedback. We show that the observed ICM properties outside cluster cores are
well-reproduced in the simulations that include cooling and star formation,
while the non-radiative simulations predict an overall shape of the ICM
profiles inconsistent with observations. In particular, we find that the ICM
entropy in our runs with cooling is enhanced to the observed levels at radii as
large as half of the virial radius. We also find that outside cluster cores
entropy scaling with the mean ICM temperature in both simulations and Chandra
observations is consistent with being self-similar within current error bars.
We find that the pressure profiles of simulated clusters are also close to
self-similar and exhibit little cluster-to-cluster scatter. The X-ray
observable-total mass relations for our simulated sample agree with the Chandra
measurements to \~10%-20% in normalization. We show that this systematic
difference could be caused by the subsonic gas motions, unaccounted for in
X-ray hydrostatic mass estimates. The much improved agreement of simulations
and observations in the ICM profiles and scaling relations is encouraging and
the existence of tight relations of X-ray observables, such as Yx, and total
cluster mass and the simple redshift evolution of these relations hold promise
for the use of clusters as cosmological probes.Comment: 14 pages, 6 figures. Matches version accepted to Ap
Normalizing the Temperature Function of Clusters of Galaxies
We re-examine the constraints which can be robustly obtained from the
observed temperature function of X-ray cluster of galaxies. The cluster mass
function has been thoroughly studied in simulations and analytically, but a
direct simulation of the temperature function is presented here for the first
time. Adaptive hydrodynamic simulations using the cosmological Moving Mesh
Hydro code of Pen (1997a) are used to calibrate the temperature function for
different popular cosmologies. Applying the new normalizations to the
present-day cluster abundances, we find for a hyperbolic universe, and for a spatially flat universe with a cosmological constant.
The simulations followed the gravitational shock heating of the gas and dark
matter, and used a crude model for potential energy injection by supernova
heating. The error bars are dominated by uncertainties in the heating/cooling
models. We present fitting formulae for the mass-temperature conversions and
cluster abundances based on these simulations.Comment: 20 pages incl 5 figures, final version for ApJ, corrected open
universe \gamma relation, results unchange
Revertant fibres and dystrophin traces in Duchenne muscular dystrophy: Implication for clinical trials
Duchenne muscular dystrophy (DMD) is characterised by the absence of dystrophin in muscle biopsies, although residual dystrophin can be present, either as dystrophin-positive (revertant) fibres or traces. As restoration of dystrophin expression is the end point of clinical trials, such residual dystrophin is a key factor in recruitment of patients and may also confound the analysis of dystrophin restoration in treated patients, if, as previously observed in the mdx mouse, revertant fibres increase with age. In 62% of the diagnostic biopsies reports of 65 DMD patients studied, traces or revertants were recorded with no correlation between traces or revertants, the patients' performance, or corticosteroids response. In nine of these patients, there was no increase in traces or revertants in biopsies taken a mean of 8.23 years (5.8-10.4 years) after the original diagnostic biopsy. This information should help in the design and execution of clinical trials focused on dystrophin restoration strategies. (C) 2010 Elsevier B.V. All rights reserved
Dependence of the Energy Spectrum of UHE Cosmic Rays on the Latitude of an Extensive Air Shower Array
Several energy spectra of cosmic rays with energies E_0 \geq 10^17 eV
measured at the Yakutsk EAS, AGASA, Haverah Park, HiRes, Auger, and SUGAR
arrays are considered. It is shown that the fairly good mutual agreement of the
spectrum shapes can be achieved if the energy of each spectrum is multiplied by
a factor K specific for each spectrum. These factors exhibit a pronounced
dependence on the latitude of the above-mentioned arrays.Comment: 4 pages, 4 figure
A Very Hot, High Redshift Cluster of Galaxies: More Trouble for Omega_0 = 1
We have observed the most distant (z=0.829) cluster of galaxies in the
Einstein Extended Medium Sensitivity Survey, with the ASCA and ROSAT
satellites. We find an X-ray temperature of 12.3 +3.1/-2.2 keV for this
cluster, and the ROSAT map reveals significant substructure. The high
temperature of MS1054-0321 is consistent with both its approximate velocity
dispersion, based on the redshifts of 12 cluster members we have obtained at
the Keck and the Canada-France-Hawaii telescopes, and with its weak lensing
signature. The X-ray temperature of this cluster implies a virial mass ~ 7.4 x
10^14 h^-1 solar masses, if the mean matter density in the universe equals the
critical value, or larger if Omega_0 < 1. Finding such a hot, massive cluster
in the EMSS is extremely improbable if clusters grew from Gaussian
perturbations in an Omega_0 = 1 universe. Combining the assumptions that
Omega_0 = 1 and that the intial perturbations were Gaussian with the observed
X-ray temperature function at low redshift, we show that the probability of
this cluster occurring in the volume sampled by the EMSS is less than a few
times 10^{-5}. Nor is MS1054-0321 the only hot cluster at high redshift; the
only two other EMSS clusters already observed with ASCA also have
temperatures exceeding 8 keV. Assuming again that the initial perturbations
were Gaussian and Omega_0 = 1, we find that each one is improbable at the <
10^{-2} level. These observations, along with the fact that these luminosities
and temperatures of the high- clusters all agree with the low-z L_X-T_X
relation, argue strongly that Omega_0 < 1. Otherwise, the initial perturbations
must be non-Gaussian, if these clusters' temperatures do indeed reflect their
gravitational potentials.Comment: 20 pages, 4 figures, To appear in 1 Aug 1998 ApJ (heavily revised
version of original preprint
The binary period and outburst behaviour of the SMC X-ray binary pulsar system SXP504
A probable binary period has been detected in the optical counterpart to the
X-ray source CXOU J005455.6-724510 = RX J0054.9-7245 = AXJ0054.8-7244 = SXP504
in the Small Magellanic Cloud. This source was detected by Chandra on 04 Jul
2002 and subsequently observed by XMM-Newton on 18 Dec 2003. The source is
coincident with an Optical Gravitational Lensing (OGLE) object in the
lightcurves of which several optical outburst peaks are visible at ~ 268 day
intervals. Timing analysis shows a period of 268.6 +/- 0.1 days at > 99%
significance. Archival Rossi X-ray Timing Explorer (RXTE) data for the 504s
pulse-period has revealed detections which correspond closely with predicted or
actual peaks in the optical data. The relationship between this orbital period
and the pulse period of 504s is within the normal variance found in the Corbet
diagram.Comment: Accepted by MNRAS. 1 LATEX page. 4 figure
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