1,748 research outputs found
Low Temperature Symmetry of Pyrochlore Oxide Cd2Re2O7
We report the X-ray study for the pyrochlore oxide Cd2Re2O7. Two
symmetry-lowering structural transitions were observed at Ts1=200K and
Ts2=120K. The former is of the second order from the ideal cubic pyrochlore
structure with space group Fd-3m to a tetragonally distorted structure with
I-4m2, while the latter is of the first order likely to another tetragonal
space group I4122. We discuss the feature of the lattice deformation.Comment: 4 pages, 4 figure
High Pressure Effects on Superconductivity in the beta-pyrochlore Oxides AOs2O6 (A=K, Rb, Cs)
Recently new pyrochlore oxides superconductors AOs2O6 (A=K,Rb,Cs) were found
and we measured the pressure dependence of magnetization up to 1.2 GPa in order
to deduce the pressure effect of Tc in the three beta-pyrochlore oxides. It is
found that the initial pressure dependence of Tc is positive for all the
compounds. Only KOs2O6 exhibits a saturation in Tc at 0.56 GPa and the downturn
at higher pressureComment: 7 pages, 2 figure
Hall effect in superconducting Fe(Se0.5Te0.5) thin films
The Hall effect is investigated for eight superconducting Fe(Se_0.5_Te_0.5_)
thin films grown on MgO and LaSrAlO_4_ substrates with different transition
temperatures (T_c_). The normal Hall coefficients (R_H_) have positive values
with magnitude of 1 - 1.5 x 10^-3^ cm^3^/C at room temperature for the all
samples. With decreasing temperature, we find two characteristic types of
behavior in R_H_(T) depending on T_c_. For thin films with lower T_c_
(typically T_c_ < 5 K), R_H_ start decreasing approximately below T = 250 K
toward a negative side, some of which shows sign reversal at T = 50 - 60 K, but
turns positive toward T = 0 K. On the other hand for the films with higher T_c_
(typically T_c_ > 9 K), R_ H_ leaves almost unchanged down to T = 100 K, and
then starts decreasing toward a negative side. Around the temperatures when
R_H_ changes its sign from positive to negative, obvious nonlinearity is
observed in the field-dependence of Hall resistance as to keep the low-field
R_H_ positive while the high-field R_H_ negative. Thus the electronic state
just above T_c_ is characterized by n_e_ (electron density) > n_h_ (hole
density) with keeping \mu_e_ < \mu_h_. These results suggest the dominance of
electron density to the hole density is an essential factor for the occurence
of superconductivity in Fe-chalcogenide superconductors.Comment: 11 pages, 4 figures, revised version for Physical Review B. accepted
for publication in Physical Review
New beta-Pyrochlore Oxide Superconductor CsOs2O6
The discovery of a new beta-pyrochlore oxide superconductor CsOs2O6 with Tc =
3.3 K is reported. It is the third superconductor in the family of
beta-pyrochlore oxides, following KOs2O6 with Tc = 9.6 K and RbOs2O6 with Tc =
6.3 K. The Tc of this series decreases with increasing the ionic radius of
alkaline metal ions, imposing negative chemical pressure upon the Os pyrochlore
lattice.Comment: submitted to J. Phys. Soc Jp
Parker-Jeans Instability of Gaseous Disks Including the Effect of Cosmic Rays
We use linear analysis to examine the effect of cosmic rays (CRs) on the
Parker-Jeans instability of magnetized self-gravitating gaseous disks. We adopt
a slab equilibrium model in which the gravity (including self-gravity) is
perpendicular to the mid-plane, the magnetic field lies along the slab. CR is
described as a fluid and only along magnetic field lines diffusion is
considered. The linearised equations are solved numerically. The system is
susceptible to Parker-Jeans instability. In general the system is less unstable
when the CR diffusion coefficient is smaller (i.e., the coupling between the
CRs and plasma is stronger). The system is also less unstable if CR pressure is
larger. This is a reminiscence of the fact that Jeans instability and Parker
instability are less unstable when the gas pressure is larger (or temperature
is higher). Moreover, for large CR diffusion coefficient (or small CR
pressure), perturbations parallel to the magnetic field are more unstable than
those perpendicular to it. The other governing factor on the growth rate of the
perturbations in different directions is the thickness of the disk or the
strength of the external pressure on the disk. In fact, this is the determining
factor in some parameter regimes.Comment: 19pages, 14figures submitted to Ap
Crustal Heating and Quiescent Emission from Transiently Accreting Neutron Stars
Nuclear reactions occurring deep in the crust of a transiently accreting
neutron star efficiently maintain the core at a temperature >5e7 K. When
accretion halts, the envelope relaxes to a thermal equilibrium set by the flux
from the hot core, as if the neutron star were newly born. For the
time-averaged accretion rates typical of low-mass X-ray transients, standard
neutrino cooling is unimportant and the core thermally re-radiates the
deposited heat. The resulting luminosity has the same magnitude as that
observed from several transient neutron stars in quiescence. Confirmation of
this mechanism would strongly constrain rapid neutrino cooling mechanisms for
neutron stars. Thermal emission had previously been dismissed as a predominant
source of quiescent emission since blackbody spectral fits implied an emitting
area much smaller than a neutron star's surface. However, as with thermal
emission from radio pulsars, fits with realistic emergent spectra will imply a
substantially larger emitting area. Other emission mechanisms, such as
accretion or a pulsar shock, can also operate in quiescence and generate
intensity and spectral variations over short timescales. Indeed, quiescent
accretion may produce gravitationally redshifted metal photoionization edges in
the quiescent spectra (detectable with AXAF and XMM). We discuss past
observations of Aql~X-1 and note that the low luminosity X-ray sources in
globular clusters and the Be star/X-ray transients are excellent candidates for
future study.Comment: 5 pages, 2 ps figures, uses AASTEX macros. To appear in ApJ letters,
10 September 1998. Revised to conform with journal; minor numerical
correction
A Comparative Study of the Parker Instability under Three Models of the Galactic Gravity
To examine how non-uniform nature of the Galactic gravity might affect length
and time scales of the Parker instability, we took three models of gravity,
uniform, linear and realistic ones. To make comparisons of the three gravity
models on a common basis, we first fixed the ratio of magnetic pressure to gas
pressure at = 0.25, that of cosmic-ray pressure at = 0.4, and
the rms velocity of interstellar clouds at = 6.4 km s, and then
adjusted parameters of the gravity models in such a way that the resulting
density scale heights for the three models may all have the same value of 160
pc. Performing linear stability analyses onto equilibrium states under the
three models with the typical ISM conditions, we calculate the maximum growth
rate and corresponding length scale for each of the gravity models. Under the
uniform gravity the Parker instability has the growth time of 1.2
years and the length scale of 1.6 kpc for symmetric mode. Under the realistic
gravity it grows in 1.8 years for both symmetric and
antisymmetric modes, and develops density condensations at intervals of 400 pc
for the symmetric mode and 200 pc for the antisymmetric one. A simple change of
the gravity model has thus reduced the growth time by almost an order of
magnitude and its length scale by factors of four to eight. These results
suggest that an onset of the Parker instability in the ISM may not necessarily
be confined to the regions of high and .Comment: Accepted for publication in ApJ, using aaspp4.sty, 18 text pages with
9 figure
Novel Cauchy-horizon instability
The evolution of weak discontinuity is investigated on horizons in the
-dimensional static solutions in the Einstein-Maxwell-scalar-
system, including the Reissner-Nordstr\"om-(anti) de Sitter black hole. The
analysis is essentially local and nonlinear. We find that the Cauchy horizon is
unstable, whereas both the black-hole event horizon and the cosmological event
horizon are stable. This new instability, the so-called kink instability, of
the Cauchy horizon is completely different from the well-known
``infinite-blueshift'' instability. The kink instability makes the analytic
continuation beyond the Cauchy horizon unstable.Comment: 6 pages, 1 figure, final version to appear in Physical Review
Unstable Nonradial Oscillations on Helium Burning Neutron Stars
Material accreted onto a neutron star can stably burn in steady state only
when the accretion rate is high (typically super-Eddington) or if a large flux
from the neutron star crust permeates the outer atmosphere. For such situations
we have analyzed the stability of nonradial oscillations, finding one unstable
mode for pure helium accretion. This is a shallow surface wave which resides in
the helium atmosphere above the heavier ashes of the ocean. It is excited by
the increase in the nuclear reaction rate during the oscillations, and it grows
on the timescale of a second. For a slowly rotating star, this mode has a
frequency of approximately 20-30 Hz (for l=1), and we calculate the full
spectrum that a rapidly rotating (>>30 Hz) neutron star would support. The
short period X-ray binary 4U 1820--30 is accreting helium rich material and is
the system most likely to show this unstable mode,especially when it is not
exhibiting X-ray bursts. Our discovery of an unstable mode in a thermally
stable atmosphere shows that nonradial perturbations have a different stability
criterion than the spherically symmetric thermal perturbations that generate
type I X-ray bursts.Comment: Accepted for publication in Astrophysical Journal, 22 pages, 14
figure
Convergence to a self-similar solution in general relativistic gravitational collapse
We study the spherical collapse of a perfect fluid with an equation of state
by full general relativistic numerical simulations. For 0, it has been known that there exists a general relativistic counterpart
of the Larson-Penston self-similar Newtonian solution. The numerical
simulations strongly suggest that, in the neighborhood of the center, generic
collapse converges to this solution in an approach to a singularity and that
self-similar solutions other than this solution, including a ``critical
solution'' in the black hole critical behavior, are relevant only when the
parameters which parametrize initial data are fine-tuned. This result is
supported by a mode analysis on the pertinent self-similar solutions. Since a
naked singularity forms in the general relativistic Larson-Penston solution for
0, this will be the most serious known counterexample against
cosmic censorship. It also provides strong evidence for the self-similarity
hypothesis in general relativistic gravitational collapse. The direct
consequence is that critical phenomena will be observed in the collapse of
isothermal gas in Newton gravity, and the critical exponent will be
given by , though the order parameter cannot be the black
hole mass.Comment: 22 pages, 15 figures, accepted for publication in Physical Review D,
reference added, typos correcte
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