13,326 research outputs found
Finite-Size Scaling Critical Behavior of Randomly Pinned Spin-Density Waves
We have performed Monte Carlo studies of the 3D model with random
uniaxial anisotropy, which is a model for randomly pinned spin-density waves.
We study simple cubic lattices, using values in the
range 16 to 64, and with random anisotropy strengths of = 1, 2, 3, 6
and . There is a well-defined finite temperature critical point, ,
for each these values of . We present results for the angle-averaged
magnetic structure factor, at for . We also use
finite-size scaling analysis to study scaling functions for the critical
behavior of the specific heat, the magnetization and the longitudinal magnetic
susceptibility. Good data collapse of the scaling functions over a wide range
of is seen for = 6 and . For our finite values of the scaled magnetization function increases with below , and
appears to approach an -independent limit for large . This suggests that
the system is ferromagnetic below .Comment: 21 pages in single column format, 20 .eps files, revised and
expanded, errors corrected, submitted to PR
Complete high-precision entropic sampling
Monte Carlo simulations using entropic sampling to estimate the number of
configurations of a given energy are a valuable alternative to traditional
methods. We introduce {\it tomographic} entropic sampling, a scheme which uses
multiple studies, starting from different regions of configuration space, to
yield precise estimates of the number of configurations over the {\it full
range} of energies, {\it without} dividing the latter into subsets or windows.
Applied to the Ising model on the square lattice, the method yields the
critical temperature to an accuracy of about 0.01%, and critical exponents to
1% or better. Predictions for systems sizes L=10 - 160, for the temperature of
the specific heat maximum, and of the specific heat at the critical
temperature, are in very close agreement with exact results. For the Ising
model on the simple cubic lattice the critical temperature is given to within
0.003% of the best available estimate; the exponent ratios and
are given to within about 0.4% and 1%, respectively, of the
literature values. In both two and three dimensions, results for the {\it
antiferromagnetic} critical point are fully consistent with those of the
ferromagnetic transition. Application to the lattice gas with nearest-neighbor
exclusion on the square lattice again yields the critical chemical potential
and exponent ratios and to good precision.Comment: For a version with figures go to
http://www.fisica.ufmg.br/~dickman/transfers/preprints/entsamp2.pd
Revision of the African caecilian genus Schistometopum Parker (Amphibia: Gymnophiona: Caeciliidae)
http://deepblue.lib.umich.edu/bitstream/2027.42/56430/4/MP187.pd
Double Electron Transitions In Collisions Between Multiply Charged Ions And Helium Atoms
A semiclassical model for He is applied to the study of multiply charged ions colliding with He at intermediate energies. Single and double-electron capture and ionisation and capture-ionisation reactions are studied for incident-ion charge states q=+1 to +50 at 1 MeV amu-1. For charge states q\u3eor approximately=+10, single electron capture is found to be dominated by a two-electron transition in which one electron is captured and the other is ionised
A Compton Up-scattering Model for Soft Lags in the Lower Kilohertz QPO in 4U1608-52
An empirical Compton up-scattering model is described which reproduces both
the fractional amplitude (RMS) vs. energy and the soft time lags in the 830 Hz
QPO observed in 4U1608-52 on Mar. 3, 1996. A combination of two coherent
variations in the coronal and soft photon temperatures (with their relative
contributions determined by enforcing energy conservation) gives rise to the
QPO's energy dependent characteristics. All input parameters to the model, save
a characteristic plasma size and the fraction of Comptonized photons impinging
on the soft photon source, are derived from the time-averaged photon energy
spectrum of the same observation. Fits to the fractional RMS and phase lag data
for this kilohertz QPO imply that the spatial extent of the plasma is in the
range from 4 to 15 km.Comment: 4 pages, 2 figure
Modeling the Optical-X-ray Accretion Lag in LMC X-3: Insights Into Black-Hole Accretion Physics
The X-ray persistence and characteristically soft spectrum of the black hole
X-ray binary LMC X-3 make this source a touchstone for penetrating studies of
accretion physics. We analyze a rich, 10-year collection of optical/infrared
(OIR) time-series data in conjunction with all available contemporaneous X-ray
data collected by the ASM and PCA detectors aboard the Rossi X-ray Timing
Explorer. A cross-correlation analysis reveals an X-ray lag of ~2 weeks.
Motivated by this result, we develop a model that reproduces the complex OIR
light curves of LMC X-3. The model is comprised of three components of
emission: stellar light; accretion luminosity from the outer disk inferred from
the time-lagged X-ray emission; and light from the X-ray-heated star and outer
disk. Using the model, we filter a strong noise component out of the
ellipsoidal light curves and derive an improved orbital period for the system.
Concerning accretion physics, we find that the local viscous timescale in the
disk increases with the local mass accretion rate; this in turn implies that
the viscosity parameter alpha decreases with increasing luminosity. Finally, we
find that X-ray heating is a strong function of X-ray luminosity below ~50% of
the Eddington limit, while above this limit X-ray heating is heavily
suppressed. We ascribe this behavior to the strong dependence of the flaring in
the disk upon X-ray luminosity, concluding that for luminosities above ~50% of
Eddington, the star lies fully in the shadow of the disk.Comment: Accepted in ApJ (12 pages long in emulateapj format
The X-ray Outburst of H1743-322: High-Frequency QPOs with a 3:2 Frequency Ratio
We observed the 2003 X-ray outburst of H1743-322 in a series of 130 pointed
observation with RXTE. We searched individual observations for high-frequency
QPOs (HFQPOs) and found only weak or marginal detections near 240 and 160 Hz.
We next grouped the observations in several different ways and computed the
average power-density spectra (PDS) in a search for further evidence of HFQPOs.
This effort yielded two significant results for those observations defined by
the presence of low-frequency QPOs (0.1-20 Hz) and an absence of
``band-limited'' power continua: (1) The 9 time intervals with the highest 7-35
keV count rates yielded an average PDS with a QPO at Hz. (; 3--35 keV); and (2) a second group with lower 7-35 keV count rates (26
intervals) produced an average PDS with a QPO at Hz (;
7--35 keV). The ratio of these two frequencies is . This finding
is consistent with results obtained for three other black hole systems that
exhibit commensurate HFQPOs in a 3:2 ratio. Furthermore, the occurrence of
H1743-322's slower HFQPO at times of higher X-ray luminosity closely resembles
the behavior of XTE J1550-564 and GRO J1655-40. We discuss our results in terms
of a resonance model that invokes frequencies set by general relativity for
orbital motions near a black-hole event horizon.Comment: 12 pages, 3 figures, submitted to Ap
Electron Capture Cross Sections For TiH⁴⁺
Electron capture cross sections have been computed for Ti4++H to Ti3++H+ and Ti3++H+ to Ti4++H, where the reactants were in their ground states. The energy range investigated was 0.1 to 10 keV amu-1. The impact parameter perturbed-stationary-state method incorporating electron translation factors were used to calculate the cross sections. The molecular wavefunctions were generated using the pseudo-potential method. At 1 keV amu-1 ( nu rel=4.4*107 cm s-1), representative electron capture cross sections for the Ti4++H and Ti3++H+ reactions are 2*10-15 cm 2 and 1*10-17 cm2, respectively. An appreciable energy dependence was observed for both cross sections
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