Finite-Size Scaling Critical Behavior of Randomly Pinned Spin-Density Waves

We have performed Monte Carlo studies of the 3D $XY$ model with random uniaxial anisotropy, which is a model for randomly pinned spin-density waves. We study $L \times L \times L$ simple cubic lattices, using $L$ values in the range 16 to 64, and with random anisotropy strengths of $D / 2 J$ = 1, 2, 3, 6 and $\infty$. There is a well-defined finite temperature critical point, $T_c$, for each these values of $D / 2 J$. We present results for the angle-averaged magnetic structure factor, $S (k)$ at $T_c$ for $L = 64$. 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 $T$ is seen for $D / 2 J$ = 6 and $\infty$. For our finite values of $D / 2 J$ the scaled magnetization function increases with $L$ below $T_c$, and appears to approach an $L$-independent limit for large $L$. This suggests that the system is ferromagnetic below $T_c$.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 $\beta/\nu$ and $\gamma/\nu$ 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 $\beta/\nu$ and $\gamma/\nu$ 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 $166 \pm 5$ Hz. ($4.1 \sigma$; 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 $242 \pm 3$ Hz ($6.0 \sigma$; 7--35 keV). The ratio of these two frequencies is $1.46 \pm 0.05$. 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