Critical Behavior of O(n)-symmetric Systems With Reversible Mode-coupling Terms: Stability Against Detailed-balance Violation

We investigate nonequilibrium critical properties of $O(n)$-symmetric models with reversible mode-coupling terms. Specifically, a variant of the model of Sasv\'ari, Schwabl, and Sz\'epfalusy is studied, where violation of detailed balance is incorporated by allowing the order parameter and the dynamically coupled conserved quantities to be governed by heat baths of different temperatures $T_S$ and $T_M$, respectively. Dynamic perturbation theory and the field-theoretic renormalization group are applied to one-loop order, and yield two new fixed points in addition to the equilibrium ones. The first one corresponds to $\Theta = T_S / T_M = \infty$ and leads to model A critical behavior for the order parameter and to anomalous noise correlations for the generalized angular momenta; the second one is at $\Theta = 0$ and is characterized by mean-field behavior of the conserved quantities, by a dynamic exponent $z = d / 2$ equal to that of the equilibrium SSS model, and by modified static critical exponents. However, both these new fixed points are unstable, and upon approaching the critical point detailed balance is restored, and the equilibrium static and dynamic critical properties are recovered.Comment: 18 pages, RevTeX, 1 figure included as eps-file; submitted to Phys. Rev.

Scaling of thermal conductivity of helium confined in pores

We have studied the thermal conductivity of confined superfluids on a bar-like geometry. We use the planar magnet lattice model on a lattice $H\times H\times L$ with $L \gg H$. We have applied open boundary conditions on the bar sides (the confined directions of length $H$) and periodic along the long direction. We have adopted a hybrid Monte Carlo algorithm to efficiently deal with the critical slowing down and in order to solve the dynamical equations of motion we use a discretization technique which introduces errors only $O((\delta t)^6)$ in the time step $\delta t$. Our results demonstrate the validity of scaling using known values of the critical exponents and we obtained the scaling function of the thermal resistivity. We find that our results for the thermal resistivity scaling function are in very good agreement with the available experimental results for pores using the tempComment: 5 two-column pages, 3 figures, Revtex

Critical Binder cumulant for isotropic Ising models on square and triangular lattices

Using Monte Carlo techniques, the critical Binder cumulant U* of isotropic nearest-neighbour Ising models on square and triangular lattices is studied. For rectangular shapes, employing periodic boundary conditions, U* is found to show the same dependence on the aspect ratio for both lattice types. Similarly, applying free boundary conditions for systems with square as well as circular shapes for both lattices, the simulational findings are also consistent with the suggestion that, for isotropic Ising models with short-range interactions, U* depends on the shape and the boundary condition, but not on the lattice structure.Comment: 7 pages, 4 figures, submitted to J. Stat. Mec

Life in the Atacama — Year 2: Geologic reconnaissance through long-range roving and implications on the search for life

The Life in the Atacama-2004 project, which included geological, morphological, and mineralogical mapping through combined satellite, field-based, and microscopic perspectives and long-range roving, led to the localization of potential habitats

Singularity in the boundary resistance between superfluid 4^4He and a solid surface

We report new measurements in four cells of the thermal boundary resistance $R$ between copper and $^4$He below but near the superfluid-transition temperature $T_\lambda$. For $10^{-7} \leq t \equiv 1 - T/T_\lambda \leq 10^{-4}$ fits of $R = R_0 t^{x_b} + B_0$ to the data yielded $x_b \simeq 0.18$, whereas a fit to theoretical values based on the renormalization-group theory yielded $x_b = 0.23$. Alternatively, a good fit of the theory to the data could be obtained if the {\it amplitude} of the prediction was reduced by a factor close to two. The results raise the question whether the boundary conditions used in the theory should be modified.Comment: 4 pages, 4 figures, revte

Spectroscopic results from the Life in the Atacama (LITA) project 2004 field season

Analysis of spectroscopy datasets from rover field tests in the Atacama Desert (Chile), focusing on the composition of the surface and identification of potential habitats for life

Recent Star Formation in Sextans A

We investigate the relationship between the spatial distributions of stellar populations and of neutral and ionized gas in the Local Group dwarf irregular galaxy Sextans A. This galaxy is currently experiencing a burst of localized star formation, the trigger of which is unknown. We have resolved various populations of stars via deep UBV(RI)_C imaging over an area with diameter \sim 5.'3. We have compared our photometry with theoretical isochrones appropriate for Sextans A, in order to determine the ages of these populations. We have mapped out the history of star formation, most accurately for times \lesssim 100 Myr. We find that star formation in Sextans A is correlated both in time and space, especially for the most recent (\lesssim 12 Myr) times. The youngest stars in the galaxy are forming primarily along the inner edge of the large H I shell. Somewhat older populations, \lesssim 50 Myr, are found inward of the youngest stars. Progressively older star formation, from \sim 50--100 Myr, appears to have some spatially coherent structure and is more centrally concentrated. The oldest stars we can accurately sample appear to have approximately a uniform spatial distribution, which extends beyond a surface brightness of \mu_B \simeq 25.9 mag arcsec^{-2} (or, a radius r \simeq 2.'3$). Although other processes are also possible, our data provides support for a mechanism of supernova-driven expansion of the neutral gas, resulting in cold gas pileup and compression along the H I shell and sequential star formation in recent times.Comment: 64 pages, 22 figures, to appear in A