Surface Critical Behavior in Systems with Absorbing States

We present a general scaling theory for the surface critical behavior of non-equilibrium systems with phase transitions into absorbing states. The theory allows for two independent surface exponents which satisfy generalized hyperscaling relations. As an application we study a generalized version of directed percolation with two absorbing states. We find two distinct surface universality classes associated with inactive and reflective walls. Our results indicate that the exponents associated with these two surface universality classes are closely connected.Comment: latex, 4 pages, to appear in PR

Statistical Laws and Mechanics of Voronoi Random Lattices

We investigate random lattices where the connectivities are determined by the Voronoi construction, while the location of the points are the dynamic degrees of freedom. The Voronoi random lattices with an associated energy are immersed in a heat bath and investigated using a Monte Carlo simulation algorithm. In thermodynamic equilibrium we measure coordination number distributions and test the Aboav-Weaire and Lewis laws.Comment: 14 pages (figures not included), LaTeX, HLRZ-26/9

Quenched noise and over-active sites in sandpile dynamics

The dynamics of sandpile models are mapped to discrete interface equations. We study in detail the Bak-Tang-Wiesenfeld model, a stochastic model with random thresholds, and the Manna model. These are, respectively, discretizations of the quenched Edwards-Wilkinson equation with columnar, point-like and correlated noise, with the constraint that the interface velocity is either zero or exactly one. The constraint, embedded in the sandpile rules, gives rise to another noise component. This term has for the Bak-Tang-Wiesenfeld model long-range on-site correlations and reveals that with open boundary conditions there is no spatial translational invariance.Comment: 4 pages, 3 figure

Active Width at a Slanted Active Boundary in Directed Percolation

The width W of the active region around an active moving wall in a directed percolation process diverges at the percolation threshold p_c as W \simeq A \epsilon^{-\nu_\parallel} \ln(\epsilon_0/\epsilon), with \epsilon=p_c-p, \epsilon_0 a constant, and \nu_\parallel=1.734 the critical exponent of the characteristic time needed to reach the stationary state \xi_\parallel \sim \epsilon^{-\nu_\parallel}. The logarithmic factor arises from screening of statistically independent needle shaped sub clusters in the active region. Numerical data confirm this scaling behaviour.Comment: 5 pages, 5 figure

Novel Quenched Disorder Fixed Point in a Two-Temperature Lattice Gas

We investigate the effects of quenched randomness on the universal properties of a two-temperature lattice gas. The disorder modifies the dynamical transition rates of the system in an anisotropic fashion, giving rise to a new fixed point. We determine the associated scaling form of the structure factor, quoting critical exponents to two-loop order in an expansion around the upper critical dimension d$_c=7$. The close relationship with another quenched disorder fixed point, discovered recently in this model, is discussed.Comment: 11 pages, no figures, RevTe

Sow body condition at weaning and reproduction performance in organic piglet production

The objective was to investigate the variation in backfat at weaning and its relations to reproduction results in organic sow herds in Denmark. The study included eight herds and 573 sows. The average backfat at weaning mean�13 mm; SD�4.2 mm) ranging from 10.5 to 17.3 mm among herds shows that it is possible to avoid poor body condition at weaning even with a lactation length of seven weeks or more. No main effect of backfat at weaning on reproduction performance was found, but the probability of a successful reproduction after weaning tended to decrease with decreasing backfat for first parity sows, whereas the opposite was the case for multiparous sows

Surface Critical Behavior in Systems with Non-Equilibrium Phase Transitions

We study the surface critical behavior of branching-annihilating random walks with an even number of offspring (BARW) and directed percolation (DP) using a variety of theoretical techniques. Above the upper critical dimensions d_c, with d_c=4 (DP) and d_c=2 (BARW), we use mean field theory to analyze the surface phase diagrams using the standard classification into ordinary, special, surface, and extraordinary transitions. For the case of BARW, at or below the upper critical dimension, we use field theoretic methods to study the effects of fluctuations. As in the bulk, the field theory suffers from technical difficulties associated with the presence of a second critical dimension. However, we are still able to analyze the phase diagrams for BARW in d=1,2, which turn out to be very different from their mean field analog. Furthermore, for the case of BARW only (and not for DP), we find two independent surface beta_1 exponents in d=1, arising from two distinct definitions of the order parameter. Using an exact duality transformation on a lattice BARW model in d=1, we uncover a relationship between these two surface beta_1 exponents at the ordinary and special transitions. Many of our predictions are supported using Monte-Carlo simulations of two different models belonging to the BARW universality class.Comment: 19 pages, 12 figures, minor additions, 1 reference adde