Drived diffusion of vector fields

A model for the diffusion of vector fields driven by external forces is proposed. Using the renormalization group and the $\epsilon$-expansion, the dynamical critical properties of the model with gaussian noise for dimensions below the critical dimension are investigated and new transport universality classes are obtained.Comment: 11 pages, title changed, anisotropic diffusion further discussed and emphasize

Microscopic models for Kitaev's sixteenfold way of anyon theories

In two dimensions, the topological order described by $\mathbb{Z}_2$ gauge theory coupled to free or weakly interacting fermions with a nonzero spectral Chern number $\nu$ is classified by $\nu \; \mathrm{mod}\; 16$ as predicted by Kitaev [Ann. Phys. 321, 2 (2006)]. Here we provide a systematic and complete construction of microscopic models realizing this so-called sixteenfold way of anyon theories. These models are defined by $\Gamma$ matrices satisfying the Clifford algebra, enjoy a global $\mathrm{SO}(\nu)$ symmetry, and live on either square or honeycomb lattices depending on the parity of $\nu$. We show that all these models are exactly solvable by using a Majorana representation and characterize the topological order by calculating the topological spin of an anyonic quasiparticle and the ground-state degeneracy. The possible relevance of the $\nu=2$ and $\nu=3$ models to materials with Kugel-Khomskii-type spin-orbital interactions is discussed.Comment: 6+9 pages, 2+1 figures, published versio

Mean-field scaling function of the universality class of absorbing phase transitions with a conserved field

We consider two mean-field like models which belong to the universality class of absorbing phase transitions with a conserved field. In both cases we derive analytically the order parameter as function of the control parameter and of an external field conjugated to the order parameter. This allows us to calculate the universal scaling function of the mean-field behavior. The obtained universal function is in perfect agreement with recently obtained numerical data of the corresponding five and six dimensional models, showing that four is the upper critical dimension of this particular universality class.Comment: 8 pages, 2 figures, accepted for publication in J. Phys.

Fresh look at randomly branched polymers

We develop a new, dynamical field theory of isotropic randomly branched polymers, and we use this model in conjunction with the renormalization group (RG) to study several prominent problems in the physics of these polymers. Our model provides an alternative vantage point to understand the swollen phase via dimensional reduction. We reveal a hidden Becchi-Rouet-Stora (BRS) symmetry of the model that describes the collapse ($\theta$-)transition to compact polymer-conformations, and calculate the critical exponents to 2-loop order. It turns out that the long-standing 1-loop results for these exponents are not entirely correct. A runaway of the RG flow indicates that the so-called $\theta^\prime$-transition could be a fluctuation induced first order transition.Comment: 4 page

Changes in metal biogeochemistry resulting from wetland creation: bioavailability, toxicity and risk "WETMAT": final report

In the vicinity of rivers, flooding of available lowlands can be considered as a possibility for temporal storage of river water during high water regimes to minimize the flooding danger of inhabited areas. The creation of wetlands for controlled flooding may also contribute to an increased ecological and biological value of the ecosystem area. Due to industrial activities, river water, sediments and soils are often contaminated with metals. The aim of this study was to determine the changes in the metal biogeochemistry under different hydrological conditions and in the presence of reed plants and how this affects the uptake of metals by plants and invertebrates. Two experimental setups were used. Lab scale experiments consisted of 42 barrels filled up with three different soils and flooded with water of different salinities. For the field scale experiment, 4 tanks filled with two soil types were placed on a platform in the river Scheldt and flooded daily by river water. Results showed that the metal mobility did not depend on the total metal content. Other factors such as soil pH, CEC, redox status and carbonate, OM and clay contents however play an important role in mediating metal behaviour. To prevent the mobilization of the metals, it is advisable to reduce redox variations in the soil. The two-year period of the experiment was too short to detect any measurable changes in most of the soil properties. The measured metal (im)mobilization kinetics can however already be incorporated in models in the near future. These models should allow to predict metal mobility changes at the oxic-anoxic interface upon changing hydrological conditions, where the already existing geochemical models fail due to the fact that they are based on steady state conditions. In the two-year experimental period, large quantities of metal contaminated sediment were found to be deposited on the soils of the tanks which were placed in the river Scheldt. This sedimentation could pose a problem, as it minimizes the water storage capacity of a flood control area. Moreover, the contamination status of alluvial soils should be used as a criterion when selecting new flood control areas as uncontaminated flood control areas are expected to be contaminated when polluted sediments originating from the surrounding surface waters are being deposited or when the soil is contacted with metal-polluted surface water. The latter can be expected in the future, as upon increasing oxygen contents of the river waters due to decreasing organic matter loads, metals can be mobilised from metal-polluted river sediments. Reduction of metal mobility upon increasing oxygen contents of river waters therefore needs further investigation. The metal accumulation in benthic organisms like T. tubifex was found to strongly depend on the inundation period. A higher Zn and Cu accumulation was recorded when substrates were allowed to oxidise. For the talitrid amphipod O. gammarellus, no clear influence was noted. Photosynthetic performance of the reed plants seemed to be mainly determined by other factors than merely metal contamination levels. There was however a possible relation between the abundance of stress enzymes and the metal contamination, making it a more valuable parameter to assess the effect of metals on reed plants in wetlands. The reed itself did not have any measurable effect on the concentration of the metals in the soil or pore water. Also no differences in the concentrations of metals in the reed biomass or differences in the reed growth were observed between the contaminated and uncontaminated soil

Correlated Initial Conditions in Directed Percolation

We investigate the influence of correlated initial conditions on the temporal evolution of a (d+1)-dimensional critical directed percolation process. Generating initial states with correlations ~r^(sigma-d) we observe that the density of active sites in Monte-Carlo simulations evolves as rho(t)~t^kappa. The exponent kappa depends continuously on sigma and varies in the range -beta/nu_{||}<=kappa<=eta. Our numerical results are confirmed by an exact field-theoretical renormalization group calculation.Comment: 10 pages, RevTeX, including 5 encapsulated postscript figure

Spontaneous Symmetry Breaking in Directed Percolation with Many Colors: Differentiation of Species in the Gribov Process

A general field theoretic model of directed percolation with many colors that is equivalent to a population model (Gribov process) with many species near their extinction thresholds is presented. It is shown that the multicritical behavior is always described by the well known exponents of Reggeon field theory. In addition this universal model shows an instability that leads in general to a total asymmetry between each pair of species of a cooperative society.Comment: 4 pages, 2 Postscript figures, uses multicol.sty, submitte

Quine, Ontology, and Physicalism

Quine's views on ontology and naturalism are well-known but rarely considered in tandem. According to my interpretation the connection between them is vital. I read Quine as a global epistemic structuralist. Quine thought we only ever know objects qua solutions to puzzles about significant intersections in observations. Objects are always accessed descriptively, via their roles in our best theory. Quine's Kant lectures contain an early version of epistemic structuralism with uncharacteristic remarks about the mental. Here Quine embraces mitigated anomalous monism, allowing introspection and the availability in principle of full physical descriptions of the perceptual states which get science off the ground. Later versions abandon these ideas. My epistemic-structural interpretation explains why. I argue first-personal introspective access to mental states is incompatible with global epistemic structuralism

Influence of humidity on granular packings with moving walls

A significant dependence on the relative humidity H for the apparent mass (Mapp) measured at the bottom of a granular packing inside a vertical tube in relative motion is demonstrated experimentally. While the predictions of Janssen's model are verified for all values of H investigated (25%< H <80%), Mapp increases with time towards a limiting value at high relative humidities (H>60%) but remains constant at lower ones (H=25%). The corresponding Janssen length is nearly independent of the tube velocity for H>60% but decreases markedly for H=25%. Other differences are observed on the motion of individual beads in the packing. For H=25%, they are almost motionless while the mean particle fraction of the packing remains constant; for H>60% the bead motion is much more significant and the mean particle fraction decreases. The dependence of these results on the bead diameter and their interpretation in terms of the influence of capillary forces are discussed.Comment: 6 pages, 6 figure