Phase Transitions in the Two-Dimensional XY Model with Random Phases: a Monte Carlo Study

We study the two-dimensional XY model with quenched random phases by Monte Carlo simulation and finite-size scaling analysis. We determine the phase diagram of the model and study its critical behavior as a function of disorder and temperature. If the strength of the randomness is less than a critical value, $\sigma_{c}$, the system has a Kosterlitz-Thouless (KT) phase transition from the paramagnetic phase to a state with quasi-long-range order. Our data suggest that the latter exists down to T=0 in contradiction with theories that predict the appearance of a low-temperature reentrant phase. At the critical disorder $T_{KT}\rightarrow 0$ and for $\sigma > \sigma_{c}$ there is no quasi-ordered phase. At zero temperature there is a phase transition between two different glassy states at $\sigma_{c}$. The functional dependence of the correlation length on $\sigma$ suggests that this transition corresponds to the disorder-driven unbinding of vortex pairs.Comment: LaTex file and 18 figure

Domain regime in two-dimensional disordered vortex matter

A detailed numerical study of the real space configuration of vortices in disordered superconductors using 2D London-Langevin model is presented. The magnetic field $B$ is varied between 0 and $B_{c2}$ for various pinning strengths $\Delta$. For weak pinning, an inhomogeneous disordered vortex matter is observed, in which the topologically ordered vortex lattice survives in large domains. The majority of the dislocations in this state are confined to the grain boundaries/domain walls. Such quasi-ordered configurations are observed in the intermediate fields, and we refer it as the domain regime (DR). The DR is distinct from the low-field and the high-fields amorphous regimes which are characterized by a homogeneous distribution of defects over the entire system. Analysis of the real space configuration suggests domain wall roughening as a possible mechanism for the crossover from the DR to the high-field amorphous regime. The DR also shows a sharp crossover to the high temperature vortex liquid phase. The domain size distribution and the roughness exponent of the lattice in the DR are also calculated. The results are compared with some of the recent Bitter decoration experiments.Comment: 9 pages, 9 figure

Constellations of identity: place-ma(r)king beyond heritage

This paper will critically consider the different ways in which history and belonging have been treated in artworks situated in the Citadel development in Ayr on the West coast of Scotland. It will focus upon one artwork, Constellation by Stephen Hurrel, as an alternative to the more conventional landscapes of heritage which are adjacent, to examine the relationship between personal history and place history and argue the primacy of participatory process in the creation of place and any artwork therein. Through his artwork, Hurrel has attempted to adopt a material process through which place can be created performatively but, in part due to its non-representational form, proves problematic, aesthetically and longitudinally, in wholly engaging the community. The paper will suggest that through variants of ‘new genre public art’ such as this, personal and place histories can be actively re-created through the redevelopment of contemporary urban landscapes but also highlight the complexities and indeterminacies involved in the relationship between artwork, people and place

Site-specific immobilization of microbes using carbon nanotubes and dielectrophoretic force for microfluidic applications

We developed a microbial immobilization method for successful applications in microfluidic devices. Single-walled nanotubes and Escherichia coli were aligned between two cantilever electrodes by a positive dielectrophoretic force resulting in a film of single-walled nanotubes with attached Escherichia coli. Because this film has a suspended and porous structure, it has a larger reaction area and higher reactant transfer efficiency than film attached to the substrate surface. The cell density of film was easily controlled by varying the cell concentration of the suspension and varying the electric field. The film showed excellent stability of enzyme activity, as demonstrated by measuring continuous reaction and long-term storage times using recombinant Escherichia coli that expressed organophosphorus hydrolase.X1133sciescopu

Omnivorousness in sport: The importance of social capital and networks

There has been for some time a significant and growing body of research around the relationship between sport and social capital. Similarly, within sociology there has been a corpus of work that has acknowledged the emergence of the omnivore–univore relationship. Surprisingly, relatively few studies examining sport and social capital have taken the omnivore–univore framework as a basis for understanding the relationship between sport and social capital. This gap in the sociology of sport literature and knowledge is rectified by this study that takes not Putnam, Coleman or Bourdieu, but Lin’s social network approach to social capital. The implications of this article are that researchers investigating sport and social capital need to understand more about how social networks and places for sport work to create social capital and, in particular, influence participating in sporting activities. The results indicate that social networks both facilitate and constrain sports participation; whilst family and friendship networks are central in active lifestyles, those who are less active have limited networks

Simultaneous Diagonal and Off Diagonal Order in the Bose--Hubbard Hamiltonian

The Bose-Hubbard model exhibits a rich phase diagram consisting both of insulating regimes where diagonal long range (solid) order dominates as well as conducting regimes where off diagonal long range order (superfluidity) is present. In this paper we describe the results of Quantum Monte Carlo calculations of the phase diagram, both for the hard and soft core cases, with a particular focus on the possibility of simultaneous superfluid and solid order. We also discuss the appearance of phase separation in the model. The simulations are compared with analytic calculations of the phase diagram and spin wave dispersion.Comment: 28 pages plus 24 figures, uuencoded Revtex+postscript file

Critical Behaviour of Superfluid 4^4He in Aerogel

We report on Monte Carlo studies of the critical behaviour of superfluid $^4$He in the presence of quenched disorder with long-range fractal correlations. According to the heuristic argument by Harris, uncorrelated disorder is irrelevant when the specific heat critical exponent $\alpha$ is negative, which is the case for the pure $^4$He. However, experiments on helium in aerogel have shown that the superfluid density critical exponent $\zeta$ changes. We hypothesize that this is a cross-over effect due to the fractal nature of aerogel. Modelling the aerogel as an incipient percolating cluster in 3D and weakening the bonds at the fractal sites, we perform XY-model simulations, which demonstrate an increase in $\zeta$ from $0.67 \pm 0.005$ for the pure case to an apparent value of $0.722\pm 0.005$ in the presence of the fractal disorder, provided that the helium correlation length does not exceed the fractal correlation length.Comment: 4 pages, RevTex, 3 postscript figures, LaTeX file and figures have been uuencoded

Universal Conductivity in the Two dimensional Boson Hubbard Model

We use Quantum Monte Carlo to evaluate the conductivity $\sigma$ of the 2--dimensional disordered boson Hubbard model at the superfluid-bose glass phase boundary. At the critical point for particle density $\rho=0.5$, we find $\sigma_{c}=(0.45 \pm 0.07) \sigma_{Q}$, where $\sigma_{Q}= e_{*}^{2} / h$ from a finite size scaling analysis of the superfluid density. We obtain $\sigma_{c}=(0.47 \pm 0.08) \sigma_{Q}$ from a direct calculation of the current--current correlation function. Simulations at the critical points for other particle densities, $\rho=0.75$ and $1.0$, give similar values for $\sigma$. We discuss possible origins of the difference in this value from that recently obtained by other numerical approaches.Comment: 20 pages, figures available upon request. Tex with jnl3.tex and reforder.tex macros. cond-mat/yymmnn

Dual superfluid-Bose glass critical point in two dimensions and the universal conductivity

We study the continuum version of the dual theory for a system of two-dimensional, zero temperature, disordered bosons, interacting with short range repulsion and at a commensurate density. The dual theory, which describes vortices in the bosonic ground state, and has a form of 3D classical scalar electrodynamics in random, correlated magnetic field, admits a new disordered critical point within RG calculation at fixed dimension. The universal conductivity and the critical exponents at the superfluid-Bose glass critical point are calculated as series in fixed-point values of the dual coupling constants, to the lowest non-trivial order: $\sigma_c = 0.25 (2e)^2 /h$, $\nu=1.38$ and $z=1.93$. The comparison with numerical results and experiments is discussed.Comment: 8 pages, LaTex, some clarifications and references adde

Three-dimensional Josephson-junction arrays in the quantum regime

We study the quantum phase transition properties of a three-dimensional periodic array of Josephson junctions with charging energy that includes both the self and mutual junction capacitances. We use the phase fluctuation algebra between number and phase operators, given by the Euclidean group E_2, and we effectively map the problem onto a solvable quantum generalization of the spherical model. We obtain a phase diagram as a function of temperature, Josephson coupling and charging energy. We also analyze the corresponding fluctuation conductivity and its universal scaling form in the vicinity of the zero-temperature quantum critical point.Comment: 9 pages, LATEX, three PostScript figures. Submitted to Phys. Rev. Let