Ground state properties of fluxlines in a disordered environment

A new numerical method to calculate exact ground states of multi-fluxline systems with quenched disorder is presented, which is based on the minimum cost flow algorithm from combinatorial optimization. We discuss several models that can be studied with this method including their specific implementations, physically relevant observables and results: 1) the N-line model with N fluxlines (or directed polymers) in a d-dimensional environment with point and/or columnar disorder and hard or soft core repulsion; 2) the vortex glass model for a disordered superconductor in the strong screening limit and 3) the Sine-Gordon model with random pase shifts in the strong coupling limit.Comment: 4 pages RevTeX, 3 eps-figures include

Nature of the vortex-glass order in strongly type-II superconductors

The stability and the critical properties of the three-dimensional vortex-glass order in random type-II superconductors with point disorder is investigated in the unscreened limit based on a lattice {\it XY} model with a uniform field. By performing equilibrium Monte Carlo simulations for the system with periodic boundary conditions, the existence of a stable vortex-glass order is established in the unscreened limit. Estimated critical exponents are compared with those of the gauge-glass model.Comment: Error in the reported value of the exponent eta is correcte

Superconductor-to-Normal Phase Transition in a Vortex Glass Model: Numerical Evidence for a New Percolation Universality Class

The three-dimensional strongly screened vortex-glass model is studied numerically using methods from combinatorial optimization. We focus on the effect of disorder strength on the ground state and found the existence of a disorder-driven normal-to-superconducting phase transition. The transition turns out to be a geometrical phase transition with percolating vortex loops in the ground state configuration. We determine the critical exponents and provide evidence for a new universality class of correlated percolation.Comment: 11 pages LaTeX using IOPART.cls, 11 eps-figures include

Numerical Study of Order in a Gauge Glass Model

The XY model with quenched random phase shifts is studied by a T=0 finite size defect energy scaling method in 2d and 3d. The defect energy is defined by a change in the boundary conditions from those compatible with the true ground state configuration for a given realization of disorder. A numerical technique, which is exact in principle, is used to evaluate this energy and to estimate the stiffness exponent $\theta$. This method gives $\theta = -0.36\pm0.013$ in 2d and $\theta = +0.31\pm 0.015$ in 3d, which are considerably larger than previous estimates, strongly suggesting that the lower critical dimension is less than three. Some arguments in favor of these new estimates are given.Comment: 4 pages, 2 figures, revtex. Submitted to Phys. Rev. Let

Application of a minimum cost flow algorithm to the three-dimensional gauge glass model with screening

We study the three-dimensional gauge glass model in the limit of strong screening by using a minimum cost flow algorithm, enabling us to obtain EXACT ground states for systems of linear size L<=48. By calculating the domain-wall energy, we obtain the stiffness exponent theta = -0.95+/-0.03, indicating the absence of a finite temperature phase transition, and the thermal exponent nu=1.05+/-0.03. We discuss the sensitivity of the ground state with respect to small perturbations of the disorder and determine the overlap length, which is characterized by the chaos exponent zeta=3.9+/-0.2, implying strong chaos.Comment: 4 pages RevTeX, 2 eps-figures include

Efficient Reverse Transcription Using Locked Nucleic Acid Nucleotides towards the Evolution of Nuclease Resistant RNA Aptamers

Background: Modified nucleotides are increasingly being utilized in the de novo selection of aptamers for enhancing their drug-like character and abolishing the need for time consuming trial-and-error based post-selection modifications. Locked nucleic acid (LNA) is one of the most prominent and successful nucleic acid analogues because of its remarkable properties, and widely explored as building blocks in therapeutic oligonucleotides. Evolution of LNA-modified RNA aptamers requires an efficient reverse transcription method for PCR enrichment of the selected RNA aptamer candidates. Establishing this key step is a pre-requisite for performing LNA-modified RNA aptamer selection

Long Range Order at Low Temperature in Dipolar Spin Ice

Recently it has been suggested that long range magnetic dipolar interactions are responsible for spin ice behavior in the Ising pyrochlore magnets ${\rm Dy_{2}Ti_{2}O_{7}}$ and ${\rm Ho_{2}Ti_{2}O_{7}}$. We report here numerical results on the low temperature properties of the dipolar spin ice model, obtained via a new loop algorithm which greatly improves the dynamics at low temperature. We recover the previously reported missing entropy in this model, and find a first order transition to a long range ordered phase with zero total magnetization at very low temperature. We discuss the relevance of these results to ${\rm Dy_{2}Ti_{2}O_{7}}$ and ${\rm Ho_{2}Ti_{2}O_{7}}$.Comment: New version of the manuscript. Now contains 3 POSTSCRIPT figures as opposed to 2 figures. Manuscript contains a more detailed discussion of the (i) nature of long-range ordered ground state, (ii) finite-size scaling results of the 1st order transition into the ground state. Order of authors has been changed. Resubmitted to Physical Review Letters Contact: [email protected]

Current-voltage scaling of chiral and gauge-glass models of two-dimensional superconductors

The scaling behavior of the current-voltage characteristics of chiral and gauge glass models of disordered superconductors, are studied numerically, in two dimensions. For both models, the linear resistance is nonzero at finite temperatures and the scaling analysis of the nonlinear resistivity is consistent with a phase transition at T=0 temperature characterized by a diverging correlation length $\xi \propto T^{-\nu_{T}}$ and thermal critical exponent $\nu_{T}$. The values of $\nu_{T}$, however, are found to be different for the chiral and gauge glass models, suggesting different universality classes, in contrast to the result obtained recently in three dimensions.Comment: 4 pages, 4 figures (included), to appear in Phys. Rev.

Defect-unbinding and the Bose-glass transition in layered superconductors

The low-field Bose-glass transition temperature in heavy-ion irradiated Bi_2Sr_2CaCu_2O_8+d increases progressively with increasing density of irradiation-induced columnar defects, but saturates for densities in excess of 1.5 x10^9 cm^-2. The maximum Bose-glass temperature corresponds to that above which diffusion of two-dimensional pancake vortices between different vortex lines becomes possible, and above which the ``line-like'' character of vortices is lost. We develop a description of the Bose-glass line that is in excellent quantitative agreement with the experimental line obtained for widely different values of track density and material parameters.Comment: 4 pages, 4 figures, submitted to Phys. Rev. Let

Properties of the Bose glass phase in irradiated superconductors near the matching field

Structural and transport properties of interacting localized flux lines in the Bose glass phase of irradiated superconductors are studied by means of Monte Carlo simulations near the matching field B_Phi, where the densities of vortices and columnar defects are equal. For a completely random columnar pin distribution in the xy-plane transverse to the magnetic field, our results show that the repulsive vortex interactions destroy the Mott insulator phase which was predicted to occur at B = B_Phi. On the other hand, for ratios of the penetration depth to average defect distance lambda/d <= 1, characteristic remnants of the Mott insulator singularities remain visible in experimentally accessible quantities as the magnetization, the bulk modulus, and the magnetization relaxation, when B is varied near B_Phi. For spatially more regular disorder, e.g., a nearly triangular defect distribution, we find that the Mott insulator phase can survive up to considerably large interaction range \lambda/d, and may thus be observable in experiments.Comment: RevTex, 17 pages, eps files for 12 figures include