5,705 research outputs found
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
Dislocations in the ground state of the solid-on-solid model on a disordered substrate
We investigate the effects of topological defects (dislocations) to the
ground state of the solid-on-solid (SOS) model on a simple cubic disordered
substrate utilizing the min-cost-flow algorithm from combinatorial
optimization. The dislocations are found to destabilize and destroy the elastic
phase, particularly when the defects are placed only in partially optimized
positions. For multi defect pairs their density decreases exponentially with
the vortex core energy. Their mean distance has a maximum depending on the
vortex core energy and system size, which gives a fractal dimension of . The maximal mean distances correspond to special vortex core
energies for which the scaling behavior of the density of dislocations change
from a pure exponential decay to a stretched one. Furthermore, an extra
introduced vortex pair is screened due to the disorder-induced defects and its
energy is linear in the vortex core energy.Comment: 6 pages RevTeX, eps figures include
Fluctuation Dissipation Ratio in Three-Dimensional Spin Glasses
We present an analysis of the data on aging in the three-dimensional Edwards
Anderson spin glass model with nearest neighbor interactions, which is well
suited for the comparison with a recently developed dynamical mean field
theory. We measure the parameter describing the violation of the
relation among correlation and response function implied by the fluctuation
dissipation theorem.Comment: LaTeX 10 pages + 4 figures (appended as uuencoded compressed
tar-file), THP81-9
Impact of Interdisciplinary Undergraduate Research in Mathematics and Biology on the Development of a New Course Integrating Five STEM Disciplines
Funded by innovative programs at the National Science Foundation and the Howard Hughes Medical Institute, University of Richmond faculty in biology, chemistry, mathematics, physics, and computer science teamed up to offer first- and second-year students the opportunity to contribute to vibrant, interdisciplinary research projects. The result was not only good science but also good science that motivated and informed course development. Here, we describe four recent undergraduate research projects involving students and faculty in biology, physics, mathematics, and computer science and how each contributed in significant ways to the conception and implementation of our new Integrated Quantitative Science course, a course for first-year students that integrates the material in the first course of the major in each of biology, chemistry, mathematics, computer science, and physics
Dynamic Scaling in Diluted Systems Phase Transitions: Deactivation trough Thermal Dilution
Activated scaling is confirmed to hold in transverse field induced phase
transitions of randomly diluted Ising systems. Quantum Monte Carlo calculations
have been made not just at the percolation threshold but well bellow and above
it including the Griffiths-McCoy phase. A novel deactivation phenomena in the
Griffiths-McCoy phase is observed using a thermal (in contrast to random)
dilution of the system.Comment: 4 pages, 4 figures, RevTe
Random antiferromagnetic quantum spin chains: Exact results from scaling of rare regions
We study XY and dimerized XX spin-1/2 chains with random exchange couplings
by analytical and numerical methods and scaling considerations. We extend
previous investigations to dynamical properties, to surface quantities and
operator profiles, and give a detailed analysis of the Griffiths phase. We
present a phenomenological scaling theory of average quantities based on the
scaling properties of rare regions, in which the distribution of the couplings
follows a surviving random walk character. Using this theory we have obtained
the complete set of critical decay exponents of the random XY and XX models,
both in the volume and at the surface. The scaling results are confronted with
numerical calculations based on a mapping to free fermions, which then lead to
an exact correspondence with directed walks. The numerically calculated
critical operator profiles on large finite systems (L<=512) are found to follow
conformal predictions with the decay exponents of the phenomenological scaling
theory. Dynamical correlations in the critical state are in average
logarithmically slow and their distribution show multi-scaling character. In
the Griffiths phase, which is an extended part of the off-critical region
average autocorrelations have a power-law form with a non-universal decay
exponent, which is analytically calculated. We note on extensions of our work
to the random antiferromagnetic XXZ chain and to higher dimensions.Comment: 19 pages RevTeX, eps-figures include
Off-Equilibrium Dynamics in Finite-Dimensional Spin Glass Models
The low temperature dynamics of the two- and three-dimensional Ising spin
glass model with Gaussian couplings is investigated via extensive Monte Carlo
simulations. We find an algebraic decay of the remanent magnetization. For the
autocorrelation function a typical
aging scenario with a scaling is established. Investigating spatial
correlations we find an algebraic growth law of
the average domain size. The spatial correlation function scales with . The sensitivity of the
correlations in the spin glass phase with respect to temperature changes is
examined by calculating a time dependent overlap length. In the two dimensional
model we examine domain growth with a new method: First we determine the exact
ground states of the various samples (of system sizes up to )
and then we calculate the correlations between this state and the states
generated during a Monte Carlo simulation.Comment: 38 pages, RevTeX, 14 postscript figure
Quantum vs. Geometric Disorder in a Two-Dimensional Heisenberg Antiferromagnet
We present a numerical study of the spin-1/2 bilayer Heisenberg
antiferromagnet with random interlayer dimer dilution. From the temperature
dependence of the uniform susceptibility and a scaling analysis of the spin
correlation length we deduce the ground state phase diagram as a function of
nonmagnetic impurity concentration p and bilayer coupling g. At the site
percolation threshold, there exists a multicritical point at small but nonzero
bilayer coupling g_m = 0.15(3). The magnetic properties of the single-layer
material La_2Cu_{1-p}(Zn,Mg)_pO_4 near the percolation threshold appear to be
controlled by the proximity to this new quantum critical point.Comment: minor changes, updated figure
Disorder Driven Critical Behavior of Periodic Elastic Media in a Crystal Potential
We study a lattice model of a three-dimensional periodic elastic medium at
zero temperature with exact combinatorial optimization methods. A competition
between pinning of the elastic medium, representing magnetic flux lines in the
mixed phase of a superconductor or charge density waves in a crystal, by
randomly distributed impurities and a periodic lattice potential gives rise to
a continuous phase transition from a flat phase to a rough phase. We determine
the critical exponents of this roughening transition via finite size scaling
obtaining , , and find
that they are universal with respect to the periodicity of the lattice
potential. The small order parameter exponent is reminiscent of the random
field Ising critical behavior in 3.Comment: 4 pages, 3 eps-figures include
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