4 research outputs found
Flux-lattice melting in two-dimensional disordered superconductors
The flux line lattice melting transition in two-dimensional pure and
disordered superconductors is studied by a Monte Carlo simulation using the
lowest Landau level approximation and quasi-periodic boundary condition on a
plane. The position of the melting line was determined from the diffraction
pattern of the superconducting order parameter. In the clean case we confirmed
the results from earlier studies which show the existence of a quasi-long range
ordered vortex lattice at low temperatures. Adding frozen disorder to the
system the melting transition line is shifted to slightly lower fields. The
correlations of the order parameter for translational long range order of the
vortex positions seem to decay slightly faster than a power law (in agreement
with the theory of Carpentier and Le Doussal) although a simple power law decay
cannot be excluded. The corresponding positional glass correlation function
decays as a power law establishing the existence of a quasi-long range ordered
positional glass formed by the vortices. The correlation function
characterizing a phase coherent vortex glass decays however exponentially
ruling out the possible existence of a phase coherent vortex glass phase.Comment: 12 pages, 21 figures, final version to appear in Phys. Rev.
Quantum Griffiths effects and smeared phase transitions in metals: theory and experiment
In this paper, we review theoretical and experimental research on rare region
effects at quantum phase transitions in disordered itinerant electron systems.
After summarizing a few basic concepts about phase transitions in the presence
of quenched randomness, we introduce the idea of rare regions and discuss their
importance. We then analyze in detail the different phenomena that can arise at
magnetic quantum phase transitions in disordered metals, including quantum
Griffiths singularities, smeared phase transitions, and cluster-glass
formation. For each scenario, we discuss the resulting phase diagram and
summarize the behavior of various observables. We then review several recent
experiments that provide examples of these rare region phenomena. We conclude
by discussing limitations of current approaches and open questions.Comment: 31 pages, 7 eps figures included, v2: discussion of the dissipative
Ising chain fixed, references added, v3: final version as publishe