Showing papers by "David A. Huse published in 1996"

Topological defects in the random-field XY model and the pinned vortex lattice to vortex glass transition in type-II superconductors.

Abstract: As a simplified model of randomly pinned vortex lattices or charge-density waves, we study the random-field {ital XY} model on square ({ital d}=2) and simple cubic ({ital d}=3) lattices. We verify in Monte Carlo simulations that the average spacing between topological defects (vortices) diverges more strongly than the Imry-Ma pinning length as the random field strength {ital H} is reduced. We suggest that for {ital d}=3 the simulation data are consistent with a topological phase transition at a nonzero critical field {ital H}{sub {ital c}} to a pinned phase that is defect free at large length scales. We also discuss the connection between the possible existence of this phase transition in the random-field {ital XY} model and the magnetic-field-driven transition from a pinned vortex lattice to a vortex glass in weakly disordered type-II superconductors. {copyright} {ital 1996 The American Physical Society.}

Microscopic coexistence of magnetism and superconductivity in ErNi2B2C

Abstract: MAGNETISM and superconductivity are manifestations of two different ordered states into which metals can condense at low temperatures In general these states are mutually exclusive1; they do not coexist at the same place in a sample The study of the interplay between these properties has recently been revitalized by the discovery2,3 of a class of compounds with formula RNi2B2C (where R is a rare-earth element) which are both antiferromagnetic and superconducting at sufficiently low temperature4 It has been suggested5 that magnetic and superconducting order can coexist in these materials on an atomic scale Here we use small-angle neutron scattering to study the structure of the superconducting vortex lattice in ErNi2B2C Our results show that the development of magnetic order causes the vortex lines to disorder and rotate away from the direction of the applied magnetic field This coupling of superconductivity and magnetism provides clear evidence for microscopic coexistence of magnetic and superconducting order, and indicates that magnetic superconductors may exhibit a range of unusual phenomena not observed in conventional superconductors

Quantum Griffiths singularities in the transverse-field Ising spin glass

Abstract: We report a Monte Carlo study of the effects of {ital fluctuations} in the bond distribution of Ising spin glasses in a transverse magnetic field, in the {ital paramagnetic} {ital phase} in the {ital T}{r_arrow}0 limit. Rare, strong fluctuations give rise to Griffiths singularities, which can dominate the zero-temperature behavior of these quantum systems, as originally demonstrated by McCoy for one-dimensional ({ital d}=1) systems. Our simulations are done on a square lattice in {ital d}=2 and a cubic lattice in {ital d}=3, for a Gaussian distribution of nearest neighbor (only) bonds. In {ital d}=2, where the {ital linear} susceptibility was found to diverge at the critical transverse field strength {Gamma}{sub {ital c}} for the order-disorder phase transition at {ital T}=0, the average {ital nonlinear} susceptibility {chi}{sub nl} diverges in the paramagnetic phase for {Gamma} well above {Gamma}{sub {ital c}}, as is also demonstrated in the accompanying paper by Rieger and Young. In {ital d}=3, the linear susceptibility remains finite at {Gamma}{sub {ital c}}, and while Griffiths singularity effects are certainly observable in the paramagnetic phase, the nonlinear susceptibility appears to diverge only rather close to {Gamma}{sub {ital c}}. These results show that Griffiths singularities remain persistent in dimensions above onemore » (where they are known to be strong), though their magnitude decreases monotonically with increasing dimensionality (there being no Griffiths singularities in the limit of infinite dimensionality). {copyright} {ital 1996 The American Physical Society.}« less

Nonlocal conductivity in the vortex-liquid regime of a two-dimensional superconductor.

Abstract: We have simulated the time-dependent Ginzburg-Landau equation with thermal fluctuations, to study the nonlocal dc conductivity of a superconducting film. Having examined points in the phase diagram at a wide range of temperatures and fields below the mean-field upper critical field, we find a portion of the vortex-liquid regime in which the nonlocal ohmic conductivity in real space is negative over a distance several times the spacing between vortices. The effect is suppressed when driven beyond linear response. Earlier work had predicted the existence of such a regime, due to the high viscosity of a strongly-correlated vortex liquid. This behavior is clearly distinguishable from the monotonic spatial fall-off of the conductivity in the higher temperature or field regimes approaching the normal state. The possibilities for experimental study of the nonlocal transport properties are discussed.