arXiv subjects

David G. Stroud

Publications and source records attributed to David G. Stroud.

3 recordsLinked to original sources

arXiv2000

High field magnetotransport in composite conductors: the effective medium approximation revisited

The self consistent effective medium approximation (SEMA) is used to study three-dimensional random conducting composites under the influence of a strong magnetic field {\bf B}, in the case where all constituents exhibit isotropic response. Asymptotic analysis is used to obtain almost closed form results for the strong field magnetoresistance and Hall resistance in various types of two- and three-constituent isotropic mixtures for the entire range of compositions. Numerical solutions of the SEMA equations are also obtained, in some cases, and compared with those results. In two-constituent free-electron-metal/perfect-insulator mixtures, the magnetoresistance is asymptotically proportional to $|{\bf B}|$ at {\em all concentrations above the percolation threshold}. In three-constituent metal/insulator/superconductor mixtures a line of critical points is found, where the strong field magnetoresistance switches abruptly from saturating to non-saturating dependence on $|{\bf B}|$, at a certain value of the insulator-to-superconductor concentration ratio. This transition appears to be related to the phenomenon of anisotropic percolation.

cond-mat.mtrl-sci↗

arXiv1994

2D-3D Vortex Line Transition in Two-Layer Josephson Arrays

We calculate the dynamical response of two-layer Josephson arrays to a bias current applied into only one layer (primary layer), in a perpendicular magnetic field. The pancake vortices in the two layers sometimes form into flux lines which move as a unit under the influence of the bias current. The lines break apart into independent pancakes in sufficiently high currents, sufficiently weak interlayer coupling, or possibly sufficiently high fields. We discuss the relevance of these results to recent experiments in high-T$_c$ superconductors in the flux transformer geometry.

cond-mat↗

arXiv1993

Vortex Motion and Vortex Friction Coefficient in Triangular Josephson Junction Arrays

The dynamical response of triangular JJA is investigated using the RCSJ model. A flux flow regime is found to extend between a lower vortex-depinning current and a higher critical current, in agreement with previous calculations for square arrays. In the flux flow regime, the dynamical response to the bias current is roughly Ohmic, and the time-dependent voltage can be well understood in terms of vortex degrees of freedom. The vortex friction coefficient $η$ depends strongly on the McCumber-Stewart parameter $β$, and at large $β$ is approximately independent of the shunt resistance $R$. To account for this, we generalize a model of Geigenmüller {\it et al} to treat energy loss from moving vortices to the phase analog of optical spin waves in a triangular lattice. The value of $η$ at all values of $β$ agrees quite well with this model in the low-density limit. The vortex depinning current is estimated as $0.042I_c$, independent of the direction of applied current, in agreement with static calculations by Lobb {\it et al}. A simple argument suggests that quantum effects in vortex motion may become important when the flux flow resistivity is of order $h/(2e)^2$ per unit frustration.

cond-mat↗