Melting of Flux Lines in an Alternating Parallel Current

TL;DR

This paper investigates how an alternating parallel current affects the melting behavior of flux lines in superconductors, revealing a frequency-dependent phase diagram and melting temperature.

Contribution

It provides an exact calculation of flux line fluctuations under alternating current and introduces a nonequilibrium phase diagram based on current magnitude and frequency.

Findings

Melting temperature vanishes at a limiting current for zero frequency.

Finite frequency results in a non-zero melting temperature.

Exact fluctuation calculations inform the phase diagram.

Abstract

We use a Langevin equation to examine the dynamics and fluctuations of a flux line (FL) in the presence of an {\it alternating longitudinal current} $J_{\parallel}(\omega)$. The magnus and dissipative forces are equated to those resulting from line tension, confinement in a harmonic cage by neighboring FLs, parallel current, and noise. The resulting mean-square FL fluctuations are calculated {\it exactly}, and a Lindemann criterion is then used to obtain a nonequilibrium `phase diagram' as a function of the magnitude and frequency of $J_{\parallel}(\omega)$. For zero frequency, the melting temperature of the mixed phase (a lattice, or the putative "Bose" or "Bragg Glass") vanishes at a limiting current. However, for any finite frequency, there is a non-zero melting temperature.