Theory and experiment testing flux-line-cutting physics

TL;DR

This paper compares five theories of flux-line behavior in type-II superconductors, presents experimental results favoring the extended elliptic critical-state model, and explores the interplay between flux cutting and depinning.

Contribution

It provides experimental validation for the extended elliptic critical-state model and clarifies the relationship between flux cutting and flux transport in superconductors.

Findings

Flux cutting and flux transport occur simultaneously when current exceeds critical value.

The ratio of effective resistivities $ ho_c/ ho_f$ influences superconductor dynamics.

Experimental data favor the extended elliptic critical-state model.

Abstract

We discuss predictions of five proposed theories for the critical state of type-II superconductors accounting for both flux cutting and flux transport (depinning). The theories predict different behaviours for the ratio $E_y/E_z$ of the transverse and parallel components of the in-plane electric field produced just above the critical current of a type-II superconducting slab as a function of the angle of an in-plane applied magnetic field. We present experimental results measured using an epitaxially grown YBCO thin film favoring one of the five theories: the extended elliptic critical-state model. We conclude that when the current density $\bm J$ is neither parallel nor perpendicular to the local magnetic flux density $\bm B$, both flux cutting and flux transport occur simultaneously when $J$ exceeds the critical current density $J_c$, indicating an intimate relationship between flux cutting and depinning. We also conclude that the dynamical properties of the superconductor when $J$ exceeds $J_c$ depend in detail upon two nonlinear effective resistivities for flux cutting ($\rho_c$) and flux flow ($\rho_f$) and their ratio $r= \rho_c/\rho_f$.