Why an ac magnetic field shifts the irreversibility line in type-II superconductors

TL;DR

This paper explains how a weak ac magnetic field applied perpendicularly to a dc field in a thin superconductor causes vortex motion, leading to voltage generation and relaxation of critical currents, affecting the irreversibility line.

Contribution

It introduces a theory describing vortex dynamics under combined ac and dc fields, explaining the shaking effect and its impact on the superconductor's phase transitions.

Findings

Ac field induces a dc voltage and decay of critical currents.

Vortex motion is driven by the ac field, not thermal activation.

The theory aligns with experimental observations of the shaking effect.

Abstract

We show that for a thin superconducting strip placed in a transverse dc magnetic field - the typical geometry of experiments with high-Tc superconductors - the application of a weak ac magnetic field perpendicular to the dc field generates a dc voltage in the strip. This voltage leads to the decay of the critical currents circulating in the strip, and eventually the equilibrium state of the superconductor is established. This relaxation is not due to thermally activated flux creep but to the "walking" motion of vortices in the two-dimensional critical state of the strip with in-plane ac field. Our theory explains the shaking effect that was used for detecting phase transitions of the vortex lattice in superconductors with pinning. Some recent experiments on this subject are discussed.