"Vortex-melting front" in thin superconductors with pinning

TL;DR

This paper reinterprets the second flux front in thin superconductors, showing it signifies the disappearance of solid vortex droplets rather than the vortex-lattice melting transition, due to electrodynamics effects.

Contribution

It provides a new understanding of the second flux front in thin superconductors, emphasizing electrodynamics' role in the phase mixture.

Findings

The second flux front does not mark vortex melting.

A nearly constant magnetic induction region exists near the transition.

The second flux front indicates disappearance of solid vortex droplets.

Abstract

Magneto-optical observations of a second flux front, which occurs at the second peak in the magnetization of Bi_2 Sr_2 CaCu_2 O_x single crystals related to the known first order ``vortex-lattice melting'', are reconsidered. We show that, in thin samples, electrodynamics necessarily leads to an extended region in which the magnetic induction adopts a nearly constant value close to that at which the phase transition occurs at thermal equilibrium. In this region a dynamical phase mixture of vortex ``solid'' and ``liquid'' should exist. Interestingly, the observed second flux front does not mark the ``melting'' front, as it was naively interpreted earlier, but indicates the disappearance of the last ``solid droplets''.