Inverse melting and re-entrant transformations of the vortex lattice in amorphous Re6Zr thin film

TL;DR

This study observes inverse melting in a 2D vortex system within amorphous Re6Zr thin films, where increasing temperature or magnetic field first orders the vortex lattice before melting it again, revealing re-entrant phase transitions.

Contribution

It provides direct imaging evidence of inverse melting and re-entrant vortex lattice transformations in a superconducting thin film, a phenomenon rarely observed experimentally.

Findings

Vortices form a pinned liquid with low mobility at low T and fields.

Increasing T or magnetic field orders vortices into a nearly perfect lattice.

Further increase causes the vortex lattice to melt into a vortex liquid again.

Abstract

Melting of a solid is one of the most ubiquitous phenomena observed in nature. Most solids, when heated, melt from a crystalline state to an isotropic liquid at a characteristic temperature. There are however situations where increase in temperature can induce a transition to a more ordered state. Broadly termed as "inverse melting", experimental realisations of such situations are rare. Here, we report such a phenomenon in the 2-dimensional vortex liquid that forms in a moderately pinned amorphous Re6Zr (a-ReZr) thin film, from direct imaging of the vortex lattice using a scanning tunnelling microscope. At low temperature and magnetic fields, we find that the vortices form a "pinned liquid" , that is characterised by a low mobility of the vortices and vortex density that is spatially inhomogeneous. As the temperature or magnetic field is increased the vortices become more ordered, eventually forming a nearly perfectly ordered vortex lattice. Above this temperature/magnetic field, the ordered vortex lattice melts again into a vortex liquid. This re-entrant transformation from a liquid to solid-like state and then back to a liquid also leaves distinct signature in the magnetotransport properties of the superconductor.