Direct observation of melting in a 2-D superconducting vortex lattice

TL;DR

This study provides the first direct imaging of the melting process in a 2-D superconducting vortex lattice, revealing intermediate phases and the transition into a vortex liquid.

Contribution

It offers the first direct, step-by-step visualization of vortex lattice melting at the individual vortex level in a 2-D superconductor.

Findings

Identification of a hexatic phase with free dislocations.

Observation of a smectic-like phase possibly due to disclination unbinding.

Direct imaging of the transition into an isotropic vortex liquid.

Abstract

Topological defects such as dislocations and disclinations are predicted to determine the twodimensional (2-D) melting transition. In 2-D superconducting vortex lattices, macroscopic measurements evidence melting close to the transition to the normal state. However, the direct observation at the scale of individual vortices of the melting sequence has never been performed. Here we provide step by step imaging through scanning tunneling spectroscopy of a 2-D system of vortices up to the melting transition in a focused-ion-beam nanodeposited W-based superconducting thin film. We show directly the transition into an isotropic liquid below the superconducting critical temperature. Before that, we find a hexatic phase, characterized by the appearance of free dislocations, and a smectic-like phase, possibly originated through partial disclination unbinding. These results represent a significant step in the understanding of melting of 2-D systems, with impact across several research fields, such as liquid crystal molecules, or lipids in membranes.