Melting of the vortex lattice through intermediate hexatic fluid in a-MoGe thin film

TL;DR

This study demonstrates the two-step melting of a 2D vortex lattice in a-MoGe thin films, revealing a transition through a hexatic fluid phase consistent with theoretical predictions, using imaging and transport measurements.

Contribution

It provides experimental evidence for the vortex lattice melting process through an intermediate hexatic phase in a 2D superconductor.

Findings

Vortex lattice melts via a hexatic fluid phase.

Transport measurements reveal signatures of phase transitions.

Constructed a vortex phase diagram for 2D superconductor.

Abstract

The hexatic fluid refers to a phase in between a solid and a liquid which has short range positional order but quasi-long range orientational order. In the celebrated theory of Berezinskii, Kosterlitz and Thouless and subsequently refined by Halperin, Nelson and Young, it was predicted that a 2-dimensional hexagonal solid can melt in two steps: first, through a transformation from a solid to a hexatic fluid which retains quasi long range orientational order and then from a hexatic fluid to an isotropic liquid. In this paper, using a combination of real space imaging and transport measurements we show that the 2-dimensional vortex lattice in a-MoGe thin film follows this sequence of melting as the magnetic field is increased. Identifying the signatures of various transitions on the bulk transport properties of the superconductor, we construct a vortex phase diagram for a two dimensional superconductor.