On the nature of the phase transitions in two-dimensional type II superconductors

TL;DR

This study uses advanced simulations to investigate phase transitions in two-dimensional type-II superconductors, revealing that larger system sizes support a continuous melting process consistent with the KTHNY theory, challenging earlier first-order transition findings.

Contribution

The paper demonstrates that larger system simulations show no first-order transition, supporting a continuous melting scenario in 2D superconductors using the optimal energy diffusion algorithm.

Findings

No evidence of first-order transition in large systems

Supports KTHNY scenario of continuous melting

Melting proceeds via hexatic phase

Abstract

We have simulated the thermodynamics of vortices in a thin film of a type-II superconductor. We make the lowest Landau level approximation, and use quasi-periodic boundary conditions. Our work is consistent with the results of previous simulations where evidence was found for an apparent first order transition between the vortex liquid state and the vortex crystal state. We show, however, that these results are just an artifact of studying systems which are too small. There are substantial difficulties in simulating larger systems using traditional approaches. By means of the optimal energy diffusion algorithm we have been able to study systems containing up to about one thousand vortices, and for these larger systems the evidence for a first order transition disappears. By studying both crystalline and hexatic order, we show that the KTHNY scenario seems to apply, where melting from the crystal is first to the hexatic liquid state and next to the normal vortex liquid, in both cases via a continuous transition.