Numerical studies of the phase diagram of layered type II superconductors in a magnetic field

TL;DR

This paper uses numerical simulations within the Lawrence-Doniach model to study the phase diagram of layered type II superconductors, revealing a first order transition between vortex liquid states that aligns with experimental observations in YBaCuO.

Contribution

It provides detailed numerical evidence of a first order phase transition in layered superconductors, clarifies its nature, and compares results with experimental data, including effects of disorder.

Findings

First order transition separates two vortex liquid states.

Transition line ends at a critical point at low fields.

Weak disorder does not eliminate the transition, but strong disorder does.

Abstract

We report on simulations of layered superconductors using the Lawrence-Doniach model in the framework of the lowest Landau level approximation. We find a first order phase transition with a $B(T)$ dependence which agrees very well with the experimental ``melting'' line in YBaCuO. The transition is not associated with vortex lattice melting, but separates two vortex liquid states characterised by different degrees of short-range crystalline order and different length scales of correlations between vortices in different layers. The transition line ends at a critical end-point at low fields. We find the magnetization discontinuity and the location of the lower critical magnetic field to be in good agreement with experiments in YBaCuO. Length scales of order parameter correlations parallel and perpendicular to the magnetic field increase exponentially as 1/T at low temperatures. The dominant relaxation time scales grow roughly exponentially with these correlation lengths. We find that the first order phase transition persists in the presence of weak random point disorder but can be suppressed entirely by strong disorder. No vortex glass or Bragg glass state is found in the presence of disorder. The consistency of our numerical results with various experimental features in YBaCuO, including the dependence on anisotropy, and the temperature dependence of the structure factor at the Bragg peaks in neutron scattering experiments is demonstrated.