Phase Diagram of Vortices in High-T_c Superconductors with a Melting Line in the deep H_{c2} Region

TL;DR

This paper models the phase diagram of vortex states in high-temperature superconductors near the upper critical field, revealing complex phase transitions including first, second, and third-order lines using a variational approach.

Contribution

It introduces a variational method to analyze vortex phase transitions in the deep H_{c2} region, identifying multiple phase transition lines and their properties.

Findings

First-order transition between Bragg-glass and vortex-glass phases.

Almost vertical third-order glass transition line near critical temperature.

Vertical second-order transition lines crossing first-order lines.

Abstract

We use a simple elastic Hamiltonian for the vortex lattice in a weak impurity background which includes defects in the form of integer-valued fields to calculate the free energy of a vortex lattice in the deep H_{c2} region. The phase diagram in this regime is obtained by applying the variational approach of M{\'e}zard and Parisi developed for random manifolds. We find a first-order line between the Bragg-glass and vortex-glass phase as a continuation of the melting line. In the liquid phase, we obtain an almost vertical third-order glass transition line near the critical temperature in the H-T plane. Furthermore, we find an almost vertical second-order phase transition line in the Bragg-glass as well as the vortex-glass phases which crosses the first-order Bragg-glass, vortex-glass transition line. We calculate the jump of the temperature derivate of the induction field across this second-order line as well as the entropy and magnetic field jumps across the first-order line.