Phase diagram of vortex matter in layered high-temperature superconductors with random point pinning

Abstract

We study the phase diagram of the superconducting vortex system in layered high-temperature superconductors in the presence of a magnetic field perpendicular to the layers and of random atomic-scale point pinning centers. We consider the highly anisotropic limit where the pancake vortices on different layer are coupled only by their electromagnetic interaction. The free energy of the vortex system is then represented as a Ramakrishnan-Yussouff free energy functional of the time-averaged vortex density. We numerically minimize this functional and examine the properties of the resulting phases. We find that, in the temperature- (T) pinning strength (s) plane at constant magnetic induction, the equilibrium phase at low T and s is a Bragg glass. As one increases s or T a first-order phase transition occurs to another phase that we characterize as a pinned vortex liquid. The weakly pinned vortex liquid obtained for high T and small s smoothly crosses over to the strongly pinned vortex liquid as T is decreased or s increased—we do not find evidence for the existence, in thermodynamic equilibrium, of a distinct vortex glass phase in the range of pinning parameters considered here. We present results for the density correlation functions, the density and defect distributions, and the local field distribution accessible via muon spin rotation experiments. These results are compared with those of existing theoretical, numerical, and experimental studies.

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