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

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 YBa2Cu3O7−δ. The transition is not associated with vortex lattice melting, but separates two vortex liquid states characterized 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 YBa2Cu3O7−δ. 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. The consistency of our numerical results with various experimental features in YBa2Cu3O7−δ, including the dependence on anisotropy, and the temperature dependence of the structure factor at the Bragg peaks in neutron scattering experiments is demonstrated.

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