Simulation of vortex matter two-step melting in an anisotropic superconductor with columnar defects

Columnar defects in high-temperature superconductors have been object of recent intense experimental and theoretical investigations. We report on the observation of a melting in two steps in a 3D vortex line system with randomly-placed columnar defects. We perform a Monte Carlo simulation using the Lawrence-Doniach model, with Bi2Sr2CaCu2O8 parameters, in a magnetic field applied perpendicular to the CuO2 planes and parallel to the columnar defects. The Bose glass phase observed at low temperatures melts, as the temperature increases, through two steps: first it depins to a distorted Abrikosov lattice, in which the vortex matter presents some degree of hexatic order; in the sequence, further increment in the temperature causes the fusion of this distorted lattice.