Roles of nano-sized pinning centers and vortex density on vortex melting

We have investigated vortex melting in superconductors using extensive molecular dynamic simulations on samples with periodic square arrays of nano-sized pinning centers. Several strengths and densities of the pinning centers were considered. The vortex filling factor, n was always kept larger than 1. A one step solid-liquid vortex transition was found for all cases. For fixed pinning densities and strengths, the melting temperature, T_m was found to decrease with increasing the magnetic field. We have found that the vortex-lattice to vortex-liquid melting line shifts up as the pinning strength or the pinning density was increased. The effect on the melting line was found to be more pronounced at small values of strength and density of pinning sites. Thus a direct numerical evidence for the significant role of both the strength and density of the nano-sized pinning centers on the position of the melting line was obtained.