Anisotropy of the low-field critical point of the melting line of twinnedYBa2Cu3O7−δsingle crystals

Abstract

We demonstrate that the anisotropy of the low-field critical point ${(H}_{\mathrm{lcp}})$ of the vortex lattice melting line in a twinned ${\mathrm{YBa}}_{2}{\mathrm{Cu}}_{3}{\mathrm{O}}_{7\ensuremath{-}\ensuremath{\delta}}$ single crystal follows the intrinsic anisotropic behavior of the melting line when the disordered phase is originated by point defects. When the vortices are pinned at twin boundaries an upwards shift of ${H}_{\mathrm{lcp}}$ is observed. The contribution of twin boundary pinning has been found to match with the change in the vortex elastic energy barriers thus supporting the identification of the low-field phase as a disordered solid vortex phase where vortex pinning overcomes the vortex-vortex interaction.