Influence of nonlocal electrodynamics on the anisotropic vortex pinning in YNi2B2C

We have studied the pinning force density Fp of YNi2B2C superconductors for various field orientations. We observe anisotropies both between the c axis and the basal plane and within the plane that cannot be explained by the usual mass anisotropy. For magnetic field H parallel c, the reorientation structural transition in the vortex lattice due to nonlocality, which occurs at a field H1∼1 kOe, manifests itself as a kink in Fp(H). When H perpendicularc, Fp is much larger and has a quite different H dependence, indicating that other pinning mechanisms are present. In this case the signature of nonlocal effects is the presence of a fourfold periodicity of Fp within the basal plane