- m at . s up r - co n ] 2 5 Ju n 20 01 Influence of nonlocal electrodynamics on the anisotropic vortex pinning in Y Ni

We have studied the pinning force density Fp of Y N i2B2C superconductors for various field orien-tations. We observe anisotropies both between the c-axis and the basal plane and within the plane, that cannot be explained by usual mass anisotropy. For magnetic field H c, the reorientation structural transition in the vortex lattice due to nonlocality, which occurs at a field H1 ∼ 1kOe, manifests itself as a kink in Fp(H). When H⊥c, 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. After Kogan and co-workers 1 showed in 1996 that non-local electrodynamics effects are relevant in high-κ su-perconductors in a broad H-T domain, several works demonstrated that nonlocality give rise to unusual properties of the vortex matter. Those studies were done mainly on the family of compounds RNi 2 B 2 C , where These borocarbides are very well suited to study nonlocal effects due to their intermediate κ values (∼ 10-20) that result in a wide field range where the extended London description holds, their high transition temperature T c and the possibility to fabricate clean single crystals with large electronic mean free path. Deviations of the equilibrium magnetization from the local London prediction, 2 structural phase transitions separating a variety of exotic rhombic and square vortex lattices (FLL), 3–10 and fourfold anisotropies in the basal plane of these tetragonal materials 11,12 provide strong support for the nonlocal scenario. Although the consequences of nonlocality on the equilibrium properties of the superconducting vortex matter is by now convincingly established, very little is known about its effects on the nonequilibrium vortex response. For both Y N i 2 B 2 C and LuN i 2 B 2 C Eskildsen et al. 13 have shown that, for H c-axis and well above the field H 2 ∼ 1kOe marking a transition from a rhombic to a square vortex lattice, the pinning properties sharply disagree with the expectations of the Larkin-Ovchinnikov collective pinning theory for a triangular lattice. The authors associated the discrepancy to the different (and to large extent unknown) elastic properties of the square lattice. However, neither the irreversible response in other field orientations, where the lattice geometry is different, nor the influence of the lattice transitions themselves on vortex …