Four-fold anisotropy of the basal plane magnetization of YNi2B2C: nonlocaleffects

We present studies of the vortex state magnetization M of a single crystal of the nonmagnetic borocarbide superconductor YNi2B2C. With magnetic field H applied in the square basal plane of the tetragonal unit cell, M has a fo ur-fold anisotropy. This pi /2 periodicity occurs deep in the superconducti ve mixed state, with an unusual field dependence, for a range of temperatur es T < T-c. The observations are well explained by a generalized London mod el incorporating non-local electrodynamics, using parameters from complemen tary experiments.