Fourfold basal plane anisotropy of the nonlocal magnetization of YNi2B2C

Studies of single crystal YNi2B2C have revealed a fourfold anisotropy of th e equilibrium maenetization in the square crystallographic basal plane. Thi s pi/2 periodicity occurs deep in the superconductive mixed state. In this crystal symmetry, an ordinary superconductive mass anisotropy (as in usual London theory) allows only a constant, isotropic response. In contrast, the experimental results are well described by generalized London theory incor porating nonlocal electrodynamics, as needed for this clean, intermediate-k appa superconductor.