Nonlocal effects in the superconductor YNi 211B 2C

Abstract

Flux lines repel each other, thus a hexagonal lattice which maximizes the distance between them is generally the lowest energy configuration. In the rare-earth nickel borocarbide superconductor YNi 2B 2C, square lattices of flux lines have been observed when the field is parallel to the c-axis. Using the c-axis penetration depth as a gauge, we find that the cross-section of an individual flux line has a four-fold symmetry, which makes square lattices energetically favored over the more prosaic hexagonal ordering. The dependence of the penetration depth within the basal plane is discussed.