Nonlocal effects in the superconductor (YNi2B2C)-B-11

Flux lines repel each other, thus a hexagonal lattice which maximizes the d istance between them is generally the lowest energy configuration. In the r are-earth nickel borocarbide superconductor YNi2B2C, square lattices of flu x 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 lattic es energetically favored over the more prosaic hexagonal ordering. The depe ndence of the penetration depth within the basal plane is discussed. (C) 19 99 Elsevier Science Ltd. All rights reserved.