VORTEX LATTICE TRANSITIONS IN BOROCARBIDES

Nonlocal corrections to the London model are used to describe vortex l attices in LuNi2B2C. Within this scheme in fields along the c axis, vo rtices should form a square lattice (rotated 45 degrees relative to th e crystal axes a and b) at large fields, should transform to a triangu lar one at a field H-2, and to another triangular lattice at a still l ower field H-1. We argue that the transition at H-2 is of second order , whereas at H-1 it is of first order. In tilted fields, H-2 is predic ted to move to higher fields. Neutron-scattering data for ErNi2B2C are discussed.