Vortex lattice transitions in borocarbides

Abstract

Nonlocal corrections to the London model are used to describe vortex lattices in ${\mathrm{LuNi}}_{2}$${\mathrm{B}}_{2}$C. Within this scheme in fields along the c axis, vortices should form a square lattice (rotated 45\ifmmode^\circ\else\textdegree\fi{} relative to the crystal axes afandfb) at large fields, should transform to a triangular one at a field ${\mathrm{H}}_{2}$, and to another triangular lattice at a still lower field ${\mathrm{H}}_{1}$. We argue that the transition at ${\mathrm{H}}_{2}$ is of second order, whereas at ${\mathrm{H}}_{1}$ it is of first order. In tilted fields, ${\mathrm{H}}_{2}$ is predicted to move to higher fields. Neutron-scattering data for ${\mathrm{ErNi}}_{2}$${\mathrm{B}}_{2}$C are discussed.