Flux Line Lattice Reorientation in the Borocarbide Superconductors with $\mathit{H}\parallel \mathit{a}$

Small angle neutron scattering studies of the flux line lattice in ${\mathrm{LuNi}}_2{\mathrm{B}}_2\mathrm{C}$ and ${\mathrm{ErNi}}_2{\mathrm{B}}_2\mathrm{C}$ induced by a field parallel to the $a$ axis reveal a first order flux line lattice reorientation transition. Below the transition the flux line lattice nearest neighbor direction is parallel to the $b$ axis, and above the transition it is parallel to the $c$ axis. This transition cannot be explained using nonlocal corrections to the London model. In addition, the anisotropy of the penetration depth $\lambda$ and the coherence length $\xi$ change at the transition.