Flux line lattice symmetries in the borocarbide superconductor LuNi2B2C

We compare the results of small angle neutron scattering on the flux line lattice (FLL) obtained in the borocarbide superconductor LuNi2B2C with the applied field along the c- and a-axes. For H parallel toc the temperature dependence of the FLL structural phase transition from square to hexagonal symmetry was investigated. Above 10 K the transition onset field, H-2(T), rises sharply, bending away from Hc(2) (T) in contradiction to theoretical predictions of the two merging. For H parallel toa a first order FLL reorientation transition is observed at H-tr = 3-3.5 kOe. Below H-tr the FLL nearest neighbor direction is parallel to the b-axis, and above H-tr to the c-axis. This transition cannot be explained using nonlocal corrections to the London model.