Temperature dependence of the flux line lattice transition into square symmetry in superconducting LuNi2B2C

We have investigated the temperature dependence of the H parallel to c flux line lattice structural phase transition from square to hexagonal symmetry , in the tetragonal superconductor LuNi2B2C (T-c = 16.6 K). At temperatures below 10 K the transition onset field, H-2(T), is only weakly temperature dependent. Above 10 K, H-2(T) rises sharply, bending away from the upper cr itical field. This contradicts theoretical predictions of H-2(T) merging wi th the upper critical field and suggests that just below the H-c2(T) curve the flux line lattice might be hexagonal.