Temperature Dependence of the Flux Line Lattice Transition into Square Symmetry in Superconducting ${\mathrm{LuNi}}_2{B}_{2}C$

We have investigated the temperature dependence of the $\mathbf{H}\parallel \mathbf{c}$ flux line lattice structural phase transition from square to hexagonal symmetry, in the tetragonal superconductor ${\mathrm{LuNi}}_2{\mathrm{B}}_2\mathrm{C}$ ( $T_{\mathrm{c}}=16.6\mathrm{}\mathrm{K}$). At temperatures below 10 K the transition onset field, $H_2\left(T\right)$, is only weakly temperature dependent. Above 10 K, $H_2\left(T\right)$ rises sharply, bending away from the upper critical field. This contradicts theoretical predictions of $H_2\left(T\right)$ merging with the upper critical field and suggests that just below the $H_{\mathrm{c}2}\left(T\right)$ curve the flux line lattice might be hexagonal.