The superconducting energy gap and vortex lattice structure in LuNi₂B₂C

We present Scanning Tunneling Microscope (STM) observations of the superconducting energy gap and vortex lattice (VL) structure of the (001) face of a LuNi₂B₂C single crystal. In zero field, the conductance versus voltage characteristics consistently show a peaked structure that can be fitted with a broadened BCS like density of states, giving a gap value of 2.2 meV and 2Δ/κT_c = 3.2 ± 0.1, implying weak coupling superconductivity. In applied magnetic fields of 0.375 T and 1.5 T, we make the first STM observation of a square VL, with its unit cell rotated 45° with respect to the crystallographic unit cell. The VL structure is analyzed with Ginzburg-Landau theory, including an additional free energy term with four-fold symmetry arising from the tetragonal crystal structure. The square VL is found to be stable if the four-fold free energy term is large enough to cause the ratio of the in-plane upper critical fields H_c2 (45°) /H_c2 (0) to be less than 0.97. Measurements of the in-plane upper critical field anisotropy by SQUID magnetometry show a ratio of 0.92 at 4.2 K, consistent with the Ginzburg-Landau analysis.