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/kTc = 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 Hc2 (45°)/Hc2 (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.