THE SUPERCONDUCTING ENERGY-GAP AND VORTEX LATTICE STRUCTURE IN LUNI2B2C

We present Scanning Tunneling Microscope (STM) observations of the sup erconducting energy gap and vortex lattice (VL) structure of the (001) face of a LuNi2B2C single crystal. In zero held, the conductance vers us voltage characteristics consistently show a peaked structure that c an be fitted with a broadened BCS Like density of states, giving a gap value of 2.2 meV and 2 Delta/kT(c) = 3.2 +/- 0.1, implying weak coupl ing 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 degrees with respect to the crystallographic unit cell. Th e VL structure is analyzed with Ginzburg-Landau theory, including an a dditional free energy term with four-fold symmetry arising from the te tragonal 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 degrees)/H-c2 (0) to be less t han 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.