Highly anisotropic gap function in borocarbide superconductor LuNi2B2C -: art. no. 237001

The thermal conductivity of borocarbide superconductor LuNi2B2C was measured down to 70 mK (T-c/200) in a magnetic field perpendicular to the heat current from H = 0 to above H-c2 = 7 T. As soon as vortices enter the sample, the conduction at T --> 0 grows rapidly, showing unambiguously that delocalized quasiparticles are present at the lowest energies. The field dependence is very similar to that Of UPt3, a heavy-fermion superconductor with a line of nodes in the gap, and very different from the exponential dependence characteristic of s-wave superconductors. This is strong evidence for a highly anisotropic gap function in LuNi2B2C, possibly with nodes.