The thermal conductivity of borocarbide superconductor $\mathrm{LuNi}{}_2{\mathrm{B}}_2\mathrm{C}$ was measured down to 70 mK $(T_c/200\mathrm{})$ in a magnetic field perpendicular to the heat current from $H=0\mathrm{}$ to above $H_{c2\mathrm{}}=7\mathrm{T}$. As soon as vortices enter the sample, the conduction at $T\to 0$ grows rapidly, showing unambiguously that delocalized quasiparticles are present at the lowest energies. The field dependence is very similar to that of $\mathrm{UPt}{}_3$, 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 $\mathrm{LuNi}{}_2{\mathrm{B}}_2\mathrm{C}$, possibly with nodes.

• Received 14 May 2001

DOI:https://doi.org/10.1103/PhysRevLett.87.237001