Critical fields and specific heat of ${\mathrm{LuNi}}_2{\mathrm{B}}_2\mathrm{C}$

We have measured the upper critical magnetic field, lower critical magnetic field, and low-temperature specific heat of ${\mathrm{LuNi}}_2{\mathrm{B}}_2\mathrm{C}.$ Our upper critical-field measurements above $\sim 2\mathrm{K}$ are consistent with earlier measurements by other groups. The upper critical field exhibits a finite slope as $\overrightarrow{T}0,$ and we discuss possible origins of this very unusual behavior. We observe a linear temperature dependence for the lower critical field of our single-crystal sample, a temperature dependence that disagrees with earlier measurements on polycrystals and suggests that the lower critical field may be anisotropic. The Sommerfeld coefficient increases with magnetic field as $H^{\epsilon }$ with $\epsilon =0.63\mathrm{}\pm 0.12$ in qualitative agreement with earlier work. The temperature dependence of the zero-field electronic specific heat is exponential well below $T_c.$ We discuss our results in light of recent proposals of both anisotropic s-wave and d-wave superconductivity for ${\mathrm{LuNi}}_2{\mathrm{B}}_2\mathrm{C}.$ Our data seem most consistent with strong-coupling superconductivity and a weakly anisotropic s-wave energy gap.