Critical fields and specific heat of LuNi2B2C -: art. no. 134519

We have measured the upper critical magnetic field, lower critical magnetic field, and low-temperature specific heat of LuNi2B2C. Our upper critical-field measurements above similar to2 K are consistent with earlier measurements by other groups. The upper critical field exhibits a finite slope as 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 +/- 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 LuNi2B2C. Our data seem most consistent with strong-coupling superconductivity and a weakly anisotropic s-wave energy gap.