Critical fields and specific heat ofLuNi2B2C

Abstract

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 $\ensuremath{\sim}2\mathrm{K}$ are consistent with earlier measurements by other groups. The upper critical field exhibits a finite slope as $\stackrel{\ensuremath{\rightarrow}}{T}0,$ 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 ${H}^{\ensuremath{\epsilon}}$ with $\ensuremath{\epsilon}=0.63\ifmmode\pm\else\textpm\fi{}0.12$ 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.