Electron density of states in the borocarbide intermetallic superconductors

We present heat-capacity measurements in a magnetic field up to 7.7 T on the recently discovered borocarbide intermetallic superconductors ${\mathrm{LuNi}}_2$ ${\mathrm{B}}_2$C and ${\mathrm{LaPt}}_{1.7}$ ${\mathrm{Au}}_{0.3}$ ${\mathrm{B}}_2$C. For ${\mathrm{LuNi}}_2$ ${\mathrm{B}}_2$C with ${\mathit{T}}_{\mathit{c}}$=16.5 K, we find a large density of states at the Fermi level, γ∼19(2) mJ/mole ${\mathrm{K}}^2$, and an effective Debye temperature ${\mathrm{\Theta }}_{\mathit{D}}$ of 345 K. In comparison, ${\mathrm{LaPt}}_{1.7}$ ${\mathrm{Au}}_{0.3}$ ${\mathrm{B}}_2$C, with ${\mathit{T}}_{\mathit{c}}$=10.2 K, has a γ of 7.5(1.5) mJ/mole ${\mathrm{K}}^2$ and a smaller Debye temperature ${\mathrm{\Theta }}_{\mathit{D}}$ of 220 K. Through comparison with the specific-heat anomaly at ${\mathit{T}}_{\mathit{c}}$, ΔC/γ${\mathit{T}}_{\mathit{c}}$, the magnetic susceptibility, and the calculated band structure, these compounds are found to be in the weak-to-intermediate coupling limit with λ≊0.5–1.0. Their position on the γ-${\mathit{T}}_{\mathit{c}}$ plot indicates dominant phonon-mediated electron pairing.