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 LuNi2B2C and LaPt1.7Au0.3B2C. For LuNi2B2C with Tc=16.5 K, we find a large density of states at the Fermi level, γ∼19(2) mJ/mole K2, and an effective Debye temperature ΘD of 345 K. In comparison, LaPt1.7Au0.3B2C, with Tc=10.2 K, has a γ of 7.5(1.5) mJ/mole K2 and a smaller Debye temperature ΘD of 220 K. Through comparison with the specific-heat anomaly at Tc, ΔC/γTc, the magnetic susceptibility, and the calculated band structure, these compounds are found to be in the weak-to-intermediate coupling limit with λ0.5-1.0. Their position on the γ-Tc plot indicates dominant phonon-mediated electron pairing.