SPECIFIC-HEAT ANALYSIS OF RARE-EARTH TRANSITION-METAL BOROCARBIDES - AN ESTIMATION OF THE ELECTRON-PHONON COUPLING STRENGTH

We analyzed the superconducting and normal-state heat capacity of supe rconducting YNi2B2C, LuNi2B2C, LaPt1.5Au0.5B2C and the nonsuperconduct ing reference compounds LaNi2B2C2 YCo2B2. The deviations of the thermo dynamic ratios as Delta C/gamma T-c, gamma T-c(2)/H-c(2)(0), H-c'(T-c) /TcHc(0), and Delta(0)/k(B)T(c), from their BCS values give an estimat e for the strong-coupling parameter T-c/omega(in) which is in the rang e of 0.06-0.1 and indicates that the phonon mediated superconductivity can be classified in the moderately strong-coupling limit. From the n ormal-state specific heat between 2 and 300 K we constructed model pho non spectra to determine the moments of the phonon density of states F (omega) and calculated the electron-phonon enhancement factor lambda w ith the Alien and Dynes formula which yields lambda values in the rang e of 0.95-1.15 for the three superconducting compounds. The comparison of these lambda values with the electron mass enhancement derived fro m the ratio of the Sommerfeld parameter gamma and the calculated densi ty of states at the Fermi level, N(E(F)), shows that band structure ca lculations overestimate N(E(F)). Furthermore we determined the upper c ritical field H-c2(T) from specific-heat measurements up to 11 T and t he normalized Ginzburg-Landau parameter k(T) which both show BCS-type behavior for LaPt1.5Au0.5B2C, but a significant upward curvature for Y Ni2B2C and especially for LuNi2B2C.