Strong electron-phonon coupling in the rare-earth carbide superconductor La2C3

Abstract

We present the results of a crystal structure determination using neutron powder diffraction as well as the superconducting properties of the rare-earth sesquicarbide La2C3 (T-c approximate to 13.4 K) by means of specific-heat and upper critical field measurements. From the detailed analysis of the specific heat and a comparison with ab initio electronic structure calculations, a quantitative estimate of the electron-phonon coupling strength and the logarithmic average phonon frequency is made. The electron-phonon coupling constant is determined to be lambda(ph)similar to 1.35. The electron-phonon coupling to low energy phonon modes is found to be the leading mechanism for superconductivity. Our results suggest that La2C3 is in the strong-coupling regime, and the relevant phonon modes are La related rather than C-C stretching modes. The upper critical field shows a clear enhancement with respect to the Werthamer-Helfand-Hohenberg prediction, consistent with the strong electron-phonon coupling. Possible effects on the superconducting properties due to the noncentrosymmetry of the crystal structure are discussed.