Evidence for strong electron-phonon coupling in MgCNi3

The title compound is investigated by specific heat measurements in the normal and superconducting states supplemented by upper critical field transport, susceptibility, and magnetization measurements. From a detailed analysis including also full potential electronic structure calculations for the Fermi surface sheets, Fermi velocities, and partial densities of states the presence of both strong electron-phonon interactions and considerable pair breaking has been revealed. The specific heat and the upper critical field data can be described to a first approximation by an effective single-band model close to the clean limit derived from a strongly coupled predominant hole subsystem with small Fermi velocities. However, in order to account also for Hall-conductivity and thermopower data in the literature, an effective general two-band model is proposed. This two-band model provides a flexible enough frame to describe consistently all available data within a scenario of phonon mediated $s$-wave superconductivity somewhat suppressed by a sizable electron-paramagnon or electron-electron Coulomb interaction. For quantitative details the relevance of soft phonons and of a Van Hove\char21{}type singularity in the electronic density of states near the Fermi energy is suggested.