•R. Heid, W. Reichardt und K.-P. Bohnen
Institut für Festkörperphysik, Forschungszentrum Karlsruhe, D-76021 Karlsruhe

Superconductivity in nickel borocarbides exhibits various unusual properties [1], which raises the question if electron-phonon coupling is the main source of pairing in this class of materials. This subject can be elucidated by modern first principles methods based on density-functional theory which provide a detailed microscopic picture of the electron-phonon interaction. We present calculations of the bandstructure, the phonon dispersion, and the electron-phonon coupling for superconducting YNi₂B₂C (T_c=15.5 K) and compare them with results for the non-superconducting reference compounds LaNi₂B₂C and Y(Ni_.75Co_.25)₂B₂C. The calculations reproduce a pronounced phonon anomaly in the vicinity of the wave vector (0.6,0,0) in a low frequency branch of D₄ symmetry that has been observed experimentally both in YNi₂B₂C and LuNi₂B₂C [2]. We find that the magnitude of the anomaly and of the electron-phonon coupling depend very sensitively on the chosen structural parameters (volume, c/a ratio). Another pronounced anomaly is predicted in an optic branch of the same symmetry with predominant B and C elongations. Although the low frequency phonon spectrum is strongly dominated by the vibrations of the heavy metal ions we find that B and C contribute more than 50% to the electron-phonon coupling constant l.