Photoemission and inverse-photoemission study of superconductingYNi2B2C: Effects of electron-electron and electron-phonon interactions

We have studied the electronic structure of the superconductor ${\mathrm{YNi}}_2$ ${\mathrm{B}}_2$C by photoemission and inverse-photoemission spectroscopy. The spectra show a prominent Ni 3d band centered around 1.5 eV below the Fermi level (${\mathit{E}}_{\mathit{F}}$), whose top crosses ${\mathit{E}}_{\mathit{F}}$, and B, C 2sp-derived states at higher binding energies, generally consistent with band-structure calculations. However, the Ni 3d-derived conduction bands are narrower than calculated and are accompanied by a satellite, signaling electron-correlation effects within the d bands. The density-of-states peak at ${\mathit{E}}_{\mathit{F}}$ predicted by band-structure calculations is suppressed and its spectral weight is transferred away from ${\mathit{E}}_{\mathit{F}}$, presumably due to an electron-phonon interaction and/or electron correlation.