Photoemission study of Ni borocarbides: SuperconductinC and nonsuperconductinC

We have studied the electronic structure of Ni borocarbides by means of photoemission and inverse-photoemission spectroscopy. The core-level and valence-band spectra of superconducting (Formula presented)(Formula presented)C and nonsuperconducting (Formula presented)(Formula presented)C are presented and are compared with band-structure calculations. The core-level spectra well reflect their highly covalent bonding character. The Ni core-level spectra show weak but distinct satellites due to two-hole bound states, indicating significant electron correlation in both compounds. Although the gross electronic structure of both compounds is in agreement with the band-structure calculations except for the two-hole bound-state satellites, spectra near the Fermi level ((Formula presented)) are quite different from those predicted by the calculations. That is, high-resolution photoemission spectra do not show a peak at (Formula presented) in (Formula presented)(Formula presented)C and that at ∼0.1 eV below (Formula presented) in (Formula presented)(Formula presented)C, which have been predicted by the calculations, indicating that electron correlation and/or electron-phonon interaction may play a significant role in the low-energy excitations in the Ni borocarbides. A similar behavior in the spectra of A15-type superconductors is also pointed out.