High-resolution photoemission study of the surface and bulk electronic structure of copper-nickel alloys

Energy-distribution curves (EDC's) of photoelectrons emitted normal to the surface of single crystals of ${\mathrm{Cu}}_{0.03}$ ${\mathrm{Ni}}_{0.97}$, ${\mathrm{Cu}}_{0.16}$ ${\mathrm{Ni}}_{0.84}$, and ${\mathrm{Cu}}_{0.24}$ ${\mathrm{Ni}}_{0.76}$ have been measured at high energy and angle resolution. Incident photon energies of 11.83, 16.85, and 21.22 eV were used. The annealed alloy samples show a surface segregation of Cu, which is essentially restricted to the outermost atomic layer. By bombarding the samples with low-energy (600-eV) Ar ions, the Cu-enriched surface layer can be removed and the Cu surface concentration is then not much different from that of the bulk. By measuring the EDC's of the annealed samples, properties of the surface electronic structure of the Cu-rich surface layer are obtained, which in the Cu $d$-electron region is dependent on the single-crystal face and the Cu concentration. The EDC's of the bombarded surfaces are believed to represent mainly the bulk electronic properties of the alloys. In this case the Ni-derived states have similarities to those of pure Ni, whereas the Cu-derived levels show distinct differences in shape and position compared to the results of pure Cu. Both effects are qualitively understood in the light of recent calculations of the complex band structure of Cu-Ni alloys.