Photoemission and Optical Studies of Cu-Ni Alloys. II. Ni-Rich and Nearly Equiatomic Alloys

We report and discuss photoemission and optical reflectivity data for Cu-Ni alloys containing ≥ 38% Ni. Our results indicate that the filled densities of states for Ni-rich alloys (those containing ≤ 20% Cu) are little changed from that of pure Ni. Data for alloys containing as little as 39% Cu give evidence that the $\mathrm{Cu} d$ states remain about 2 eV below the Fermi energy, and it is suggested that the $\mathrm{Cu} d$ states are well below the Fermi energy for lower Cu concentrations as well, in agreement with the assumptions of the minimum-polarity model, proposed by Lang and Ehrenreich, for Ni-Cu alloys. The $\mathrm{Ni} d$-state density is modified significantly, in both width and shape, as the Ni content in Cu-Ni alloys increases. On the basis of the optical and photoemission data for pure Cu, pure Ni, and all alloys studied, it is suggested that structure in the optical transition strength $\omega \sigma$ near $h\nu =5.\mathrm{}0$ eV is due to transitions from deep hybridized $d$ states to states just above the Fermi level, these transitions occurring with enhanced matrix elements.