Photoemission studies of the surface and bulk electronic structure of the Cu-Ni alloys

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We report new photoemission studies on single-crystal Cu-Ni alloys in the photon energy range $6\ensuremath{\le}h\ensuremath{\nu}\ensuremath{\le}21.2$ eV. The short electron escape length at 21.2 eV allows the electronic structure of the first few atomic layers (i.e., surface electronic structure) to be studied. Data are presented on the annealed and sputtered surfaces of the (100) oriented 50-at.%-Cu-50-at.%-Ni and 10-at.%-Cu-90-at.%-Ni bulk composition samples, as well as alloy surfaces of different compositions prepared by interdiffusing a sandwich structure of a thin layer of $\mathrm{Cu}(\ensuremath{\simeq}0.5 \ensuremath{\mu}\mathrm{m})$ on top of a single-crystal Ni (100) substrate. A first-order model density of states was deduced for alloy surfaces of arbitrary composition. It consists of two broad peaks 2 eV apart, corresponding to contributions from the Cu and Ni. The positions of maxima are independent of surface composition or condition, but the relative peak amplitude is a function of the surface composition. At lower photon energies, the escape length increases considerably, thus allowing the electronic structure of the inner atomic layers (i.e., bulk density of states) to be studied. In addition to the Ni $d$ peak at the Fermi level, a Ni derived peak at approximately 1 eV below ${E}_{F}$ was resolved for both the 50-at.%-Cu-50-at.%-Ni and 10-at.%-Cu-90-at.%-Ni samples. The deduced models of the bulk density of states for these two compositions are in accord with theories based on the coherent-potential approximation and the average $T$-matrix approximation. High-resolution data for the 10-at.%-Cu-90-at.%-Ni sample, in the energy range $10\ensuremath{\le}h\ensuremath{\nu}\ensuremath{\le}25$ eV, taken using synchrotron radiation, are also presented.