Angle-resolved photoemission study of (100), (110), and (111) surfaces of ${\mathrm{Cu}}_{0.9}$${\mathrm{Al}}_{0.1}$: Bulk and surface electronic structure of the alloy

We present angle-resolved photoemission spectra from the low-index faces of $\mathrm{Cu}\mathrm{Al}$ single crystals using 16.85- and 21.22-eV radiation, together with the computed bulk electronic structure of the alloy within the framework of the muffin-tin coherent-potential approximation (CPA). The $d$-band complex of Cu is found to suffer shifts of less than 0.1 eV on alloying, whereas the states of $s-p$ symmetry are lowered in energy by as much as on the order of 1 eV. The dispersion curve of the uppermost valence band is measured and with the extrapolation of this curve to the Fermi energy ($E_F$), the Fermi-surface radii along two directions in the (100) mirror plane are deduced in Cu and $\mathrm{Cu}\mathrm{Al}$. Of the known surface states in Cu, the (100) Tamm state persists in the alloy but the (111) Tamm state appears only as a weak shoulder in the alloy spectra. The Shockley-state bands pérsist on the (110) as well as the (111) alloy face and are estimated to accommodate approximately 0.03 electrons/atom more in the alloy compared to Cu. The Shockley states are lowered (in relation to $E_F$) by 0.3-0.4 eV as a result of alloying but the Tamm states are lowered by less than 0.1 eV. The CPA computations are found to be in an overall quantitative accord with the bulk measurements and these calculations also lead to a qualitative understanding of some of the experimental results concerning surface states in the alloy.