Interband optical properties and electronic structure ofα-phase disordered copper-aluminum alloys

Interband optical absorption spectra of $\alpha$-phase Cu-Al concentrations in the range 0-10 at.% have been obtained from near-normal-incidence reflectivity measurements in the spectral region 1-6 eV and Kramers-Kronig analysis. The development of the spectra in the disordered alloys are interpreted using a sinking-conduction-band model. The experimentally obtained sinking rate of 0.1 eV per at.% Al indicates an increase of the Fermi-surface dimensions consistent with available positron-annihilation data. At the highest concentrations, where short-range order tends to form, drastic changes of the optical properties are noted which are indicative of interesting electronic as well as metallurgical effects. Numerical computations of the joint density of states for Cu bands and alloy-model bands have also been performed using the Hodges-Ehrenreich combined interpolation scheme to generate energy eigenvalues at a large number of points in the Brillouin zone and varying the input parameters to simulate the effects of alloying in the disordered alloy. The theoretical and experimental results are compared to obtain qualitative information about the relative strength of matrix elements and about the possible effects of alloying on the over-all width of the $d$ bands.