Comparative study of the electronic structure of noble-metal-noble-metal alloys by optical spectroscopy

Transmittance and reflectance measurements have been performed between 0.5 and 6 eV on thin films of noble-metal-noble-metal alloys over the whole concentration range. Optical absorption clearly shows that these alloys are of two types: On the one hand, in Au-Ag and Au-Cu alloys, the absorption spectrum moves and modifies its shape gradually from a constituent to the other one; on the other hand, in Ag-Cu alloys, the absorption spectrum has essentially the shape of the matrix spectrum, with a supplementary absorption band due to impurities. This suggests that in the first case, the $d$ bands of the constituents strongly overlap, while in the second case they overlap only slightly. All these results will be discussed in the framework of existing theories: the coherent-potential approximation and the virtual-bound-state theory. Ordering effects in Au-Cu and clustering effects in Ag-Cu are also investigated.