Faraday Effect and Fermi Surfaces of the Silver-Gold Alloy System

Measurements of the polar-reflection Faraday effect (PRFE) have been made in the visible and near infrared regions of the spectrum on three solid solutions of gold and silver, of 22.5, 47.0, and 71.5 at.% gold concentration. The PRFE measures bulk properties, is quite sensitive to contact of the Fermi surface with the zone faces, and gives a quantitative estimate of the extent of such contact. Further, since measurements are made at optical frequencies, the PRFE is not subject to the usual limitation of long electron lifetime, and is therefore applicable to nondilute solid solutions. These measurements yield the first direct information on the Fermi-surface geometry in nondilute disordered alloys. Because both silver and gold are monovalent, have the same atomic size, and are miscible in all proportions, the changes in the Fermi surface can be related to changes in the effective lattice potential alone. The results indicate that Fermi surface contact area and shape vary linearly from pure silver to pure gold. This result confirms the assumption that such an alloy can be treated by averaging the potentials of the pure constituents, and indicates that the symmetries of the conduction-band wave functions at the center of the $〈111〉$ zone faces are the same for both gold and silver.