Electron Momentum Densities in Metals and Alloys

Abstract

Among the methods available for the study of the electronic structure of solids, the investigation of the electron momentum density with the aid of low-energy positron annihilation takes a place of increasing importance. During the 35 years that have elapsed since the first observation of the angular correlation between the two annihilation quanta by BERINGER and MONTGOMERY [2.1] the positron annihilation technique has developed into a method capable of yielding relevant information about various aspects of the electronic structure such as Fermi surface dimensions and wave functions. So far, the technique has been applied mainly to pure metals which have served as a practicing ground. For metals, however, the dimensions of the Fermi surface may be obtained by other methods such as the de Haas-van Alphen (dHvA) effect, the magnetoresistance effect, the rf size effect, etc., that are often capable of a greater precision than positron annihilation. Lately, nondilute disordered alloys and high-temperature phases of ordered materials have been gaining interest. In these systems excessive electron scattering due to the short mean free path of the electrons precludes the use of the above-mentioned methods at solute concentrations ≧1%. At higher concentrations, other methods like the measurement of Kohn anomalies in X-ray diffuse scattering [2.2] or inelastic neutron scattering [2.3], and the Faraday effect [2.4,5] can still be used for investigations of the Fermi surface geometry. The Compton effect [2.6] is able, in principle, to provide direct information about the entire electron momentum distribution, but cannot compete in resolution with positron annihilation.