Positron-annihilation study of the electronic structure ofCu91Ge9

Abstract

A novel effect of alloying on the electron momentum density in a disordered a-Cu9iGe9 alloy is reported. The effect, which is the appearance of a bump in the derivative spectra at low momenta, is predicted by theory employing the coherent-potential approximation and is observed in two-dimensional angular correction experiments on the alloy. It reflects the presence of an s-p impurity band far below the Fermi level. Accurate experimental values for the Fermi-surface radii /cioo and k no and for the associated disorder-induced smearings in CuGe are also reported for the first time.

Modern Band Theory of Disordered Alloys: Basic Concepts Including a Discussion of Momentum Densities

“…17 and 18 show that very substantial changes occur in g(p) as Cu is alloyed with Ge or Pd. gip) in Cu is flat and featureless in the first BZ, but in CuGe it develops a pronounced peak and in CuPd a dip at p = 0 [38,40]. These effects contrast sharply with the case of CuNi alloys discussed in Fig.…”

“…In the limit of low Ge concentration (< 0.5 at.% Ge), the neck as well as the belly radii increase at roughly half the rigid-band rate for quadrivalent impurities, in accord with the dHvA measurements [36,37], By contrast, in the high concentration regime (> 5 at.% Ge), the rate of change of the various FS radii corresponds to an effective Ge valence of somewhat less than 4. More recently, [38] reports a high-resolution 2 D-ACAR investigation of the CuGe alloys, finding a good accord with KKR-CPA predictions of both the FS radii and their disorder-induced widths.…”

“…Ref. [38] in particular shows that a bump near p = 0 appears in the theoretical as well as the experimental 2 D-ACAR derivative spectra in CuGe. This bump is not present in Cu, and arises from the aforementioned peak in g(p) near p = 0 and constitutes a signature of the Gederived impurity band of s-p character (the lower A x band in the inset in Fig.…”

“…Contributions to g(p) in the alloy for E > 0 (dashed) and E < 0 (chain curve) are shown. The inset gives the complex band structure of Cu 90 Ge 10 (after [38]). g(p) at p = 0 arises from states around the valence band edge; in particular, the d-bands do not contribute to g(p) at p = 0.…”

“…g(p) at p = 0 arises from states around the valence band edge; in particular, the d-bands do not contribute to g(p) at p = 0. The electron states at the valence band edge, which lie some 10 eV below the Fermi energy £ F , are difficult to access experimentally; in photoemission, for example, these states possess The question of the extent to which the 2 D-ACAR spectra in CuGe and CuPd reflect the behavior of g {p) near p = 0 is considered in [38] and [40]. Ref.…”