A novel effect of alloying on the electron momentum density in a disordered α-${\mathrm{Cu}}_{91}$${\mathrm{Ge}}_9$ 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 correlation 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 ${\mathrm{k}}_{100}$ and ${\mathrm{k}}_{110}$ and for the associated disorder-induced smearings in CuGe are also reported for the first time.

• Received 30 April 1987

DOI:https://doi.org/10.1103/PhysRevLett.59.720