Nonlinear Composition Dependence of the Fermi-Surface Dimensions in Alpha-Phase Copper-Germanium Alloys

In considering the $\alpha -{\mathrm{CU}}_{100-x}{\mathrm{Ge}}_x$ alloys on the basis of a realistic muffin-tin Hamiltonian, the most notable feature of the electronic spectra is found to be the appearance of an impurity band below the host conduction band. In accord with relevant experiments, a Fermi surface is predicted to grow at approximately half the rigid-band rate for $x<\mathrm{}0.5\mathrm{}$ at.% Ge and essentially at the rigid-band rate for $x>\mathrm{}5\mathrm{}$ at.% Ge. This effect is related to a transition in the nature of states associated with the impurity band; these states are localized atomic $s$-like states for $x<~0.5$ and become delocalized in concentrated alloys.