Breakdown of the virtual-crystal approximation inGe2x(GaAs)1−x

New information on the electronic structure of ${\mathrm{Ge}}_{2x}{\left(\mathrm{GaAs}\right)}_{1-x}$ is obtained by use of the coherent potential approximation (CPA) and x-ray photoelectron spectroscopy (XPS). Two extreme structural models are considered. In each case, the CPA valence-band density of states differs significantly from that of the virtual-crystal approximation (VCA) but is consistent with measured XPS spectra. Large deviations from VCA behavior are also observed in the band-gap region where the present CPA results strongly support the conclusions of previous recursion calculations concerning the sensitivity of the gap to short-range order.