Phonons in isotopically disordered Ge

We report Raman- and neutron-scattering, infrared-transmission, and photoluminescence experiments in a series of isotopically enriched samples of Ge, including natural Ge and an alloy ${}_{}{}^{70}{}_{}{}^{}\mathrm{Ge}_{0.5}^{}{}_{}{}^{}$ ${}_{}{}^{76}{}_{}{}^{}\mathrm{Ge}_{0.5}^{}{}_{}{}^{}$. Emphasis is put on the study of disorder-induced effects on the phonon spectra. Results are compared to exhaustive theoretical calculations using the coherent-potential and self-consistent Born approximations. Reasonably good agreement is found between theory and experiment for the former. Data for the dependence of the energy of the ${\mathit{E}}_1$ interband electronic gap on isotopic mass are also presented.