We present Raman spectra of natural Ge, Ge70, and Ge0.570 Ge0.576 in the TO phonon region. The spectrum of the isotopically pure Ge70 shows only the Raman-active phonon with k=0 (), as required by k conservation. The two isotopically disordered crystals, however, show spectra at the low energy side of the phonon which correspond to isotopic disorder-induced scattering by TO phonons with k0 (band modes). The line shape and the strength of the observed spectra agree with calculations based on the coherent potential approximation and an ansatz for the Raman polarizability. The novel Ge0.570 Ge0.576 thus proves to be an ideal crystal to investigate mass disorder effects.
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