INFRARED-ABSORPTION IN GE-76 AND NATURAL GE CRYSTALS - EFFECTS OF ISOTOPIC DISORDER ON Q-NOT-EQUAL-0 PHONONS

Self-energy effects (frequency shifts and line broadenings) due to isotopic disorder on the off-zone-center (q not-equal 0) phonon spectra in natural Ge have been investigated by comparing infrared transmission spectra of an isotopically enriched germanium crystal (86% Ge-76) and a natural Ge crystal at temperatures of 25, 100, and 300 K. Because selection rules only allow for the observation of two-phonon (and higher-order) absorption in Ge, q not-equal 0 phonons at Brillouin-zone points with high two-phonon density of states were investigated. Besides the trivial dependence of the phonon energies on the average isotopic mass mBAR (proportional to mBAR-1/2), isotopic disorder leads to additional energy shifts and line broadenings in natural Ge. The observed self-energy effects depend strongly on the phonon energy and are only significant for the transverse optical vibrations at the zone boundary (L, K, W, and X critical points), in agreement with coherent-potential-approximation calculations.