Phonon anharmonicity in Ge

The transverse acoustic phonons at the X point in Si, Ge and some III-V semiconductors (GaAs, InSb ...) exhibit a negative Gruneisen parameter and are responsible for the anomalous negative thermal expansion coefficients at low temperatures.
The IN20 spin-echo (TASSE) measurements on an isotopic crystal of ⁷⁴Ge have revealed their metastable character (line-width close to zero) below 100 K. This was confirmed by subsequent ab initio calculations: the phonons required by the only possible difference process of decay are not thermally populated at such low temperatures.

This behaviour is qualitatively different from that of optical phonons at the centre of the Brillouin zone, the only ones accessible to line-width measurements by Raman spectroscopy.