Thermal variation of phonon frequency and line width in Ge studied by TAS spin-echo

We report on the use of the spin-echo option of a thermal neutron beam thre e-axis spectrometer to investigate the anharmonic behaviour of a zone-bound ary 'transverse acoustic' phonon in germanium. The intrinsic line width at room temperature is inferred to be 110 (20) mu eV, while the phonon frequen cy appears to scale with the harmonic vibrational energy of the crystal rat her than follow the crystal volume variation as predicted by the quasiharmo nic approach. (C) 2001 Elsevier Science B.V. All rights reserved.