Crystal Dynamics of Gallium Arsenide

The dispersion relation for the normal modes of vibration propagating along the [ζ00], [ζζζ], and [0ζζ] directions in a single crystal of pure gallium arsenide at 296^∘{}K has been determined by neutron spectrometry. The frequencies (units 10¹² cps) of specific modes are (a) (0,0,0): TP 8.02±0.08, LO 8.55±0.20; (b) (1,0,0): TO 7.56±0.08, TA 2.36±0.015, LO 7.22±0.15, LA 6.80±0.06; (c) (0.5,0.5,0.5): TO 7.84±0.12, TA 1.86±0.02, LO 7.15±0.07, LA 6.26±0.10. The results are analysed in terms of the dipole approximation model. Although a fairly complex version of this model provides a satisfactory description of most of the data, certain significant discrepancies remain. These indicate that the neglect of quadrupole interactions is a serious weakness of the dipole approximation model.