Supersonic surface acoustic waves on the 001 and 110 surfaces of cubic crystals

Criteria are reported here for the existence of supersonic surface acoustic waves (SSAW) on the (001) and (110) surfaces of cubic crystals. These are the common crystal cuts for which SSAW have been observed experimentally using surface Brillouin scattering and other techniques. Two categories of SSAW are distinguished. Symmetry protected SSAW exist by virtue of being located in high symmetry crystallographic directions for which the coupling to the phase matched bulk wave, which would otherwise result in their attenuation, is suppressed by symmetry. Secluded SSAW occur in lower-symmetry directions, where the reason for the vanishing of their coupling to their phase matched bulk wave is less evident. The stability domain for the elastic constant ratios a=C11/C44 and b=C12/C44 is subdivided into a number of regions in which various symmetry protected and secluded SSAW exist. Some of the boundaries between these regions are expressible in analytical form, others have been established purely numerically.