Acoustic phonon modes of free-standing rectangular wires

We theoretically investigate acoustic phonon modes of free-standing square and rectangular wires of cubic crystals, using a resonant ultrasonic method. The normal modes consist of a compressional made termed the dilatational mode and non-compressional modes referred to as torsional, shear and flexural modes. The dispersion relations of the dilatational mode are obtained for square and rectangular GaAs wires, taking account of elastic anisotropy of the material, showing non-linear dispersion relations with subband structure. The change of the dispersion relations due to the difference in the cross-sectional shape is investigated. We also examine the displacement of these modes and show the existence of acoustic edge modes. (C) 1997 Academic Press Limited.