THE SLIDING MODE IN INCOMMENSURATE INTERGROWTH

COMPOUNDS. Sander van Smaalen, Lab. of Crystallography, University of Bayreuth, D-95440 Bayreuth, Germany, and D. Schmicker, Atomic Force Microscopes, Gneisenaustrasse 214, D-47057 Duisburg, Germany.

Phasons in incommensurately modulated structures and the sliding mode in incommensurate intergrowth compounds are zero-frequency modes of crystal vibrations, which can exist in addition to ordinary acoustic phonons. Direct experimental observation of these vibrations is possible by inelastic scattering techniques, like inelastic neutron scattering and Brillouin scattering, or by observation of the propagation of ultrasonic waves. All these techniques probe the crystal vibrations at different frequencies, and a qualitatively different behavior is expected pertaining to the sliding modes and phasons.

In the present contribution we give an overview of the experimental information about the sliding mode and phasons. A phenomenological theory is given of the long-wavelength vibrations in intergrowth crystals, within the framework of the theory of elasticity. A complete set of restrictions has been derived on the components of the extended tensor of elasticity for all (3+1)-dimensional superspace groups. The differences are discussed between the sliding mode, phasons, and acoustic phonons, and particular attention is given to pinning, damping, and the polarisation of these modes.