Abstract:
To study the interaction of acoustic waves with other excitations in a dielectric crystal it is necessary to have a consistent set of coupled dynamic equations for them. A particularly important consequence of those equations is the energy conservation law. A Lagrangian-based theory has been used to produce the most general energy conservation statement yet obtained in a dielectric. It includes acoustic waves, electromagnetic waves, spin waves, and optic modes of ionic, electronic, and excitonic origin. Further, the excitations can be arbitrarily nonlinear in any field or combination of fields, can interact with any multipole order of the bound charge density and its current density, can involve any derivative of a field, and can occur in crystals of any symmetry. It is applied to a soliton that is a mixed mode (a ``polariton'') of an acoustic wave and a ``soft'' optic mode at a temperature just above a ferroelastic phase transition. Such an interaction involves the acoustic coupling to the optic mode turning the quadratic potential energy of the latter negative and so needs a quartic energy to stabilize it. Allowed soliton velocities fill the gap of linear wave velocities produced by the polaritonic interaction. [Work supported by NSF.]