## 4pPA2. A general energy conservation law in dielectrics.

### Session: Thursday Afternoon, December 5

### Time: 2:15

**Author: Donald F. Nelson**

**Location: WPI, Worcester, MA 01609**

**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.]

ASA 132nd meeting - Hawaii, December 1996