Present Trends in Collective Excitations in Solids

Abstract

Excitations of solids are in general identified as collective because of the many-body interactions. Single particle descriptions are therefore approximations, and well-known quasi-particle excitations such as phonons, excitons and plasmons are themselves approximations. The exact many-body excitations are designated “collectons.” The formulation is further generalized to include probes, sources and applied stresses in interpreting phenomena. Examples include the polariton which results from coupling of the electromagnetic probe with transverse polarization in solids, atomic beams coupled to surface phonons, and solid bodies coupled to high pressure apparatus. General trends in research on collective excitation are reviewed, including extremal conditions for experiments, applied research and sophisticated formulations such as quantum field analyses. Finally, trends specific to particular classes of collective excitations are reviewed.

Supported in part by a grant from the Army Research Office and in part by a Humboldt Prize (Senior U.S. Scientist Award).