  A material which is an insulator entirely because of interaction effects is
called a correlated insulator. Examples are trans-polyacetylene and the cuprate
high temperature superconductors. Whereas doping of a band insulator results in
a shift of the chemical potential into the conduction or valence band, doping
of a correlated insulator produces fundamental changes in the electronic
density of states itself. We have found that a general feature of doping a
correlated insulator is the generation of topological defects; solitons in
one-dimension and anti-phase domain walls in higher dimensions. We review the
well known features of this process in polyacetylene, and describe the
experimental evidence that the analogous features are seen in the cuprate
superconductors. We also distinguish the case in which the doping-induced
features can be viewed as a Fermi surface instability, as in polyacetylene, and
the more usual case in which they are a consequence of a Coulomb frustrated
electronic tendency to phase separation.
  Paper presented at the Sixtieth Birthday Symposium for Alan J. Heeger,
January 20, 1996

Condensed Matter