  Polarons are composite quasiparticles comprising electronic charge carriers
taken together with the alterations they induce in surrounding condensed
matter. Strong-coupling polarons form when electronic charge carriers become
self-trapped: bound within potential wells stabilized by carriers presence.
Distinctively, exciting these bound carriers generates broad absorption bands.
Strong-coupling polarons are slow and massive since moving them requires atomic
motion. Their transport differs qualitatively from that of conventional
electronic charge carriers. Large (strong-coupling) polarons move coherently
with mobilities that fall with rising temperature. These massive quasiparticles
very weak scatterings by phonons produce much lower room-temperature mobilities
than those permitted of conventional electronic charge carriers. Moreover, the
long scattering times associated with large polarons weak scattering relegates
their principal ac (Drude) transport to below phonon frequencies. Small
(strong-coupling) polarons move incoherently with even lower thermally assisted
mobilities. Strikingly, a magnetic field often deflects small polarons in the
opposite sense than it does conventional charge carriers, thereby producing
anomalously signed Hall Effects. In exceptional circumstances charge carriers
self-trap in pairs thereby forming large and small bipolarons. Some transport
features distinguish them from polarons. Interference between carrier-induced
atomic displacement patterns produce attractive interactions between
like-charged polarons and short-range repulsive interactions between oppositely
charged polarons.
