Polarons in condensed matter

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.