Polarons

A conduction electron (or hole) together with its self-induced polarisation in a polar semiconductor or an ionic crystal forms a quasi-particle, which is called a polaron. The polaron concept is of interest, not only because it describes the particular physical properties of charge carriers in polarizable solids but also because it constitutes an interesting field theoretical model of a fermion interacting with a scalar boson field. The early work on polarons was concerned with general theoretical formulations and approximations, which now constitute the standard polaron theory, and with experiments on cyclotron resonance and transport properties. Because of the more recent interest in the two-dimensional electron gas, the study of the polaron in two dimensions became important. Again cyclotron resonance, and therefore the behaviour of polarons in magnetic fields, was a key issue. When two electrons (or two holes) interact with each other simultaneously through the Coulomb force and via the electron-phonon-electron interaction, either two independent polarons can occur or a bound state of two polarons - a bipolaron - can arise. Bipolarons have been considered to possibly play a role in high-Tc superconductivity. The polaron concept has been extended to several systems where one or many fermions interact with a bath of bosons, e. g., small polaron, piezopolaron, polaronic exciton, spin - or magnetic - polaron, "ripplonic polaron", "plasmaron", "hydrated polarons" etc.