Polarons and Excitonic Polarons

Abstract

An electron accompanied by optical phonons in polar crystals was originally called a polaron. As all atomic sites in a basis do not have the same charge in such crystals, the interaction of an electron with ions creates a polarized electric field which moves with it. A polarized field interacts with optical phonons which move with the electron. Earlier work in this area was developed by Mott and Gurney⁽¹⁾ and Fröhlich,⁽²⁾ who studied the motion of a slow electron in polar crystals. As will be shown later, the polaron state can arise from both optical and acoustic phonons and in polar and nonpolar crystals. It is therefore more appropriate to call electrons accompanied by optical phonons optical polarons and electrons accompanied by acoustic phonons acoustic polarons. Energetically the state of a polaron has lower energy than that of a free electron, and the effective mass of a polaron is larger than that of a free electron. Polarons can be formed with electrons in the ground state as well as in excited states. In the limit of strong coupling between an electron and phonons, the electron may get localized, and this is known as a self-trapped electron. The case of self-trapping will be addressed in the next chapter.