Abstract
Abstract It was Landau (1933) and, later, Pekar (1946, 1949) who speculated that an electron moving within a polar crystal should experience self-trapping due to strongly interacting with the polarization fields arising from the lattice perturbations created by the electron. These speculations later led to the concept of the polaron as a quasi particle, identified as an electron, dressed in its own polarization cloud and moving within the polar crystal. Here we shall not dwell on the polaron problem (Devreese 1972) since our main concern will be with the weak coupling (perturbative) regime between the electron and the lattice polarization fields. In ionic or partially ionic lattices, electric polarization is largely associated with anion-cation pairs whose relative motion characterizes the optical modes of vibrations. The quanta corresponding to the optical modes, also called optical phonons, have relatively large energies compared with the acoustic modes and so the scattering process in which the electron and the lattice exchange a longitudinal optical (LO) phonon involve large energy changes. In particular, electron energy loss is much more efficient when effected by emission of polar optical phonons. This fact makes the electron-LO phonon interaction an important scattering mechanism for charge carriers at room temperature and for most of the polar semiconductor crystals of interest (Ridley 1993a).
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