Electron-phonon (“e-ph”) interactions of second or higher order can for almost all cases of practical interest be considered as minor scattering-mechanisms for charge carriers in metals and semiconductors. It is quite obvious that any strong contribution of such processes would necessarily imply a failure of the adiabatic principle and therefore of the usually quite successful description of the dynamics of electrons and phonons in the frame-work of perturbation theory. On the other hand this perturbative approach in terms of free carriers and phonons in the initial and final states of a scattering event can only be justified for sufficiently weak e-ph-coupling. The well-known exception to this frequently encountered “weak-coupling” situation are strongly ionic substances, where each charge carrier continuously interacts with the polarization-field of the polar lattice modes; the corresponding quasi-particle, the polaron, can in the terminology of quantum field theory be considered as a bare electron, dressed by a cloud of virtual phonons. For not too strong e-ph-coupling, the lattice-distortion by the moving charge carrier can be reasonably well described by the usual e-ph-dynamics, and the average number of virtual phonons accompanying the carrier can be estimated, and has been found to reach values of 3 or more for the most interesting case of the alkali-halides.
KeywordsDeformation Potential Phonon Process Move Charge Carrier Wave Funetion Maximum Energy Loss
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