Electron Interaction in Solids. The Nature of the Elementary Excitations

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Abstract
Possible elementary excitations in solids are studied with the aid of the general theoretical approach developed in the preceding papers of this series. Particular attention is paid to the basic theoretical justification for the individual-particle-like elementary excitations ("effective" electrons). It is concluded that good qualitative arguments may now be given for the existence of effective electrons in solids, but that a detailed quantitative deduction has yet to be made. The presence of an energy gap is shown to be a necessary condition for the existence of strong spatial correlations between minority carriers in solids (excitons, conduction electron plasmons in semiconductors, etc.) and the nature of such correlated minority electron excitations is discussed. The plasmon spectrum of various solids is discussed and compared with experiment.