Dispersion relations for hot electrons

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Abstract
The analytical properties of the linear response functions, for perturbations of a system in a nonequilibrium steady state, are discussed with application to semiconductor hot-electron phenomena. The characteristics of the frequency dependence of the harmonic response coefficients are considered in terms of dispersion relations which are consequences of ‘‘causality,’’ and in terms of properties of the time-domain response functions. The possibility of materials with a real-part differential conductance which is negative in some range of frequencies, while remaining positive at zero frequency, is discussed.