Elementary excitations, electronic correlation functions and sum rule in noble-metal chalcogenides

Noble-metal chalcogenides are known as both electronic and ionic conductors. Physics in superionic conductors is investigated on the basis of the idea of elementary excitations. First, the semiconducting properties of noble-metal chalcogenides are investigated by preparing the full Hamiltonian for conduction electrons and phonons. The influence of electron interactions on the longitudinal acoustic wave frequencies and the matrix element for the electron–phonon interaction are investigated. Three cases of ω >> kυF, ω < kυF and ω = 0 are investigated for polar semiconductors like noble-metal chalcogenides. Next, the structure factors, and the f-sum rule of conductivity are investigated in silver chalcogenides by making use of a continuum model. The structure factors See, SAe and SBe with which electrons are connected are expressed symmetrically in terms of the structure factors SAA, SBB and SAB of ions in the long-wavelength limit using the fluctuation–dissipation theorem and the Kramers–Kronig relation. The obtained conductivity satisfies the f-sum rule.