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Phonon mechanism in the most dilute superconductor: n-type SrTiO3

Superconductivity of doped SrTiO3 is proven to be a particular case of the broader concept of the non-adiabatic pairing mediated by phonons with frequency comparable or larger the Fermi energy. We argue that, for carrier concentrations exceeding that of the mobility edge, the superconductivity of doped SrTiO3 is mediated by interaction of electrons with several longitudinal (LO) optical polar phonons. The electronic spectrum of SrTiO3 consists at low temperatures of three conduction bands which are successively doped. Each band contributes to the Cooper instability and exhibits a superconducting gap in the energy spectrum. The theory presented below predicts maxima in dependence of Tc(n)-the transition temperature on n, the number of electrons owing to the following mechanism. Doping by electrons increases density of states at the Fermi surface and Tc initially grows up. At the same time, screening on the part of accumulating charges tends to reduce amplitude of the electrical fields inherent in LO phonon modes and at larger concentrations the matrix element of interaction between electrons and LO phonons decreases. The compromise between the two tendencies leads to maxima in the Tc(n)-dependence providing interpretation to one of the most intriguing experimental findings in Xiao Lin et al [Phys. Rev. Lett. 112, 207002 (2014)]. Having reached a maximum in the third band, the superconducting transition finally decreases, rounding out the Tc(n)-dome, the three maxima in Tc(n)with accompanying superconducting gaps emerging consecutively as electrons fill successive bands. This arises from attributes of the LO optical phonon pairing mechanism. More generally, the mechanism opens prospect of increasing temperature of the superconducting transition in transition-metals oxides and other polar crystals.