Electron-phonon coupling and superconductivity in SrTiO3

Doped STO is a superconductor with a maximal Tc rising from zero at zero doping to 400 mK around 0.005 electrons per Ti atom, and dropping to zero above 0.02 electrons per Ti atom. Substituting the heavier O-18 isotope for the natural O-16 in the samples results in a strong increase of Tc with an isotope coefficient some 20 times larger and of opposite sign as predicted by the BCS model. This effect was theoretically anticipated based on a model where the superconducting pairing in STO is mediated by the optical phonons which are associated to the para- to ferro-electric phase transition in in the undoped insulating compound, the so-called TO1 mode. Here we demonstrate from experimental data that the spectral weight of the TO1 mode is anomalously large, providing a strong indication for the so-called "charged phonon" effect predicted by MJ Rice. The experimentally observed spectral weight of mode TO1 is used to determine the nature and strength of its' coupling to the conduction electrons, and implications of these numbers are presented for the corresponding channel of superconducting pairing.