Tuning of the electronic screening and electron-phonon coupling in doped SrTiO3 and WO3

The electronic structure and the electronic part of the electron-phonon cou pling lambda is calculated for doped SrTiO3 and WO3 systems in the perovski te structure. Dilute doping is treated by supercells and the virtual crysta l approximation. The long-range part of the coupling is found to be weakly screened for low doping, when the material is close to insulating. The low- q part of lambda becomes large despite the low density of states. This is c onsistent with the appearance of superconductivity for dilute doping.