The Influence of Electron Transfer on T_c in Superconductors

Increase of T_c in Al_1？x(SiO₂)_x cermets with increasing x is caused by electron transfer from the Al grains to the SiO₂ phase occupying surface states, expressed by $T_c/T_{c,\max }=1？\gamma ？n^2$ (*), where n is the electron density in the Al phase and $\gamma$ a characteristic parameter. Decrease of T_c in Pb-Cu-sandwiches is attributed to the electron transfer from the Cu film to the Pb film. $\gamma$ and $T_{c,\max }$ in equation (*) stands for the influence of the electron-phonon interaction and $n^2$ for the influence of the electron-electron Coulomb repulsion on T_c. The result that equation (*) holds for both hole-doped cuprate high-temperature superconductivity (HTSC) and Al_1？x(SiO₂)_x cermets is an important indication that common mechanisms underlie HTSC and classical superconductors. The difference between the two is that in HTSC, electron transfer occurs between different electronic bands, but in Al_1？x(SiO₂)_x cermets between different phases.