Electric-field modulation of low electron density thin-film superconductors.

Past investigations of electric-field modulation of the normal- state and superconducting properties of thin films are reviewed and compared with recent work on amorphous-composite In/InO sub x thin films with electron carrier density as low as 10 sup 20 cm sup (-3). Electric charge induced in the In/InO sub x films by a capacitatively-coupled gate electrode has a particularly strong influence when the disorder (resistivity) is increased towards a critical value where superconductivity rapidly disappears. The experimentally-measured field-effect mobility, the unperturbed electron density, and the transition temperature are strongly dependent on oxide content, substrate material, and dielectric capping overlayers. The field-effect measurements confirm the importance of electronic states at the metal-dielectric interfaces and enable a description of "interface dominated superconductivity" in this and possibly other low electron density superconductors. Implications for the high T sub c oxides are also presented.