THE CAUSE OF UNUSUAL CHARGING DEPENDENCE OF TC IN ELECTROSTATIC CHARGING EXPERIMENTS ON HIGH-TEMPERATURE SUPERCONDUCTING FILMS

The unexpected disagreement between the dependence of the superconducting transition temperature T(c) on the carrier density n obtained from oxygen doping experiments (the "universal linear relation" of Uemura) and that from charging experiments on YBaCuO films, is treated in terms of the electrical field effect theory of superconductors. The results show that the effect of charging is influenced by an oxygen distribution within the film, and must be large in high quality samples which have an increased oxygen content near the superconductor-insulator interface. A small residual nonstoichiometry or disorder near the surface suppresses the modulation of T(c). A conducting layer between superconducting film and insulator may change the sign of the charging effect. The experimental results analyzed show the possibility of a charging investigation of the T(c) dependence on the electronic properties of superconducting materials by scanning of the Fermi level within the energy bands of the screening layer, which may both elucidate the mechanism of high-temperature superconductivity and open the way to modem device applications.