Influence of an external electric field on high-T-c superconductivity

The surface superconductivity induced by applying an electric field perpendicular to the surface of a high-T-c superconductor covered by an insulator and a gate electrode is studied theoretically. Taking the idea into account that the change of the surface carrier density confined in a thin screening layer modulates the transport properties of superconductors, we develop a simple method based on the Chen and Yang model to study the T-c shift dependence on the applied voltage in the frameworks of a Thomas-Fermi approximation and BCS theory. In such a case a novel boundary condition is introduced to modify the G-L equations. Solving the modified one, the T-c shift derived by the present model is similar in qualitative behavior to the Shapiro result, but much simpler in form. In addition, it is shown to be proportional to the applied field (the square of applied field) for thin film (thick film) for a given material. Based on our simple results, the critical current density J(c) and nucleation field H-c3 as functions of the applied voltage are easily derived.