Effect of an electric field on oxygen ordering and superconducting temperature of YBa2Cu3O6+x thin films

The effect of an external electric field on the superconducting temperature T-c of YBa2Cu3O6+x thin films is studied by the cluster-variation method. We present a model in which the field effect on T-c is related to the oxygen ordering. Electric dipole moments of oxygen occur due to charge transfer from the O-Cu-O chains in the CuOx layers to the CuO2 layers. The polarization in the external field is assumed to depend on the length of these chains. A notable enhancement of T-c is found at the border between the tetragonal (semiconducting) and orthorhombic (metallic) phases (0.3 less than or similar to x less than or similar to 0.4). It is also demonstrated that moderate external fields do not increase the length of the chains and do not affect T-c at high values of the oxygen concentration x.