The effect of an external electric field on the superconducting temperature ${\mathit{T}}_{\mathit{c}}$ of ${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_{6+\mathit{x}}$ thin films is studied by the cluster-variation method. We present a model in which the field effect on ${\mathit{T}}_{\mathit{c}}$ is related to the oxygen ordering. Electric dipole moments of oxygen occur due to charge transfer from the O-Cu-O chains in the ${\mathrm{CuO}}_{\mathit{x}}$ layers to the ${\mathrm{CuO}}_2$ layers. The polarization in the external field is assumed to depend on the length of these chains. A notable enhancement of ${\mathit{T}}_{\mathrm{c}}$ is found at the border between the tetragonal (semiconducting) and orthorhombic (metallic) phases (0.3≲x≲0.4). It is also demonstrated that moderate external fields do not increase the length of the chains and do not affect ${\mathit{T}}_{\mathit{c}}$ at high values of the oxygen concentration x. © 1996 The American Physical Society.

• Received 8 September 1995

DOI:https://doi.org/10.1103/PhysRevB.53.425