Ion migration in high-T-c superconductors and the nature of the conductingstate in d-wave superconductors

The available experimental data on the ion migration in superconductingY-12 3 and Bi-2223 films under the effect of high transport currents are summari zed. Irrespective of structural perfection, electrostimulated diffusion of heavy ions and oxygen gives rise to the formation of new, stable phases and partial degradation of superconductivity. A model taking into account the proximity of the superconducting, structural, and magnetic transitions in h igh-T-c materials is used to explain ion migration as the response of the s ystem of ions and coherent d electrons to the external electric field. The d-wave symmetry superconducting state is shown to possess combined, rotatio nal + translational invariance, rotational symmetry being due to the motion of electrons in the curl field of the ions, playing the role of topologica l defects.