Implantation, damage, and regrowth of high T_c superconductors

We have observed superconductivity in thin films of (La_1-xSr_x)₂CuO_y which are fabricated by implanting evaporated La/Cu multilayer films with Sr and annealing in oxygen. The films are insulating and partially transparent as-implanted, but they darken and become conducting at annealing temperatures as low as 500°C. The resistive behavior is very sensitive to the annealing conditions and the superconducting layer appears to be buried beneath the surface. Similarly, implantation of F, O, and Ne into single crystals and high quality (χ_min< 30%) thin films of YBa₂Cu₃O_7-δ creates a buried damage layer and renders the films insulating. From Rutherford backscattering and channeling analysis, we find that some regrowth of the damaged layer does occur on subsequent annealing. Studies of the transport properties of these films as a function of MeV-ion-beam-induced damage show that the critical current can be controllably reduced at low fluences. The metal-to-insulator transition, which occurs at higher fluences, seems to result from a reduction in carrier mobility rather than carrier freeze-out.