Two-dimensional superconductor-insulator transition in bulk single-crystal YBa2Cu3O6.38.

The deleterious effects of disorder are amplified in lower dimensions, where long-range coherence is, at best, only marginally stable. A case in point is the destruction of superconductivity in ultrathin granular' and continuous metal films. The disorder is parametrized by the film sheet resistance and the data suggest a zero-temperature threshold for quenching global superconducting coherence near h/4e =6.45 kO/&. Experimental variables for adjusting the disorder include film thickness for elemental metals ' and composites, ' stoichiometry or doping for compounds, including oxide superconductor single crystals, " and ion bombardment. The application of a magnetic field to a disordered superconducting film leads to a fundamentally different type of T =0 superconductor-insulator transition. ' Fieldinduced vortices Bose condense, resulting in a transition from a state with zero resistance and pinned vortices ("vortex glass" ) to a state with zero conductivity and localized Cooper pairs ("electron glass" ). The finitetemperature scaling predictions of the Fisher theory ' have been verified in amorphous-composite InO thin films, ' ' and values for universal critical exponents predicted, but not numerically determined by the theory were measured. We report here evidence that the field-induced two-dimensional superconductor-insulator transition persists in a highly anisotropic three-dimensional superconductor, namely oxygen-deficient YBa2Cu307 —s (8 =0.62). We find the same dynamical exponent describing the critical behavior of the sheet resistance in field as measured for a-lnO„, but with a very different scale for the transition. Our results on barely superconducting YBa2Cu307 —b crystals permit a test of the field-induced superconductor-insulator scenario in a regime not accessible to disordered films as well as helping elucidate the nature of the incipient superconducting state in relatively pristine, but essentially decoupled Cu02 planes. Decreasing the oxygen content of single-crystal YBazCu307 & preferentially creates vacancies in the chains, reducing the superconducting-transition temperature T, and increasing the anisotropy. ' Moreover, when thermally equilibrated and quenched from elevated temperatures Tq below 200 C, the vacancies disorder, lowering T, while keeping the total oxygen content of the crystal constant. ' By adjusting Tg, T,. can be precisely controlled. This unique property of YBa2Cu307 —& allowed us to fine tune T,. suSciently close to the superconductorinsulator boundary that a magnetic field could drive the transition. We started with a high-quality twinned single crystal of YBa2Cu30q b with a superconducting-transition temperature of 13 K and a 10%-90% width of 1 K as measured