Spin-polarized quasiparticle transport in cuprate superconductors

The effects of spin-polarized quasiparticle transport in superconducting YBa2Cu3O_{7-\delta} (YBCO) epitaxial films are investigated by means of current injection into perovskite ferromagnet-insulator-superconductor (F-I-S) heterostructures. These effects are compared with the injection of simple quasiparticles into control samples of perovskite non-magnetic metal-insulator-superconductor (N-I-S). Systematic studies of the critical current density as a function of the injection current density, temperature and the thickness of the superconductor reveal drastic differences between the F-I-S and N-I-S heterostructures, with strong suppression of the critical currents and a rapidly increasing characteristic transport length near the superconducting transition temperature only in the F-I-S samples. The temperature dependence of the injection efficiency in the F-I-S samples is also in sharp contrast to that in the N-I-S samples. The characteristic times estimated from our studies are suggestive of anisotropic spin relaxation processes, possibly with spin-orbit interaction dominating the c-axis spin transport and exchange interaction prevailing within the CuO2 planes. Several alternative scenarios attempted to account for the suppression of critical currents in F-I-S samples are also critically examined, and are found to be neither compatible with experimental data nor with the established theory of nonequilibrium superconductivity.