The effects of spin-polarized quasiparticle transport in superconducting YBa 2 Cu 3 O 7Ϫ␦ ͑YBCO͒ epitaxial films are investigated by means of current injection into perovskite ferromagnet-insulator-superconductor ͑F-I-S͒ heterostructures. These effects are compared with the injection of simple quasiparticles into control samples of perovskite nonmagnetic metal-insulator-superconductor ͑N-I-S͒. Systematic studies of the critical current density (J c ) as a function of the injection current density (J in j ), temperature (T), and the thickness ͑d͒ of the superconductor reveal drastic differences between the F-I-S and N-I-S heterostructures, with strong suppression of J c and a rapidly increasing characteristic transport length near the superconducting transition temperature T c only in the F-I-S samples. The temperature dependence of the efficiency (ϵ⌬J c /J in j ; ⌬J c : the suppression of critical current due to finite J in j ͒ in the F-I-S samples is also in sharp contrast to that in the N-I-S samples, suggesting significant redistribution of quasiparticles in F-I-S due to the longer lifetime of spin-polarized quasiparticles. Application of conventional theory for nonequilibrium superconductivity to these data further reveal that a substantial chemical potential shift * in F-I-S samples must be invoked to account for the experimental observation, whereas no discernible chemical potential shift exists in the N-I-S samples, suggesting strong effects of spin-polarized quasiparticles on cuprate superconductivity. 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 CuO 2 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.

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The local density of states (LDOS) around two nonmagnetic impurities which are located at different sites is studied within the two-dimensional t-J-U model. The order parameters are determined in a self-consistent way with the Gutzwiller projected mean-field approximation and the Bogoliubov-de Gennes theory. When the two impurities are located one or two sites away, we find the superconductivity coherence peaks are more strongly suppressed and the zero-energy peak (ZEP) has split into two peaks. Whereas when the two impurities are located next to each other, the ZEP vanished, and LDOS does not change a lot compared with the case away from the impurities.

The pairing symmetiy and the superconducting gap in high-temperature superconducting cuprates are investigated as a function of the hole doping level (x) and temperature (7), using directional scanning tunneling spectroscopy (STS). It is found that the predominant pairing symmetry is which is insensitive to the irariations in T and x. In contrast, the maximum superconducting gap (zld) in YBa2Cu3O74 and La2SrCuO44 scales with the superconducting transition temperature (Ta), &1d the ratio of (2A,/kBT) increases with decreasing doping level. The dominance of d22 pairing is consistent with strong spatial variations in the local quasiparticle spectra near non-magnetic impurities such as Zn and Mg in a (Zn,Mg)-doped YBa2Cu3O74 single crystal. To further elucidate the nature of the pairing state, the c-axis spin-polarized quasiparticle transport in the superconducting state of YBa2Cu3O74 is investigated by studying the critical currents and STS under the injection of electrical currents from the underlying ferromagnetic La7Sr03MnO3 layer in various ferromagnetinsulator-superconductor (F-I-S) heterostructures. The temperature dependent spin diffusion length (5) and signatures of nonequilibrium quasiparticle distribution under spin injection in d-wave superconductors are determined for the first time.