Pairing symmetry and spin-polarized quasiparticle transport in high-temperature superconducting cuprates

Abstract

The pairing symmetry and the superconducting gap in high- temperature superconducting cuprates are investigated as a function of the hole doping level (x) and temperature (T), using directional scanning tunneling spectroscopy (STS). It is found that the predominant pairing symmetry is (d_(x^2 - y^2)), which is insensitive to the variations in T and x. In contrast, the maximum superconducting gap (Δ_d) in YBa_2Cu_3O_(7-∂) and La_(2-x)Sr_xCuO_(4-∂) scales with the superconducting transition temperature (T_c), and the ratio of (2Δ_d/k_BT)c) increases with decreasing doping level. The dominance of d_(x^2 - y^2) 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 YBa_2Cu_3O_(7-∂) single crystal. To further elucidate the nature of the pairing state, the c-axis spin- polarized quasiparticle transport in the superconducting state of YBa_2Cu_3O_(7-∂) is investigated by studying the critical currents and STS under the injection of electrical currents from the underlying ferromagnetic La_(0.7)Sr_(0.3)MnO_3 layer in various ferromagnet-insulator-superconductor (F-I-S) heterostructures. The temperature dependent spin diffusion length (∂_s) and signatures of nonequilibrium quasiparticle distribution under spin injection in d-wave superconductors are determined for the first time.