Spin-polarized quasiparticle transport in cuprate superconductors

The effects of spin-polarized quasiparticle transport in superconducting YBa₂ Cu₃O_7−δ (YBCO) epitaxial films are investigated by means of current injection into perovskite ferromagnet-insulator-superconductor (F-I-S) heterostructures. Transport and magnetic properties of these CMR perovskites are first investigated by inducing lattice distortions using lattice mismatching substrates. The half-metallic nature of these perovskites provides an epitaxially grown heterostructure, ideal for injection of spin-polarized current. 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 (J_c) as a function of the injection current density (J_inj), temperature (T), and the thickness (d) of the superconductor demonstrate the “self-injection effect” and reveal dramatic 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 (η = ΔJ_c/ J_inj; ΔJ_c: the suppression of critical current due to finite J_inj) 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 reveals that a substantial chemical potential shift μ* 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 CuO₂ planes.