Spin-Polarized Quasiparticle Transport in Cuprate Superconductors

Nonequilibrium superconductivity;spin-injection;YBCO

Fu, Chu-Chen ; Yeh, Nai-Chang

University:California Institute of Technology

Department:Physics, Mathematics and Astronomy

关键词: Nonequilibrium superconductivity;    spin-injection;    YBCO;   

Others  :  https://thesis.library.caltech.edu/4227/1/Thesis02_03.pdf

美国|英语

来源: Caltech THESIS

【 摘 要 】

The effects of spin-polarized quasiparticle transport in superconducting YBa2Cu3O(7-d) (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 (Jc) as a function of the injection current density, temperature, and the thickness 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 Jc and a rapidly increasing characteristic transport length near the superconducting transition temperature Tc only in the F-I-S samples. The temperature dependence of the efficiency 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 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.