Spin Injection Effect And Vortex Dynamics In High Temperature Superconducting Heterostructure

In recent years, heterostructures based on high temperature superconductors (HTS), especially the bilayers composed of HTS and other strongly correlated electron systems, have attracted extensive attention. These bilayers often show a series of rich and novel physical phenomena due to the co-existence and competition between different types of order states (such as superconductivity, ferromagnetism, ferroelectricity and so on). The investigation on heterostructures involving HTS can realize the practical application of spintronic devices based on high temperature superconductors, and also can help us intensively understand the physical mechanism in HTS.In this dissertation, effect of injected spins with different polarized orientations on the superconductivity, pinning mechanism and vortex phase transition in cuprate/manganite are studied. Furthermore, the rectifying characteristics of oxide Schottky junctions consisted of HTSs are also investigated. The dissertation can be divided into five parts as follows:In chapter1, we make a brief introduction to the superconductivity and vortex dynamics in HTS, especially the different pinning mechanisms and vortex phase transitions. Combining the research progresses of cuprate/manganite heterostructures, we discuss the influences of spin injection on superconducting and pinning properties. At last, the achievement of Schottky junctions based on HTS is also introduced.In chapter2, we fabricated epitaxial La0.7Sr0.3MnO3/La1.85Sr0.15CuO4(LSMO/LSCO) composite thin films and realized the spin injection effects on the superconductivity by different electrode configurations on the same specimen. Using this method, we investigate the influence of spin-polarized quasi-particle injection with different orientations on the superconductivity and find that the upper critical field and flux pinning potential are reduced by the spin injection, especially for the magnetic field parallel to ab-plane of LSCO. This suggests the different suppression on the superconducting pairing strength due to the different interaction between injected spins and the local antiferromagnetic spin fluctuations in La1.85Sr0.15CuO4.In chapter3, the pinning mechanisms in LSMO/LSCO heterostructure are systematically investigated. The angle dependent Jc(θ) near H//c can be scaled by the well known Ginzburg-Landau (G-L) expression, while for H//ab the intrinsic pinning drives the data to deviate from the G-L fitting. For the pinning dominated by randomly distributed defects, the two pinning mechanisms associated with disorders with the spatial variations in the Tc throughout the material(δTc pinning) or from spatial variations in the charge carrier mean free path l near lattice defects(δl pinning) coexist in our bilayer. Besides, what the most important is that injected spin-polarized quasiparticles can increase the contribution of δTc pinning by breaking the time-reversal symmetry of the Cooper pairs.In chapter4, through the scaling analysis of the I-V characteristics of LSMO/LSCO bilayer with different electrode configurations, the vortex glass transition and vortex phase diagram in this bilayer are studied. It is found that all the I-V curves in various magnetic fields can be scaled with a three dimensional (3D) vortex glass model, and a strong spin injection configuration can induce a better3D scaling behavior which is closely related to the decrease of the anisotropy parameter. Using temperature dependences of the upper critical field Hc2and the vortex glass transition field Hg determined from the scaling analyses of the I-V curves, the vortex phase diagram of the LSMO/LSCO bilayer with spin injection is constructed. We find that the spin injection shifts both Hg and Hc2to lower temperatures and fields due to the Cooper pair-breaking effect, especially for H//ab.In chapter5, the rectifying characteristics are investigated for the epitaxial YBa2Cu3O7-δ/Nb-doped SrTiO3(YBCO/NSTO) Schottky junctions. From the Ⅰ-Ⅴ curves with different temperature, we obtain the temperature dependent diffusion voltage. When superconducting transition of YBCO occur, the diffusion voltage show a visible reduction from its original track, which is due to the opening of superconducting gap. Thus we get the superconducting energy gap values by the relative change of diffusion voltage. Furthermore, we also find the energy gap is anisotropy with different magnetic field orientations. Based on the relationship between superconducting energy gap and upper critical field, we obtained the anisotropy parameter in YBCO, which is very close to the result obtained from the resistance broadening on the same sample, confirming the reliability for monitoring the superconducting gap of HTS in this kind of Schottky junctions.