Theoretical Study On Electron Transport In High-Tc Superconductors And In Ferromagnet-superconductor Heterostructure Junctions

Recent years, with the development of spin electronics and utilization of the information storage and quantum transmission devices, the transportation, control and manipulation of the spin polarized current have attracted many interesting. We consider that two ways can be used to achieve the above purposes: One way is to use the spin-dependent transport phenomena—the Spin Hall Effect caused by spin-orbit (SO) coupling interaction to produce and transport spin-polarized current. Another way, in ferromagnet/superconductor (F/S) tunnel junctions and its multilayered structures, is to realize the conversion from the even-frequency singlet Cooper pairs in superconductor to the odd-frequency equal-spin triplet pairs in ferromagnet caused by the interface spin-flip scattering and finally achieved the manipulation for equal-spin current in the ferromagnet.In the framework of the Bogoliubov-de Gennes theory, we investigate theoretically the influence of the SO interaction to the vortex charge sign and the local density of states (LDOS) on different doping high-temperature superconductors (HTSCs). In addition, study and disscuss the different structures of the ferromagnet/metallic superconductor heterojunction aiming at the present arguments. Main results are summarized as follows:(1) The effect of the SO coupling interaction on the antiferromagnetism, vortex charges and LDOS in the vortex core for doped HTSCs is investigated by self-consistently diagonalizing a model Hamiltonian with competing antiferromagnetic (AF) and d-wave superconductivity (DSC) interactions. Our research shows that SO- coupling can generate both spin-split and spin-flip, which can lead to a transfer between spin-up and spin-down channels. It also can lead to a double-peak spectrum of the vortex-core state, and vortex charge sign change. Scanning tunneling microscopy spectra on differential conductance and nuclear magnetic resonance experiments on vortex charges for YBa₂Cu₃O_7-x are interpreted. (2) The LDOS of a ferromagnet/superconductor/ferromagnet (F/S/F) structure changes with the exchange field is investigated by diagonalizing a mean-field model Hamiltonian. Our research shows that (a). When exchange field I=0 (F becomes a normal metal) , f 3 = (↑↓-↓↑) 2can penetrate into superconductor in a superconducting coherence length, which can lead to a minigap in the normal mental. In addition, the energy gap in superconductor will decrease by the reverse proximity effect (b). When I=0.3Δ0 (Δ0 is superconducting pair potential), the minigap in the normal metal will split up because of the Zeeman Effect, it lead to the LDOS exhibiting 4 peaks. (c). When I=0.74Δ0,Since f3 will oscillate in ferromagnet, it will realize the transition between "0 state" and "Ï€" in the LDOS. (d). As I increase continually, f3 undergoes rapid damped oscillations in ferromagnet. When I is large enough, f3 penetrate the interface and spread out a very short distance in the ferromagnet. It will decay to 0 at last, which lead the supercurerent not to pass through the F/S/F junction.(3) We investigate density of states (DOS) spectra of an F/S/F structure by including the interface spin-flip scattering potential into the existing mean-field Hamiltonian. We show that the spin-flip scattering can lead to electron spin-mixing in the vicinity of an FS interface, enabling s-wave singlet and odd-frequency triplet pairs conversion and the existence of Andreev bound states with subgap energy (±εb) via the intra- and inter-band Andreev reflections. This adds spin-resolved subgap peaks to the DOS spectra. The relevant elastic co-tunneling at±εb enhances the local conductance, while its competition with the crossed Andreev reflection will lead to a sign change in the nonlocal differential conductance.(4) We investigate spin-triplet pairs tunneling through a Holmium (Ho)-Cobalt(Co)-Ho ferromagnetic trilayer sandwiched between two conventional s-wave superconducting leads by diagonalizing a mean-field model Hamiltonian. We show that the Ho layer behaving as a spin-active source with a spin polarized scattering potential activates the spin degree of freedom in the transport of Cooper pairs and controls the conversion between spin-singlet pairs and odd-frequency triplet pairs, leading to systematic dependences of the equal-spin triplet supercurrent on the magnetization profile of Ho ferromagnet. We find that a symmetric magnetization texture that leads to identical spin polarized scattering potentials in Ho layers can enhance the equal-spin triplet pair tunneling process, whereas the anti-symmetric one that splits electron spin band oppositely at the left and right Ho/Co interfaces declines the triplet pair tunneling. The triplet supercurrent amplitude shows sharp peaks with Ho thickness, and a platform with Co thickness as long as the phase coherence of the triplet pairs is maintained, in good agreement with related experimental observations.