Theoretical Study on Superconductor - Magnet Multilayers

对由非常规超导体和流动铁/反铁磁体组成的多层材料的电子特性进行了理论研究。我们处理方形晶格上的紧密结合模型，在最近邻位点（现场）之间具有吸引（排斥）相互作用来描述超导性（磁性）。然后用平均场近似推导了描述超导序参量（SCOP）和磁化强度的空间变化的Bogoliubov de Gennes方程。这些BdG方程以一种完全自洽的方式得到了数值解。研究发现：(1)在超导体与（反）铁磁体界面附近，由于I邻近效应，出现了SCOP和磁化共存的表面态。(2)在接触面附近，产生了与体体不同对称性的超细流场。特别是，即使自旋-单重态对称存在于体中，自旋-三重态的自旋-单重态也会发生。我们还研究了杂质的影响，因为它们总是存在于实际系统中，因此有必要研究它们对超导态的影响，以便将理论结果与实验结果进行比较。(3)在杂质周围诱导出了与体体不同对称性的粒子场。自发自旋电流也可以出现在具有自旋轨道相互作用的磁性杂质附近。

Theoretical study has been carried out on the electronic properties of multilayers composed of unconventional superconductors and itinerant ferro/antiferromagnets. We treat a tight binding model on a square lattice with attractive (repulsive) interactions between nearest-neighbor sites (on-site) to describe the superconductivity (magnetism). Then we derive the Bogoliubov de Gennes (BdG) equations which describe the spatial variations of superconducting order parameters (SCOP) and magnetization using mean-field approximation. These BdG equations are solved numerically in a fully self-consistent way. We found the following :(1) Near the interface between the superconductor and the (anti)ferromagnet, surface state where SCOP and magnetization coexist appears due to the I proximity effect.(2) Near the interface the SCOP's with different symmetries from that of the bulk are induced. In particular, even if the SCOP has a spin-singlet symmetry in the bulk, spin-triplet SCOP can occur.We have also examined the effect of impurities, since they always exist in real systems, and so it is necessary to examine the effect on the superconducting state in order to compare the theoretical results with experiments.(3) It turns out the SCOP s with different symmetries from that of the bulk is induces around the impurity. Also spontaneous spin currents can appear near a magnetic impurity with spin-orbit interactions.