Ferromagnetic / Ferromagnetic / S _± - Wave Superconductor Junctions Spin Transport Properties

Iron-based high-temperature superconductor (SC) is a new type of material discovered recently in superconducting research field, which has attracted much attention. As other newly discovered unconventional SCs, the iron-based SC is also faced with one of the most important problem——the determination of its pairing symmetry. For a long time, ones can obtain the information about different kinds of superconducting pairing mechanisms and energy gaps by measuring some basic parameters of superconducting tunneling junctions.In this paper, we study ferromagnet (FM)/FM/,s±-wave SC tunneling junctions by using an extended BTK method, in which Bogoliubov-de Gennes(BDG) equation is extended into eight-component and the essence of interband coupling strength in iron-based SCs is correctly captured. Our attention is focused on the effects of the magnetic configuration, the gap ratio and interband coupling strength of the two bands in the iron-based SC, and the spin polarization of the FM on the spin transport of tunneling junctions, including on the spin-triplet and differential conductance. Then we compare the results with those of the corresponding systems consisting of other SCs, which can be resultantly used to experimentally probe and identify the antiphase s-wave pairing in iron-based SCs.We have studied the spin transport properties including pairing state and differential conductance under the condition of collinear and noncollinear magnetic configurations. In the noncollinear magnetic configuration, there exist twofold novel Andreev reflections (ARs) and twofold usual ARs due to the existence of two bands in the iron-based SC. For the former, the incident electron and the twofold Andreev-reflected holes come from the different spin subbands, forming twofold spin-singlet pairing states. For the latter, the incident electron and the twofold novel Andreev-reflected holes belong to the same spin subband, leading twofold spin-triplet pairing states. The gap ratio and interband coupling strength γ of the two bands in the iron-based SC and the spin polarization P of FM have an important influence on the spin-singlet and triplet pairing states, which means that they also have important influence on the properties of spin-polarized transport. It is shown that the conversions of the conductance not only between the peak and valley at zero energy but also between the peaks and dips at two gap energies are strongly dependent on both the interband coupling strength γ in the SC and the spin polarization P in the FM. Furthermore, we find that, in the collinear magnetic configuration, the antiparallel magnetic configuration always suppresses the appearance of the characteristics of differential conductance spectrum. The interband coupling strength γ and the spin polarization P compete with each other to exert the influence on differential conductance, and the results mainly depend on their values and magnetic configurations. In particular, for a given P, whether it is parallel or antiparallel magnetic configuration, the variation of Y can change conductance feature at zero energy, but cannot change those at two gap energies.