Tunneling conductance and twofold spin-singlet Andreev reflections in ferromagnet/ferromagnet/iron pnictide superconductor hybrid structures with collinear magnetizations

Iron-based superconductors, as some other high-temperature superconducting materials such as the cuprates, are confronted with uncovering the unconventional pairing symmetry, although most researchers favor the so-called s±-wave pairing state. Herein, we theoretically investigate the tunneling conductance of clean ferromagnet (FM)/FM/iron pnictide superconductor (SC) hybrid structures with the SC having s±(two energy gaps have phase difference) pairing symmetry. Novel twofold spin-singlet pairing states near the FM/SC interface in collinear magnetizations emerge due to the presence of two bands in the SC. Conversions of the differential conductance in the ferromagnetic alignment of the two FMs between the peak and valley are shown to be much different from those in the antiferromagnetic alignment. More importantly, two rather different properties in contrast with tunneling into a conventional s-wave SC and an s++-wave SC (two energy gaps have the same sign) are also exhibited, which can be experimentally used to probe and identify the s±pairing symmetry in the iron pnictide SC.