Twofold spin-triplet pairing states and tunneling conductance in ferromagnet/ferromagnet/iron pnictide superconductor heterojunctions

• An eight-component Bogoliubov–de Gennes (BDG) equation.
• Twofold novel ARs and twofold usual ARs.
• Conversions of conductance between the zero-bias peak and valley at zero energy.
• Conversions of conductance between peaks and dips at two gap energies.
• The importance of the interband coupling strength in the SC.

By applying an extended eight-component Bogoliubov–de Gennes equation, we study theoretically the tunneling conductance in clean ferromagnet/ferromagnet/iron pnictide superconductor (FM/FM/iron-based SC) heterojunctions. Under the condition of noncollinear magnetizations, twofold novel Andreev reflections exist due to the existence of two bands in the SC, in which the incident electron and the two Andreev-reflected holes, belonging to the same spin subband, form twofold spin-triplet pairing states near the FM/iron-based SC interface. It is shown that the conversions of the conductance not only between the zero-bias 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 in the SC and the spin polarization in the FM. The qualitative differences from tunneling into a conventional ss-wave SC are also presented, which may help with experimentally probing and identifying the antiphase ss-wave pairing symmetry in the iron-based SC.