Study of spin polarization in graphene-based unconventional superconductor junctions

Abstract

In this paper, we study spin polarization in ferromagnetic-unconventional superconductor graphene junction, where we consider three different Cooper pairing states: one unconventional dx2-y2-wave and two time-reversal symmetry broken dx2-y2+is and dx2-y2+idxy . In the absence of Fermi wave mismatch (FWM), a high-spin polarization is achievable if we increase the exchange field of ferromagnet close to the Fermi energy values in the ferromagnetic region. In the presence of FWM, the angle between the normal to the interface and the x-axis of superconducting crystal, i. e. β plays a crucial role in spin polarization. For β=π/8, the polarization is totally negative especially in the sub-gap region when modified conventional Andreev reflection (MCAR) governs, and it is completely positive when modified specular Andreev reflection (MSAR) dominates on the tunneling junction. If we rotate β from β=π/8 to β=π/4, the spin polarization shows three different behaviors for the mentioned Cooper pairing states. Therefore, not only is it possible to discriminate MCAR from MSAR which is significant especially from a condensed-matter perspective but also one can find high spin polarization by tuning the junction parameters which is important in spintronic applications.