Tunneling conductance of graphene ferromagnet-insulator-superconductor junctions

Abstract

We study the transport properties of a graphene ferromagnet-insulator-superconductor (FIS) junction within the Blonder-Tinkham-Klapwijk formalism by solving spin-polarized Dirac-Bogoliubov-de-Gennes equation. In particular, we calculate the spin polarization of tunneling current at the I-S interface and investigate how the exchange splitting of the Dirac fermion bands influences the characteristic conductance oscillation of the graphene junctions. We find that the retro- and specular Andreev reflections in the graphene FIS junction are drastically modified in the presence of exchange interaction and that the spin polarization of tunneling current can be tuned from the positive to negative value by bias voltage . In the thin-barrier limit, the conductance of a graphene FIS junction oscillates as a function of barrier strength . Both the amplitude and phase of the conductance oscillation varies with the exchange energy . For (Fermi energy), the amplitude of oscillation decreases with . For , the amplitude of oscillation increases with , where ( is the applied electrostatic potential on the superconducting segment of the junction). For , the amplitude of oscillation decreases with again. Interestingly, a universal phase difference of in exists between the curves for and . Finally, we find that the transitions between retro- and specular Andreev reflections occur at and , and hence the singular behavior of the conductance near these bias voltages results from the difference in transport properties between specular and retro-Andreev reflections.