Novel Andreev reflection and differential conductance of a ferromagnet/ferromagnet/superconductor junction on graphene

Via numerical calculation of the spin-dependent Dirac-Bogoliubov-de Gennes equation, the differential conductance is obtained for a ferromagnet/ferromagnet/superconductor (F/F/S) junction on graphene where the two F layers are undoped. If the two F layers have noncollinear magnetizations, the spin-flipped scattering at the F/F interface leads to the novel Andreev reflection (AR), in which the spin directions of an incident electron and the reflected hole are opposite to each other. When the exchange energy is larger than the superconducting gap, this novel AR manifests itself as sub-gap differential conductance peaks because of the formation of spin-flipped Andreev bound states in the intermediate F layer, whereas for the parallel and anti-parallel configurations no such peaks can be found. In the transitional regime with the exchange energy close to the gap, for noncollinear configurations, the round-trip path supporting the formation of those bound states is broken and a differential conductance dip can be found near the point where the external bias equals the exchange energy.