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

Abstract

Via numerical calculation of the spin-dependent Dirac–Bogoliubov–de Gennes equation, thedifferential 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 havenoncollinear magnetizations, the spin-flipped scattering at the F/F interface leads to thenovel Andreev reflection (AR), in which the spin directions of an incident electron and thereflected hole are opposite to each other. When the exchange energy is larger thanthe superconducting gap, this novel AR manifests itself as sub-gap differentialconductance peaks because of the formation of spin-flipped Andreev bound states in theintermediate F layer, whereas for the parallel and anti-parallel configurations nosuch peaks can be found. In the transitional regime with the exchange energyclose to the gap, for noncollinear configurations, the round-trip path supportingthe formation of those bound states is broken and a differential conductancedip can be found near the point where the external bias equals the exchangeenergy.