Superconductivity and ferromagnetism in graphene

The interplay between the two competing effects of the superconductivity and ferromagnetism has been the subject of general interest since the discovery of the superconductivity. In this talk we address this subject in graphene by studying the proximity effect in a ferromagnet(F)-superconductor(S) junction which is realized in a graphene sheet [1,2]. We show that for a weakly doped F graphene with an exchange field h larger than its Fermi energy EF, the superconducting electron-hole conversion of massless Dirac fermions at FS junction is associated with a spin Klein tunnelling through an exchange field p-n barrier between two spin-split conduction and valance subbands. This Andreev-Klein process results in an enhancement of the subgap conductance of FS junction by h up to the point at which the conductance at low voltages is greater its value for the corresponding non-ferromagnetic junction. This effect is in striking contrast to the common view that the ferromagnetic ordering and the spin-singlet superconducting correlations are exclusive phenomena. We present a map of angle of Andreev reflection showing that depending on the value of h/EF, the spin and energy states of the incident electron the reflection can be retro or specular types in both convergent and divergent ways with the reflection direction aligned, respectively, closer to and farther from the normal to the junction as compared to the incident direction.