Spin-switch effect from crossed Andreev reflection in superconducting graphene spin valves

We consider the nonlocal quantum transport properties of a graphene superconducting spin valve. It is shown that one may create a spin-switch effect between perfect elastic cotunneling (CT) and perfect crossed Andreev reflection (CAR) for all bias voltages in the low-energy regime by reversing the magnetization direction in one of the ferromagnetic layers. This opportunity arises due to the possibility of tuning the local Fermi level in graphene to values equivalent to a weak magnetic exchange splitting, thus reducing the Fermi surface for minority spins to a single point and rendering graphene to be half metallic. Such an effect is not attainable in a conventional metallic spin valve setup, where the contributions from CT and CAR tend to cancel each other and noise measurements are necessary to distinguish these processes.

Figure 1

(Color online) Proposed experimental setup for the spin-switch effect between crossed Andreev reflection and elastic cotunneling. Ferromagnetism and superconductivity are induced by the proximity effect to a host material. The induced exchange fields in the nonsuperconducting graphene regions are oriented either parallel or antiparallel with respect to each other. In the parallell alignment, the density of states vanishes for both normal Andreev reflection and crossed Andreev reflection processes such that only elastic cotunneling contributes to nonlocal transport. In the antiparallel alignment, the density of states vanishes for both normal Andreev reflection and elastic cotunneling, leaving only crossed Andreev reflection as the nonlocal transport channel.

Figure 2

(Color online) Plot of the conductance for CT processes $G_{\text{CT}}/G_F$ versus bias voltage in the upper panel and versus length of the $S$ region in the lower panel. Here, we consider the P alignment and ${\mu }_F=h_0$ such that $G_{\text{CAR}}\to 0$.

Figure 3

(Color online) Plot of the conductance for CAR processes $G_{\text{CAR}}/G_F$ versus bias voltage in the upper panel and versus length of the $S$ region in the lower panel. Here, we consider the AP alignment and ${\mu }_F=h_0$ such that $G_{\text{CT}}\to 0$.