Spin And Valley Polarization Electron Transport Properties Of Graphene - Based Superconducting Junctions

In recent years, the discovery of graphene (G), a single layer of graphite, has been attracting considerable attention. Besides the spin and charge degrees of freedom, electrons in G have an additional valley degree of freedom like spin one, which is related to the time-reversal symmetry. Hence, the Josephson junctions based on G can exhibit novel supercurrent effect. Furthermore, for the zigzag G nanoribbon (ZR), due to the even or odd of the chain number leading to the different pesudoparities of valleys, the superconducting junctions based on the ZR may present lots of outstanding features.In this article, applying the method based on Green’s function theory, we study the spin-polarized and valley-polarized transport properties in the superconducting junctions composed of superconductor (S) and G.Firstly, we study the supercurrent properties in Josephson junctions based on a valley polarized G. It is shown that in the junctions including the superconducting metal electrode with a direct contact of the ZR along the current direction (Bulk-S), there exists an anomalous Josephson effect. When the phase difference is 0 or π, the supercurrent is nonzero. The reason can be given as follows. In the context of the interplay of the static staggered potentials and the off-resonant circularly polarized light field, the valley polarization could be induced and the valley-mixing scattering at the interface between Bulk-S and valley polarized G take place, which gives rise to the breaking of time-reversal symmetry.Secondly, we study the spin polarized transport properties in the ZR superconducting junctions. The variations of nonlocal Andreev reflection and elastic contunneling probabilities with the length L of the S and bias voltage eVL of left ZR electrode, are investigated. It is found that when one of the ZR is magnetized (MZR) and L of the S layer is very small, we can obtain the current along opposite directions with opposite spin orientations by adjusting the bias voltage polarity of the left ZR electrode. This indicates that the structure can be used to manufacture a spin diode with bidirectional conductibility and opposite spin directions. As both of the ZRs are magnetized, in the parallel magnetic configuration, the current with one spin direction can turn into that with the other spin direction with the exchange field increased. And thus, the system can be used to make the device with converter of spin filtering. In the antiparallel magnetic configuration, for the small exchange field, there only exists the spin-down nonlocal Andreev reflection, which means that pure spin entangled electron pairs can be obtained. However, for the large exchange field, both the tunneling probabilities are almost zero and there is no current, indicating 100% tunneling magnetoresistance with high magnetic storage efficiency.