▎ 摘　　要

The Josephson current in an asymmetric graphene-based SG(1)/F-B/SG(2) junction where SG(1.2) are graphene. layers which have been induced into two superconducting states having order parameters Delta(1) and Delta(2) (Delta(1) not equal Delta(2)) on the left and right sides of a ferromagnetic barrier F-B of thickness d, respectively is studied. The presence of the exchange energy E-ex and the gate potential V-G; in the barrier F-B are taken into account. For the case of k(parallel to)/0, we find that the Josephson current depends on the exchange energy but is independent of the gate voltage. We find that increasing Delta(2) can induce the junction to become a pi-junction. This does not occur in the case of similar junctions having conventional superconductors in them. The critical current at zero temperature for Delta(2) - infinity has the form I-C(Delta(2) -> infinity) = 2e Delta(1)/h = 2I(C)(Delta(2) = Delta(1)). This behavior of the Josephson current in a graphene junction is quite different from that of the supercurrent in conventional asymmetric junctions. For those junctions, it is predicted that I-C alpha Delta(1) Delta(2), leading to I-C -> infinity as Delta(2) -> infinity. A transition from a 0-junction to a pi-junction is observed as chi(ex) (chi(ex) similar to 2E(ex)d/h nu(F)) is increased. In a 0-junction, the spin supercurrents arising from the spin dependent Andreev energy levels do not vanish. This leads to is = I-s + I-up arrow + I-down arrow= 0 but with I-up arrow = I-down arrow =-I-down arrow not equal 0. (C) 2009 Elsevier B.V. All rights reserved.