Josephson current through a quantum-dot ring: π-junction transition and persistent current magnification

By means of the exact diagonalization approach, the Josephson and persistent currents in a superconductor/quantum-dot ring/superconductor (S/QDR/S) structure are theoretically investigated. The ground state is obtained within zero bandwidth approximation in which the superconductors are replaced by effective local pairing potentials. It is found that Josephson current can flow through this structure in the presence of various electron correlations. Furthermore, in the half-filled case, a novel 0-π transition behavior is observed, which arises from the interplay of interdot antiferromagnetic coupling and electron correlations. When the symmetry of the two arms in the QDR is broken down, the quantum interference efficiently causes the persistent current magnification, even in the case of equilibrium and zero magnetic flux.