Supercurrent and its Fano effect in a Josephson Aharonov-Bohm ring

The Josephson current through an Aharonov-Bohm (AB) interferometer, in which a quantum dot (QD) is situated on one arm and a magnetic flux Φ threads through the ring, has been investigated. In the presence of the magnetic flux, the relation between the Josephson current and the superconductor phase is complex, and the system can be adjusted to π junction by either modulating the magnetic flux or the QD’s energy level ε_d. Due to the electron-hole symmetry, the Josephson current I has the property I(ε_d,Φ)=I(-ε_d,Φ+π). The Josephson current exhibits a jump when a pair of Andreev bound states aligns with the Fermi energy. The condition for the current jump is given. Particularly, we find that the position of the current jump and the position of the maximum value of the critical current I_c are identical. Due to the interference between the two paths, the critical current I_c versus the QD’s level ε_d shows a typical Fano shape, which is similar to the Fano effect in the corresponding normal device. However they also show some differences. For example, the critical current never reaches zero for any parameters, while the current in the normal device can reach zero at the destruction point.