Josephson effect through a multilevel quantum dot near a singlet-triplet transition

We investigate the Josephson effect through a two-level quantum dot with an exchange coupling between two dot electrons. We compute the superconducting phase relationship and construct the phase diagram in the superconducting gap-exchange coupling plane in the regime of the singlet-triplet transition driven by the exchange coupling. In our study two configurations for the dot-lead coupling are considered: one where effectively only one channel couples to the dot and the other where the two dot orbitals have opposite parities. Perturbative analysis in the weak-coupling limit reveals that the system experiences transitions from 0 to pi (negative critical current) behavior, depending on the parity of the orbitals and the spin correlation between dot electrons. The strong coupling regime is tackled with the numerical renormalization group method, which first characterizes the Kondo correlations due to the dot-lead coupling and the exchange coupling in the absence of superconductivity. In the presence of superconductivity, many-body correlations such as two-stage Kondo effect compete with the superconductivity and the comparison between the gap and the relevant Kondo temperature scales allows to predict a rich variety of phase diagrams for the ground state of the system and for the Josephson current. Numerical calculations predict that our system can exhibit Kondo-driven 0-pi-0 or pi-0-pi double transitions and, more interestingly, that if proper conditions are met a Kondo-assisted pi junction can arise, which is contrary to a common belief that the Kondo effect opens a resonant level and makes the 0 junction. Our predictions could be probed experimentally for a Buckminster fullerene sandwiched between two superconductors.