Fluctuation-exchange approximation theory of the nonequilibrium singlet-triplet transition

As a continuation of a previous work [B. Horváth , Phys. Rev. BPLRBAQ1098-012110.1103/PhysRevB.82.165129 82, 165129 (2010)], here we extend the so-called fluctuation exchange approximation (FLEX) to study the nonequilibrium singlet-triplet transition. We show that, while being relatively fast and a conserving approximation, FLEX is able to recover all important features of the transition, including the evolution of the linear conductance throughout the transition, the two-stage Kondo effect on the triplet side, and the gradual opening of the singlet-triplet gap on the triplet side of the transition. A comparison with numerical renormalization-group calculations also shows that FLEX captures rather well the width of the Kondo resonance. FLEX thus offers a viable route to describe correlated multilevel systems under nonequilibrium conditions, and in its rather general form, as formulated here, it could find a broad application in molecular electronics calculations.