Fluctuation-exchange approximation theory of the non-equilibrium singlet-triplet transition

TL;DR

This paper extends the Fluctuation Exchange Approximation (FLEX) to analyze non-equilibrium singlet-triplet transitions in correlated systems, successfully capturing key features like conductance evolution and Kondo effects, with potential applications in molecular electronics.

Contribution

The paper develops a non-equilibrium FLEX method that efficiently models singlet-triplet transitions, matching key features of more computationally intensive approaches.

Findings

FLEX accurately reproduces the conductance evolution during the transition.

FLEX captures the two-stage Kondo effect on the triplet side.

FLEX aligns well with NRG results on Kondo resonance width.

Abstract

As a continuation of a previous work [B. Horv\'ath et al., Phys. Rev. B {\bf 82}, 165129 (2010)], here we extend the so-called Fluctuation Exchange Approximation (FLEX) to study the non-equilibrium 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 multi-level systems under non-equilibrium conditions, and, in its rather general form, as formulated here, it could find a broad application in molecular electronics calculations.