Flavor fluctuations in three-level quantum dots: Generic SU(3) Kondo fixed point in equilibrium and non-Kondo fixed points in nonequilibrium

Abstract

We study a $3$-level quantum dot in the singly occupied cotunneling regime coupled via a generic tunneling matrix to several multi-channel leads in equilibrium or nonequilibrium. We derive an effective model where also each reservoir has three channels labelled by the quark flavors $u$, $d$ and $s$ with an effective d.o.s. polarized w.r.t. an eight-dimensional $F$-spin corresponding to the eight generators of $SU(3)$. In equilibrium we perform a standard poor man scaling analysis and show that tunneling via virtual intermediate states induces flavor fluctuations on the dot which become $SU(3)$-symmetric at a characteristic and exponentially small low-energy scale $T_K$. Using the numerical renormalization group (NRG) we study in detail the linear conductance and confirm the $SU(3)$-symmetric Kondo fixed point with universal conductance $G=2.25 e^2/h$ for various tunneling setups by tuning the level spacings on the dot. In contrast to the equilibrium case, we find in nonequilibrium that the fixed point model is not $SU(3)$-symmetric but characterized by rotated $F$-spins for each reservoir with total vanishing sum. At large voltage we analyse the $F$-spin magnetization and the current in golden rule as function of a magnetic field for the isospin of the up/down quark and the level spacing to the strange quark. As a smoking gun to detect the nonequilibrium fixed point we find that the curve of zero $F$-spin magnetization has a particular shape on the dot parameters. We propose that our findings can be generalized to the case of quantum dots with an arbitrary number $N$ of levels.