Full counting statistics for orbital-degenerate impurity Anderson model with Hund's rule exchange coupling

TL;DR

This paper analyzes non-equilibrium current fluctuations in a quantum dot with Hund's rule coupling, revealing how spin pairs influence noise and deriving a formula for the Fano factor based on orbital degeneracy.

Contribution

It provides a detailed theoretical analysis of current fluctuations in an orbital-degenerate Anderson model with Hund's coupling, including new formulas for the Fano factor and insights into crossover behavior.

Findings

Spin-triplet and spin-singlet quasiparticle pairs enhance current fluctuations.

Fano factor formula depending on orbital degeneracy M.

Crossover behavior between small and large Hund's coupling J.

Abstract

We study non-equilibrium current fluctuations through a quantum dot, which includes a ferromagnetic Hund's rule coupling $J$, in the low-energy Fermi liquid regime using the renormalized perturbation theory. The resulting cumulant for the current distribution in the particle-hole symmetric case, shows that spin-triplet and spin-singlet pairs of quasiparticles are formed in the current due to the Hund's rule coupling and these pairs enhance the current fluctuations. In the fully screened higher-spin Kondo limit, the Fano factor takes a value $F_b = (9M+6)/ (5M+4)$ determined by the orbital degeneracy $M$. We also investigate crossover between the small and large $J$ limits in the two-orbital case M=2, using the numerical renormalization group approach.