Theory of a.c. spin current noise and spin conductance through a quantum dot in the Kondo regime I: The equilibrium case

TL;DR

This paper investigates the equilibrium frequency-dependent spin current noise and conductance through a quantum dot in the Kondo regime, revealing universal behaviors, low-temperature suppression, and resonance phenomena related to spin correlations.

Contribution

It introduces a detailed analysis of spin current correlations in the Kondo regime, highlighting universal scaling functions and novel low-frequency anomalies.

Findings

Spin cross-correlations are suppressed below the Kondo temperature.

A dynamical spin accumulation resonance occurs at the Kondo energy.

Low-temperature anomalies related to spin conservation are observed at high temperatures.

Abstract

We analyze the equilibrium frequency-dependent spin current noise and spin conductance through a quantum dot in the local moment regime. Spin current correlations behave markedly differently from charge correlations. Equilibrium spin correlations are characterized by two universal scaling functions in the absence of an external field: one of them is related to charge correlations, while the other one describes cross-spin correlations. We characterize these functions using a combination of perturbative and non-perturbative methods. We find that at low temperatures spin cross-correlations are suppressed at frequencies below the Kondo scale, $T_K$, and a dynamical spin accumulation resonance is found at the Kondo energy, $\omega \sim T_K$. At higher temperatures, $T>T_K$, surprising low-frequency anomalies related to overall spin conservation appear in the spin noise and spin conductance, and the Korringa rate is shown to play a distinguished role. The transient spin current response also displays universal and singular properties.