Nonequilibrium Spin Dynamics in the Ferromagnetic Kondo Model

TL;DR

This paper studies the nonequilibrium spin relaxation in the ferromagnetic Kondo model, revealing rapid long-time relaxation and deviations from thermal equilibrium in steady state magnetization through analytical and numerical methods.

Contribution

It introduces combined analytical and numerical approaches to analyze real-time spin dynamics in the ferromagnetic Kondo model, providing new insights into relaxation processes.

Findings

Long-time relaxation is faster with anisotropic couplings.

Steady state magnetization significantly deviates from equilibrium.

Analytical results agree with numerical simulations.

Abstract

Motivated by recent experiments on molecular quantum dots we investigate the relaxation of pure spin states when coupled to metallic leads. Under suitable conditions these systems are well described by a ferromagnetic Kondo model. Using two recently developed theoretical approaches, the time-dependent numerical renormalization group and an extended ow equation method, we calculate the real-time evolution of a Kondo spin into its partially screened steady state. We obtain exact analytical results which agree well with numerical implementations of both methods. Analytical expressions for the steady state magnetization and the dependence of the long-time relaxation on microscopic parameters are established. We find the long-time relaxation process to be much faster in the regime of anisotropic Kondo couplings. The steady state magnetization is found to deviate significantly from its thermal equilibrium value.