Nonequilibrium Kondo model: Crossover from weak to strong coupling

TL;DR

This paper introduces a novel real-time renormalization group approach to analyze the nonequilibrium Kondo model, revealing universal behaviors in spin relaxation and conductance across coupling regimes.

Contribution

It develops a new formulation of the real-time renormalization group method using the Laplace variable as the flow parameter, providing insights into the crossover from weak to strong coupling.

Findings

Universal line shape for nonlinear conductance derived

Long-time spin dynamics show exponential decay with a power-law pre-factor

Multichannel models exhibit non-Fermi-liquid power-law decay

Abstract

We analyze the nonequilibrium Kondo model at finite voltage and temperature by using a new formulation of the real-time renormalization group method with the Laplace variable as the flow parameter. We evaluate the energy-dependent spin relaxation rate and nonlinear conductance, and derive an approximate form for the universal line shape for the latter in the whole crossover regime from weak to strong coupling. The results are shown to agree well with exact methods in equilibrium, Fermi-liquid theory, weak-coupling expansions, and recent experiments. For the transient spin dynamics we find a universal exponential decay in the long-time limit along with a truncation-dependent pre-exponential power law. For multichannel models a pure power-law decay typical for non-Fermi-liquid behaviour is predicted.