Non-equilibrium steady state in a periodically driven Kondo model

TL;DR

This paper analyzes a periodically driven Kondo model, deriving exact results for steady-state spin dynamics and revealing that fast driving induces novel quantum states beyond equilibrium descriptions.

Contribution

It provides exact analytical solutions for the steady state of a driven Kondo model and demonstrates the emergence of non-equilibrium quantum states at fast driving frequencies.

Findings

Algebraic long-time behavior of spin correlations remains unaffected by driving.

Steady state under slow driving is equivalent to a single quench.

Fast driving leads to non-equilibrium states not describable by equilibrium Hamiltonians.

Abstract

We investigate the Kondo model with time-dependent couplings that are periodically switched on and off. On the Toulouse line we derive exact analytical results for the spin dynamics in the steady state that builds up after an infinite number of switching periods. Remarkably, the algebraic long time behavior of the spin-spin correlation function remains completely unaffected by the driving. In the limit of slow driving the dynamics become equivalent to that of a single interaction quench. In the limit of fast driving one can show that the steady state cannot be described by some effective equilibrium Hamiltonian since a naive implementation of the Trotter formula gives wrong results. As a consequence, the steady state in the limit of fast switching serves as an example for the emergence of new quantum states not accessible in equilibrium.