Nonequilibrium Dynamics Across an Impurity Quantum Critical Point

TL;DR

This paper investigates how a quantum impurity coupled to a fermionic reservoir reaches equilibrium after local quenches, revealing complex dynamics linked to quantum critical points and the Kondo effect.

Contribution

It introduces a variational wave function approach to study nonequilibrium dynamics across a quantum critical point in the Anderson Impurity model.

Findings

Equilibration is connected to the Kondo effect at low energies.

Nontrivial dynamical behavior emerges near the quantum critical point.

The variational method qualitatively captures the physics of the system.

Abstract

Whether a small quantum mechanical system is able to equilibrate with its environment once an external local perturbation drives it out of thermal equilibrium is a central question which cuts across many different fields of science. Here we consider such a problem for a correlated quantum impurity coupled to a fermionic reservoir and driven out of equilibrium by local quantum quenches such as those recently realized in optical absorption experiments on single quantum dots. We argue that equilibration in this problem is deeply connected to the occurrence of Kondo Effect at low energy and that a highly non trivial dynamical behavior may emerge whenever a local quantum critical point intrudes between a conventional Kondo screened phase and a Kondo unscreened one. We discuss this issue in the context of the Anderson Impurity model coupled to a pseudo-gap reservoir by using a correlated time dependent variational wave function that is able to qualitatively describe this physics.