Quantum criticality out of equilibrium in the pseudogap Kondo model

TL;DR

This paper studies the non-equilibrium quantum phase transition in the pseudogap Kondo model, revealing universal critical behaviors and how bias voltage influences quantum criticality in a system with power-law density of states.

Contribution

It provides a controlled RG analysis of non-equilibrium criticality in the pseudogap Kondo model, including analytical and numerical results for conductance and susceptibility near critical points.

Findings

Identification of universal non-equilibrium scaling laws

Analytical and numerical computation of conductance and susceptibility

Discovery of new non-equilibrium quantum critical behaviors

Abstract

We theoretically investigate the non-equilibrium quantum phase transition in a generic setup: the pseudogap Kondo model where a quantum dot couples to two-left (L) and right (R)-voltage-biased fermionic leads with power-law density of states (DOS) with respect to their Fermi levels {\mu}_L/R, {\rho}_c,L(R) ({\omega}) \propto |{\omega} - {\mu}_L(R) |r, and 0 < r < 1. In equilibrium (zero bias voltage) and for 0 < r < 1/2, with increasing Kondo correlations, in the presence of particle-hole symmetry this model exhibits a quantum phase transition from a unscreened local moment (LM) phase to the Kondo phase. Via a controlled frequency-dependent renormalization group (RG) approach, we compute analytically and numerically the non-equilibrium conductance, conduction electron T-matrix and local spin susceptibility at finite bias voltages near criticality. The current-induced decoherence shows distinct nonequilibrium scaling, leading to new universal non-equilibrium quantum critical behaviors in the above observables. Relevance of our results for the experiments is discussed.