Kondo Breakdown via Fractionalization in a Frustrated Kondo Lattice Model

TL;DR

This paper investigates a model of Dirac electrons coupled to frustrated spins, revealing a non-Fermi liquid phase with fractionalization and Kondo breakdown, characterized by a violation of Luttinger theorem and observable through spectral and mutual information measurements.

Contribution

It introduces a sign-free quantum Monte Carlo study of a frustrated Kondo lattice model showing fractionalized Fermi liquid behavior and Kondo breakdown in a non-Fermi liquid phase.

Findings

In the ferromagnetic phase, Dirac electrons become gapped.

In the spin liquid phase, electrons remain massless due to Kondo breakdown.

The non-Fermi liquid phase violates the Luttinger theorem and exhibits fractionalization.

Abstract

We consider Dirac electrons on the honeycomb lattice Kondo coupled to spin-1/2 degrees of freedom on the kagome lattice. The interactions between the spins are chosen along the lines of the Balents-Fisher-Girvin model that is known to host a $\mathbb{Z}_2$ spin liquid and a ferromagnetic phase. The model is amenable to sign free auxiliary field quantum Monte Carlo simulations. While in the ferromagnetic phase the Dirac electrons acquire a gap, they remain massless in the $\mathbb{Z}_2$ spin liquid phase due to the breakdown of Kondo screening. Since our model has an odd number of spins per unit cell, this phase is a non-Fermi liquid that violates the conventional Luttinger theorem which relates the Fermi surface volume to the particle density in a Fermi liquid. This non-Fermi liquid is a specific realization of the so called fractionalized Fermi liquid proposed in the context of heavy fermions. We probe the Kondo breakdown in this non-Fermi liquid phase via conventional observables such as the spectral function, and also by studying the mutual information between the electrons and the spins.