Hidden non-Fermi liquid behavior due to crystal field quartet

TL;DR

This paper investigates how crystal field effects induce non-Fermi liquid behavior in a Kondo model with quartet ground states, revealing a crossover to Fermi liquid behavior influenced by singlet excited states.

Contribution

It introduces a realistic Kondo model incorporating both magnetic and quadrupolar couplings and demonstrates how singlet excited states lead to non-Fermi liquid behavior and a sensitive crossover to Fermi liquid states.

Findings

Non-Fermi liquid behavior observed in quadrupolar susceptibility.

Crossover to Fermi liquid state is driven by singlet excited states.

Kondo temperature in magnetic susceptibility is less sensitive to non-magnetic coupling.

Abstract

We study a realistic Kondo model for crystal field quartet ground states having magnetic and non-magnetic (quadrupolar) exchange couplings with conduction electrons, using the numerical renormalization group method. We focus on a local effect dependent on singlet excited states coupled to the quartet, which reduces the non-magnetic coupling significantly and drives non-Fermi liquid behavior observed in the calculated quadrupolar susceptibility. A crossover from the non-Fermi liquid state to the Fermi liquid state is characterized by a small energy scale very sensitive to the non-magnetic coupling. On the other hand, the Kondo temperature observed in the magnetic susceptibility is less sensitive. The different crystal-field dependence of the two exchange couplings may be related to the different $x$ dependence of quadrupolar and magnetic ordering temperatures in Ce$_x$La$_{1-x}$B$_6$.