Kondo Effect of a Jahn-Teller Ion Vibrating in a Cubic Anharmonic Potential

TL;DR

This paper investigates the Kondo effect in a Jahn-Teller ion system with anharmonic vibrations, revealing entropy plateaus and a novel screening process using numerical renormalization group analysis.

Contribution

It introduces a detailed analysis of the Kondo effect in a Jahn-Teller system with anharmonic vibrations, highlighting entropy changes and effective models for the phenomenon.

Findings

Observation of a $ ext{log} 3$ entropy plateau due to quasi-triple degeneracy.

Identification of a $ ext{log} 2$ entropy change from vibronic rotational degrees of freedom.

Explanation of the Kondo temperature via an effective anisotropic $s$-$d$ model.

Abstract

We discuss the Kondo effect in a spinless two-orbital conduction electron system coupled with anharmonic Jahn-Teller vibration by employing a numerical renormalization group technique. When a temperature $T$ is decreased, we encounter a plateau of $\log 3$ entropy due to quasi-triple degeneracy of local low-energy states, composed of vibronic ground states and the first excited state with an excitation energy of $\Delta E$. Around at $T$$\approx$$\Delta E$, we observe an entropy change from $\log 3$ to $\log 2$. This $\log 2$ entropy originates from the rotational degree of freedom of the vibronic state and it is eventually released due to the screening by orbital moments of conduction electrons, leading to the Kondo effect of a Jahn-Teller ion. The Kondo temperature is explained by the effective $s$-$d$ model with anisotropic exchange interactions.