Field-induced valence fluctuation in YbB$_{12}$

TL;DR

This study investigates how high magnetic fields induce valence fluctuations in YbB$_{12}$, affecting its elastic properties and revealing field-induced electronic instabilities through ultrasonic measurements and theoretical analysis.

Contribution

It provides the first detailed experimental evidence of field-induced valence fluctuations in YbB$_{12}$ and explains their impact on elastic constants using a two-band model and multipole susceptibility analysis.

Findings

Field-induced valence fluctuations cause softening of the bulk modulus.

Valence fluctuations are enhanced by magnetic fields, leading to instability.

Elastic constants show characteristic changes at the insulator-metal transition.

Abstract

We performed high-magnetic-field ultrasonic experiments on YbB$_{12}$ up to 59 T to investigate the valence fluctuations in Yb ions. In zero field, the longitudinal elastic constant $C_{11}$, the transverse elastic constants $C_{44}$ and $\left( C_{11} - C_{12} \right)/2$, and the bulk modulus $C_\mathrm{B}$ show a hardening with a change of curvature at around 35 K indicating a small contribution of valence fluctuations to the elastic constants. When high magnetic fields are applied at low temperatures, $C_\mathrm{B}$ exhibits a softening above a field-induced insulator-metal transition signaling field-induced valence fluctuations. Furthermore, at elevated temperatures, the field-induced softening of $C_\mathrm{B}$ takes place at even lower fields and $C_\mathrm{B}$ decreases continuously with field. Our analysis using the multipole susceptibility based on a two-band model reveals that the softening of $C_\mathrm{B}$ originates from the enhancement of multipole-strain interaction in addition to the decrease of the insulator energy gap. This analysis indicates that field-induced valence fluctuations of Yb cause the instability of the bulk modulus $C_\mathrm{B}$.