We performed high-magnetic-field ultrasonic experiments on ${\mathrm{YbB}}_{12}$ 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 $C_{\mathrm{B}}$.

中文翻译：

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YbB12的场致价态涨落

我们在 $B_{12}$最高59 T来研究Yb离子的化合价波动。在零场时，纵向弹性常数$C_{11}$，横向弹性常数 $C_{44}$ 和 $\left(C_{11}-C_{12}\right)/\mathrm{2个}$，以及体积模量 $C_{\mathrm{乙}}$显示出在约35 K处具有曲率变化的硬化，表明化合价波动对弹性常数的贡献很小。在低温下施加强磁场时，$C_{\mathrm{乙}}$在场感应的绝缘体-金属过渡信号上方表现出软化，表明场感应的化合价涨落。此外，在高温下，磁场引起的软化$C_{\mathrm{乙}}$ 发生在更低的领域 $C_{\mathrm{乙}}$随场而连续减小。我们使用基于两频带模型的多极磁化率进行分析，结果表明，$C_{\mathrm{乙}}$除了减少绝缘子能隙外，还来自多极应变相互作用的增强。该分析表明，Yb的场致价态波动会引起体积模量的不稳定性$C_{\mathrm{乙}}$。