Field-induced valence fluctuations in YbB12

We performed high-magnetic-field ultrasonic experiments on YbB12 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 (C-11 - C-12)/2, and the bulk modulus C-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-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-B takes place at even lower fields and C-B decreases continuously with field. Our analysis using the multipole susceptibility based on a two-band model reveals that the softening of C-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-B.