Pressure-induced changes of valence fluctuation in β−YbAlB4 probed by x-ray absorption spectroscopy

We report detailed valence fluctuation phenomena in the unique quantum-critical compound $\ensuremath{\beta}\text{\ensuremath{-}}{\mathrm{YbAlB}}_{4}$ under pressures and at low temperatures. We directly observed the drastic change in the pressure dependence of the mean Yb valence at about 3 GPa with no pressure-induced structural transition. Below 3 GPa, the anomaly in the temperature dependences of the mean Yb valence was observed at about 55 K under ambient pressure, corresponding to a previously observed reduced Kondo coherence temperature, and this temperature rapidly decreases to 22 K at 2.2 GPa, in contrast to the monotonous decrease of the high Kondo temperature ($\ensuremath{\sim}250\phantom{\rule{0.16em}{0ex}}\mathrm{K}$ at ambient pressure). The present results reveal that this reduced Kondo coherence temperature is responsible for the unconventional quantum critical behaviors in $\ensuremath{\beta}\text{\ensuremath{-}}{\mathrm{YbAlB}}_{4}$. Above 3 GPa, it was found that the temperature dependence of the mean Yb valence exhibits a discontinuous change at low temperature (10.5 and 9 K under 4.8 and 5.5 GPa, respectively).