Intermediate valence state in YbB4 revealed by resonant x-ray emission spectroscopy

Abstract

We report the temperature dependence of the Yb valence in the geometrically frustrated compound YbB4 from 12 to 300 K using resonant x-ray emission spectroscopy at the Yb L-alpha 1 transition. We find that the Yb valence, v, is hybridized between the v = 2 and v = 3 valence states, increasing from v = 2.61 +/- 0.01 at 12 K to v = 2.67 +/- 0.01 at 300 K, confirming that YbB4 is a Kondo system in the intermediate valence regime. This result indicates that the Kondo interaction in YbB4 is substantial, and is likely to be the reason why YbB4 does not order magnetically at low temperature, rather than this being an effect of geometric frustration. Furthermore, the zero-point valence of the system is extracted from our data and compared with other Kondo lattice systems. The zero-point valence seems to be weakly dependent on the Kondo temperature scale, but not on the valence change temperature scale T-v.