Intermediate Valence State in $\mathrm{YbB_4}$ Revealed by Resonant X-ray Emission Spectroscopy

Abstract

We report the temperature dependence of the Yb valence in the geometrically frustrated compound YbB 4 from 12 to 300 K using resonant X-ray emission spectroscopy at the Yb Lα 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 YbB 4 is a Kondo system in the intermediate valence regime. This result indicates that the Kondo interaction in YbB 4 is substantial, and is likely to be the reason why YbB 4 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 Tv.