Intermediate Valence state in YbB₄revealed by Resonant X-Ray Emission Spectroscopy (RXES)

In crystal systems with competing, incongruous, anti-ferromagnetic exchange interactions, geometric frustration is found and often leads to the suppression of long-range magnetic order. On the other hand, in Yb-based systems where the Kondo interaction between 4f and conduction electrons is dominant, hybridization between these also results in the suppression of magnetic order. When the Kondo interaction is strong enough physical hybridization between the 4f and conduction electrons occurs, resulting in a quantum mechanically degenerate electronic ground-state, a so-called intermediate valence (IV) state. YbB₄ is a rare system where both mechanisms are plausible explanations for the lack of magnetic order [1]. YbB₄ crystallizes into a tetragonal crystal structure (space group P4/mbm) mapped to the well known geometrically frustrated Shastry-Sutherland Lattice within the ab plane [2]. YbB₄ has also been proposed as a Kondo-dominated system residing in the IV regime but has to date lacked direct confirmation of such via spectroscopic means [3,4]. We study the existence of an IV state in YbB₄ using RXES at the Yb L(α1) transition and study the temperature dependence of the Yb valence from 12 to 300 K. We confirm that YbB₄ exists in an IV state at all temperatures, with the Yb valence increasing from v=2.61±0.01 at 12 K to v=2.67±0.01 at 300 K.

[1] J. Etourneau et al., Journal of the Less-Common Metals 67, 531 (1979).

[2] D. Okuyama et al., Journal of the Physical Society of Japan 74, 2434 (2005).

[3] J. Y. Kim et al., Journal of Applied Physics 101, 09D501 (2007).

[4] A. S. Panfilov et al., Low Temperature Physics 41, 193 (2015).