Similar temperature scale for valence changes in Kondo lattices with different Kondo temperatures

TL;DR

This study investigates how valence changes with temperature in Yb Kondo lattices, revealing a universal temperature scale for valence increase that challenges existing theoretical predictions and excludes a quantum critical valence transition.

Contribution

It demonstrates a universal temperature scale for valence changes across different Kondo temperatures, contradicting current theoretical models and prompting further theoretical investigation.

Findings

Valence at low temperature scales with T_K as predicted.

The temperature for valence increase is nearly the same across materials.

No evidence of quantum critical valence transition in YbRh2Si2.

Abstract

The Kondo model predicts that both the valence at low temperatures and its temperature dependence scale with the characteristic energy T_K of the Kondo interaction. Here, we study the evolution of the 4f occupancy with temperature in a series of Yb Kondo lattices using resonant X-ray emission spectroscopy. In agreement with simple theoretical models, we observe a scaling between the valence at low temperature and T_K obtained from thermodynamic measurements. In contrast, the temperature scale T_v at which the valence increases with temperature is almost the same in all investigated materials while the Kondo temperatures differ by almost four orders of magnitude. This observation is in remarkable contradiction to both naive expectation and precise theoretical predictions of the Kondo model, asking for further theoretical work in order to explain our findings. Our data exclude the presence of a quantum critical valence transition in YbRh2Si2.