Abstract
The Kondo model predicts that both the valence at low temperatures and its temperature dependence scale with the characteristic energy TK 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 TK obtained from thermodynamic measurements. In contrast, the temperature scale Tv 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.
Highlights
The Kondo model predicts that both the valence at low temperatures and its temperature dependence scale with the characteristic energy TK of the Kondo interaction
In these systems the occupation of the f shell, i.e. the number of 4f electrons nf for Ce or 4f holes nh for Yb, shows a weak decrease with increasing hybridization and the valence v starts to deviate from the trivalent state nf,h = 1
Comparing with the data on other Yb Kondo lattices that are available in the literature, we find that this behavior is observed in all of them and even seems to extend into the mixed-valent regime
Summary
The Kondo model predicts that both the valence at low temperatures and its temperature dependence scale with the characteristic energy TK of the Kondo interaction. As a result the local moments gets screened by the conduction electrons, the magnetic order disappears, leaving a paramagnetic ground state with strongly renormalized conduction electrons, referred to as Kondo lattices and heavy fermion systems In these systems the occupation of the f shell, i.e. the number of 4f electrons nf for Ce or 4f holes nh for Yb, shows a weak decrease with increasing hybridization and the valence v starts to deviate from the trivalent state nf,h = 1. In the limit of strong hybridization, one enters the intermediate valence regime where a strong quantum mechanical mixing of different valence state occurs and real charge fluctuations are dominant[1, 2] This regime can be described with the Anderson model, which takes both the large Coulomb repulsion between f electrons and the hybridization between 4f and conduction electrons explicitly into account and allows for both spin and charge fluctuations. Since in Kondo lattices the deviation from the trivalent state is small and typical changes in nf between 3 and 300 K are only of the order of a few percent, these studies of the T-
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