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

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.

Possible room-temperature signatures of unconventional 4f -electron quantum criticality in YbMn6Ge6−xSnEichenberger1, Magnette2, Malterre3 et al. 2020Phys. Rev. B91270View full textAdd to dashboardCite

“…In YbMn 6 Ge 6−x Sn x materials, the Mn sublattice orders magnetically near 300 K, and the strong Mn-Yb interaction shifts the Yb magnetic instability deep inside the mixed-valent regime. At x c ∼ 5.23, ν is measured to be 2.77, which yields T K of at least ∼250 K based on the almost linear scaling between these two quantities [34]. Such Kondo temperature is considerably higher than that of most of the 4 f quantum-critical materials presently known.…”

Large Fermi surface expansion through anisotropic mixing of conduction and f electrons in the semimetallic Kondo lattice CeBi

“…As stated in the introduction, such temperature dependence of the RE valence in KL systems appears to be quite natural [6,35,36]. In low carrier density systems, the many-body Kondo effect is largely diminished due to the stringent requirement for the conduction electron density and, therefore, the valence fluctuation originating in the substantial c-f mixing can be resolved via the FS expansion.…”

Kondo-Induced Giant Isotropic Negative Thermal Expansion

“…The valence decrease by ∼0.2 in g−Sm 0.77 Y 0.23 S at low temperatures is stronger by about an order of magnitude than the typical valence change displayed by Kondo materials, which are most often Ce or Yb-based [24][25][26][27][28]. Particularly interesting is the comparison with an Sm-based Kondo system that has recently drawn a lot of interest, SmB 6 , and which is also in the strongly intermediate-valence regime.…”