Interplay between Kondo behavior and charge density waves in YbPd

Cerium and ytterbium compounds are systems that exhibit original magnetic, electronic and thermodynamic properties at low temperatures such as the Kondo effect, heavy fermions, intermediate valence and charge density wave. These properties are due to the very strong electron-electron interactions of the f states as well as to the hybridization of the f states with the conduction electrons[1,2].

We investigate the temperature(T)-dependent electronic structure of YbPd, a Kondo mixed-valent cubic compound at high-T which exhibits incommensurate and commensurate charge-density-wave(CDW) ordering in the tetragonal phase below T$_1$ = 130 K and T$_2$ = 105 K, respectively. Below 105 K, magnetic ${Yb}^{3+}$ and nonmagnetic ${Yb}^{2.6+}$ are arranged alternatively along the c axis, in a tetragonal structure exhibiting valence order[3]. Hard x-ray photoemission spectroscopy of Yb 3$d$ states show T-independent Yb$^{2+}$ and Yb$^{3+}$ features indicating single-Yb site dynamic valence fluctuations above T$_1$, and a clear T-dependent valence change of the two crystallographic Yb sites in the CDW phase. Simplified single-impurity Anderson model calculations of the Yb 3$d$ and 4$f$ states show[4] good agreement with the T-dependent valency change and provide site-dependent Kondo temperatures. The results indicate an evolution from dynamic mixed-valence in the C-phase to long range static CDW order in the T'-phase, which is driven by the difference in Kondo energies of the two phases.