Dual Kondo effect charge ordering and zero thermal expansion in a correlated intermetallic

Abstract

The possibility that valency changes due to the Kondo effect induce a charge-density-wave (CDW) transition and lead to zero-thermal-expansion by compensating the accompanying structural changes is appealing from both a fundamental and applied physics perspective. Theoretical studies have predicted CDW-order caused by the Kondo effect, whereby a material would exhibit a temperature-dependent dual Kondo effect comprising of two sublattices with different single-ion Kondo temperatures, but its experimental realization remains elusive. Here, we show direct evidence of a dual Kondo effect providing the electronic energy gain for a CDW accompanied by zero-thermal-expansion, in a strongly correlated f-electron material. YbPd undergoes a cubic to tetragonal transition with an incommensurate-CDW below T1 = 130 K, which becomes commensurate below T2 = 105 K. Bulk-sensitive spectroscopy reveals temperature-independent ytterbium single-site mixed-valence above T1, and a clear temperature-dependent mixed-valence charge-disproportionation of two crystallographic ytterbium sites in the CDW phases. Simplified single-impurity Anderson model calculations prove existence of a dual Kondo mixed-valency coupled to the CDW changes associated with the two ytterbium sites, and quantify site-dependent single-ion Kondo temperatures. The dual Kondo temperatures track the evolution of lattice parameters, resulting in a cell-volume compensated Kondo-CDW phase. The results provide a route to develop room temperature intermetallic zero-thermal-expansion materials.