From mixed valence to the Kondo lattice regime

Abstract

Abstract. Many heavy fermion materials are known to crossover from the Kondo lattice regime to the mixed-valent regime or vice-versa as a function of pressure or doping. We study this crossover theoretically by employing the periodic Anderson model within the framework of the dynamical mean field theory. Changes occurring in the dynamics and transport across this crossover are highlighted. As the valence is decreased (increased) relative to the Kondo lattice regime, the Kondo resonance broadens significantly, while the lower (upper) Hubbard band moves closer to the Fermi level. The resistivity develops a two peak structure in the mixed valent regime: a low temperature coherence peak and a high temperature 'Hubbard band' peak. These two peaks merge yielding a broad shallow maximum upon decreasing the valence further. The optical conductivity, likewise exhibits an unusual absorption feature (shoulder) in the deep mid-infrared region, which grows in intensity with decreasing valence. The involvement of the Hubbard bands in dc transport, and of the effective f -level in the optical conductivity are shown to be responsible for the anomalous transport properties. A two-band hybridization-gap model, which neglects incoherent effects due to many-body scattering, commonly employed to understand the optical response in these materials is shown to be inadequate, especially in the mixed-valent regime. Comparison of theory with experiment carried out for (a) dc resistivities of CeRhIn 5 , Ce 2 Ni 3 Si 5 , CeFeGe 3 and YbIr 2 Si 2 ; (b) pressure dependent resistivity of YbInAu 2 and CeCu 6 ; and (c) optical conductivity measurements in YbIr 2 Si 2 yields excellent agreement.

Strongly hybridized electronic structure of YbAl2: An angle-resolved photoemission study

“…In the case of YbAl 2 , the Yb valence is nearly divalent +2.2 as a consequence of the strong c-f hybridization 14 . With decreasing hybridization strength toward Yb 3+ , the resonance peak will move closer to E F and become sharper, providing a well-defined Kondo resonance peak 31,32 . Such an evolution of Kondo resonance may bring about the drastically modified FS topology and the highly renormalized band structure compared with the LDA calculation.…”

Kondo-hole substitution in heavy fermions: Dynamics and transport

“…Evaluation of local self-energy Σ f ↑/↓ and the selfconsistency of DMFT is carried out in the manner discussed in detail in previous works 31,32 for the clean case. For disordered systems, the CPA Green's functions are used to evaluate transport properties which have been discussed in the next section.…”

Dynamics of Valence Fluctuations in the Extended Periodic Anderson Model

“…is the Feenberg selfenergy, that may be calculated self-consistently through following relations [15] )…”

“…The agreement has been excellent. The LMA+DMFT investigations of the PAM have also been very successful in describing the spectral and transport properties of a large range of heavy fermion [13,14] and mixed-valent materials [15]. In this work, we have investigated the EPAM using LMA.…”