Meta-orbital Transition in Heavy-fermion Systems: Analysis by Dynamical Mean Field Theory and Self-consistent Renormalization Theory of Orbital Fluctuations

摘要

We investigate a two-orbital Anderson lattice model with Ising orbitalintersite exchange interactions by means of dynamical mean field theorycombined with the static mean field approximation of the intersite orbitalinteractions. Focusing on Ce-based heavy-fermion compounds, we examine theorbital crossover between the two orbital states, when the total f-electronnumber per site n_f is n_f ~ 1. We show that a "meta-orbital" transition, atwhich the occupancy of the two orbitals changes steeply, occurs when thehybridization between the ground-state f-electron orbital and conductionelectrons are smaller than that between the excited f-electron orbital andconduction electrons. Near the meta-orbital critical end point, the orbitalfluctuations are enhanced, and couple with the charge fluctuations. A criticaltheory of the meta-orbital fluctuations is also developed by applying theself-consistent renormalization theory of itinerant electron magnetism to theorbital fluctuations. The critical end point, first-order transition andcrossover are described within Gaussian approximations of orbital fluctuations.We discuss the relevance of our results to CeAl2, CeCu2Si2, CeCu2Ge2 and therelated compounds, which all have low-lying crystalline-electric-field excitedstates.