Meta-Orbital Transition in Heavy-Fermion Systems: Analysis by Dynamical Mean Field Theory and Self-Consistent Renormalization Theory of Orbital Fluctuations

Abstract

We investigate a two-orbital Anderson lattice model with Ising orbital\nintersite exchange interactions by means of dynamical mean field theory\ncombined with the static mean field approximation of the intersite orbital\ninteractions. Focusing on Ce-based heavy-fermion compounds, we examine the\norbital crossover between the two orbital states, when the total f-electron\nnumber per site n_f is n_f ~ 1. We show that a "meta-orbital" transition, at\nwhich the occupancy of the two orbitals changes steeply, occurs when the\nhybridization between the ground-state f-electron orbital and conduction\nelectrons are smaller than that between the excited f-electron orbital and\nconduction electrons. Near the meta-orbital critical end point, the orbital\nfluctuations are enhanced, and couple with the charge fluctuations. A critical\ntheory of the meta-orbital fluctuations is also developed by applying the\nself-consistent renormalization theory of itinerant electron magnetism to the\norbital fluctuations. The critical end point, first-order transition and\ncrossover are described within Gaussian approximations of orbital fluctuations.\nWe discuss the relevance of our results to CeAl2, CeCu2Si2, CeCu2Ge2 and the\nrelated compounds, which all have low-lying crystalline-electric-field excited\nstates.\n