Cooperative Effect of Coulomb Interaction and Electron–Phonon Coupling on the Heavy Fermion State in the Two-Orbital Periodic Anderson Model

Abstract

We investigate the two-orbital periodic Anderson model, where the local orbital fluctuations of f -electrons couple with a two-fold degenerate Jahn–Teller phonon, by using the dynamical mean-field theory. It is found that the heavy fermion state caused by the Coulomb interaction between f -electrons U is largely enhanced due to the electron–phonon coupling g , in contrast to the case with the single-orbital periodic Anderson model where the effects of U and g compete to each other. In the heavy fermion state for large U and g , both the orbital and lattice fluctuations are enhanced, while the charge (valence) and spin fluctuations are suppressed. In the strong coupling regime, a sharp soft phonon mode with a large spectral weight is observed for small U , while a broad soft phonon mode with a small spectral weight is observed for large U . The cooperative effect of U and g for half-filling with two f -electrons per atom n f =2 is more pronounced than that for quarter-filling with n f =1.