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

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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 $n_{f}=1$.
2010-05-15