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Mass enhancement in an extended periodic Anderson model with valence fluctuations

Kubo, Katsunori 

In recent years, valence fluctuation mediated superconductivity has been discussed. Such valence fluctuations are inferred, e.g., for CeCu$$_2$$Si$$_2$$, from a rapid change of the effective mass $$m^*$$ through the relation: $$m^*/m=(1-n_f/2)/(1-n_f)$$, where $$m$$ is the free-electron mass and $$n_f$$ is the number of $$f$$ electrons per site. However, this relation is derived for the periodic Anderson model (PAM), in which a sharp valence change does not occur. In addition, $$m^*$$ varies non-monotonically in CeCu$$_2$$Si$$_2$$ under pressure, which is not expected from that equation. Thus, we extend this relation to a model with a sharp valence change. We study an extended PAM with the Coulomb interaction $$U_{cf}$$ between the conduction and $$f$$ electrons by the Gutzwiller method. Then, we find that $$m^*$$ becomes large in the intermediate valent regime by an effect of $$U_{cf}$$, and $$m^*$$ changes non-monotonically as a function of $$n_f$$. Thus our theory seems relevant to CeCu$$_2$$Si$$_2$$.

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