  A new microscopic model of the charge Kondo effect is proposed, and is
studied on the basis of a numerical renormalization group calculations. It is
shown that the charge Kondo effect is caused by a pair hopping interaction
between the conduction band and the localized band. It is found that the
formation of a charge Kondo-Yosida singlet due to its mechanism corresponds to
the appearance of valence skipping phenomenon, and that the charge Kondo-Yosida
singlet and the spin Kondo-Yosida singlet state coexist for a tuned parameter
set, because the charge and spin degrees of freedom are separated in this
model. Then, it is also found that the Sommerfeld coefficient is enhanced by
the competition between these two singlet states.
