New developments of superconducting mechanism. Valence skipping and charge-transfer fluctuations

As a possible superconducting mechanism with higher transition temperature, the valence skipping mechanism and that due to charge transfer fluctuations are attracted renewal attention. In this report, I review a recent development concerning a microscopic origin of valence skipping phenomenon observed in ions with high principal quantum number, such as Tl and Bi, in relation to the so-called charge Kondo effect and superconductivity, and briefly describe quite recent results on a possibility of charge transfer order in the d-p model with the Coulomb repulsion between electrons on d-orbital at Cu-site and p-orbital at O-site. (author)