Kenji Yonemitsu, Jun-ichiro Kishine and Takuhiro Ogawa
[Special Issue of Rev. High Press. Sci. Tech. 6, 360 (1997)]
Different crossovers and transitions are studied for two-band systems by various techniques. Incomplete interference between the Cooper and Peierls channels leads to spin-density wave, d-wave superconductor, and metallic phases with decreasing dominance of the Peierls channel. The discrepancy between the spin gap observed in the static susceptibility and that in the NMR for undoped spin ladder systems is resolved by using free massive Majorana fermions. Metal-insulator transitions in (pi)-d hybrid systems are successfully described by the 1/N correction to the mean-field slave-boson approach.