Several suggested mechanism for high temperature superconductivity

Based on the existing theoretical works on high-temperature superconducting (HTSC) oxides, the materials can be separated into three types. The first type of HTSC oxides has no Cu atom (for example, BaPb1-xBixO3, Ba1-xKxBiO3, and Li1+xTi2-xO4) and exhibits 3-dimensional isotropic superconducting (SC) properties. It has been pointed out that the missing valence in these HTSC materials (for example, in BaPb1-xBixO3 the valence of Pb ion is either +2 or +4, but not +3; in Ba1-xKxBiO3 the valence of Bi ion is ether +3 or +5, but not +4; and Li1+xTi2-xO4 the valence of Ti ion is either +2 or +4, but not +3) is a strong indication of the existence of the negative-U centers. That is, when two quasiparticles of same charge localize in the same region, they attract each with an effective interaction energy U < O. The physics of negative-U centers was investigated using the negative-U extended Hubbard model. It was found that in this model the local-pairing SC phase and the charge-density-wave phase compete with each other, a phenomenon observed also in HYSC oxides. The calculated SC transition temperature Tc (in relative scale not given) and the measured Tc have common characteristic feature. While theoretical analysis along this line was reactivated recently, in this paper the author does not discuss more of this type of HTSC materials