Charge fluctuation mechanisms for high temperature superconductivity.

Some general arguments affecting the likelihood that charge fluctuations can promote superconductivity are discussed, and a calculational framework is presented. Simple exciton and plasmon mechanisms are suggested to be unlikely candidates, as are intraband charge fluctuations nearby a charge-density wave instability. The most successful mechanisms are argued to involve strongly-localized interband, or charge-transfer excitations, and a specific model of a low lying A sub (1g)- symmetry mode is discussed in detail.