The two-band model including intra-atomic repulsion on Cu and near-neighbor Cu-O repulsion for the high-temperature superconductors is supported by a variety of experiments and theoretical calculations as the minimum necessary. The specific mechanism for high Tc through charge-transfer resonances was proposed because the known alternative mechanisms—phonons and magnetic excitations—were believed unlikely. The case for charge-transfer-resonance-induced high Tc has, however, not yet been proven. Equally important, the various anomalies in the metallic state are not yet understood. However, calculations on the model do show a charge-transfer-gap insulating state which is antiferromagnetic at and near 1/2 filling, a metallic state for intermediate filling with effective particle-particle attraction, and a charge-transfer instability beyond a certain filling.
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