= Charge-transfer insulator
A correlated transition-metal compound whose lowest charge excitation primarily transfers an electron between a ligand orbital and a metal orbital. Its gap is controlled by charge-transfer and Coulomb energies rather than a weakly interacting single-band picture alone. Many cuprate parent compounds are usefully described this way. This distinguishes the orbital character of the low-energy excitation from an idealized single-band <Mott insulator>.
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