 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.