  Experiments have shown that the families of cuprate superconductors that have
the largest transition temperature at optimal doping also have the largest
oxygen hole content at that doping. They have also shown that a large
charge-transfer gap, a quantity accessible in the normal state, is detrimental
to superconductivity. We solve the three-band Hubbard model with cellular
dynamical mean-field theory and show that both of these observations follow
from the model. Cuprates play a special role amongst doped charge-transfer
insulators of transition metal oxides because copper has the largest covalent
bonding with oxygen.
