Charge transfer insulators at half-filling in multiband models of cuprates

Self-consistent mean-field three-band and four-band Hubbard models are used to study the collapse of the Mott gap in doped cuprates. While no set of doping-independent parameters can explain the observed gaps for the entire doping range, the experimental results are consistent with a weakly doping dependent Hubbard U. A key finding is that, when the Cu-O splitting energy $\Delta$ is large, the cuprates behave as Mott insulators. However, for small $\Delta$, the cuprates become charge transfer insulators.