SINGLE-BAND, TRIPLE-BAND, OR MULTIPLE-BAND HUBBARD MODELS

The relevance of different models, such as the one-band t-J model and the three-band Emery model, as a realistic description of the electronic structure of high-T(c) materials is discussed. Starting from a multiband approach using cluster calculations and an impurity approach, the following questions are addressed: What is the stabilization energy of the Zhang-Rice singlet and triplet with respect to other symmetry levels (such as the d3z2-r2, etc.)? How does the oxygen 2p-band width (t(pp)) influence the stability of the Zhang-Rice singlet? How large is the dispersion of the singlet state? It is argued that, if the one-band model does not give a satisfactory description of the low-energy physics, then the three-band model will be incomplete as well.