Van Hove Singularities around the Fermi level in YBa2Cu3O7: The importance of the chains

We have reproduced band structure calculations from the literature and have used them to analyze in detail the energy landscape around the Fermi level. We found three Van Hove singularities, two below (-230, -54 meV) and one above the Fermi level (+27 meV). We have studied the composition of each one of them and found that states comming from the chain do contribute in a very importan way. The contribution from the planes are indeed important and, therefore, we find that a 2D description includes the most important contributions. Nevertheless, the contribution from states out of the planes (the chains and the apical oxygen) is by no means negligable. We find that it is possible that in YBaCuO part of the condensate lies in the chains, a fact that would agree with some recent evidences concerning PrBaCuO. Our general conclusion is that the 2D description of 123-compounds might be insuficient to explain all the experimental details and that a 3D description seems compulsory to fully account for the phenomenom of superconductivity in YBaCuO.