Superconductivity in cuprates: the Van Hove scenario

Abstract

Many recent experiments of angular resolved photoemission spectroscopy (ARPES) have confirmed the existence of saddle points (van Hove singularity or vHs.) close to the Fermi level in five different copper oxide compounds by three different groups, in Stanford', in Argonne2 and in Wisconsin3. These observations have been made in the following compounds : Bi2Sr2CuO6 (Bi 2201), Bi2Sr2CaCu2O8 (Bi 2212), YBa2Cu3O7 (Y123), YBa2Cu4O8 (Y124) and Nd2Ce,CuO4÷ (NCCO). These experiments establish a general feature : in very high T superconductors cuprates (T 90 K) van Hove singularities are present close to the Fermi level. This is probably not purely accidental and we think that any theoretical model must take into account these experimental facts. The origin of high T in the cuprates is still controversial and the role of these singularities in the mechanism of high T superconductivity is not yet established, but we want to stress that the model of 2D itinerant electrons in presence of v.H. singularities in the band structure has already explained a certain number of experimental facts, i.e. high L's, anomalous isotope effect4, marginal Fermi liquid effects5 and the very small values of the coherence length6. It was also been shown that the singularity is in the middle of a wide band and that in these circumstances, the Coulomb repulsion j.t is renormalized and .t is replaced by a smaller number, the effective electronphonon coupling is Xeff l.L and remains positive7. We think that this fact explains the very low T observed in Sr2RuO4, where a very narrow band has been determined by ARPES