EXPERIMENTALLY OBSERVED EXTENDED SADDLE-POINT SINGULARITY IN THE ENERGY-SPECTRUM OF YBA2CU3O6.9 AND YBA2CU4O8 AND SOME OF THE CONSEQUENCES

Angle-resolved photoemission experiments show that the high temperature superconductors YBa2Cu3O6.9 and YBa2Cu4O8 have a spectral density which exhibits an ''extended'' saddle point singularity along the GAMMA-Y symmetry direction, centered on the Y point. This, in turn, leads to a van Hove singularity in the density of states with a divergence stronger than the well-known logarithmic one. We examine the consequences of this singularity in several limiting cases on the superconducting transition temperature, the ratio 2DELTA(O)/T(c), and the isotope effect. We conclude that this singularity alone, although it can possibly lead to sufficiently high transition temperatures and a vanishingly small isotope effect, does not explain the ratio of 2DELTA(O)/T(c) approximately 6 observed experimentally.