Aa. Abrikosov et al., EXPERIMENTALLY OBSERVED EXTENDED SADDLE-POINT SINGULARITY IN THE ENERGY-SPECTRUM OF YBA2CU3O6.9 AND YBA2CU4O8 AND SOME OF THE CONSEQUENCES, Physica. C, Superconductivity, 214(1-2), 1993, pp. 73-79
Angle-resolved photoemission experiments show that the high temperatur
e superconductors YBa2Cu3O6.9 and YBa2Cu4O8 have a spectral density wh
ich 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 str
onger 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 effe
ct. We conclude that this singularity alone, although it can possibly
lead to sufficiently high transition temperatures and a vanishingly sm
all isotope effect, does not explain the ratio of 2DELTA(O)/T(c) appro
ximately 6 observed experimentally.