Observation of an "extended" Van Hove singularity in YBa2Cu4O8 by ultrahigh energy resolution angle-resolved photoemission

We characterize the nature of the Van Hove singularity near the Fermi energy (EF) in YBa2Cu4O8 using ultrahigh energy resolution (10 meV) angle-resolved photoemission spectroscopy, together with corresponding first-principles spectra intensity computations. The singularity, related to CuO2 planes, is located about 19 meV below EF at the Y point in the Brillouin zone and arises from a band which is nearly dispersionless along the Γ to Y direction. Such an "extended" saddle point would render the band structure quasi-1D and yield a power law divergence in the density of states, allowing the occurrence of Tc's of order 100 K even in a weak coupling BCS scheme.