Observation of an "Extended" Van Hove Singularity in YBa2Cu4O8 by Ultrahigh Energy Resolution Angle-Resolved Photoemission

Abstract

We characterize the nature of the Van Hove singularity near the Fermi energy ($E_F$) in Y${\mathrm{Ba}}_2$ ${\mathrm{Cu}}_4$ ${\mathrm{O}}_8$ using ultrahigh energy resolution (10 meV) angle-resolved photoemission spectroscopy, together with corresponding first-principles spectra intensity computations. The singularity, related to Cu${\mathrm{O}}_2$ planes, is located about 19 meV below $E_F$ at the $Y$ point in the Brillouin zone and arises from a band which is nearly dispersionless along the $\Gamma$ 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 $T_c\text{'}\mathrm{s}$ of order 100 K even in a weak coupling BCS scheme.