Evidence for a surface-derived electronic state on ${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_{6.8}$

From angle-resolved photoemission spectroscopy performed on cleaved surfaces of twinned ${\mathrm{YBa}}_2$ ${\mathrm{Cu}}_3$ ${\mathrm{O}}_{6.8}$ single crystals, we found strong evidence for the existence of an intrinsic surface-derived electronic band in this high-temperature superconductor. It is observable only near the X (Y) point of the Brillouin zone, showing slight ${\mathit{k}}_{\mathrm{?}}$—and almost no ${\mathit{k}}_{\mathrm{\perp }}$—dispersion and a very pronounced sensitivity to surface contamination. We discuss the relation of this state to possible crystal terminations and compare to a calculation of the surface electronic structure.