Angle-resolved photoemission on untwinned ${\mathrm{YBa}}_2{\mathrm{Cu}}_3{\mathrm{O}}_{6.95}$. I. Electronic structure and dispersion relations of surface and bulk bands

Angle-resolved photoemission studies of the dispersion relations and polarization dependence of bands in the near $E_F$ region of untwinned ${\mathrm{YBa}}_2{\mathrm{Cu}}_3{\mathrm{O}}_{7-\delta }$ (Y123) single crystals reveal previously unseen details of the electronic structure and significantly expand our understanding of this system. We find that the narrow, intense feature seen along $\Gamma Y$ in Y123 and ${\mathrm{YBa}}_2{\mathrm{Cu}}_4{\mathrm{O}}_8$ (Y124) is best explained as a surface termination effect related to the quasi-one-dimensional ${\mathrm{CuO}}_3$ chains, masking underlying electronic states at the Fermi energy. Many of the previously puzzling photoemission results from these materials are readily understood within this context, including the failure to observe a superconducting gap, the weak effect of deoxygenation on the photoemission spectra for $\delta <~0.5$, the weak dependence of this feature on Pr doping, and the strong photon energy dependence of the photoemission spectra. Our data, taken in the superconducting state, reveal the bilayer splitting and $c$-axis dispersion of the ${\mathrm{CuO}}_2$ plane bands. We also find evidence for a one-dimensional chain Fermi surface parallel to the $\Gamma X$ direction, but see no indication of the BaO derived “stick” pocket centered on the $S$ point.