Superconducting-gap anisotropy in ${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_{7\mathrm{-}\mathrm{\delta }}$:mPhotoemission results on untwinned crystals

Polarization-dependent angle-resolved photoemission studies of untwinned single crystals of ${\mathrm{YBa}}_2$ ${\mathrm{Cu}}_3$ ${\mathrm{O}}_{6.95}$ (Y123) reveal modifications of the near-Fermi-edge spectral weight which are indicative of an anisotropic superconducting gap. The measured magnitude of the leading edge shift increases monotonically from the Γ-S line toward the zone boundary, reaching a maximum value of approximately 25 meV, similar to the behavior seen in ${\mathrm{Bi}}_2$ ${\mathrm{Sr}}_2$ ${\mathrm{CaCu}}_2$ ${\mathrm{O}}_{8+\mathrm{\delta }}$ (Bi2212). Within experimental uncertainty, we find variation which is consistent with that expected for a ${\mathrm{d}}_{{\mathrm{x}}^2\mathrm{-}{\mathrm{y}}^2}$ order parameter, providing additional evidence in support of the d-wave pairing hypothesis.