Gap anisotropy in Bi 2 Sr 2 CaCu 2 O 8 + δ by ultrahigh-resolution angle-resolved photoemission

Ultrahigh-resolution (\ensuremath{\Delta}E up to 10 meV) angle-resolved photoemission spectroscopy on single crystals of ${\mathrm{Bi}}_{2}$${\mathrm{Sr}}_{2}$${\mathrm{CaCu}}_{2}$${\mathrm{O}}_{8+\mathrm{\ensuremath{\delta}}}$ (Bi-2212) show a highly anisotropic superconducting energy gap: the gap is minimized, with a value close to zero, along the \ensuremath{\Gamma}-X and \ensuremath{\Gamma}-Y symmetry directions, while a large gap (\ensuremath{\Delta}=22 meV) is observed along the \ensuremath{\Gamma}-M\ifmmode\bar\else\textasciimacron\fi{} (Cu-O bond) direction. However, the observation of gap anisotropy may depend on sample conditions; in a few cases the gap becomes more isotropic. A dip in the spectral weight at 65 meV below the Fermi energy is clearly observed only in samples with an anisotropic gap.