Gap anisotropy in Bi[sub 2]Sr[sub 2]CaCu[sub 2]O[sub 8+[delta]] by ultrahigh-resolution angle-resolved photoemission

Ultrahigh-resolution ([Delta][ital E] up to 10 meV) angle-resolved photoemission spectroscopy on single crystals of Bi[sub 2]Sr[sub 2]CaCu[sub 2]O[sub 8+[delta]] (Bi-2212) show a highly anisotropic superconducting energy gap: the gap is minimized, with a value close to zero, along the [Gamma]-[ital X] and [Gamma]-[ital Y] symmetry directions, while a large gap ([Delta]=22 meV) is observed along the [Gamma]-[ital [bar M]] (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.