Phenomenological models for the gap anisotropy of Bi2Sr2CaCu2O8 as measured by angle-resolved photoemission spectroscopy

Abstract

Recently, high-resolution angle-resolved photoemission spectroscopy has been used to determine the detailed momentum dependence of the superconducting gap in the high-temperature superconductor ${\mathrm{Bi}}_2$ ${\mathrm{Sr}}_2$ ${\mathrm{CaCu}}_2$ ${\mathrm{O}}_8$. In this paper, we first describe tight-binding fits to the normal-state dispersion and superlattice modulation effects. We then discuss various theoretical models in light of the gap measurements. We find that the simplest model which fits the data is the anisotropic s-wave gap cos(${\mathit{k}}_{\mathit{x}}$)cos(${\mathit{k}}_{\mathit{y}}$), which within a one-band BCS framework suggests the importance of next-near-neighbor Cu-Cu interactions. Various alternative interpretations of the observed gap are also discussed, along with the implications for microscopic theories of high-temperature superconductors.