Momentum Dependence of the Superconducting Gap inBi2Sr2CaCu2O8

Abstract

We measure the momentm dependence of the superconducting gap along the Fermi surface of ${\mathrm{Bi}}_{2}{\mathrm{Sr}}_{2}{\mathrm{CaCu}}_{2}{\mathrm{O}}_{8}$ by high resolution angle-resolved photoemission spectroscopy. The gap is large in the vicinity of the $\overline{M}$ point (where $\ensuremath{\gamma}$- $\overline{M}$ is along the Cu-O bond), and small along the $\ensuremath{\gamma}$- $X$ and $\ensuremath{\gamma}$- $Y(\ensuremath{\pi},\ensuremath{\pi})$ directions. However, the gap is not zero along these directions, but, within our accuracy, becomes zero about ${10}^{\ensuremath{\circ}}$ on either side of the $(\ensuremath{\pi},\ensuremath{\pi})$ directions. We discuss the implications of these results for the symmetry of the order parameter.