Momentum Dependence of the Superconducting Gap inBi2Sr2CaCu2O8

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 $\gamma$- $\overline{M}$ is along the Cu-O bond), and small along the $\gamma$- $X$ and $\gamma$- $Y(\pi ,\pi )$ directions. However, the gap is not zero along these directions, but, within our accuracy, becomes zero about ${10}^{\circ }$ on either side of the $(\pi ,\pi )$ directions. We discuss the implications of these results for the symmetry of the order parameter.