Angular dependence of the in-plane energy gap by tunneling spectroscopy

A technique for tunneling into the a-b plane of the layered cuprate superconductors is described and applied to study the in-plane gap anisotropy of the high-(Formula presented) superconductor (Formula presented)(Formula presented)(Formula presented)(Formula presented). The results show unambiguously that the in-plane energy gap is highly anisotropic with a gap minimum along the Cu-(Formula presented) bond direction. Using these results, we present an angular mapping of the in-plane gap anisotropy and compare our results to those from other experiments. Effects of the gap anisotropy on the tunneling spectra will also be discussed.