Observation of a-b plane gap anisotropy in ${\mathrm{Bi}}_2$${\mathrm{Sr}}_2$${\mathrm{CaCu}}_2$${\mathrm{O}}_8$ with a low temperature scanning tunneling microscope

Significant gap anisotropy within the a-b plane of ${\mathrm{Bi}}_2$ ${\mathrm{Sr}}_2$ ${\mathrm{CaCu}}_2$ ${\mathrm{O}}_8$ has been observed with a low temperature scanning tunneling microscope. The energy gap is largest along the a-axis direction, with ${\mathrm{\Delta }}_{\mathit{p}\mathrm{-}\mathit{p}}$=36.6 meV and decreases gradually to a value of 23 meV along the direction 45° from the a axis. The tunneling data have been fit well using a simple model which assumes a d-wave-pairing energy gap, and allows tunneling in all directions close to the normal of the surface.