Gap anisotropy of Bi2Sr2CaCu2O8 measured by low-temperature scanning tunneling microscopy

Energy gaps of single-crystal Bi2Sr2CaCu2O8 in the c-axis and the ab-plane directions are measured directly by the scanning-tunneling-microscopy method. Due to the short coherence length along the c axis, conductance curves in this direction are seriously smeared and do not fit the conventional BCS density of states. In the ab direction, the shapes of the conductance curves are different from that of c-axis tunneling and show sharper gap features. The energy gap in the ab direction is measured to be 35 meV, which is 10 meV larger than the value in the c-axis direction.