ATOMIC IMAGES AND TUNNELING SPECTRA ON BI2SR2CACU2O8+DELTA CLEAVED SURFACE BY STM

Atomic images of the Bi-O and Cu-O planes were selectively observed in STM experiments on slightly underdoped Bi2Sr2CaCu2O8+delta by changin g bias voltage, and tunneling spectra were measured while observing th e Cu-O plane at three tip heights (h(1) > h(2) > h(3)). The Cu-O plane images indicate that the conduction holes go mainly into O 2p(sigma), and Cu 3d(x2-y2) 2 orbitals. The spectra for h(1) and h(2) are consis tent with a gap having lines of nodes at the directions 45 degrees fro m the Cu-O bonds, as shown in our previous work. On the other hand, th e spectra for h(3), where the tip seems to be very close to the specim en surface, are quite similar to expectations for the pair-breaking du e to non-magnetic impurities in a d-wave superconductor. This, combine d with the result for the node directions, strongly supports a d(x2-y2 ) superconducting gap.