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 changing bias voltage, and tunneling spectra were measured while observing the 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 consistent with a gap having lines of nodes at the directions 45 degrees from the Cu-O bonds, as shown in our previous work. On the other hand, the spectra for h(3), where the tip seems to be very close to the specimen surface, are quite similar to expectations for the pair-breaking due to non-magnetic impurities in a d-wave superconductor. This, combined with the result for the node directions, strongly supports a d(x2-y2) superconducting gap.