Atomic Images and Tunneling Spectra on Bi₂Sr₂CaCu₂O₈+δ Cleaved Surface by STM

Abstract

Atomic images of the Bi--O and Cu--O planes were selectively observed in STM experiments on slightly underdoped Bi₂Sr₂CaCu₂O_8+δ by changing bias voltage, and tunneling spectra were measured while observing the Cu--O plane at three tip heights (h₁>h₂>h₃). The Cu--O plane images indicate that the conduction holes go mainly into O 2p_σ and Cu 3d_x²_-y² orbitals. The spectra for h₁ and h₂ are consistent with a gap having lines of nodes at the directions 45° from the Cu--O bonds, as shown in our previous work. On the other hand, the spectra for h₃, 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.