STM study of the electronic density of states in the superconducting phase of Bi 2Sr 2CaCu 2O 8

Abstract

Tunneling spectroscopic measurement was carried out in the superconducting phase of Bi 2Sr 2CaCu 2O 8 with use of a low temperature scanning tunneling microscope. Both the cleaved Bi-O surface and the lateral surface of a single crystal were investigated. The tunneling conductance shows an explicit superconducting gap structure at both surfaces. For the lateral surface the conductance curve is independent of the tip distance from the sample surface and gives directly the electronic density of states in the superconducting phase. The obtained gap structure is in good agreement with the BCS prediction, although it seems that the gap is slightly anisotropic. The obtained gap-ratio 2Δ/k BT c=6.1 indicates the strong-coupling nature of this superconductor. For the Bi-O surface, on the other hand, the normalized differential conductance increases with increasing the tip distance, especially at the outside of the superconducting gap as reported by Hasegawa et al. Such a dependence on the tip distance is attributed to series barriers of Bi-O, Sr-O layers and vacuum for the tunneling between the Cu-O layer and the tip metal.