Atom-site specified tunneling spectroscopy on oxide superconductors and charge density wave materials

Abstract

Room temperature and cryogenic scanning tunneling microscopes have been developed with which local tunneling spectra have been obtained at specified positions with atomic resolution. Thus obtained tunneling spectra on the basal and cross-sectional planes of the layered structure of Bi2Sr2CaCu2Oy single crystal have indicated that: (1) the CuO2 layer has finite density of states at EF but the spectrum exhibits two linear branches across EF differing from a typical metal. The slopes of the two branches systematically change with the tip-to-surface distance. (2) The BiO monolayer on the cleaved surface and (BiO)2 layer exposed at the edge on the cross-sectional plane are both insulative with finite energy gaps which decrease with increase in the oxygen content of the crystal. (3) The spectrum is essentially the same at different locations on the cleaved surface. (4) The superconducting gap structure is simple with a low inner-gap conductance indicating the opening of a finite gap. The gap is significantly larger than the BCS weak coupling value, 2Δ/kBTc ~ 9. Observation on the Ta(S, Se)2 cleaved surface has revealed that the excitation spectrum is more "metallic" at the crest and more "semiconductive" at the trough of CDW.