LT-STM/STS observations on electrical field etched surfaces of YBa2Cu3O7 − δ single crystal

Surface degradation of YBa2Cu3O7 − δ makes it difficult to get a reliable and reproducible tunneling spectra. Therefore, most of tunneling spectra have shown some smeared superconducting gap structures with extremely high conductance levels at the zero biased voltage. In this study, to investigate the stability of YBa2Cu3O7 − δ crystal surface, and to get more reliable spectra, tunneling spectrum observations were carried out on in situ prepared crystal surface by electrical field etching technique with use of a scanning tunneling microscope. As a result, it is found that the crystal surface obtained by this treatment is enough stable at 4.2 K for a long time (at least for 2 h), and that the tunneling spectra observed on the locally etched surface show a fairly reproducible superconducting gap structure with a low conductance level at the zero biased voltage. The tunneling spectra also showed asymmetrical bump structures below the Fermi level outside the gap region, as observed on Bi2Sr2CaCu2O8 + δ and YBa2Cu4O8 − δ. This result shows a possibility that the asymmetric bump structure may be closely related with superconducting electronic states of CuO2 plane commonly to all of HTSs.