Current imaging tunneling spectroscopy characterization of YBa_2Cu_3O_(7-X) and Bi_2Sr_2CaCu_2O_(8+X) single crystals

A combined scanning electron microscope-scanning tunneling microscope (SEM-STM) system has been used to investigate surface topographic and electronic properties of YBa2Cu3O7-x (YBCO) and Bi2Sr2Ca-Cu2O8+x (Bi-2212) single crystals. Current imaging tunneling spectroscopy (CITS) measurements reveal the coexistence of areas with metallic and semiconducting characteristics in regions close to micron-sized growth steps imaged by SEM in the YBCO and Bi-2212 samples. On the contrary, a metallic behaviour is always found by CITS in extended areas of the crystals far from the growth steps. This inhomogeneous electronic behaviour is attributed to the defects related to the surface oxygen stoichiometry or distribution. Modifications induced by means of voltage pulses applied with the STM have also been investigated. We report on the threshold voltages necessary to modify the surface of YBCO and Bi-2212 single crystals. Furthermore, CITS measurements have been carried out in the modified areas in order to analyze the surface alterations caused in these materials