Etch pits and tunnel barrier in chemically etched YBa2Cu3O7-δ crystals by scanning tunneling microscopy

By imaging in air the (001) surfaces of YBa2Cu3O7-δ single crystals etched in dilute CH3OH : Br2 solution with scanning tunneling microscopy we have observed etch pits and followed their modification with time. The current-voltage characteristics of the junction are measured and described in terms of electron tunneling through a trapezoidal potential barrier of average thickness (≈ 25 Å). Such a barrier results from the passivated layer formed on the superconductor surface exposed to room air after the etching. The superconductor surface is metallic. The normalized conductance G(V) measured at room temperature is that of a metal-insulator-metal junction except at small bias. Sharp peaks assigned to local density-of-states features within 1 eV from the Fermi level are observed.