Energy gap and surface structure of ${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_{7\mathrm{-}\mathit{x}}$ probed by scanning tunneling microscopy

We report the results of scanning tunneling microscopy (STM) and spectroscopy (STS) studies on high-quality single crystals of ${\mathrm{YBa}}_2$ ${\mathrm{Cu}}_3$ ${\mathrm{O}}_{7\mathrm{-}\mathit{x}}$ (YBCO) which were cleaved in situ at 20 K prior to measurement. STS reveals an energy gap with 2Δ/${\mathit{kT}}_{\mathit{c}}$=6–8 and conductance curves with small zero bias conductance. STM images show for the first time the complex atomic structure of the YBCO surface; atomic resolution disappears after heating to 70 K. Large features may be due to the electronic effects of oxygen disorder, and YBCO is shown to cleave between the BaO and CuO chain layers.