Surface structures and energy gap of YBa₂Cu₃O_y/PrBa₂Cu₃O_y superlattices and YBa₂Cu₃O_y thin films probed with a scanning tunneling microscope and a scanning force microscope

We report here the results of investigations using a scanning tunneling microscope (STM) and a scanning force microscope (STM) on YBa₂Cu₃O_y/PrBa₂Cu₃O_y (YBCO/PrBCO) superlattices and YBCO thin films. STM and SFM measurements were performed in air at 295 K while the tunneling spectroscopy was carried out in vacuum (< 10^-3 Torr) at 4.3 K. STM images were observed to exhibit the atomic structure of [(YBCO)_n/(PrBCO_m)]_l superlattices where the subscripts n and m refer to the numbers of unit cells of the individual YBCO or PrBCo layers respectively and l is the number of repetitions of the (YBCO)_n/(PrBCO)_m cell. The STM images of a [(YBCO)_l/(PrBCO)_l]₄₀ superlattice show atoms aligned with a lattice parameter of 3.81 Å along the crystal a-axis and 3.89 Å along the crystal b-axis. SFM images show a periodic variation on the YBCO surface. In the tunneling spectra, the current-voltage characteristics at 4.3 K show Coulomb staircases. The tunneling conductance at 4.3 K reveals conductance peaks at 2 Δ = 3.5, 17.0; 23.5 and 41 meV and the curve dI/dV versus V has a zero bias conductance. The tunneling spectra of YBCO thin films showed semiconducting or superconducting behaviors depending on the tip-sample distance.