Superconducting Properties of Ultrathin YBa 2 CU 3 O 7 Films

Ultrathin YBa 2 CU 3 O 7 (YBCO) films were epitaxially grown on non-superconducting PrBa 2 CU 3 O 7 (PrBCO) buffer layer by reactive evaporation, in which the crystal growth was controlled through the monitoring of reflection high energy electron diffraction (RHEED) specular intensity oscillations. Atomic force microscope (AFM) observation reveals that the steps in the surface of ultrathin YBCO films (∼100Aa) have one-unit-cell height, implying a unit cell-by-unit cell growth manner. One-unit-cell thick (1-UCT) YBCO layer exhibits superconductivity when charge reservoir layers of BaO-CuO-BaO are located above and below the CuO 2 bilayer interposed with an Y layer. Essential transport properties of the normal state in bulk YBCO, such as T-linear dependence of resistivity and T -1 dependence Of Hall resistivity, are found to be inherent in the individual CuO 2 bilayer. Study of diagonal and Hall resistivities provides evidence of Kosterlitz-Thouless transition to be occurred in 1-UCT YBCO.