ULTRATHIN FILMS AND SUPERLATTICES OF HIGH-TC SUPERCONDUCTORS CONTROLLED IN THE UNIT-CELL SCALE USING RHEED OSCILLATIONS

Isolated ultrathin films of YBa2Cu3O7-x (YBCO) on PrBa2Cu3O7-x (PrBCO) and YBCO/PrBCO superlattices were epitaxially grown on SrTiO3 by reactive coevaporation with measurement of reflection high energy electron diffraction (RHEED) specular intensity. X-ray diffraction profiles and cross sectional electron microscopic (TEM) observation reveal that the heterostructures are excellent crystallinity with no interface disorder and fit for the purpose of the study of superconductivity of the 1 unit-cell layer of YBCO. The 1 unit-cell layer of YBCO sandwiched between PrBCO layers exhibits a superconducting transition with the onset temperature at 70K and the zero resistance at 30K. A reduction of the onset temperature is found to be mainly due to a decrease of the hole carriers but not due to the absence of interlayer coupling. It was demonstrated that the 1 unit-cell layer of YBCO needed the overlayer of PrBCO to give rise to superconductivity. The overlayer of PrBCO may have the role of supply of carriers for the underlying YBCO layer. The resistive transition for the ultrathin YBCO layer below 4 unit-cells thick was independent of the magnetic field up to 10T applied parallel to the CuO2 layer.