EXTREMELY SMOOTH YBCO-FILMS ON MGO WITH HIGH CRITICAL CURRENT DENSITIES

We have prepared segregation free, extremely smooth YBCO films on MgO substrates by our technique of thermal co-evaporation. The two key ingredients were selection of high quality substrates by RHEED, and a very close control of the stoichiometry. The absolute composition of the films was determined by heavy-ion-RBS with an accuracy of 1 % for the content of each metal. Deviations from stoichiometry by 2 - 3 % manifest themselves by characteristic surface morphologies while films with ideal stoichiometry reveal no surface structure at all with FE-SEM but a residual roughness of about 5 monolayers (5,5 nm) with STM. All films near stoichiometry have T(c)'s of 90 K and j(c)'s of 3.10(6) A/cm2 at 77 K.