Copper composition dependence of the structure and microwave properties in Y-Ba-Cu-O films

The correlation between the copper composition and the microwave properties of YBa/sub 2/Cu/sub 3/O/sub x/ (YBCO) films is studied using coplanar waveguide resonators at 5 GHz. The c-axis lattice constant increases and the axial ratio b/a-the degree of orthorhombicity-decreases as the copper composition of the YBCO films decreases. The microwave properties of these films correlate well with these structural changes. The films with copper-rich composition have a low surface resistance R/sub s/ of 60 /spl mu//spl Omega/ at 50 K and a short magnetic penetration depth /spl lambda//sub L/(0) of 200 nm. The values of these quantities increase with decreasing copper composition. The films with less copper also show a gradual decrease in R/sub s/ below T/sub c,zero/, which is different from the tendency expected from the two-fluid model. These unusual microwave properties can be explained by the inhomogeneous superconductor model in which the regions with locally lowered T/sub c/ are distributed in the YBCO phase. The copper deficiencies that occurred in the films with less copper composition cause these inhomogeneities and thus they are an important cause of high R/sub s/ values of these YBCO films.