EFFECTS OF COPPER DEFICIENCY ON THE STRUCTURE AND MICROWAVE PROPERTIES OF YBA2CU3O7-DELTA FILMS DEPOSITED BY LASER-ABLATION

The work reported here evaluated the correlation between the copper co mposition and the structural and microwave properties of YBa2Cu3O7-del ta (YBCO) films deposited on MgO substrates. Detailed x-ray diffractio n measurements revealed that the superconducting YBCO phase in the fil ms has a solubility range on the copper-poor composition side. The str ucture of the YBCO phase depends on copper composition: a lower copper composition is associated with a larger c-axis lattice constant and a decreased degree of orthorhombicity. These structural changes correla ted well with increases in normal resistivity, decreases of transition temperature T-c, and increases in microwave surface resistance R-s. T he films with less copper composition also show a more gradual decreas e in R-s below T-c than do copper-rich films. These results suggest th at changes in these properties are closely connected with copper defic iency that can occur more easily when the copper composition is low. T he copper composition is, therefore, a crucial factor in developing YB CO films with low microwave loss. (C) 1998 American Institute of Physi cs. [S0021-8979(98)05116-0].