CORRELATION BETWEEN THE MICROWAVE SURFACE-RESISTANCE AND THE VOLUMIC FRACTION OF A-AXIS GRAINS IN YBA2CU3O7-X FILMS

A quantitative study of the effect of a-axis grains (the a-axis is per pendicular to the substrate plane) on the microwave surface resistance in epitaxial YBa2Cu3O7-x films is reported. The films were grown by M etal Organic Chemical Vapor Deposition on MgO and LaAlO3 substrates. T he fraction of a-axis grains %a has been varied in the range 0-70% by changing the oxygen partial pressure during the growth. A clear correl ation between the surface resistance R-s(0.85T(c)), measured at 87 GHz , and the volumic fraction %a is found. At variance, the residual surf ace resistance R-s(4.2 K) does not correlate to %a. (C) 1998 Elsevier Science B.V. All rights reserved.