Granularity effect on microwave surface resistance in high-Tc YBa2Cu3O7-x bulk and thin film superconductor

We report the effect of variation of Josephson coupling strength in YBa2Cu3O7-x (YBCO) superconductor due to grain enlargement, grain orientation and magnetic field on microwave surface resistance (Rs). The coupling strength in the bulk samples has been increased by increasing the sample density from 4.4 to 5.3 g/cm3, whereas in thin films, the same could be increased by increasing the c-axis orientation of the grains. The value of Rs (10 GHz, 65 K) in bulk samples has been found to decrease from 52 to 4 mΩ with the increase of the coupling strength from 0.06 to 0.43 and in thin films from 930 to 600 μΩ with increase of the coupling strength from 0.92 to 2.43. The effect of grain decoupling on microwave surface resistance was studied under dc and microwave magnetic fields. The surface resistance increases gradually with the application of dc and microwave magnetic fields due to grain decoupling and finally gets saturated beyond a certain critical field. (author)