Microwave Surface Impedance of High-Temperature Superconducting YBa₂Cu₃O₇-δ Films in DC Magnetic Field (Method of Coplanar Resonator)

The field and temperature dependences of microwave surface impedance Z(s)(T, H-0) = Rs(T, H-0) + iX(s)(T, H-0) in superconducting perfect single-crystalline YBCO films, deposited by magnetron sputtering on sapphire-CeO2-sublayer (YSZ) substrate, are obtained by use of coplanar resonator technique at frequency f = 5.25 GHz in the presence of applied weak dc magnetic field H, (0 <H-0 <<12 mT). The microwave measurements are performed within the temperature range, 13 <T <80 K; they allow to obtain the surface resistance R,(T, H-0) as well as the penetration depth 5(T, H,) dependences on temperature and dc magnetic field values and to extract the contribution of oscillating Abrikosov vortices to the microwave response of the film. The power level P-in and amplitude of the microwave field H-similar to used in these experiments are rather small at all measurements thus providing the obtained results correspondent to the linear response regime. Experiments of two kinds in applied de magnetic field H, are performed, namely, 'field cooled' (FC) and 'zero field cooled' (ZFC) measurements of Z(s)(T, H-0). These experiments have revealed a significant difference of the microwave vortex response and corresponding dependences of R,(T, H-0) for FC and ZFC regimes. The vortex contribution to the surface resistance and possible explanation of obtained experimental results are discussed within the framework of the Coffey-Clem and Brandt phenomenological theories of ac linear vortex response and flux penetration inside a superconducting strip.