Microwave surface resistance and vortices in high-Tcsuperconductors: Observation of flux pinning and flux creep

History dependence and logarithmic time dependence of the microwave surface resistance have been observed in polycrystalline ${\mathrm{YBa}}_2$ ${\mathrm{Cu}}_3$ ${\mathrm{O}}_{7\mathrm{-}\mathit{x}}$ in a magnetic field. We interpret our data in terms of granularity and use the model of Gittleman and Rosenblum to relate the surface resistance to the number of vortices in the sample. At a given field, we find that the surface resistance of a zero-field-cooled sample is higher than that of a field-cooled sample, suggesting that the former contains more intergranular vortices. It is also observed that once the external dc field is removed, flux creep out of the sample results in a logarithmic time decay of the surface resistance, from which we estimate a thermal activation energy at 77 K of about 0.1 eV.