Low-field dependence of the microwave absorption in superconducting $\mathrm{Y}{\mathrm{Ba}}_2{\mathrm{Cu}}_3{\mathrm{O}}_{7-x}$ near $T_c$

Novel microwave absorption and dispersion measurements have been performed on polycrystalline samples and well-characterized single-crystal platelets of high-$T_c$ superconductors $\mathrm{Y}{\mathrm{Ba}}_2{\mathrm{Cu}}_3{\mathrm{O}}_{7-x}$. The results are explained in terms of fluxoids and rapid variation of the penetration depth near and below $T_c$. The microwave absorption increases as a function of the magnetic field as a result of increase in the surface area of the normal-conducting region. By applying a modulating field in a particular manner and from thermodynamic considerations there are strong indications of energy absorption by fluxoids.