Magnetically and thermally modulated microwave absorption in Y-Ba-Cu-O single crystal near T_c

The microwave absorption R in the Y-Ba-Cu-O single crystals was investigated near {T_c \approx 92 K} and in the external magnetic field {0<H \leq 9 kOe}. A modified ESR spectrometer was used in the experiment. The method of temperature modulation, along with the usual method of magnetic-field modulation, was first applied in studying of the microwave response of these crystals. Peaks in the temp erature dependencies of the signals {\partial R}/ {\partial H} and {\partial R} /{\partial T} observed in the vicinity of {T_c} were differently shaped and slightly shifted one with respect to another. The evolution of the peaks with variation of the magnetic field and angle {\theta} between {\bf H} and the {\bf c}-axis was traced. It has been shown that the observed difference of the temperature dependencies of the derivatives {\partial R}/{\partial H} and {\partial R}/{\partial T} occures due to the field-induced broadening of the superconducting transition, which is inherent in the high-{T_c} superconductors.