Microwave study of ${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_{\mathrm{x}}$ single crystals

Absorption of microwaves (9.3 GHz) was measured as a function of an external applied magnetic field in single crystals of ${\mathrm{YBa}}_2$ ${\mathrm{Cu}}_3$ ${\mathrm{O}}_{\mathrm{x}}$. Three distinct types of signals were observed successively as the temperature was lowered from $T_c$ to 2.4 K. They differ in widths, angular dependences, and hysteretic properties. We discuss possible microwave loss mechanisms and a model based on internal Josephson junctions to explain our observation.