Mechanisms of direct microwave absorption in Y1Ba2Cu3O7−x

Direct microwave absorption was studied as a function of magnetic field up to 8 kG and in a temperature range between 10 K and 150 K in Y₁Ba₂Cu₃O_7−x. Large microwave absorption was observed below T_c over a wide magnetic field range. The microwave power absorption is approximately described by I=A(T) H+C(T), where A(T) and C(T) are temperature dependent constants. A(T) is proportional to the square root of the microwave power, while C(T) shows a different behaviour. We propose different origins for A(T) and C(T), namely fluxons in grains and weak Josephson links. Moreover, also flux trapping was observed in the superconducting state. Possible mechanisms for the absorption are discussed.