Microwave heating-induced static magnetic flux penetration in YBa ₂Cu ₃O _7-δ superconducting thin films

The magneto-optical imaging technique is used to visualize the penetration of the magnetic induction in YBa ₂Cu ₃O _7-δ thin films during surface resistance measurements. The in situ surface resistance measurements were performed at 7 GHz using the dielectric resonator method. When only the microwave magnetic field H _rf is applied to the superconductor, no H _rf-induced vortex penetration is observed, even at high rf power. In contrast, in the presence of a constant magnetic field superimposed on H _rf, we observe a progression of the flux front as H _rf is increased. A local thermometry method based on the measurement of the resonant frequency of the dielectric resonator placed on the YBa ₂Cu ₃O _7-δ thin film shows that the H _rf-induced flux penetration is due to the increase of the film temperature.