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Current-assisted and thermoassisted non-linearity in radio-frequency absorption in high-Tc materials

A model describing the modulated microwave ( omega /2 pi approximately 1010 Hz) and moderate frequency (104-107 Hz) absorption in high-Tc superconductors is suggested. At microwaves the response of the absorption to the low-frequency modulation is attributed to the interaction of the microwave field with the flux lines oscillating near the pinning centres. The non-linearity is caused by the fluxon displacement due to the shielding supercurrent. It is shown that the experimental data on low-field modulated microwave absorption are consistent with the pinning potential U(u) varies as -(1+cos(au)) similar to that for Josephson weak links. At moderate frequencies the non-linear response is explained by the thermoassisted fluxon jumps over the potential barriers at omega tau <<1, where tau is the characteristic time of the jump.