The Nonlinear Microwave Surface Impedance of HTS Cuprate YBA2CU3O7-δ Thin Film Patterned as a Stripline Resonator

Our measurements performed at low input power levels (P_in ≪ P_pit) have revealed the nonlinear character of the surface resistance R_s(I₀) of the YBCO film under study. It is shown that at low enough amplitudes I₀ of the microwave current in the resonator R_s(I₀) ˜ log(I₀), while at high enough I₀ amplitudes (which are less than the critical current value I_c for the constriction) the R_s(I₀) dependence is quadratic: R_s(I₀) = α(ω,j_cw,d)I₀₂ with a proportionality coefficient α, depending on the microwave frequency ω, critical current density j_c, resonator width w and thickness of the film d: α(ω, j_c, w, d) ˜ ω(j_c ² d²w). This type of nonlinear Rs(I₀) dependence (at least at high enough I₀ values) allows to assume that the origin of nonlinearity is related to the entry of microwave induced Abrikosov vortices through the film edges.