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
(I
₀
) of the YBCO film under study. It is shown that at low enough amplitudes I
₀
of the microwave current in the resonator R
_s
(I
₀
) ˜ log(I
₀
), while at high enough I
₀
amplitudes (which are less than the critical current value I
_c
for the constriction) the R
_s
(I
₀
) dependence is quadratic: R
_s
(I
₀
) = α(ω,j
_c
w,d)I
₀₂
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
d
²
w). This type of nonlinear Rs(I
₀
) dependence (at least at high enough I
₀
values) allows to assume that the origin of nonlinearity is related to the entry of microwave induced Abrikosov vortices through the film edges.