Frequency dispersion of nonlinear response of thin superconducting films*

Effect of microwave radiation on transport properties of $La_{2-x}Sr_xCuO_4$ atomically thin films grown by Molecular Beam Epitaxy were studied. Resistance changes induced by the applied microwaves with variable frequencies (0.1-20GHz) and powers were measured at different temperatures near the superconducting transition ($\sim 8-15$K). Strong drop of three orders of magnitude of the nonlinear response is found within a few GHz of a cutoff frequency ($\omega_{cut}\sim 2GHz$). Expected frequency dependence vastly underestimates the sharpness of this drop. Numerical simulations considering an $ac$ response which follow the $dc$ I-V characteristics of the films replicate the low frequency behavior, but fail above the threshold frequency $\omega_{cut}$. The observed phenomenon suggests significant decrease of the effectiveness of vortex-antivortex dissociation induced by the oscillating superconducting condensate.