The resistance anomaly in the surface layer of Bi${}_2$Sr${}_2$CaCu${}_2$O${}_{8+x}$ single crystals under radio-frequency irradiation

We observed that radio-frequency (rf) irradiation significantly enhances the $c$-axis resistance near and below the superconducting transition of the CuO${}_2$ layer in contact with a normal-metal electrode on the surface of Bi${}_2$Sr${}_2$CaCu${}_2$O${}_{8+x}$ single crystals. We attribute the resistance anomaly to the rf-induced charge-imbalance nonequilibrium effect in the surface CuO${}_2$ layer. The relaxation of the charge-imbalance in this highly anisotropic system is impeded by the slow quasiparticle recombination rate, which results in the observed excessive resistance.