Specific features of the nonlinear microwave response of the (Na$_{0.3}$K$_{0.7}$)$_{x}$Fe$_{2-y}$Se$_{2}$ sodium–potassium ferroselenide-based multiband superconductors

Temperature dependences of the third-harmonic power for the (Na$_{0.3}$K$_{0.7}$)$_{x}$Fe$_{2-y}$Se$_{2}$ sodium–potassium ferroselenide single crystals have been investigated by nonlinear near-field microwave microscopy. The temperature dependence of magnetic susceptibility at a frequency of 100 kHz for this compound has been measured. The experimental results suggest a double-gap structure of the (Na$_{0.3}$K$_{0.7}$)$_{x}$Fe$_{2-y}$Se$_{2}$ superconductor.