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

Abstract—Temperature dependences of the third-harmonic power for the (Na0.3K0.7)xFe2 – ySe2 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 (Na0.3K0.7)xFe2 – ySe2 superconductor.