Using the method of near-field microwave microscopy, we investigated the temperature dependence of the third harmonic power P3ω (T) of superconductor – normal metal MoN / Al structures with proximity effect. The presence of an additional (low-temperature) maximum and vanishing in the dependence P3ω (T) for the MoN / Al structure, which disappear when a weak perpendicular magnetic field is applied.

AbstractAn unusual decrease in the slope of the upper critical field near T _ c 0 at a gradual increase in the ion implantation dose has been experimentally observed in narrow bridges formed on the base of thin HTSC YBa_2Cu_3O_7 – _ x films, while an increase in the defect concentration usually leads to an increase in the local slope of the phase transition line H _ c 2( T ). In addition, it has been found that the temperature dependence of the upper critical field has a positive curvature near T _ c 0. A possible theoretical interpretation of the results is proposed. It is based on the modified Ginzburg–Landau theory with a nonuniform length of superconducting correlations.

Abstract —Temperature dependences of the third-harmonic power for the (Na_0.3K_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_0.3K_0.7)_ x Fe_2 – _ y Se_2 superconductor.