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Using the self-flux technique we grew superconducting$(K_{1-z}Na_z)_{x}Fe_{2-y}Se_2$ (z = 0.3) single crystals. The EDX mappingrevealed the uniform elements distribution on the crystal surface while the XRDmeasurements indicate that the crystals are compositionally inhomogenous onnanoscale. The physical properties of the as-prepared sample are characterizedby electrical resistivity, magnetization and specific heat measurements.Resistivity measurements show the onset of the superconducting transition at 33K and zero resistivity at 31.7 K. The large upper critical field $H_{c2}$(0)was estimated as high as about of 140 T for the in-plane field and 38 T for theout-of-plane field. The anisotropy of $H_{c2}^{ab}(0)/H_{c2}^{c}(0)$ andcoherence lengths $\xi^{ab}(0)/\xi^{c}$(0) was found to be around 3.7. Thepioneer studies by multiple Andreev reflections effect spectroscopy("break-junction" technique) revealed the presence of two anisotropicsuperconducting gaps $\Delta_L\,=\,(9.3\pm1.5)\,meV$,$\Delta_S\,=\,(1.9\pm0.4)\,meV$, and provided measuring of the $\Delta_L$(T)temperature dependence. The BCS-ratio for the large gap$2\Delta_L/k_BT_c^{bulk}\,\approx\,6.3$ points to a strong electron-bosoncoupling in the "driving" condensate characterized by $\Delta_L$ orderparameter.

Crystal growth, transport phenomena and two gap superconductivity in the mixed alkali metal $(K_{1-z}Na_z)_{x}Fe_{2-y}Se_2$ iron selenide