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

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 mapping revealed the uniform elements distribution on the crystal surface while the XRD measurements indicate that the crystals are compositionally inhomogenous on nanoscale. The physical properties of the as-prepared sample are characterized by electrical resistivity, magnetization and specific heat measurements. Resistivity measurements show the onset of the superconducting transition at 33 K 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 the out-of-plane field. The anisotropy of $H_{c2}^{ab}(0)/H_{c2}^{c}(0)$ and coherence lengths $ξ^{ab}(0)/ξ^{c}$(0) was found to be around 3.7. The pioneer studies by multiple Andreev reflections effect spectroscopy ("break-junction" technique) revealed the presence of two anisotropic superconducting gaps $Δ_L\,=\,(9.3\pm1.5)\,meV$, $Δ_S\,=\,(1.9\pm0.4)\,meV$, and provided measuring of the $Δ_L$(T) temperature dependence. The BCS-ratio for the large gap $2Δ_L/k_BT_c^{bulk}\,\approx\,6.3$ points to a strong electron-boson coupling in the "driving" condensate characterized by $Δ_L$ order parameter.

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