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We report evidence for multiband transport and an insulating low-temperaturenormal state in superconducting K$_{0.50}$Na$_{0.24}$Fe$_{1.52}$Se$_{2}$ with$T_{c}\approx 20$ K. The temperature-dependent upper critical field, $H_{c2}$,is well described by a two-band BCS model. The normal-state resistance,accessible at low temperatures only in pulsed magnetic fields, shows aninsulating logarithmic temperature dependence as $T \rightarrow 0$ aftersuperconductivity is suppressed. This is similar as for high-$T_{c}$ copperoxides and granular type-I superconductors, suggesting that thesuperconductor-insulator transition observed in high magnetic fields is relatedto intrinsic nanoscale phase separation.

Multiband behavior and non-metallic low-temperature state of K$_{0.50}$Na$_{0.24}$Fe$_{1.52}$Se$_{2}$