Multiband behavior and non-metallic low-temperature state of K₀.₅₀Na₀.₂₄Fe₁.₅₂Se₂

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We report evidence for multiband transport and an insulating low-temperature normal state in superconducting K₀.₅₀Na₀.₂₄Fe₁.₅₂Se₂ with Tc≈ 20 K. The temperature-dependent upper critical field, Hc₂, is well described by a two-band BCS model. The normal-state resistance, accessible at low temperatures only in pulsed magnetic fields, shows an insulating logarithmic temperature dependence as T → 0 after superconductivity is suppressed. This is similar as for high-Tc copper oxides and granular type-I superconductors, suggesting that the superconductor-insulator transition observed in high magnetic fields is related to intrinsic nanoscale phase separation.