Multiband transport and nonmetallic low-temperature state of K_0.50Na_0.24Fe_1.52Se₂

We report evidence for multiband transport and an insulating low-temperature normal state in superconducting K_0.50Na_0.24Fe_1.52Se₂ with T_c approximate to 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 an insulating logarithmic temperature dependence as T -> 0 after superconductivity is suppressed. This is similar as for high-T_c 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.