Multiband behavior and non-metallic low-temperature state of   K$_{0.50}$Na$_{0.24}$Fe$_{1.52}$Se$_{2}$
  We report evidence for multiband transport and an insulating low-temperature normal 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 an insulating logarithmic temperature dependence as $T \rightarrow 0$ 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. 
Hyejin Ryu, F. Wolff-Fabris, J. B. Warren, M. Uhlarz, J. Wosnitza, C.   Petrovic
cond-mat.supr-con cond-mat.mtrl-sci cond-mat.str-el