Granular metallicity as a minimal normal state for superconductivity

We report the evolution of electrical transport properties in insulating FeSe films with electron doping induced by the ionic liquid gating technique. Superconductivity never emerges in the strong insulators with variable-range hopping behavior but is shown to arise once the resistance of the normal state varies as ln(1/T) indicating that this behavior corresponds to the minimal conducting character for developing superconductivity. Our work...