Magnetic-field-induced log-T insulating behavior in the resistivity of fluorine-doped SmFeAsO_1−xF_x

We report the resistivity of a series of nominally fluorine-doped SmFeAsO_1−xF_x polycrystalline superconductors in magnetic fields up to 60 T. For samples where x<0.15, the low-temperature resistive state is characterized by pronounced magnetoresistance including an upturn at low temperatures. The “insulating behavior” is characterized by a log-T divergence observed over a decade in temperature, a behavior strikingly similar to the underdoped cuprates. The normal state for samples with doping x>0.15 behaves very differently: metallic behavior with little magnetoresistance, where intense magnetic fields broaden the superconducting transition rather than significantly suppressing T_c. The doping at which the insulating behavior disappears coincides with the reported collapse of the structural phase transition in the phase diagram for SmFeAsO_1−xF_x series.