Magnetic-field-induced log-T insulating behavior in the resistivity of fluorine-doped SmFeAsO1-x Fx

We report the resistivity of a series of nominally fluorine-doped SmFeAsO1-x Fx 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 Tc. The doping at which the insulating behavior disappears coincides with the reported collapse of the structural phase transition in the phase diagram for SmFeAsO1-x Fx series.