Doping dependence of the upper critical field and Hall resistivity of LaFeAsO_1−xF_x (x=0, 0.025, 0.05, 0.07, 0.11, and 0.14)

The electrical resistivity (ρ_xx) and Hall resistivity (ρ_xy) of LaFeAsO_1−xF_x have been measured over a wide fluorine-doping range 0≤x≤0.14 using 60 T pulsed magnets. While the superconducting phase diagram (T_c,x) displays the classic dome-shaped structure, we find that the resistive upper critical field (H_c2) increases monotonically with decreasing fluorine concentration, with the largest H_c2≥75 T for x=0.05. This is reminiscent of the composition dependence in high-T_c cuprates and might correlate with opening of a pseudogap in the underdoped region. Furthermore, the temperature dependence of H_c2(T) for superconducting samples can be understood in terms of multiband superconductivity. ρ_xy data for nonsuperconducting samples show nonlinear field dependence, which is also consistent with a multicarrier scenario.