Hall effect and magnetoresistance in single crystals of NdFeAsO1-xFx (x=0 and 0.18)

Hall effect and magnetoresistance have been measured on single crystals of NdFeAsO1-xFx with x=0 (T-c=0 K) and x=0.18 (T-c=50 K). For the undoped samples, strong Hall effect and magnetoresistance with strong temperature dependence were found below about 150 K. The magnetoresistance was found to be as large as 30% at 15 K at a magnetic field of 9 T. From the transport data we found that the transition near 155 K was accomplished in two steps: first one occurs at 155 K which may be associated with the structural transition, the second one takes place at about 140 K which may correspond to the spin-density-wave-like transition. In the superconducting sample with T-c=50 K, it is found that the Hall coefficient also reveals a strong temperature dependence with a negative sign. But the magnetoresistance becomes very weak and does not satisfy Kohler's scaling law. These dilemmatic results (strong Hall effect and very weak magnetoresistance) prevent understanding of the normal-state electric conduction by a simple multi-band model by taking into account the electron and hole pockets. Detailed analysis further indicates that the strong temperature dependence of R-H cannot be easily understood with the simple multi-band model either. A picture concerning a suppression to the density of states at the Fermi energy in lowering temperature is more reasonable. A comparison between the Hall coefficient of the undoped sample and the superconducting sample suggests that the doping may remove the nesting condition for the formation of the spin-density wave order, since both samples have very similar temperature dependence above 175 K.