Hall effect and magnetoresistance in single crystals of NdFeAsO$_{1-x}$F$_{x}$

摘要

Hall effect and magnetoresistance have been measured on single crystals of$NdFeAsO_{1-x}F_{x}$ with x = 0 ($T_c$ = 0 $ $K) and x = 0.18 ($T_c$ = 50 $$K). For the undoped samples, strong Hall effect and magnetoresistance withstrong temperature dependence were found below about 150 K. Themagnetoresistance was found to be as large as 30% at 15 K at a magnetic fieldof 9 T. From the transport data we found that the transition near 155 K wasaccomplished in two steps: first one occurs at 155 K which may be associatedwith the structural transition, the second one takes place at about 140 K whichmay correspond to the spin-density wave like transition. In the superconductingsample with $T_c$ = 50 $ $K, it is found that the Hall coefficient also revealsa strong temperature dependence with a negative sign. But the magnetoresistancebecomes very weak and does not satisfy the Kohler's scaling law. Thesedilemmatic results (strong Hall effect and very weak magnetoresistance) preventto understand the normal state electric conduction by a simple multi-band modelby taking account the electron and hole pockets. Detailed analysis furtherindicates that the strong temperature dependence of $R_H$ cannot be easilyunderstood with the simple multi-band model either. A picture concerning asuppression to the density of states at the Fermi energy in loweringtemperature is more reasonable. A comparison between the Hall coefficient ofthe undoped sample and the superconducting sample suggests that the doping mayremove the nesting condition for the formation of the SDW order, since bothsamples have very similar temperature dependence above 175 K.