Upper critical fields and magnetic phase diagram of F-doped LaFeAsO

The high-T_c superconductivity in ReFeAsO_1−xF_x (Re=Rare earths) with a maximum T_c of about 55 K has attracted great interest. LaFeAsO_1−xF_x is the first-discovered superconducting compound in this class of materials, and has been intensively studied so far. We have performed resistivity measurements on polycrystalline samples of LaFeAsO_1−xF_x with different T_c’s and obtained the superconducting-to-normal state transition at various temperatures with 60 T pulsed magnetic fields. At the normal state, the under-doped samples show a positive magnetoresistivity ~ 30% at 60 T while for the optimally-doped sample, the magnetoresitivity effect is almost negligible. In the superconducting state, the magnetoresistance shows a broad transition, which is often observed in polycrystalline samples consisting of randomly oriented anisotropic grains. The temperature dependence of H_c2 (T) for all LaFeAsO_1−xF_x samples with different T_c’s do not follow the conventional Werthammer-Helfand-Hohenberg (WHH) behavior, and shows an almost linear temperature dependence down to ~ 0.1 T_c. A strong deviation of H_c2 (T) from the WHH prediction will be discussed in terms of a multiband effect and a Pauli paramagnetic limiting effect.