Transport and infrared properties of SmFeAs(01-xFx): from SDW to superconducting ordering

We report measurements of resistivity, magnetoresistivity, Hall effect, Seebeck coefficient and infrared reflectivity of undoped SmFeAsO and lightly doped SmFeAs(O0.93F0.07) oxypnictides. All the properties measured on SmFeAsO are characterized by clear signatures of the magnetic instability. A self-consistent picture emerges in which carrier condensation occurs below the magnetic transition, due to the opening of a spin density wave (SDW) gap. This is accompanied by the mobility increase of not-gapped carriers due to the suppression of electron-electron scattering. SmFeAs(O0.93F0.07) exhibits an increase of the metallic character on cooling consistent with electron doping, even though at room temperature values of all the properties nearly overlap with those of SmFeAsO. However, with a decrease in temperature all anomalies related to the SDW instability are missing and the superconducting transition occurs. This suggests that doping abruptly breaks the symmetries of the Fermi surface, inhibiting the SDW formation in favor of the superconducting transition, with no substantial changes in the density of states or in the effective mass.