Transport and superconducting properties of Fe-based superconductors: a comparison between SmFeAs(O1-xFx) and Fe1+y(Te1-x,Sex)

In this paper we carry out a direct comparison between transport and superconducting properties--namely resistivity, magnetoresistivity, Hall effect, Seebeck effect, thermal conductivity, upper critical field--of two different families of Fe-based superconductors, which can be viewed in many respects as end members: SmFeAsO 1 - x F x with the largest T c and the largest anisotropy and Fe 1 + y Te 1 - x Se x , with the largest H c2 , the lowest T c and the lowest anisotropy. In the case of the SmFeAsO 1 - x F x series, we find that a single-band description allows us to extract an approximate estimation of band parameters such as carrier density and mobility from experimental data, although the behaviour of the Seebeck effect as a function of doping demonstrates that a multiband description would be more appropriate. On the contrary, experimental data for the Fe 1 + y ( Te 1 - x , Se x ) series exhibit a strongly compensated behaviour, which can be described only within a multiband model. In the Fe 1 + y ( Te 1 - x , Se x ) series, the role of the excess Fe, tuned by Se stoichiometry, is found to be twofold: on one hand it dopes electrons in the system and on the other hand it introduces localized magnetic moments, responsible for Kondo like scattering and likely pairbreaking of Cooper pairs. Hence, Fe excess also plays a crucial role in determining superconducting properties such as the T c and the upper critical field H c2 . The huge H c2 values of the Fe 1 + y Te 1 - x Se x samples are described by a dirty limit law, opposed to the clean limit behaviour of the SmFeAsO 1 - x F x samples. Hence, magnetic scattering by excess Fe seems to drive the system in the dirty regime, but its detrimental pairbreaking role seems not to be as severe as predicted by theory. This issue has yet to be clarified, addressing the more fundamental issue of the interplay between magnetism and superconductivity.