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Transport and superconducting properties of Fe-based superconductors: SmFeAs(O1-x Fx) versus Fe1+y (Te1-x, Sex)
We present 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: SmFeAs(O1-xFx) with the largest
Tc and the largest anisotropy and Fe1+y(Te1-x,Sex), with the largest Hc2, the
lowest Tc and the lowest anisotropy. In the case of the SmFeAs(O1-xFx) series,
we find that a single band description allows to extract an approximated
estimation of band parameters such as carrier density and mobility from
experimental data, although the behaviour of Seebeck effect as a function of
doping demonstrates that a multiband description would be more appropriate. On
the contrary, experimental data of the Fe1+y(Te1-x,Sex) series exhibit a
strongly compensated behaviour, which can be described only within a multiband
model. In the Fe1+y(Te1-x,Sex) series, the role of the excess Fe, tuned by Se
stoichiometry, is found to be twofold: it dopes electrons in the system and it
introduces localized magnetic moments, responsible for Kondo like scattering
and likely pair-breaking of Cooper pairs. Hence, excess Fe plays a crucial role
also in determining superconducting properties such as the Tc and the upper
critical field Bc2. The huge Bc2 values of the Fe1+y(Te1-x,Sex) samples are
described by a dirty limit law, opposed to the clean limit behaviour of the
SmFeAs(O1-xFx) 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