Transport and superconducting properties of Fe-based superconductors: a comparison between SmFeAsO_1−xF_xand Fe_1+yTe_1−xSe_x

Abstract

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.

Theoretical and experimental investigation of magnetotransport in iron chalcogenides

“…At the transition it undergoes an abrupt jump to negative values of approximately −2 × 10 -9 m 3 C −1 . Contrary to the FePn case, there is no evidence of exponential drop of carrier concentration below T N [15]. In the Fe(Se 0.5 Te 0.5 ) sample R H changes sign from positive ((1-2) × 10 -9 m 3 C −1 ) to negative at ∼50 K. Also R H of the FeSe 0.95 sample is negative at low temperature, becomes positive at 60-90 K, and then turns again negative at higher temperatures.…”

“…This is clearly seen for the Fe 1+y Te compound, which exhibits kinks in all the transport curves and a change in sign of the Hall effect below T N . This behavior differs from the progressive decrease of carrier concentration in the FePn parent compounds [15], which is related to the opening of the SDW gap.…”

“…Fe 1.05 Te and Fe(Se 0.5 Te 0.5 ) samples were prepared by solid-state synthesis as described elsewhere [15]. FeSe 0.95 was produced by a multistep technique based on melting and subsequent annealing as described in [16].…”

From antiferromagnetism to superconductivity inFe1+yTe1−xSex

“…With decreasing temperature the resistivity of all the samples exhibit a logarithmic upturn which progressively flattens with increasing x (see ref. [10] for a more detailed analysis). At T ~ 77 K, FeTe shows a steep drop in the resistivity and the behaviour become quite metallic below T ~ 70 K. Samples with up to 7.5% of Se content, the effect of Se substitution results in a broadening and a shifting to lower temperature of the drop.…”

Fluctuation conductivity in two-band superconductor SmFeAsO0.8F0.2

“…The Fermi surface * E-mail: imasker@eng.ankara.edu.tr of SmFeAsO 1−x F x is formed by three concentric hole cylinders at the Γ point of Brilloun zone and a small hole cylinder near the zone boundary [6,7]. Many-band character of superconducting state of SmFeAsO 1−x F x was confirmed by ARPES [8] and de Haas-van Alphen experiments [9].…”