Orbital and spin effects for the upper critical field in As-deficient disordered Fe pnictide superconductors

Abstract

We report upper critical field B c2 (T ) data for LaO 0.9 F 0.1 FeAs 1−δ in a wide temperature and field range up to 60 T. The large slope of B c2 ≈ -5.4 to -6.6T/K near an improved T c ≈ 28.5 K of the in-plane B c2 (T ) contrasts with a flattening starting near 23 K above 30 T we regard as the onset of Pauli-limited behavior (PLB) with B c2 (0) ≈ 63 to 68 T. We interpret a similar hitherto unexplained flattening of the B c2 (T ) curves reported for at least three other disordered closely related systems as the Co-doped BaFe 2 As 2 , the (Ba,K)Fe 2 As 2 , or the NdO 0.7 F 0.3 FeAs (all single crystals) for applied fields H (a, b) also as a manifestation of PLB. Their Maki parameters have been estimated analyzing their B c2 (T ) data within the Werthamer-Helfand-Hohenberg approach. The pronounced PLB of (Ba,K)Fe 2 As 2 single crystals obtained from a Sn-flux is attributed also to a significant As deficiency detected by wave length dispersive x-ray spectroscopy as reported by Ni N et al. 2008 Phys. Rev. B 78 014507. Consequences of our results are discussed in terms of disorder effects within conventional (CSC) and unconventional superconductivity (USC). USC scenarios with nodes on individual Fermi surface sheets (FSS), e.g. p-and d-wave SC, can be discarded for our samples. The increase of dB c2 /dT | Tc by sizeable disorder provides evidence for an important intraband (intra-FSS) contribution to the orbital upper critical field. We suggest that it can be ascribed either to an impurity driven transition from s ± USC to CSC of an extended s ++ -wave state or to a stabilized s ± -state provided As-vacancies cause predominantly strong intraband scattering in the unitary limit. We compare our results with B c2 data from the literature which show often no PLB for fields below 60 to 70 T probed so far. A novel disorder related scenario of a complex interplay of SC with two different competing magnetic instabilities is suggested.

As vacancies, local moments, and Pauli limiting in LaFeAs1−δO0.9F0.1superconductors

“…At first glance one might expect an AV-gradient within the LaO 0.9 F 0.1 FeAs 1−δ samples because the vacancies start to be formed at the surface of the samples. Nevertheless, the sharp superconducting transition width comparable with that of As-stoichiometric reference samples 9 and the surprising temperature dependence of the nuclear spinlattice relaxation rate T…”

“…The analysis of the reflectivity supports this assumption. 7,9 Second, due to the same charge redistribution the distance of As atoms from the basal Fe plane in LaO 0.9 F 0.1 FeAs 1−δ increases as compared to LaO 0.9 F 0.1 FeAs.…”

“…On the other hand, upper critical field measurements at high magnetic fields have shown that many of the iron-based superconductors are limited by the Pauli paramagnetism. 8,9 This limitation should be related to a large paramagnetic spin susceptibility of the conducting electrons in the normal state which mediates the pair-breaking of singlet Cooperpairs. 9 So far, to the best of our knowledge, the expected relationship between the Pauli limiting behavior and an enhanced spin susceptibility in the normal state has not yet been confirmed experimentally for the Fe-pnictide superconductors…”

“…LaO 0.9 F 0.1 FeAs 1−δ samples were obtained by wrapping the samples in a Ta foil during the annealing procedure. 8,9 According to energy dispersive x-ray (EDX) analysis, an As/Fe ratio of about 1.0 was found in the reference sample annealed without a Ta foil and of about 0.9 to 0.95 in the As-deficient samples. At first glance one might expect an AV-gradient within the LaO 0.9 F 0.1 FeAs 1−δ samples because the vacancies start to be formed at the surface of the samples.…”

Multiple phase transitions in Pauli-limited iron-based superconductors

“…[19] reported possibly the FFLO phase in the multi-band iron-based superconductor KFe 2 As 2 . Although it should be noted that hints about the FFLO phase in pnictides had been reported in other experimental [20][21][22][23][24][25] and theoretical [26] works previously.…”