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.
Linear transverse magnetoresistance is commonly observed in many material systems including semimetals, narrow band-gap semiconductors, multi-layer graphene and topological insulators. It can originate in an inhomogeneous conductor from distortions in the current paths induced by macroscopic spatial fluctuations in the carrier mobility and it has been explained using a phenomenological semiclassical random resistor network model. However, the link between the linear magnetoresistance and the microscopic nature of the electron dynamics remains unknown. Here we demonstrate how the linear magnetoresistance arises from the stochastic behaviour of the electronic cycloidal trajectories around low-mobility islands in high-mobility inhomogeneous conductors and that this process is only weakly affected by the applied electric field strength. Also, we establish a quantitative link between the island morphology and the strength of linear magnetoresistance of relevance for future applications.
This Letter was published online on 4 December 2008 with a production error in the text of the abstract. The last sentence of the abstract should read as ''. . .unconventional superconducting pairings.'' The Letter has been corrected as of 5 December 2008. The text is correct in the printed version of the journal.
Superconducting LaFeAsO1-xFx thin films were grown on single crystalline LaAlO3 substrates with critical temperatures (onset) up to 28 K. Resistive measurements in high magnetic fields up to 40 T reveal a paramagnetically limited upper critical field mu{0}H{c2}(0) around 77 T and a remarkable steep slope of -6.2 T K-1 near T{c}. From transport measurements we observed weak-link behavior in low magnetic fields and evidence for a broad reversible regime.
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