2 results
Search Results
Now showing 1 - 2 of 2
- ItemOrbital and spin effects for the upper critical field in As-deficient disordered Fe pnictide superconductors(Milton Park : Taylor & Francis, 2009) Fuchs, G.; Drechsler, S.-L.; Kozlova, N.; Bartkowiak, M.; Hamann-Borrero, J.E.; Behr, G.; Nenkov, K.; Klauss, H.-H.; Maeter, H.; Amato, A.; Luetkens, H.; Kwadrin, A.; Khasanov, R.; Freudenberger, J.; Köhler, A.; Knupfer, M.; Arushanov, E.; Rosner, H.; Büchner, B.; Schultz, L.We report upper critical field Bc2(T) data for LaO0.9F0.1FeAs1- δ in a wide temperature and field range up to 60 T. The large slope of Bc2≈- 5.4 to -6.6 T K-1 near an improved Tc≈28.5 K of the in-plane Bc2(T) contrasts with a flattening starting near 23 K above 30 T we regard as the onset of Pauli-limited behaviour (PLB) with Bc2(0)≈63–68 T. We interpret a similar hitherto unexplained flattening of the Bc2(T) curves reported for at least three other disordered closely related systems, Co-doped BaFe2As2, (Ba,K) Fe2As2 and NdO0.7F0.3FeAs (all single crystals), for applied fields H∥(a,b), also as a manifestation of PLB. Their Maki parameters have been estimated by analysing their Bc2(T) data within the Werthamer–Helfand–Hohenberg approach. The pronounced PLB of (Ba, K)Fe2As2 single crystals obtained from an Sn flux is attributed also to a significant As deficiency detected by wavelength dispersive x-ray spectroscopy as reported by Ni et al (2008 Phys. Rev. B 78 014507). Consequences of our results are discussed in terms of disorder effects within conventional superconductivity (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 dBc2/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 Bc2 data from the literature, which often show no PLB for fields below 60–70 T probed so far. A novel disorder-related scenario of a complex interplay of SC with two different competing magnetic instabilities is suggested.
- ItemTuning functional properties by plastic deformation(Milton Park : Taylor & Francis, 2009) Kwon, A.R.; Neu, V.; Matias, V.; Hänisch, J.; Hühne, R.; Freudenberger, J.; Holzapfel, B.; Schultz, L.; Fähler, S.It is well known that a variation of lattice constants can strongly influence the functional properties of materials. Lattice constants can be influenced by external forces; however, most experiments are limited to hydrostatic pressure or biaxial stress. Here, we present an experimental approach that imposes a large uniaxial strain on epitaxially grown films in order to tune their functional properties. A substrate made of a ductile metal alloy covered with a biaxially oriented MgO layer is used as a template for growth of epitaxial films. By applying an external plastic strain, we break the symmetry within the substrate plane compared to the as-deposited state. The consequences of 2% plastic strain are examined for an epitaxial hard magnetic Nd2Fe14B film and are found to result in an elliptical distortion of the in-plane anisotropy below the spin-reorientation temperature. Our approach is a versatile method to study the influence of large plastic strain on various materials, as the MgO(001) layer used is a common substrate for epitaxial growth.