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.

2009, Kroll, T., Roth, F., Koitzsch, A., Kraus, R., Batchelor, D.R., Werner, J., Behr, G., Büchner, B., Knupfer, M.

The electronic structure of various rare-earth oxypnictides has been investigated by performing Fe L2, 3 x-ray absorption spectroscopy, and Fe 2p and valence band x-ray photoemission spectroscopy. As representative samples the non-superconducting parent compounds LnFeAsO (Ln=La, Ce, Sm and Gd) have been chosen and measured at 25 and 300 K, i.e. below and above the structural and magnetic phase transition at ~150 K. We find no significant change of the electronic structure of the FeAs layers when switching between the different rare-earth ions or when varying the temperature below and above the transition temperatures. Using a simple two-configuration model, we find qualitative agreement with the Fe 2p3/2 core-level spectrum, which allows for a qualitative explanation of the experimental spectral shapes.

2012, Zabolotnyy, V.B., Carleschi, E., Kim, T.K., Kordyuk, A.A., Trinckauf, J., Geck, J., Evtushinsky, D., Doyle, B.P., Fittipaldi, R., Cuoco, M., Vecchione, A., Büchner, B., Borisenko, S.V.

We present an angle-resolved photoemission study of the surface and bulk electronic structure of the single layer ruthenate Sr2RuO4. As the early studies by photoemission and scanning tunneling microscopy were confronted with a problem of surface reconstruction, surface ageing was previously proposed as a possible remedy to access the bulk states. Here, we suggest an alternative way by demonstrating that, in the case of Sr2RuO4, circularly polarized light can be used to disentangle the signals from the bulk and surface layers, thus opening the possibility to investigate many-body interactions both in bulk and surface bands. The proposed procedure results in improved momentum resolution, which enabled us to detect an unexpected splitting of the surface β band. We discuss the origin of the splitting of the β band and the possible connection with the Rashba effect at the surface.

2009, Evtushinsky, D.V., Inosov, D.S., Zabolotnyy, V.B., Viazovska, M.S., Khasanov, R., Amato, A., Klauss, H.-H., Luetkens, H., Niedermayer, Ch., Sun, G.L., Hinkov, V., Lin, C.T., Varykhalov, A., Koitzsch, A., Knupfer, M., Büchner, B., Kordyuk, A.A., Borisenko, S.V.

Here we present a calculation of the temperature-dependent London penetration depth, λ(T), in Ba1-xKxFe 2As2 (BKFA) on the basis of the electronic band structure (Zabolotnyy et al 2009 Nature 457 569, Zabolotnyy et al 2009 Physica C 469 448) and momentum-dependent superconducting gap (Evtushinsky et al 2009 Phys. Rev. B 79 054517) extracted from angleresolved photoemission spectroscopy (ARPES) data. The results are compared to the direct measurements of λ(T) by muon spin rotation (μSR) (Khasanov et al 2009 Phys. Rev. Lett. 102 187005). The value of λ(T = 0), calculated with no adjustable parameters, equals 270 nm, while the directly measured one is 320 nm; the temperature dependence λ(T) is also easily reproduced. Such agreement between the two completely different approaches allows us to conclude that ARPES studies of BKFA are bulk-representative. Our review of the available experimental studies of the superconducting gap in the new ironbased superconductors in general allows us to state that most of them bear two nearly isotropic gaps with coupling constants 2ΔkBTc = 2.5±1.5 and 7±2.

2015, Zwiebler, M., Hamann-Borrero, J.E., Vafaee, M., Komissinskiy, P., Macke, S., Sutarto, R., He, F., Büchner, B., Sawatzky, G.A., Alff, L., Geck, J.

The analysis of x-ray reflectivity data from artificial heterostructures usually relies on the homogeneity of optical properties of the constituent materials. However, when the x-ray energy is tuned to the absorption edge of a particular resonant site, this assumption may no longer be appropriate. For samples realizing lattice planes with and without resonant sites, the corresponding regions containing the sites at resonance will have optical properties very different from regions without those sites. In this situation, models assuming homogeneous optical properties throughout the material can fail to describe the reflectivity adequately. As we show here, resonant soft x-ray reflectivity is sensitive to these variations, even though the wavelength is typically large as compared to the atomic distances over which the optical properties vary. We have therefore developed a scheme for analyzing resonant soft x-ray reflectivity data, which takes the atomic structure of a material into account by 'slicing' it into atomic planes with characteristic optical properties. Using LaSrMnO4 as an example, we discuss both the theoretical and experimental implications of this approach. Our analysis not only allows to determine important structural information such as interface terminations and stacking of atomic layers, but also enables to extract depth-resolved spectroscopic information with atomic resolution, thus enhancing the capability of the technique to study emergent phenomena at surfaces and interfaces.

2009, Urbanik, G., Hänke, T., Hess, C., Büchner, B., Ciszewski, A., Hinkov, V., Lin, C.T., Keimer, B.

We performed scanning tunneling microscopy and spectroscopy on untwinned crystals of underdoped YBa2Cu3O7- δ at δ = 0.4. A comprehensive statistical analysis of our topographic data indicates a doping dependent cleaving behavior of this material. We find in particular that at δ = 0.4 the material primarily cleaves in multiples of one unit cell along the c-axis with a high corrugation of the topmost layer. Our data suggest that the low temperature cleaving mainly results in a disruption of the CuO chain layers involving a redistribution of the layer atoms onto the two cleaving planes. In a few instances, fractional step heights (in terms of the c-axis lattice constant) are observed as well. Scanning tunneling spectroscopy reveals that such fractional steps connect surfaces which differ significantly in their tunneling conductance.