Upper critical fields and thermally-activated transport of Nd(O_0.7F_0.3)FeAs single crystal

We present measurements of the resistivity and the upper critical field H_c2 of Nd(O_0.7F_0.3)FeAs single crystals in strong DC and pulsed magnetic fields up to 45 T and 60 T, respectively. We found that the field scale of H_c2 is comparable to ~100 T of high T_c cuprates. H_c2(T) parallel to the c-axis exhibits a pronounced upward curvature similar to what was extracted from earlier measurements on polycrystalline samples. Thus this behavior is indeed an intrinsic feature of oxypnictides, rather than manifestation of vortex lattice melting or granularity. The orientational dependence of H_c2 shows deviations from the one-band Ginzburg-Landau scaling. The mass anisotropy decreases as T decreases, from 9.2 at 44K to 5 at 34K. Spin dependent magnetoresistance and nonlinearities in the Hall coefficient suggest contribution to the conductivity from electron-electron interactions modified by disorder reminiscent that of diluted magnetic semiconductors. The Ohmic resistivity measured below T_c but above the irreversibility field exhibits a clear Arrhenius thermally activated behavior over 4-5 decades. The activation energy has very different field dependencies for H||ab and H\perp ab. We discuss to what extent different pairing scenarios can manifest themselves in the observed behavior of H_{c2}, using the two-band model of superconductivity. The results indicate the importance of paramagnetic effects on H_c2(T),which may significantly reduce H_c2(0) as compared toH_c2(0)~200-300 T based on extrapolations of H_c2(T) near T_c down to low temperatures.Comment: 11 pages, 16 figure

Photo-induced dynamics of the heme centers in cytochrome bc 1

The ultrafast response of cytochrome bc1 is investigated for the first time, via transient absorption spectroscopy. The distinct redox potentials of both c1- and b-hemes allow for a clear differentiation of their respective signals. We find that while the c1-heme photo-product exhibits the characteristics of a 5-coordinated species, the b-hemes presumably undergo photo-oxidation at a remarkably high quantum yield. The c1-heme iron–ligand recombination time is 5.4 ps, in agreement with previous reports on homologous cytochromes. The suggested photo-oxidized state of the b-hemes has a lifetime of 6.8 ps. From this short life-time we infer that the electron acceptor must be within van der Walls contact with the heme, which points to the fact that the axial histidine residue is the electron acceptor. The different heme-responses illustrate the flexibility of the c1-heme ligation in contrast to the more rigid b-heme binding, as well as the higher electronic reactivity of the b-hemes within the bc1 complex. This study also demonstrates the remarkable connection between the heme local environment and its dynamics and, therefore, biological functio

Centering Communities of Color in the Modernization of a Public Health Survey System: Lessons from Oregon

Context: Public health survey systems are tools for informing public health programming and policy at the national, state, and local levels. Among the challenges states face with these kinds of surveys include concerns about the representativeness of communities of color and lack of community engagement in survey design, analysis, and interpretation of results or dissemination, which raises questions about their integrity and relevance. Approach: Using a data equity framework (rooted in antiracism and intersectionality), the purpose of this project was to describe a formative participatory assessment approach to address challenges in Oregon Behavioral Risk Factor Surveillance System (BRFSS) and Student Health Survey (SHS) data system by centering community partnership and leadership in (1) understanding and interpreting data; (2) identifying strengths, gaps, and limitations of data and methodologies; (3) facilitating community-led data collection on community-identified gaps in the data; and (4) developing recommendations. Results: Project team members’ concerns, observations, and critiques are organized into six themes. Throughout this engagement process, community partners, including members of the project teams, shared a common concern: that these surveys reproduced the assumptions, norms, and methodologies of the dominant (White, individual centered) scientific approach and, in so doing, created further harm by excluding community knowledges and misrepresenting communities of color. Conclusions: Meaningful community leadership is needed for public health survey systems to provide more actionable pathways toward improving population health outcomes. A data equity approach means centering communities of color throughout survey cycles, which can strengthen the scientific integrity and relevance of these data to inform community health efforts

Status of high temperature superconductor based magnets and the conductors they depend upon

This paper reviews the status of high temperature superconductors for high field magnets for future devices such as a high energy LHC or a muon collider. Some of the primary challenges faced for the implementation of systems are discussed. Two conductor technologies, Bi$_2$Sr$_2$CaCu$_2$O$_{8+x}$ and YBa$_2$Cu$_3$O$_{7-\delta}$, have emerged as high field conductor options, but their relative advantages and disadvantages for high field magnets are quite different. These are reviewed from an engineering perspective, including coil manufacturing, electromechanical behaviour and quench behaviour. Lastly, the important roles of "system pull" upon conductor and magnet technology development, and of interactions between the materials and magnet communities for accelerating development, are discussed.Comment: 11 pages, contribution to the EuCARD-AccNet-EuroLumi Workshop: The High-Energy Large Hadron Collider, Malta, 14 -- 16 Oct 2010; CERN Yellow Report CERN-2011-003, pp. 59-6

Upper critical field measurements up to 60 T in arsenic-deficient LaO_(0.9)F_(0.1)FeAs_(1-delta): Pauli limiting behaviour at high fields vs improved superconductivity at low fields

We report resistivity and upper critical field B_c2(T) data for As deficient LaO_(0.9)F_(0.1)FeAs_(1-delta) in a wide temperature and high field range up to 60 T. These disordered samples exhibit a slightly enhanced superconducting transition at T_c = 29 K and a significantly enlarged slope dB_(c2))/dT = -5.4 T/K near T_c which contrasts with a flattening of B_(c2)(T) starting near 23 K above 30 T. This flattening is interpreted as Pauli limiting behaviour (PLB) with B_(c2)(0) approx 63 T. We compare our results with B_(c2)(T)-data reported in the literature for clean and disordered samples. Whereas clean samples show no PLB for fields below 60 to 70 T, the hitherto unexplained flattening of B_(c2)(T) for applied fields H || ab observed for several disordered closely related systems is interpreted also as a manifestation of PLB. Consequences of our results are discussed in terms of disorder effects within the frame of conventional and unconventional superconductivity.Comment: 4 pages, 3 figures, submitted to RHMF09 (9th International Conference on the Research in High Magnetic Fields), Dresden, July 22-25, 200

Evidence for Pauli-limiting behaviour at high fields and enhanced upper critical fields near T_c in several disordered FeAs based Superconductors

We report resistivity and upper critical field B_c2(T) data for disordered (As deficient) LaO_0.9F_0.1FeAs_1-delta in a wide temperature and high field range up to 60 T. These samples exhibit a slightly enhanced superconducting transition at T_c = 28.5 K and a significantly enlarged slope dB_c2/dT = -5.4 T/K near T_c which contrasts with a flattening of B_c2(T) starting near 23 K above 30 T. The latter evidences Pauli limiting behaviour (PLB) with B_c2(0) approximately 63 T. We compare our results with B_c2(T)-data from the literature for clean and disordered samples. Whereas clean samples show almost no PLB for fields below 60 to 70 T, the hitherto unexplained pronounced flattening of B_c2(T) for applied fields H II ab observed for several disordered closely related systems is interpreted also as a manifestation of PLB. Consequences are discussed in terms of disorder effects within the frames of (un)conventional superconductivity, respectively.Comment: 2 pages, 3 figures, submitted to M2S Tokyo 0

Systematic Study on Fluorine-doping Dependence of Superconducting and Normal State Properties in LaFePO1-xFx

We have investigated the fluorine-doping dependence of lattice constants, transports and specific heat for polycrystalline LaFePO1-xFx. F doping slightly and monotonically decreases the in-plane lattice parameter. In the normal state, electrical resistivity at low temperature is proportional to the square of temperature and the electronic specific heat coefficient has large value, indicating the existence of moderate electron-electron correlation in this system. Hall coefficient has large magnitude, and shows large temperature dependence, indicating the low carrier density and multiple carriers in this system. Temperature dependence of the upper critical field suggests that the system is a two gap superconductor. The F-doping dependence of these properties in this system are very weak, while in the FeAs system (LaFeAsO), the F doping induces the large changes in electronic properties. This difference is probably due to the different F-doping dependence of the lattice in these two systems. It has been revealed that a pure effect of electron doping on electronic properties is very weak in this Fe pnictide compound.Comment: 8 pages, 5 figures, accepted for publication in J. Phys. Soc. Jp

Evidence for two distinct anisotropies in the oxypnictide superconductors SmFeAsO_(0.8)F_(0.2) and NdFeAsO_(0.8)F_(0.2)

Single crystals of the oxypnictide superconductors SmFeAsO_(0.8)F_(0.2) and NdFeAsO_(0.8)F_(0.2) with T_c in the range of 44 K to 48 K were investigated by torque magnetometry. An analysis of the data in terms of a recently proposed model for the anisotropic magnetization in the superconducting state, treating the penetration depth anisotropy differently than the upper critical field anisotropy, provides evidence that in the oxypnictide superconductors two distinct anisotropies are present. As a result the penetration depth anisotropy differs significantly in magnitude and in temperature dependence from the upper critical field anisotropy, analogous to MgB_2 but with a reversed sign of slope. This scenario strongly suggests a new multi-band mechanism in the novel class of oxypnictide high-temperature superconductors.Comment: published online in J. Supercond. Nov. Mag

Advantageous grain boundaries in iron pnictide superconductors

High critical temperature superconductors have zero power consumption and could be used to produce ideal electric power lines. The principal obstacle in fabricating superconducting wires and tapes is grain boundaries-the misalignment of crystalline orientations at grain boundaries, which is unavoidable for polycrystals, largely deteriorates critical current density. Here, we report that High critical temperature iron pnictide superconductors have advantages over cuprates with respect to these grain boundary issues. The transport properties through well-defined bicrystal grain boundary junctions with various misorientation angles (thetaGB) were systematically investigated for cobalt-doped BaFe2As2 (BaFe2As2:Co) epitaxial films fabricated on bicrystal substrates. The critical current density through bicrystal grain boundary (JcBGB) remained high (> 1 MA/cm2) and nearly constant up to a critical angle thetac of ~9o, which is substantially larger than the thetac of ~5o for YBCO. Even at thetaGB > thetac, the decay of JcBGB was much smaller than that of YBCO.Comment: to appear in Nature Communication