417 research outputs found

    Evidence-based medical equipment management: a convenient implementation

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    Doping and critical-temperature dependence of the energy gaps in Ba(Fe_{1-x}Co_x)_2As_2 thin films

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    The dependence of the superconducting gaps in epitaxial Ba(Fe_{1-x}Co_{x})_2As_2 thin films on the nominal doping x (0.04 \leq x \leq 0.15) was studied by means of point-contact Andreev-reflection spectroscopy. The normalized conductance curves were well fitted by using the 2D Blonder-Tinkham-Klapwijk model with two nodeless, isotropic gaps -- although the possible presence of gap anisotropies cannot be completely excluded. The amplitudes of the two gaps \Delta_{S} and \Delta_{L} show similar monotonic trends as a function of the local critical temperature T_{c}^{A} (measured in the same point contacts) from 25 K down to 8 K. The dependence of the gaps on x is well correlated to the trend of the critical temperature, i.e. to the shape of the superconducting region in the phase diagram. When analyzed within a simple three-band Eliashberg model, this trend turns out to be compatible with a mechanism of superconducting coupling mediated by spin fluctuations, whose characteristic energy scales with T_{c} according to the empirical law \Omega_{0}= 4.65*k_{B}*T_{c}, and with a total electron-boson coupling strength \lambda_{tot}= 2.22 for x \leq 0.10 (i.e. up to optimal doping) that slightly decreases to \lambda_{tot}= 1.82 in the overdoped samples (x = 0.15).Comment: 8 pages, 5 color figure

    Independent determination of the two gaps by directional point-contact spectroscopy in MgB_2 single crystals

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    Directional point-contact spectroscopy measurements were performed for the first time in state-of-the-art MgB_2 single crystals. The selective suppression of the superconductivity in the "pi" band by means of a suitable magnetic field allowed separating the partial contribution of each band to the total point-contact conductance. By fitting the partial conductance curves sigma_sigma(V) and sigma_pi(V), we got an independent determination of the two gaps, Delta_sigma and Delta_pi, with a strong reduction of the experimental uncertainty. Their temperature dependence was found to agree well with the predictions of the two-band models for MgB_2.Comment: 6 pages, 4 eps figures. References added, abstract rewritten, text slightly changed. Proceedings of the BOROMAG Conference, June 17-19, Genoa, Ital

    Resistivity in Co-doped Ba-122: comparison of thin films and single crystals

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    The temperature dependence of the resistivity of epitaxial Ba(Fe_(1-x)Co_x)2As2 thin films (with nominal doping x = 0.08, 0.10 and 0.15) has been analyzed and compared with analogous measurements on single crystals taken from literature. The rho(T) of thin films looks different from that of single crystals, even when the cobalt content is the same. All rho(T) curves can be fitted by considering an effective two-band model (with holes and electrons bands) in which the electrons are more strongly coupled with the bosons (spin fluctuations) than holes, while the effect of impurities is mainly concentrated in the hole band. Within this model the mediating boson has the same characteristic energy in single crystals and thin films, but the shape of the transport spectral function at low energy has to be very different, leading to a "hardening" of the electron-boson spectral function in thin films, associated with the strain induced by the substrate.Comment: 13 pages, 4 figure

    ab-plane resistivity and possible charge stripe ordering in strongly underdoped La2x_{2-x}Srx_{x}CuO4_{4} single crystals

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    We have measured the ab-plane resistivity of La2x_{2-x}Srx_xCuO4_4 single crystals with small Sr content (x=0.052 ÷\div 0.075) between 4.2 and 300 K by using the AC Van der Pauw technique. As recently suggested by Ichikawa et al., the deviation from the linearity of the ρab(T)\rho_{\mathrm{ab}}(T) curve starting at a temperature Tch_{\mathrm{ch}} can be interpreted as due to a progressive slowing down of the fluctuations of pre-formed charge stripes. An electronic transition of the stripes to a more ordered phase could instead be responsible for some very sharp anomalies present in the ρab(T)\rho_{\mathrm{ab}}(T) of superconducting samples just above TcT_{\mathrm{c}}.Comment: M2S-HTSC-VI Conference paper (2 pages, 2 figures), using Elsevier style espcrc2.st

    Can one extract the electron-phonon-interaction from tunneling data in case of the multigap superconductor MgB2_2?

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    In the present work we calculate the tunneling density of states (DOS) of MgB% 2_{2} for different tunneling directions by directly solving the two-band Eliashberg equations (EE) in the real-axis formulation. This procedure reveals the fine structures of the DOS due to the optical phonons. Then we show that the numeric inversion of the standard \emph{single-band} EE (the only available method), when applied to the \emph{two-band} DOS of MgB2_{2}, may lead to wrong estimates of the strength of certain phonon branches (e.g. the E2gE_{2g}) in the extracted electron-phonon spectral function α2F(ω)\alpha^{2}F(\omega). The fine structures produced by the two-band interaction at energies between 20 and 100 meV turn out to be clearly observable only for tunneling along the abab planes, when the extracted α2F(ω)\alpha ^{2}F(\omega) contains the combination α2Fσσ(ω)\alpha ^{2}F_{\sigma \sigma}(\omega)\textbf{+}α2Fσπ(ω)\alpha ^{2}F_{\sigma \pi }(\omega), together with a minor α2Fππ(ω)\alpha ^{2}F_{\pi \pi}(\omega )\textbf{+}α2Fπσ(ω)\alpha ^{2}F_{\pi \sigma} (\omega) component. Only in this case it is possible to extract information on the σ\sigma-band contribution to the spectral functions. For any other tunneling direction, the π\pi-band contribution (which does not determine the superconducting properties of MgB2_{2}) is dominant and almost coincides with the whole α2F(ω)\alpha^2F(\omega) for tunneling along the c axis. Our results are compared with recent experimental tunneling and point-contact data.Comment: 5 pages, 3 figures. Submitted to Phys. Rev. B (Brief Reports

    Penetration and early colonization in basidiospore-derived infection of Melampsora pulcherrima (Bub.) Maire on Mercurialis annua L.

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    SUMMARYThe early phases of basidiospore-derived infection of Melampsora pulcherrima (Bub.) Maire on the leaves of Mercurialis annua L. were studied by light microscopy, SEM and TEM. The fine morphology of the basidiospore germling penetration and intraepidermal infection structures is discussed in comparison with that of other rusts recently described. The direct penetration through the epidermal cell wall, characteristic of the rust basidiospore-derived germlings, is confirmed. The absence of an extrahyphal matrix around the intraepidermal vesicle and the presence of a collar around the vesicle neck are pointed out

    Control of bulk superconductivity in a BCS superconductor by surface charge doping via electrochemical gating

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    The electrochemical gating technique is a powerful tool to tune the surface conduction properties of various materials by means of pure charge doping, but its efficiency is thought to be hampered in materials with a good electronic screening. We show that, if applied to a metallic superconductor (NbN thin films), this approach allows observing reversible enhancements or suppressions of the bulk superconducting transition temperature, which vary with the thickness of the films. These results are interpreted in terms of proximity effect, and indicate that the effective screening length depends on the induced charge density, becoming much larger than that predicted by standard screening theory at very high electric fields

    A Study of Carbon Substitutions in MgB_2 within the two-band Eliashberg theory

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    We study the effects of C substitutions in MgB_2 within the two-band model in the Eliashberg formulation. We use as input the B-B stretching-mode frequency and the partial densities of states N_{sigma}(EF) and N_{pi}(EF), recently calculated for Mg(B_{1-x}C_{x})_2 at various x values from first-principles density functional methods. We then take the prefactor in the Coulomb pseudopotential matrix, mu, and the interband scattering parameter, Gamma^{sigma pi}, as the only adjustable parameters. The dependence on the C content of Tc and of the gaps (Delta_{sigma} and Delta_{pi}) recently measured in Mg(B_{1-x}C_{x})_2 single crystals indicate an almost linear decrease of mu on increasing x, with an increase in interband scattering that makes the gaps merge at x=0.132. In polycrystals, instead, where the gap merging is not observed, no interband scattering is required to fit the experimental data.Comment: 7 pages, 8 figures, RevTex4. Detailed discussion of the results adde
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