352 research outputs found

    Microwave Penetration Depth and Quasiparticle Conductivity in PrFeAsO_1-y Single Crystals : Evidence for a Full-Gap Superconductor

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    In-plane microwave penetration depth λab\lambda_{ab} and quaiparticle conductivity at 28 GHz are measured in underdoped single crystals of the Fe-based superconductor PrFeAsO1−y_{1-y} (Tc≈35T_c\approx 35 K) by using a sensitive superconducting cavity resonator. λab(T)\lambda_{ab}(T) shows flat dependence at low temperatures, which is incompatible with the presence of nodes in the superconducting gap Δ(k)\Delta({\bf k}). The temperature dependence of the superfluid density demonstrates that the gap is non-zero (Δ/kBTc≳1.6\Delta/k_BT_c\gtrsim 1.6) all over the Fermi surface. The microwave conductivity below TcT_c exhibits an enhancement larger than the coherence peak, reminiscent of high-TcT_c cuprate superconductors.Comment: 4 pages, 3 figures. Version accepted for publication in Phys. Rev. Lett. For related results of hole-doped 122 system, see arXiv:0810.350

    Microwave Surface-Impedance Measurements of the Magnetic Penetration Depth in Single Crystal Ba1-xKxFe2As2 Superconductors: Evidence for a Disorder-Dependent Superfluid Density

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    We report high-sensitivity microwave measurements of the in-plane penetration depth λab\lambda_{ab} and quasiparticle scattering rate 1/τ1/\tau in several single crystals of hole-doped Fe-based superconductor Ba1−x_{1-x}Kx_xFe2_2As2_2 (x≈0.55x\approx 0.55). While power-law temperature dependence of λab\lambda_{ab} with the power ∼2\sim 2 is found in crystals with large 1/τ1/\tau, we observe exponential temperature dependence of superfluid density consistent with the existence of fully opened two gaps in the cleanest crystal we studied. The difference may be a consequence of different level of disorder inherent in the crystals. We also find a linear relation between the low-temperature scattering rate and the density of quasiparticles, which shows a clear contrast to the case of d-wave cuprate superconductors with nodes in the gap. These results demonstrate intrinsically nodeless order parameters in the Fe-arsenides.Comment: 4 pages, 4 figures, 1 table. Accepted for publication in Phys. Rev. Lett. Changed title as suggested by the PRL editor

    Line nodes in the energy gap of high-temperature superconducting BaFe_2(As_{1-x}P_x)_2 from penetration depth and thermal conductivity measurements

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    We report magnetic penetration depth and thermal conductivity data for high-quality single crystals of BaFe2_2(As1−x_{1-x}Px_{x})2_2 (Tc=30T_c=30\,K) which provide strong evidence that this material has line nodes in its energy gap. This is distinctly different from the nodeless gap found for (Ba,K)Fe2_2As2_2 which has similar TcT_c and phase diagram. Our results indicate that repulsive electronic interactions play an essential role for Fe-based high-TcT_c superconductivity but that uniquely there are distinctly different pairing states, with and without nodes, which have comparable TcT_c.Comment: 4 pages, 3 figures, revised version to be published in Phys. Rev. B Rapid Communicatio

    Superconducting Gap Structure of LaFePO Studied by Thermal Conductivity

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    The superconducting gap structure of LaFePO (Tc=7.4T_c=7.4 K) is studied by thermal conductivity (κ\kappa) at low temperatures in fields HH parallel and perpendicular to the c axis. A clear two-step field dependence of κ(H)\kappa(H) with a characteristic field Hs(∼350H_s(\sim 350 Oe) much lower than the upper critical field Hc2H_{c2} is observed. In spite of large anisotropy of Hc2H_{c2}, κ(H)\kappa(H) in both HH-directions is nearly identical below HsH_s. Above HsH_s, κ(H)\kappa(H) grows gradually with HH with a convex curvature, followed by a steep increase with strong upward curvature near Hc2H_{c2}. These results indicate the multigap superconductivity with active two-dimensional (2D) and passive 3D bands having contrasting gap values. Together with the recent penetration depth results, we suggest that the 2D bands consist of nodal and nodeless ones, consistent with the extended s-wave symmetry

    Long-Term Results of Cell-Free Biodegradable Scaffolds for In Situ Tissue-Engineering Vasculature: In a Canine Inferior Vena Cava Model

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    We have developed a new biodegradable scaffold that does not require any cell seeding to create an in-situ tissue-engineering vasculature (iTEV). Animal experiments were conducted to test its characteristics and long-term efficacy. An 8-mm tubular biodegradable scaffold, consisting of polyglycolide knitted fibers and an L-lactide and ε-caprolactone copolymer sponge with outer glycolide and ε-caprolactone copolymer monofilament reinforcement, was implanted into the inferior vena cava (IVC) of 13 canines. All the animals remained alive without any major complications until euthanasia. The utility of the iTEV was evaluated from 1 to 24 months postoperatively. The elastic modulus of the iTEV determined by an intravascular ultrasound imaging system was about 90% of the native IVC after 1 month. Angiography of the iTEV after 2 years showed a well-formed vasculature without marked stenosis or thrombosis with a mean pressure gradient of 0.51±0.19 mmHg. The length of the iTEV at 2 years had increased by 0.48±0.15 cm compared with the length of the original scaffold (2–3 cm). Histological examinations revealed a well-formed vessel-like vasculature without calcification. Biochemical analyses showed no significant differences in the hydroxyproline, elastin, and calcium contents compared with the native IVC. We concluded that the findings shown above provide direct evidence that the new scaffold can be useful for cell-free tissue-engineering of vasculature. The long-term results revealed that the iTEV was of good quality and had adapted its shape to the needs of the living body. Therefore, this scaffold would be applicable for pediatric cardiovascular surgery involving biocompatible materials

    Structural properties and superconductivity of SrFe2As2-xPx and CaFe2As2-yPy

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    The SrFe2As2-xPx and CaFe2As2-yPy materials were prepared by a solid state reaction method. X-ray diffraction measurements indicate the single-phase samples can be successfully obtained for SrFe2As2-xPx and CaFe2As2-yPy samples. Clear contraction of the lattice parameters are clearly determined due to the relatively smaller P ions substation for As. The SDW instability associated with tetragonal to orthorhombic phase transition is suppressed visibly in both systems following with the increase of P contents. The highest superconducting transitions are respectively observed at about 27 K in SrFe2As1.3P0.7 and at about 13 K in CaFe2As1.7P0.3.Comment: 11 pages, 5 figures, 2 table
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