465 research outputs found
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
We report magnetic penetration depth and thermal conductivity data for
high-quality single crystals of BaFe(AsP) (\,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)FeAs which has similar and phase diagram. Our results
indicate that repulsive electronic interactions play an essential role for
Fe-based high- superconductivity but that uniquely there are distinctly
different pairing states, with and without nodes, which have comparable .Comment: 4 pages, 3 figures, revised version to be published in Phys. Rev. B
Rapid Communicatio
Microwave Penetration Depth and Quasiparticle Conductivity in PrFeAsO_1-y Single Crystals : Evidence for a Full-Gap Superconductor
In-plane microwave penetration depth and quaiparticle
conductivity at 28 GHz are measured in underdoped single crystals of the
Fe-based superconductor PrFeAsO ( K) by using a
sensitive superconducting cavity resonator. shows flat
dependence at low temperatures, which is incompatible with the presence of
nodes in the superconducting gap . The temperature dependence
of the superfluid density demonstrates that the gap is non-zero
() all over the Fermi surface. The microwave
conductivity below exhibits an enhancement larger than the coherence
peak, reminiscent of high- 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
We report high-sensitivity microwave measurements of the in-plane penetration
depth and quasiparticle scattering rate in several
single crystals of hole-doped Fe-based superconductor
BaKFeAs (). While power-law temperature
dependence of with the power is found in crystals with
large , 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
Superconducting Gap Structure of LaFePO Studied by Thermal Conductivity
The superconducting gap structure of LaFePO (K) is studied by
thermal conductivity () at low temperatures in fields parallel and
perpendicular to the c axis. A clear two-step field dependence of
with a characteristic field Oe) much lower than the upper
critical field is observed. In spite of large anisotropy of ,
in both -directions is nearly identical below . Above
, grows gradually with with a convex curvature, followed
by a steep increase with strong upward curvature near . 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
Structural properties and superconductivity of SrFe2As2-xPx and CaFe2As2-yPy
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
Long-Term Results of Cell-Free Biodegradable Scaffolds for In Situ Tissue-Engineering Vasculature: In a Canine Inferior Vena Cava Model
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 reorganization of cylindrical nanoparticles triggered by polylactide stereocomplexation
YesCo-crystallization of polymers with different configurations/tacticities provides access to materials with enhanced performance. The stereocomplexation of isotactic poly(L-lactide) and poly(D-lactide) has led to improved properties compared with each homochiral material. Herein, we report the preparation of stereocomplex micelles from a mixture of poly(L-lactide)-b-poly(acrylic acid) and poly(D-lactide)-b-poly(acrylic acid) diblock copolymers in water via crystallization-driven self-assembly. During the formation of these stereocomplex micelles, an unexpected morphological transition results in the formation of dense crystalline spherical micelles rather than cylinders. Furthermore, mixture of cylinders with opposite homochirality in either THF/H2O mixtures or in pure water at 65 °C leads to disassembly into stereocomplexed spherical micelles. Similarly, a transition is also observed in a related PEO-b-PLLA/PEO-b-PDLA system, demonstrating wider applicability. This new mechanism for morphological reorganization, through competitive crystallization and stereocomplexation and without the requirement for an external stimulus, allows for new opportunities in controlled release and delivery applications.University of Warwick, Swiss National Science Foundation and the EPSRC. The Royal Society - an Industry Fellowship to A.P.D. The Engineering and Physical Sciences Research Council (EP/G004897/1) - funding to support postdoctoral fellowships for A.P.B. as well as funding for J.S. and M.A.D. through the Warwick Centre for Analytical Science (EP/F034210/1). The Science City Research Alliance and the HEFCE Strategic Development Fund - funding support. Some items of equipment that were used in this research were funded by Birmingham Science City, with support from Advantage West Midlands and part-funded by the European Regional Development Fund
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