1,247 research outputs found
Influence of substitutional disorder on the electrical transport and the superconducting properties of FeTeSeS
We have carried out an investigation of the structural, magnetic, transport
and superconducting properties of FeTeSeS ceramic
compounds, for and some specific Se (0 x 0.5) and S (0
y 0.12) contents. The incorporation of Se and S to the FeTe
structure produces a progressive reduction of the crystallographic parameters
as well as different degrees of structural disorder associated with the
differences of the ionic radius of the substituting cations. In the present
study, we measure transport properties of this family of compounds and we show
the direct influence of disorder in the normal and superconductor states. We
notice that the structural disorder correlates with a variable range hopping
conducting regime observed at temperatures 200 K. At lower temperatures,
all the samples except the one with the highest degree of disorder show a
crossover to a metallic-like regime, probably related to the transport of
resilient-quasi-particles associated with the proximity of a Fermi liquid state
at temperatures below the superconducting transition. Moreover, the
superconducting properties are depressed only for that particular sample, in
accordance to the condition that superconductivity is affected by disorder when
the electronic localization length becomes smaller than the coherence
length .Comment: 23 pages, 9 figure
Pressure study of the new iron-based superconductor K0.8Fe2Se2
We investigated pressure effects on transition temperature (Tc) of the new
iron-based superconductor K0.8Fe2Se2 using a BeCu/NiCrAl hybrid-type clamped
piston-cylinder cell. The Tc(onset) was 33K at 0.85 GPa. With increasing
pressure, Tc(onset) gradually increased and reached 36.6 K at 2.03 GPa.Comment: 8 pages, 3 figure
Field-Induced Magnetostructural Transitions in Antiferromagnetic Fe1+yTe1-xSx
The transport and structural properties of Fe1+yTe1-xSx (x=0, 0.05, and 0.10)
crystals were studied in pulsed magnetic fields up to 65 T. The application of
high magnetic fields results in positive magnetoresistance effect with
prominent hystereses in the antiferromagnetic state. Polarizing microscope
images obtained at high magnetic fields showed simultaneous occurrence of
structural transitions. These results indicate that magnetoelastic coupling is
the origin of the bicollinear magnetic order in iron chalcogenides.Comment: 5 pages, 5 figures, accepted for publication in Journal of the
Physical Society of Japa
Precise Pressure Dependence of the Superconducting Transition Temperature of FeSe: Resistivity and ^77Se--NMR Study
We report the precise pressure dependence of FeSe from a resistivity
measurement up to 4.15 GPa. Superconducting transition temperature (T_c)
increases sensitively under pressure, but shows a plateau between 0.5-1.5 GPa.
The maximum T_c, which is determined by zero resistance, is 21 K at
approximately 3.5 GPa. The onset value reaches ~37 K at 4.15 GPa. We also
measure the nuclear spin-lattice relaxation rate 1/T_1 under pressure using
77Se--NMR measurement. 1/T_1 shows that bulk superconductivity is realized in
the zero-resistance state. The pressure dependence of 1/T_1T just above T_c
shows a plateau as well as the pressure dependence of T_c, which gives clear
evidence of the close relationship between 1/T_1T and T_c. Spin fluctuations
are suggested to contribute to the mechanism of superconductivity.Comment: 4pages, 6figures: to be published in J. Phys. Soc. Jpn. Vol.78 No.6
(2009
The chemotherapeutic agent DMXAA as a unique IRF3-dependent type-2 vaccine adjuvant
5,6-Dimethylxanthenone-4-acetic acid (DMXAA), a potent type I interferon (IFN) inducer, was evaluated as a chemotherapeutic agent in mouse cancer models and proved to be well tolerated in human cancer clinical trials. Despite its multiple biological functions, DMXAA has not been fully characterized for the potential application as a vaccine adjuvant. In this report, we show that DMXAA does act as an adjuvant due to its unique property as a soluble innate immune activator. Using OVA as a model antigen, DMXAA was demonstrated to improve on the antigen specific immune responses and induce a preferential Th2 (Type-2) response. The adjuvant effect was directly dependent on the IRF3-mediated production of type-I-interferon, but not IL-33. DMXAA could also enhance the immunogenicity of influenza split vaccine which led to significant increase in protective responses against live influenza virus challenge in mice compared to split vaccine alone. We propose that DMXAA can be used as an adjuvant that targets a specific innate immune signaling pathway via IRF3 for potential applications including vaccines against influenza which requires a high safety profile
s-wave pairing in the optimally-doped LaO0.5F0.5BiS2 superconductor
We report on the magnetic and superconducting properties of LaO0.5F0.5BiS2 by
means of zero- (ZF) and transverse-field (TF) muon-spin spectroscopy
measurements (uSR). Contrary to previous results on iron-based superconductors,
measurements in zero field demonstrate the absence of magnetically ordered
phases. TF-uSR data give access to the superfluid density, which shows a marked
2D character with a dominant s-wave temperature behavior. The field dependence
of the magnetic penetration depth confirms this finding and further suggests
the presence of an anisotropic superconducting gap
Synthesis, structural and physical properties of -FeSe
We report on synthesis, structural characterization, resistivity, magnetic
and thermal expansion measurements on the as yet unexplored -phase of
FeSe, here synthesized under ambient- (AP) and high-pressure (HP)
conditions. We show that in contrast to -FeSe, monophasic
superconducting -FeSe can be obtained in off-stoichiometric
samples with excess Fe atoms preferentially residing in the van der Waals gap
between the FeSe layers. The AP -FeSe sample studied here
( 8.5\,K) possesses an unprecedented residual resistivity ratio
RRR 16. Thermal expansion data reveal a small feature around
90\,K, which resembles the anomaly observed at the structural and
magnetic transitions for other Fe-based superconductors, suggesting that some
kind of "magnetic state" is formed also in FeSe. %indicative of a fluctuating
magnetic ordering. For HP samples (RRR 3), the disorder within the
FeSe layers is enhanced through the introduction of vacancies, the saturated
magnetic moment of Fe is reduced and only spurious superconductivity is
observed.Comment: 7 pages, 8 figures, published versio
Local structural studies of BaKFeAs using atomic pair distribution function analysis
Systematic local structural studies of BaKFeAs system are
undertaken at room temperature using atomic pair distribution function (PDF)
analysis. The local structure of the BaKFeAs is found to be
well described by the long-range structure extracted from the diffraction
experiments, but with anisotropic atomic vibrations of the constituent atoms
( = ). The crystal unit cell parameters, the
FeAs tetrahedral angle and the pnictogen height above the Fe-plane are seen
to show systematic evolution with K doping, underlining the importance of the
structural changes, in addition to the charge doping, in determining the
properties of BaKFeAs
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