1,847 research outputs found
Uniaxial strain detwinning of CaFe2As2 and BaFe2As2: optical and transport study
TThe parent compounds of iron-arsenide superconductors, FeAs
(=Ca, Sr, Ba), undergo a tetragonal to orthorhombic structural transition at
a temperature in the range 135 to 205K depending on the
alkaline earth element. Below the free standing crystals
split into equally populated structural domains, which mask intrinsic,
in-plane, anisotropic properties of the materials. Here we demonstrate a way of
mechanically detwinning CaFeAs and BaFeAs. The
detwinning is nearly complete, as demonstrated by polarized light imaging and
synchrotron -ray measurements, and reversible, with twin pattern restored
after strain release. Electrical resistivity measurements in the twinned and
detwinned states show that resistivity, , decreases along the
orthorhombic -axis but increases along the orthorhombic -axis in
both compounds. Immediately below the ratio = 1.2 and 1.5 for Ca and Ba compounds, respectively. Contrary to
CaFeAs, BaFeAs reveals an anisotropy in the nominally
tetragonal phase, suggesting that either fluctuations play a larger role above
in BaFeAs than in CaFeAs, or that
there is a higher temperature crossover or phase transition.Comment: extended versio
Effect of tensile stress on the in-plane resistivity anisotropy in BaFe2As2
The effect of uniaxial tensile stress and the resultant strain on the
structural/magnetic transition in the parent compound of the iron arsenide
superconductor, BaFeAs, is characterized by temperature-dependent
electrical resistivity, x-ray diffraction and quantitative polarized light
imaging. We show that strain induces a measurable uniaxial structural
distortion above the first-order magnetic transition and significantly smears
the structural transition. This response is different from that found in
another parent compound, SrFeAs, where the coupled structural and
magnetic transitions are strongly first order. This difference in the
structural responses explains the in-plain resistivity anisotropy above the
transition in BaFeAs. This conclusion is supported by the
Ginzburg-Landau - type phenomenological model for the effect of the uniaxial
strain on the resistivity anisotropy
Anisotropic Impurity-States, Quasiparticle Scattering and Nematic Transport in Underdoped Ca(Fe1-xCox)2As2
Iron-based high temperature superconductivity develops when the `parent'
antiferromagnetic/orthorhombic phase is suppressed, typically by introduction
of dopant atoms. But their impact on atomic-scale electronic structure, while
in theory quite complex, is unknown experimentally. What is known is that a
strong transport anisotropy with its resistivity maximum along the crystal
b-axis, develops with increasing concentration of dopant atoms; this
`nematicity' vanishes when the `parent' phase disappears near the maximum
superconducting Tc. The interplay between the electronic structure surrounding
each dopant atom, quasiparticle scattering therefrom, and the transport
nematicity has therefore become a pivotal focus of research into these
materials. Here, by directly visualizing the atomic-scale electronic structure,
we show that substituting Co for Fe atoms in underdoped Ca(Fe1-xCox)2As2
generates a dense population of identical anisotropic impurity states. Each is
~8 Fe-Fe unit cells in length, and all are distributed randomly but aligned
with the antiferromagnetic a-axis. By imaging their surrounding interference
patterns, we further demonstrate that these impurity states scatter
quasiparticles in a highly anisotropic manner, with the maximum scattering rate
concentrated along the b-axis. These data provide direct support for the recent
proposals that it is primarily anisotropic scattering by dopant-induced
impurity states that generates the transport nematicity; they also yield simple
explanations for the enhancement of the nematicity proportional to the dopant
density and for the occurrence of the highest resistivity along the b-axis
Genetic Resistance to Malaria Is Associated With Greater Enhancement of Immunoglobulin (Ig)M Than IgG Responses to a Broad Array of Plasmodium falciparum Antigens
Background. People of the Fulani ethnic group are more resistant to malaria compared with genetically distinct ethnic groups, such as the Dogon people, in West Africa, and studies suggest that this resistance is mediated by enhanced antibody responses to Plasmodium falciparum antigens. However, prior studies measured antibody responses to <0.1% of P falciparum proteins, so whether the Fulani mount an enhanced and broadly reactive immunoglobulin (Ig)M and IgG response to P falciparum remains unknown. In general, little is known about the extent to which host genetics influence the overall antigen specificity of IgM and IgG responses to natural infections. Methods. In a cross-sectional study in Mali, we collected plasma from asymptomatic, age-matched Fulani (n = 24) and Dogon (n = 22) adults with or without concurrent P falciparum infection. We probed plasma against a protein microarray containing 1087 P falciparum antigens and compared IgM and IgG profiles by ethnicity. Results. We found that the breadth and magnitude of P falciparum-specific IgM and IgG responses were significantly higher in the malaria-resistant Fulani versus the malaria-susceptible Dogon, and, unexpectedly, P falciparum-specific IgM responses more strongly distinguished the 2 ethnic groups. Conclusions. These findings point to an underappreciated role for IgM in protection from malaria, and they suggest that host genetics may influence the antigen specificity of IgM and IgG responses to infection
Vortex phase diagram of Ba(Fe0.93Co0.07)2As2 single crystals
Detailed measurements of the global and local electromagnetic properties of
Ba(FeCo)As single crystals are reported. Analysis
of the irreversible magnetic response provides strong evidence for similar
vortex physics in this Fe-based pnictide superconductor to the high-
cuprates, such as Y-Ba-Cu-O or Nd-Ce-Cu-O. In particular, we have found a
nonmonotonic "fishtail" magnetization in loops and its signature
is also present in scans. The supercurrent density is evaluated
by using several techniques, including direct transport measurements. At 5 K we
estimate its value to be a moderate
A/cm. Analysis of the magnetic relaxation is consistent with the collective
pinning and creep models (weak pinning and fast creep) and suggests a crossover
from the collective to the plastic creep regime in fields exceeding the value
corresponding to the maximum in fishtail magnetization
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