1,604 research outputs found
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
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
Statistics of a parallel Poynting vector in the auroral zone as a function of altitude using Polar EFI and MFE data and Astrid-2 EMMA data
We study the wave-related (AC) and static (DC) parallel Poynting vector (Poynting energy flux) as a function of altitude in auroral field lines using Polar EFI and MFE data. The study is statistical and contains 5 years of data in the altitude range 5000–30000 km. We verify the low altitude part of the results by comparison with earlier Astrid-2 EMMA Poynting vector statistics at 1000 km altitude. The EMMA data are also used to statistically compensate the Polar results for the missing zonal electric field component. We compare the Poynting vector with previous statistical DMSP satellite data concerning the electron precipitation power. We find that the AC Poynting vector (Alfvén-wave related Poynting vector) is statistically not sufficient to power auroral electron precipitation, although it may, for <i>K<sub>p</sub></i>&gt;2, power 25–50% of it. The statistical AC Poynting vector also has a stepwise transition at <i>R</i>=4 <i>R<sub>E</sub></i>, so that its amplitude increases with increasing altitude. We suggest that this corresponds to Alfvén waves being in Landau resonance with electrons, so that wave-induced electron acceleration takes place at this altitude range, which was earlier named the Alfvén Resonosphere (ARS). The DC Poynting vector is ~3 times larger than electron precipitation and corresponds mainly to ionospheric Joule heating. In the morning sector (02:00–06:00&nbsp;MLT) we find that the DC Poynting vector has a nontrivial altitude profile such that it decreases by a factor of ~2 when moving upward from 3 to 4 <i>R<sub>E</sub></i> radial distance. In other nightside MLT sectors the altitude profile is more uniform. The morning sector nontrivial altitude profile may be due to divergence of the perpendicular Poynting vector field at <i>R</i>=3–4 <i>R<sub>E</sub></i>. <p><b>Keywords.</b> Magnetospheric physics (Auroral phenomena; Magnetosphere-ionosphere interactions) – Space plasma physics (Wave-particle interactions
Sign-reversal of the in-plane resistivity anisotropy in hole-doped iron pnictides
The in-plane anisotropy of the electrical resistivity across the coupled
orthorhombic and magnetic transitions of the iron pnictides has been
extensively studied in the parent and electron-doped compounds. All these
studies universally show that the resistivity across the long
orthorhombic axis - along which the spins couple antiferromagnetically
below the magnetic transition temperature - is smaller than the resistivity
of the short orthorhombic axis , i. e. .
Here we report that in the hole-doped compounds
BaKFeAs, as the doping level increases, the
resistivity anisotropy initially becomes vanishingly small, and eventually
changes sign for sufficiently large doping, i. e. . This
observation is in agreement with a recent theoretical prediction that considers
the anisotropic scattering of electrons by spin-fluctuations in the
orthorhombic/nematic state.Comment: This paper has been replaced by the new version offering new
explanation of the experimental results first reported her
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
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
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