245 research outputs found
Anisotropic charge dynamics in detwinned Ba(FeCo)As
We investigate the optical conductivity as a function of temperature with
light polarized along the in-plane orthorhombic - and -axes of
Ba(FeCo)As for =0 and 2.5 under uniaxial pressure.
The charge dynamics at low frequencies on these detwinned, single domain
compounds tracks the anisotropic transport properties across their
structural and magnetic phase transitions. Our findings allow us to estimate
the dichroism, which extends to relatively high frequencies. These results are
consistent with a scenario in which orbital order plays a significant role in
the tetragonal-to-orthorhombic structural transition
Anisotropic in-plane optical conductivity in detwinned Ba(Fe1-xCox)2As2
We study the anisotropic in-plane optical conductivity of detwinned
Ba(Fe1-xCox)2As2 single crystals for x=0, 2.5% and 4.5% in a broad energy range
(3 meV-5 eV) across their structural and magnetic transitions. For temperatures
below the Neel transition, the topology of the reconstructed Fermi surface,
combined with the distinct behavior of the scattering rates, determines the
anisotropy of the low frequency optical response. For the itinerant charge
carriers, we are able to disentangle the evolution of the Drude weights and
scattering rates and to observe their enhancement along the orthorhombic
antiferromagnetic a-axis with respect to the ferromagnetic b-axis. For
temperatures above Ts, uniaxial stress leads to a finite in-plane anisotropy.
The anisotropy of the optical conductivity, leading to a significant dichroism,
extends to high frequencies in the mid- and near-infrared regions. The
temperature dependence of the dichroism at all dopings scales with the
anisotropy ratio of the dc conductivity, suggesting the electronic nature of
the structural transition. Our findings bear testimony to a large nematic
susceptibility that couples very effectively to the uniaxial lattice strain. In
order to clarify the subtle interplay of magnetism and Fermi surface topology
we compare our results with theoretical calculations obtained from density
functional theory within the full-potential linear augmented plane-wave method.Comment: 17 pages, 9 figure
Charge dynamics of the Co-doped BaFeAs
We report on a thorough optical investigation over a broad spectral range and
as a function of temperature of the charge dynamics in
Ba(CoFe)As compounds for Co-doping ranging between 0 and
18%. For the parent compound as well as for =0.025 we observe the opening of
a pseudogap, due to the spin-density-wave phase transition and inducing a
reshuffling of spectral weight from low to high frequencies. For compounds with
0.051 0.11 we detect the superconducting gap, while at =0.18 the
material stays metallic at all temperatures. We describe the effective metallic
contribution to the optical conductivity with two Drude terms, representing the
combination of a coherent and incoherent component, and extract the respective
scattering rates. We establish that the transport properties in the normal
phase are dominated by the coherent Drude term for 00.051 and by the
incoherent one for 0.0610.18, respectively. Finally through spectral
weight arguments, we give clear-cut evidence for moderate electronic
correlations for 00.061, which then crossover to values appropriate
for a regime of weak interacting and nearly-free electron metals for
0.11
Microhardness and friction coefficient of multi-walled carbon nanotube-yttria-stabilized ZrO2 composites prepared by spark plasma sintering
Multi-walled carbon nanotubes (eight walls) are mixed with an yttria-stabilized ZrO2 powder. The specimens are densified by spark plasma sintering. Compared to ZrO2, there is a 3.8-fold decrease of the friction coefficient against alumina upon the increase in carbon content. Examinations of the friction tracks show that wear is very low when the carbon content is sufficient. Exfoliation of the nanotubes due to shearing stresses and incorporation of the debris into a lubricating film over the contact area is probable
Microstructure of (Hf-Ta-Zr-Nb)C high-entropy carbide at micro and nano/atomic level
Support from the projects APVV-15-0469, APVV-15-0621, VEGA 2/0163/16, and VEGA 2/0082/17 is acknowledged. MJR and EGC acknowledge the support of EPSRC grant XMAT (EP/K008749/2)
Distribution of Optical Spectral Weight in Detwinned Ba(Fe1− x Co x )2As2
We analyze our recent optical investigation on detwinned Ba(Fe1−x Co x )2As2 materials in the underdoped regime from the perspective of the spectral weight (SW) distribution. We identify its evolution for both in-plane crystallographic axes as a function of temperature across the structural tetragonal-orthorhombic phase transition. We can thus disentangle the anisotropy of SW occurring in the orthorhombic magnetic phase, from where we identify the relevant energy scales arising from interactions with spin fluctuation
Breaking of four-fold lattice symmetry in a model for pnictide superconductors
We investigate the interplay of onsite Coulomb repulsion and various
mechanisms breaking the fourfold lattice symmetry in a three-band model for the
iron planes of iron-based superconductors. Using cluster-perturbation theory
allows us to locally break the symmetry between the x- and y-directions without
imposing long-range magnetic order. Previously investigated anisotropic
magnetic couplings are compared to an orbital-ordering field and anisotropic
hoppings. We find that all three mechanisms for a broken rotational symmetry
lead to similar signatures once onsite interactions are strong enough to bring
the system close to a spin-density wave. The band distortions near the Fermi
level are independent of differences between the total densities found in xz
and yz orbitals
Uptake, Transport, and Toxicity of Pristine and Weathered Micro- and Nanoplastics in Human Placenta Cells
BACKGROUND: The first evidence of micro- and nanoplastic (MNP) exposure in the human placenta is emerging. However, the toxicokinetics and toxicity of MNPs in the placenta, specifically environmentally relevant particles, remain unclear. OBJECTIVES: We examined the transport, uptake, and toxicity of pristine and experimentally weathered MNPs in nonsyncytialized and syncytialized BeWo b30 choriocarcinoma cells. METHODS: We performed untargeted chemical characterization of pristine and weathered MNPs using liquid chromatography high-resolution mass spectrometry to evaluate compositional differences following particle weathering. We investigated cellular internalization of pristine and weathered polystyrene (PS; 0.05 - 10 μ m ) and high-density polyethylene (HDPE; 0 - 80 μ m ) particles using high-resolution confocal imaging and three-dimensional rendering. We investigated the influence of particle coating with human plasma on the cellular transport of PS particles using a transwell setup and examined the influence of acute MNP exposure on cell viability, damage to the plasma membrane, and expression of genes involved in steroidogenesis. RESULTS: Chemical characterization of MNPs showed a significantly higher number of unique features in pristine particles in comparison with weathered particles. Size-dependent placental uptake of pristine and weathered MNPs was observed in both placental cell types after 24 h exposure. Cellular transport was limited and size-dependent and was not influenced by particle coating with human plasma. None of the MNPs affected cell viability. Damage to the plasma membrane was observed only for 0.05 μ m PS particles in the nonsyncytialized cells at the highest concentration tested ( 100 μ g / mL ). Modest down-regulation of hsd17b1 was observed in syncytialized cells exposed to pristine MNPs. DISCUSSION: Our results suggest that pristine and weathered MNPs are internalized and translocated in placental cells in vitro. Effects on gene expression observed upon pristine PS and HDPE particle exposure warrant further examination. More in-depth investigations are needed to better understand the potential health risks of MNP and chemicals associated with them under environmentally relevant exposure scenarios. https://doi.org/10.1289/EHP10873
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