Anisotropic charge dynamics in detwinned Ba(Fe1−x_{1-x}Cox_x)2_2As2_2

We investigate the optical conductivity as a function of temperature with light polarized along the in-plane orthorhombic $a$- and $b$-axes of Ba(Fe$_{1-x}$Co$_x$)$_2$As$_2$ for $x$=0 and 2.5$\%$ under uniaxial pressure. The charge dynamics at low frequencies on these detwinned, single domain compounds tracks the anisotropic $dc$ 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

Charge dynamics of the Co-doped BaFe2_2As2_2

We report on a thorough optical investigation over a broad spectral range and as a function of temperature of the charge dynamics in Ba(Co$_x$Fe$_{1-x}$)$_2$As$_2$ compounds for Co-doping ranging between 0 and 18%. For the parent compound as well as for $x$=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$\le x \le$ 0.11 we detect the superconducting gap, while at $x$=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 $dc$ transport properties in the normal phase are dominated by the coherent Drude term for 0$\le x \le$0.051 and by the incoherent one for 0.061$\le x \le$0.18, respectively. Finally through spectral weight arguments, we give clear-cut evidence for moderate electronic correlations for 0$\le x \le$0.061, which then crossover to values appropriate for a regime of weak interacting and nearly-free electron metals for $x\ge$0.11

Strength and hardness enhancement and slip behaviour of high-entropy carbide grains during micro-compression and nanoindentation

Bulk polycrystalline high-entropy carbides are a newly developed group of materials that increase the limited compositional space of ultra-high temperature ceramics, which can withstand extreme environments exceeding 2000°C in oxidizing atmospheres. Since the deformability of grains plays an important role in macromechanical performance, we studied the strength and slip behaviour of grains of a spark-plasma sintered (Hf-Ta-Zr-Nb)C high-entropy carbide in a specific orientation during micropillar compression. Additionally, the hardness of grains of different orientations was investigated by nanoindentation. For comparison, identical measurements were carried out on the monocarbides HfC and TaC. Four micropillars were fabricated by focused ion beam (FIB) in visibly pore free regions of large (Hf-Ta-Zr-Nb)C, HfC and TaC grains of a specific orientation (Φ~14° and φ2~45°) selected by electron backscatter diffraction (EBSD). This resulted in equal Schmid factors for both the and slip systems, which were reported to operate in HfC and TaC. It was revealed that (Hf-Ta-Zr-Nb)C had a significantly enhanced yield and failure strength compared to the corresponding base monocarbides, while maintaining a similar ductility to the least brittle monocarbide (TaC) during the operation of slip systems (Fig. 1). Nanoindentation investigations revealed a significant enhancement in hardness (~30%) of the high entropy (Hf-Ta-Zr-Nb)C material compared to that calculated according to the rule of mixtures from the base monocarbides (HfC, TaC, ZrC, NbC) and in comparison to the hardest monocarbide (HfC). Additionally, it was concluded that the much larger strength enhancement of micropillars compared to the average nanohardness of randomly oriented grains is attributed to the different slip systems. For (Hf-Ta-Zr-Nb)C, the operation of was identified in micropillar experiments, but the dominant slip system in nanoindentation is assumed to be the , possibly via the activation of partial dislocations, which is attributed to the different Schmid factors due to the different stress fields between nanoindentation and micropillar compression. Please click Additional Files below to see the full abstract

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

Processing and Properties of High-Entropy Ultra-High Temperature Carbides

The research was supported by the EPSRC Programme Grant XMAT [EP/K008749/2]. The authors gratefully acknowledge the financial support from projects: APVV-15-0469 & VEGA 2/0163/16

Plasticity of ZrB2 grains during micropillar compression: The effect of anisotropy, temperature and dislocations

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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)

Identification of known and novel nonpolar endocrine disruptors in human amniotic fluid

Background Prenatal exposure to endocrine-disrupting compounds (EDCs) may contribute to endocrine-related diseases and disorders later in life. Nevertheless, data on in utero exposure to these compounds are still scarce. Objectives We investigated a wide range of known and novel nonpolar EDCs in full-term human amniotic fluid (AF), a representative matrix of direct fetal exposure. Methods Gas chromatography high-resolution mass spectrometry (GC-HRMS) was used for the targeted and non-targeted analysis of chemicals present in nonpolar AF fractions with dioxin-like, (anti-)androgenic, and (anti-)estrogenic activity. The contribution of detected EDCs to the observed activity was determined based on their relative potencies. The multitude of features detected by non-targeted analysis was tentatively identified through spectra matching and data filtering, and further investigated using curated and freely available sources to predict endocrine activity. Prioritized suspects were purchased and their presence in AF was chemically and biologically confirmed with GC-HRMS and bioassay analysis. Results Targeted analysis revealed 42 known EDCs in AF including dioxins and furans, polybrominated diphenyl ethers, pesticides, polychlorinated biphenyls, and polycyclic aromatic hydrocarbons. Only 30% of dioxin activity and <1% estrogenic and (anti-)androgenic activity was explained by the detected compounds. Non-targeted analysis revealed 14,110 features of which 3,243 matched with library spectra. Our data filtering strategy tentatively identified 121 compounds. Further data mining and in silico predictions revealed in total 69 suspected EDCs. We selected 14 chemicals for confirmation, of which 12 were biologically active and 9 were chemically confirmed in AF, including the plasticizer diphenyl isophthalate and industrial chemical p,p'-ditolylamine. Conclusions This study reveals the presence of a wide variety of nonpolar EDCs in direct fetal environment and for the first time identifies novel EDCs in human AF. Further assessment of the source and extent of human fetal exposure to these compounds is warranted.NIEHS Training in Environmental Pathology T32 progra