The observation of clean 2D superconductivity and bosonic Landau levels below 0.85 K in superlattice van der Waals material ${\mathrm{Ba}}_{6}{\mathrm{Nb}}_{11}{\mathrm{S}}_{28}$ have received much interest. Here we report the crystal structure, Hall resistivity, and 2D superconductivity of its sister compound, ${\mathrm{Ba}}_{6}{\mathrm{Nb}}_{11}{\mathrm{Se}}_{28}$. The atomic-resolution transmission electron microscopy images reveal an alternate stacking of $2H\text{\ensuremath{-}}{\mathrm{NbSe}}_{2}$ with a 3 \ifmmode\times\else\texttimes\fi{} 3 commensurate block layer, which is isostructural to ${\mathrm{Ba}}_{6}{\mathrm{Nb}}_{11}{\mathrm{S}}_{28}$ and with a larger $\mathrm{Nb}{X}_{2}$ ($X=\mathrm{S}$, Se) interlayer distance (12.8 \AA{}). Transport property measurements demonstrate a twofold enhancement in superconducting temperature $({{T}_{\mathrm{c}}}^{0}=2.2\phantom{\rule{0.16em}{0ex}}\mathrm{K})$, meanwhile the anisotropy in resistivity and carrier mobility in ${\mathrm{Ba}}_{6}{\mathrm{Nb}}_{11}{\mathrm{Se}}_{28}$ decrease drastically compared to its sulfur counterpart, driving the superconductor towards the dirty limit. Further measurements of angular-dependent upper critical field suggest the superconductivity in ${\mathrm{Ba}}_{6}{\mathrm{Nb}}_{11}{\mathrm{Se}}_{28}$ be Pauli-limit breaking (2 times the ${\ensuremath{\mu}}_{0}{H}_{P}$), and can be well described by the 2D Tinkham form. Our findings offer a valuable platform to investigate 2D superconductivity in bulk superlattice materials based on the transition-metal dichalcogenides.
Abstract The ash‐related issues hinder the safe utilization of Zhundong coal, closely relevant to its ash compositions. Previous studies mainly focused on the experimental methods and the selection of ash composition benchmark was restricted to a specific coal sample with limited element types, ignoring the comprehensive elemental interaction. Here, the impacts of eight compositions on ash fusion and crystal phase transformation behaviors were taken into consideration. Moreover, FactSage software was employed to further elucidate the transformation mechanism of ash composition with the increasing temperature. The results showed that the refractory forsterite and merwinite disappeared but more magnesioferrite was present with an increase in ferric oxide content, resulting in the decrease of ash fusion temperatures. The refractory substances, such as forsterite, perovskite, periclase, and larnite, occurred successively with the increasing ratio of basic oxide to acid oxide. The slag content had reached its maximum at 1200°C, up to 90% as the content of CaO was 16%, which was corresponding to that the flowing temperature descended to the lowest point around 1200°C with 16% CaO. With the rising temperature, more slag appeared and the composition species declined, related with the generation of fusible compounds containing Fe and Na at high temperature. The present work can play a guiding role in safe and clean use of Zhundong coal.
Abstract High pressure has become a powerful platform for creating and controlling novel states of matter, including high temperature ( T c ) superconductivity. However, the emergent phenomena generally disappear as high pressure is removed and cloud prospects for future applications. Here, from a distinguishing perspective, FeSe 1− x S x is reported as 2D van der Waals materials with extraordinary high‐ T c at ambient pressure, where the superconductivity is boosted by extreme “chemical pressure” inside the materials. Superior to external high pressure, isovalent S substitution in FeSe leads to a much greater compression rate within the superconducting iron‐chalcogenide layer, which guarantees an unabridged superconducting dome that peaked at 37.5 K. Density functional theory calculations reveal that the decreased lattice and structural parameters contribute together for the shift of Fe 3d x 2− y 2 orbital, which creates a new hole‐pocket at the Fermi level that intimately correlated with the enhanced superconductivity. This study demonstrates the design of materials with optimized superconductivity by introducing chemical pressure.
It is known that a crystal structure and symmetry determine the physical properties of materials. Lattice distortion can strongly affect the symmetry of the crystal structure. Polar materials show changes in polarization with temporal fluctuations of temperature due to the asymmetry. As a polar crystal, hemimorphite shows excellent pyroelectric properties. However, to date, there are a few studies on its intrinsic physical properties, and the mechanism of its pyroelectricity remains unclear. In this paper, single-crystal x-ray diffraction measurement was carried out to obtain the atomic positions at 100–400 K. Furthermore, the electric dipole moments of [ZnO4] and [SiO4] polyhedrons along a, b, and c axes have been calculated. The calculated pyroelectric coefficient derived from the intrinsic electric dipole moment was compared with the experimental measurement. The results indicate that the pyroelectric coefficients of hemimorphite at different temperatures mainly come from the variation of the electric dipole moment of [ZnO4] and [SiO4] polyhedrons along the c axis. The electric dipole moment changes as a function of temperature from 100 to 400 K, which is induced by the random lattice distortion. It is found that pyroelectricity is strongly correlated with the random lattice distortion. The establishment of the relationship between lattice distortion and pyroelectricity helps us to regulate the specific electrical parameters of the material, which may lead to future work in energy harvesting and further properties.
Abstract C 15 H 15 Cl 9 I 3 N 3 Sb 2 , monoclinic, C 2/ c (no. 15), a = 28.2633(7) Å, b = 12.4964(3) Å, c = 9.1542(3) Å, β = 99.5840(10)°, V = 3188.04(15) Å 3 , Z = 4, R gt ( F ) = 0.0274, wR ref ( F 2 ) = 0.0687, T = 273.15 K.
This work successfully synthesized S 0.66(2) WS 2 bulk single crystals, confirming the successful intercalation of sulfur in the M-WS 2 interlayer space.
We have successfully achieved the nonpolar-polar transition to a polar junction in apatite materials by using size-differentiated atomic substitution to induce spontaneous polarization, and prepared a material with excellent pyroelectric properties.
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