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تسجيل مستخدم جديدNatural hyperbolicity in the layered hexagonal crystal structure
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Discovering the physical requirements for meeting the indefinite permittivity in natural material as well as proposing a new natural hyperbolic media offer a possible route to significantly improve our knowledge and ability to confine and controlling light in optoelectronic devices. We demonstrate the hyperbolicity in a class of materials with hexagonal P6/mmm and P6$_{3}$/mmc layered crystal structures and its physical origin is thoroughly investigated. By utilizing density functional theory and solving the Bethe-Salpeter equation (BSE), we find that the layered crystal structure and symmetry imposed constraints in Li$_{3}$N gives rise to an exceedingly strong anisotropy in optical responses along in- and out-of-plane directions of the crystals making it a natural hyperbolic in a broad spectral range from the visible spectrum to the ultraviolet. More excitingly, the hyperbolicity relation to anisotropic interband absorption in addition to the impressive dependency of the conduction band to the lattice constant along the out-of-plane direction provide the hyperbolicity tunability in these hexagonal structures under strain, doping, and alloying. Our findings not only suggest a large family of real hexagonal compounds as a unique class of materials for realization of the highly tunable broad band hyperbolicity but also offers an approach to search for new hyperbolic
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Hexagonal layered crystalline materials, such as graphene, boron nitride, tungsten sulfate, and so on, have attracted enormous attentions, due to their unique combination of atomistic structures and superior thermal, mechanical, and physical properti
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We have investigated the crystal structure of LaOBiPbS3 using neutron diffraction and synchrotron X-ray diffraction. From structural refinements, we found that the two metal sites, occupied by Bi and Pb, were differently surrounded by the sulfur atom
s. Calculated bond valence sum suggested that one metal site was nearly trivalent and the other was nearly divalent. Neutron diffraction also revealed site selectivity of Bi and Pb in the LaOBiPbS3 structure. These results suggested that the crystal structure of LaOBiPbS3 can be regarded as alternate stacks of the rock-salt-type Pb-rich sulfide layers and the LaOBiS2-type Bi-rich layers. From band calculations for an ideal (LaOBiS2)(PbS) system, we found that the S bands of the PbS layer were hybridized with the Bi bands of the BiS plane at around the Fermi energy, which resulted in the electronic characteristics different from that of LaOBiS2. Stacking the rock-salt type sulfide (chalcogenide) layers and the BiS2-based layered structure could be a new strategy to exploration of new BiS2-based layered compounds, exotic two-dimensional electronic states, or novel functionality.
The calculated quasiparticle band structure of bulk hexagonal boron nitride using the all-electron GW approximation shows that this compound is an indirect-band-gap semiconductor. The solution of the Bethe-Salpeter equation for the electron-hole two-
particle Green function has been used to compute its optical spectra and the results are found in excellent agreement with available experimental data. A detailed analysis is made for the excitonic structures within the band gap and found that the excitons belong to the Frenkel class and are tightly confined within the layers. The calculated exciton binding energy is much larger than that obtained by Watanabe {it et al} using a Wannier model to interpret their experimental results and assuming that h-BN is a direct-band-gap semiconductor.
We have synthesized a new layered oxychalcogenide La2O2Bi3AgS6. From synchrotron X-ray diffraction and Rietveld refinement, the crystal structure of La2O2Bi3AgS6 was refined using a model of the P4/nmm space group with a = 4.0644(1) {AA} and c = 19.4
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rs hosted by bulk hBN have not been reported to date. In this work we study the emission properties of hBN crystals in the red spectral range using sub-bandgap optical excitation. Quantum emission from defects is observed at room temperature and characterized in detail. Our results advance the use of hBN in quantum nanophotonics technologies and enhance our fundamental understanding of its optical properties.
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