660 research outputs found
Fragmentation and OB Star Formation in High-Mass Molecular Hub-Filament System
Filamentary structures are ubiquitously seen in the interstellar medium. The
concentrated molecular mass in the filaments allows fragmentation to occur in a
shorter timescale than the timescale of the global collapse. Such hierarchical
fragmentation may further assist the dissipation of excessive angular momentum.
It is crucial to resolve the morphology and the internal velocity structures of
the molecular filaments observationally.
We perform 0".5-2".5 angular resolution interferometric observations toward
the nearly face-on OB cluster forming region G33.92+0.11. Observations of
various spectral lines as well as the millimeter dust continuum emission,
consistently trace several 1 pc scale, clumpy molecular arms. Some of the
molecular arms geometrically merge to an inner
3.0\,, 0.6
pc scale central molecular clump, and may directly channel the molecular gas to
the warm (50 K) molecular gas immediately surrounding the centrally
embedded OB stars. The NH spectra suggest a medium turbulence line width
of FWHM2\,km\,s in the central molecular clump, implying a
10 times larger molecular mass than the virial mass. Feedbacks from
shocks and the centrally embedded OB stars and localized (proto)stellar
clusters, likely play a key role in the heating of molecular gas and could lead
to the observed chemical stratification. Although (proto)stellar feedbacks are
already present, G33.92+0.11 chemically appears to be at an early evolutionary
stage given by the low abundance limit of SO observed in this region.Comment: 37 pages, 23 figure
Non-Hermitian guided modes and exceptional points using loss-free negative-index materials
We analyze the guided modes in coupled waveguides made of negative-index
materials without gain or loss. We show that it supports non-Hermitian
phenomenon on the existence of guided mode versus geometric parameters of the
structure. The non-Hermitian effect is different from parity-time (PT)
symmetry, and can be explained by a simple coupled-mode theory with an anti-PT
symmetry. The existence of exceptional points and slow-light effect are
discussed. This work highlights the potential of loss-free negative-index
materials in the study of non-Hermitian optics
Higher-order exceptional points in loss-free waveguide arrays with negative-index materials
Negative-index materials (NIMs) are shown to support optical anti-parity-time
(anti-PT) symmetry even when they are lossless. Here we prove the feasibility
in achieving higher-order exceptional points (EPs) in loss-free waveguide
arrays by utilizing the anti- symmetry induced by NIM. Numerical
simulation about a third-order EP fits well with the coupled-mode theory. A
scheme of achieving fourth-order EPs is also discussed. This work highlights
the potential of loss-free NIMs in the study of non-Hermitian optics
A new AgI complex based on 1-[(1H-benzimidazol-1-yl)methÂyl]-1H-1,2,4-triazole
In the title complex, bisÂ{μ-1-[(1H-benzimidazol-1-yl)methÂyl]-1H-1,2,4-triazole}disilver(I) dinitrate, [Ag2(C10H9N5)2](NO3)2, the AgI ion is nearly linearly coordinated [N—Ag—N angle is 155.72 (14)°] by two 1-[(1H-benzimidazole-1-yl)methÂyl]-1H-1,2,4-triazole (bmt) ligands. In addition, two bmt ligands link two AgI ions, forming a dinuclear unit with an Ag⋯Ag distance of 5.0179 (15) Å. The whole complex is generated by an inversion centre. The dinuclear units and the NO3
− counter-ions are connected by N—H⋯O hydrogen bonds and weak Ag⋯O interÂactions [2.831 (5), 2.887 (5) and 2.908 (5) Å], leading to a three-dimensional structure
Light Harvesting Mechanism of Photosystem II in Photosynthesis:
As one of the most important chemical reactions on the earth, the photosynthetic reaction has gained much attention. For example, foliage and algae possess superior abilities to harvest luminous energy from sunlight in photosynthetic reactions by capturing lights with the light-harvesting complex (LHC) and transferring the energy from LHC to the reaction center (RC) in Photosystem II (PSII), to realize the continuous and efficient transformation from luminous energy to chemistry energy. In this article, the progresses in the studies on the crystal structure of PSII, the energy and electron transfer mechanism, and artificial simulation on photo-induced electron transfer are reviewed. Additionally, the initiating mechanism of hydrogen-abstraction photoinitiators, and the inspiration of the principles and mechanisms of photo-induced electron transfer in macromolecular photoinitiators were also discussed. It is believed that properly choosing covalent chains of appropriate types and length as the bridge between electron donor and electron acceptor is crucial for improving the initiating efficiency of photoinitiators. Keywords: photosynthesis; photosystem; light harvesting; photoinitiator; electron transfe
1-[Bicyclo[4.2.0]octa-1(6),2,4-trien-3-yl]-3-[bicyclo[4.2.0]octa-1(6),2,4-trien-3-ylÂmethyl]imidazolium hexaÂfluoroÂphosÂphate
In the title compound, C20H19N2
+·PF6
−, the two benzocycloÂbutene units are essentially planar and they form dihedral angles of 38.0 (2) and 72.7 (2)°, with the central imidazolium ring. In the crystal structure, weak C—H⋯π and Ï€-–π stacking interÂactions [centroid–centroid distance = 3.742 (2) Å] contribute to the stability of the crystal structure. The PF6
− ion is disordered over two positions with site occupancies of 0.869 (9) and 0.131 (9)
Optimization of fermentation conditions for pristinamycin production by immobilized Streptomyces pristinaespiralis using response surface methodology
Abstract Response surface methodology was used to optimize the
fermentation conditions for the production of pristinamycin by
immobilization of Streptomyces pristinaespiralis F213 in shaking
flask cultivation. Seed medium volume, fermentation medium volume and
shaking speed of seed culture were found to have significant effects on
pristinamycin production by the Plackett-Burman design. The steepest
ascent method was adopted to approach the vicinity of optimum space,
followed by central composite design for further optimization. A
quadratic model was built to fit the pristinamycin production. The
optimum conditions were found to be seed medium volume of 29.5 ml,
fermentation medium volume of 28.8 ml, and shaking speed of seed
culture at 204 rpm. At the optimum conditions, a production of 213 mg/l
was obtained, which was in agreement with the maximum predicted
pristinamycin yield of 209 mg/l. This is the first report on
pristinamycins production by immobilized S. pristinaespiralis using
response surface methodology
High-quality multi-wavelength quantum light sources on silicon nitride micro-ring chip
Multi-wavelength quantum light sources, especially at telecom band, are
extremely desired in quantum information technology. Despite recent impressive
advances, such a quantum light source with high quality remains challenging.
Here we demonstrate a multi-wavelength quantum light source using a silicon
nitride micro-ring with a free spectral range of 200 GHz. The generation of
eight pairs of correlated photons is ensured in a wavelength range of 25.6 nm.
With device optimization and noise-rejecting filters, our source enables the
generation of heralded single-photons - at a rate of 62 kHz with
, and the generation of energy-time entangled
photons - with a visibility of in the Franson interferometer.
These results, at room temperature and telecom wavelength, in a CMOS compatible
platform, represent an important step towards integrated quantum light devices
for the quantum networks.Comment: 7 pages, 4 figure
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