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The detection of the 11.3-micron emission feature characteristic of the Si--C stretch in carbon-rich evolved stars reveals that silicon carbide (SiC) dust grains are condensed in the outflows of carbon stars. SiC dust could be a significant constituent of interstellar dust since it is generally believed that carbon stars inject a considerable amount of dust into the interstellar medium (ISM). The presence of SiC dust in the ISM is also supported by the identification of presolar SiC grains of stellar origin in primitive meteorites. However, the 11.3-micron absorption feature of SiC has never been seen in the ISM and oxidative destruction of SiC is often invoked. In this work we quantitatively explore the destruction of interstellar SiC dust through oxidation based on molecular dynamics simulations and density functional theory calculations. We find that the reaction of an oxygen atom with SiC molecules and clusters is exothermic and could cause CO-loss. Nevertheless, even if this is extrapolable to bulk SiC dust, the destruction rate of SiC dust through oxidation could still be considerably smaller than the (currently believed) injection rate from carbon stars. Therefore, the lack of the 11.3-micron absorption feature of SiC dust in the ISM remains a mystery. A possible solution may lie in the currently believed stellar injection rate of SiC (which may have been overestimated) and/or the size of SiC dust (which may actually be considerably smaller than submicron in size)., 7 pages, 5 figures; accepted for publication in MNRAS

Vortex $\ensuremath{\gamma}$-ray beams with large orbital angular momentum (OAM) have significant potential applications in material science, nuclear detection, and astrophysics. An all-optical scheme for production of intense vortex $\ensuremath{\gamma}$-ray beams using relativistic laser-plasma interactions is proposed, where the OAM of $\ensuremath{\gamma}$-photons is converted from the spin angular momentum of circularly polarized (CP) lasers. The spin-to-orbital angular momentum conversion is achieved by two processes: resonant acceleration of electrons in CP laser-plasma interactions and nonlinear Compton scattering (NCS) of another linearly polarized laser from the former. Three-dimensional particle-in-cell simulations show that vortex $\ensuremath{\gamma}$-ray beams with a large and controllable OAM of $2.5\ifmmode\times\else\texttimes\fi{}{10}^{18}\ensuremath{\hbar}$ and energy up to hundreds of $\mathrm{MeV}$ can be obtained by using two lasers at intensities of approximately ${10}^{22}\phantom{\rule{0.2em}{0ex}}{\mathrm{W}/\mathrm{cm}}^{2}$, which is much more practical and more efficient than previously proposed NCS of a Laguerre-Gaussian beam.

Global climate change is projected to increase both the intensity and frequency of extreme precipitation events (EPEs), which are considered to have stronger impacts on ecosystem functions than gradual changes in mean precipitation conditions. In this study, a consecutive 20-day extreme precipitation event (282 mm) was applied during the mid- and late-growing season periods in a semiarid steppe for three years to investigate the effects of extreme large precipitation events on aboveground net primary productivity (ANPP) and ecosystem carbon dioxide (CO 2 ) fluxes, including net ecosystem carbon absorption (NEE), gross primary productivity (GPP) and ecosystem respiration (Re). Although soil moisture was significantly increased by extreme precipitation, and even exceeded field capacity during the treatment periods, ANPP remained stable across all the treatments. There was also little change in mean growing season ecosystem CO 2 fluxes under the two precipitation treatments, despite GPP rates decreased by 34.4 and 26.3%, and NEE rates were suppressed by 77 and 68% during the mid- and late-season treatment periods, respectively. The stable CO 2 fluxes could be attributed to the recovery of GPP and NEE in 7 and 12 days after the end of EPEs. Our study demonstrated that both ANPP and CO 2 fluxes in this semiarid steppe were very stable in the face of extreme large precipitation events, regardless of the timing of events occur. Nevertheless, future, long-term studies need to investigate the potential tipping points or thresholds for ecosystem function shifts, as an increasing occurrence of EPEs has been forecasted in future climate change scenarios.

Short, brilliant pulses of MeV-level gamma rays with controllable orbital angular momentum (OAM) and small divergence angle would be useful for precision microscopy, nuclear imaging, radiography, micromanipulation, and more. These pulses are still unavailable, though, due to the unavoidable laser-induced damage associated with traditional methods. The authors show how to make such a gamma-ray pulse (shown in red), based on tailored interaction of an intense OAM-carrying Laguerre-Gaussian laser pulse (green) with underdense plasma. The topological structure, divergence angle, and OAM of the resulting gamma-ray pulse can be controlled by selecting the topological charge of the laser.

The ground state properties of Cr7C3 cell are studied in this paper by using first-principles calculations; the covalent bond in this compound is a kind of p and d hybridization between carbon (C) and chromium (Cr). The Fermi surface of projected density of states (PDOS) is dominated by d bands of Cr atoms and the whole structure has a metallic character. The calculated formation enthalpy is −116.55 and −120 kJ/mol for Pnma and Pmcn, respectively. We use two empirical equations of state (EOS) to investigate the lattice properties. The obtained average bulk modulus value is 308 GPa and the estimated mean shear modulus is 143.85 GPa, both of these two values are larger than Fe3C and Fe2B. The population analysis reveals some destabilization factors exist in Fe2B and Fe3C, but the attractive interactions for Cr7C3 are dispersed in the whole crystal. Due to the above reasons, we conclude that mechanical property and chemical stability of Cr7C3 are both much better than Fe2B and Fe3C.

First principles calculations were conducted to investigate the stabilities of six multi-component carbides of Cr 7 C 3 by calculating the cohesive energy and formation enthalpy of them. The theoretical predictions were compared with the experimental results and they were in agreement with each other. The electronic structures of the six carbides were also calculated in order to provide more information about the relationship between the stability and crystal compositions at atomic scale.

Detecting signals hidden beneath the noise floor is a challenging task. As the signal-to-noise ratio (S/N) dips below 0, false alarms and detection misses become a serious problem. Furthermore, to satisfy the real-time or near real-time requirement, detection schemes that are computationally intensive do not enjoy wide-spread adoption. In this paper, we present a new detection algorithm consisting of phase-space reconstruction technique and principal components analysis. The goal is to achieve the detection of weak signals in noisy environments. With the new algorithm, our study shows that in addition to detection, the frequency of the signal can be extracted even when the S/N reaches negative value and the FFT power spectrum shows no trace of its spectral characteristics. The signal detection scheme is insensitive to the nature of the background noise, making it viable to achieve good performance in various signal application domains. In this paper, we chose to report on the results pertaining to the analysis of time series from IPIX radar. The new detection algorithm is also computationally lean, thus enabling its use in real-time applications.

A kinetic model describing the energy deposition of fast electrons is established by considering both binary collisions and contributions due to collective process. The collision term is exactly simplified by taking into account relativistic effects within the context of fast ignition, and the explicit formulation of relativistic kinetic equation in three-dimensional momentum space is obtained with the help of spherical harmonic expansion. The corresponding numerical code is developed and benchmarked physically. Typical case studies in fast ignition are also presented.