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LAMOST DR5 released more than 200,000 low resolution spectra of early-type stars with S/N>50. Searching for metallic-line (Am) stars in such a large database and study of their statistical properties are presented in this paper. Six machine learning algorithms were experimented with using known Am spectra, and both the empirical criteria method (Hou et al. 2015) and the MKCLASS package(Gray et al. 2016) were also investigated. Comparing their performance, the random forest (RF) algorithm won, not only because RF has high successful rate but also it can derives and ranks features. Then the RF was applied to the early type stars of DR5, and 15,269 Am candidates were picked out. Manual identification was conducted based on the spectral features derived from the RF algorithm and verified by experts. After manual identification, 9,372 Am stars and 1,131 Ap candidates were compiled into a catalog. Statistical studies were conducted including temperature distribution, space distribution, and infrared photometry. The spectral types of Am stars are mainly between F0 and A4 with a peak around A7, which is similar to previous works. With the Gaia distances, we calculated the vertical height Z from the Galactic plane for each Am star. The distribution of Z suggests that the incidence rate of Am stars shows a descending gradient with increasing |Z|. On the other hand, Am stars do not show a noteworthy pattern in the infrared band. As wavelength gets longer, the infrared excess of Am stars decreases, until little or no excess in W1 and W2 bands.

The fundamental stellar atmospheric parameters (T eff and log g) and 13 chemical abundances are derived for medium-resolution spectroscopy from LAMOST Medium-Resolution Survey (MRS) data sets with a deep-learning method. The neural networks we designed, named as SPCANet, precisely map LAMOST MRS spectra to stellar parameters and chemical abundances. The stellar labels derived by SPCANet are with precisions of 119 K for T eff and 0.17 dex for log g. The abundance precision of 11 elements including [[Fe/H], and [Ni/H] are 0.06∼0.12 dex, while of [Cu/H] is 0.19 dex. These precisions can be reached even for spectra with signal-to-noise as low as 10. The results of SPCANet are consistent with those from other surveys such as APOGEE, GALAH and RAVE, and are also validated with the previous literature values including clusters and field stars. The catalog of the estimated parameters is available at http://paperdata.china-vo.org/LAMOST/MRS parameters elements.csv.

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Abstract

Obtaining structural information of uranyl species at an atomic/molecular scale is a critical step to control and predict their physical and chemical properties. To obtain such information, experimental and theoretical L3-edge X-ray absorption near-edge structure (XANES) spectra of uranium were studied systematically for uranyl complexes. It was demonstrated that the bond lengths (R) in the uranyl species and relative energy positions (ΔE) of the XANES were determined as follows: ΔE1 = 168.3/R(U-Oax)(2) - 38.5 (for the axial plane) and ΔE2 = 428.4/R(U-Oeq)(2) - 37.1 (for the equatorial plane). These formulae could be used to directly extract the distances between the uranium absorber and oxygen ligand atoms in the axial and equatorial planes of uranyl ions based on the U L3-edge XANES experimental data. In addition, the relative weights were estimated for each configuration derived from the water molecule and nitrate ligand based on the obtained average equatorial coordination bond lengths in a series of uranyl nitrate complexes with progressively varied nitrate concentrations. Results obtained from XANES analysis were identical to that from extended X-ray absorption fine-structure (EXAFS) analysis. XANES analysis is applicable to ubiquitous uranyl-ligand complexes, such as the uranyl-carbonate complex. Most importantly, the XANES research method could be extended to low-concentration uranyl systems, as indicated by the results of the uranyl-amidoximate complex (∼40 p.p.m. uranium). Quantitative XANES analysis, a reliable and straightforward method, provides a simplified approach applied to the structural chemistry of actinides.

In the absence of inversion symmetry along the c-direction, group III-nitride materials on sapphire substrates (AlN, GaN, InN) and their alloys can exhibit either an N-polar or a metal-(Al, Ga, In) polar surface. [1] Here, N-polar or metalpolar refers to the situation where the epilayer grows along the c or −c direction. Although having identical bulk properties (e.g., refractive index, bandgap, etc.), the two polarities exhibit noticeable differences in surface energy, [2] growth mode, [3] nonlinear optical property, [4] and susceptibility to chemicals. [5] Manipulating the polarity, therefore, is a fertile playground for discovering novel device concepts in form of heteropolar junctions, such as nonlinear optical devices, [3,4,6] polar discontinuity devices, [7] and high-performance GaN high-electron-mobility transistors (HEMT). [8] Particularly, a number of The performance of nitride devices is strongly affected by their polarity. Understanding the polarity determination and evolution mechanism of polar wurtzite nitrides on nonpolar substrates is therefore critically important. This work confirms that the polarity of AlN on sapphire prepared by metalorganic chemical vapor deposition is not inherited from the nitrides/sapphire interface as widely accepted, instead, experiences a spontaneous polarity inversion during the growth. It is found that at the initial growth stage, the interface favors the nitrogen-polarity, rather than the widely accepted metal-polarity or randomly coexisting. However, the polarity subsequently converts into the metal-polar situation, at first locally then expanding into the whole area, driven by the anisotropy of surface energies, which results in universally existing inherent inverse grain boundaries. Furthermore, vertical two-dimensional electron accumulation originating from the lattice symmetry breaking at the inverse grain boundary is first revealed. This work identifies another cause of high-density defects in nitride epilayers, besides lattice mismatch induced dislocations. These findings also offer new insights into atomic structure and determination mechanism of polarity in nitrides, providing clues for its manipulation toward the novel hetero-polarity devices.