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排序方式: 共有350条查询结果,搜索用时 15 毫秒
241.
采用基于第一性原理的GGA+U方法研究了BaFe18O27的晶体结构和基态电子结构. 以实验数据为初始结构的离子弛豫显示,由于稳定结构中离子半径的差异和2d位Fe的存在,位于BaO层中6h位的O离子脱离了实验结构中原胞的"表面"位置,产生畸变. 计算得到晶体磁矩为28 μB/f.u.,与实验相符. 电子态密度及能带计算表明该材料具有微弱的半金属特性,而且与c轴平行方向和垂直方向的能带色散关系有着很大不同,6g位Fe在该材料的输运特性中起着关键作用,它们形成一种"导电层",导致垂直电导率和平行电导率出现非常大的差异.
关键词:
第一性原理
W形六角铁氧体
电子结构
导电性 相似文献
242.
Shou Ri SHENG* Xiao Ling LIU Wei ZHOU Xian HUANG Institutes of Chemistry Jiangxi Normal University Nanchang Department of Chemistry Zhejiang University 《中国化学快报》2005,(3)
Polymer-supported organic reagents have been applied to the rapid preparation of smallorganic molecules because they can provide attractive and practical methods forcombinatorial chemistry and solid-phase synthesis1. Vinylphosphonates bearing anelectron withdrawing groups at the α-position are versatile intermediates for the synthesisof some hetero- and carbocyclic compounds2. Many methods have been developed forpreparing their parent compounds, 1-alkenephosphonates. Among those methods, anim… 相似文献
243.
In this paper, by introducing a two-stages polarization mode dispersion (PMD) compensator after a optical fiber link with a large PMD, we compensated over 270ps first-order and 2000ps2 high-order PMD in a optical fiber link with super high PMD. Our experimental results shows that, the compensators based on the two-stages of compensator can be used to PMD compensation in a 20Gb/s OTDM system with 60 km high PMD fiber. Before compensation,270ps DGD is became into max. 7ps after compensation. At simultaneity, the tunable FBG have a function of dispersion compensation. 相似文献
244.
Shou‐Ri Sheng Hai‐Rong Luo Wu‐Kang Sun Xiao‐Ling Liu Qin Xin Qiu‐Ying Wang 《合成通讯》2013,43(22):2839-2845
Vinyl ethers were prepared with good yields in a one‐pot, two‐step transformation by O‐alkylation reaction of phenyl 2‐hydroxyalkyl selenides with primary or secondary organic halides followed by oxidation elimination with 30% hydrogen peroxide. 相似文献
245.
Coumarin derivatives were prepared by the von Pechmann reaction of PEG‐bound acetoacetate reagent with phenols in the presence of TiCl4 in excellent yield and purity with a facile workup procedure. The polymer reagent could be recycled two to four times without diminishing the yield or purity. 相似文献
246.
N. A. Ri T. M. Khlebodarova V. V. Kogai S. I. Fadeev V. A. Likhoshvai 《Journal of Applied and Industrial Mathematics》2013,7(2):252-258
Some mathematical model is analyzed describing nitrite utilization by E. coli cells grown in a flow chemostat. The system is of interest from the standpoint of the analysis of the mechanisms by which a cell utilizes toxic substrates in respiration. It is demonstrated that the system admits the appearance of two stationary nitrite concentrations; thus, it becomes bistable. The range of the nitrite supply rate into the chemostat for which the model is bistable is determined. The model predicts the possibility of a spontaneous death of the culture in transition between stationary states. 相似文献
247.
Size-exclusion chromatography has been developed for the separation of large biological molecules including proteins, polymers, peptides, nucleic acids, and polysaccharide according to their molecular size. This study determined the retention factors for dextran (5, 25, 50, 270, 670, and 1100?kDa) and polysaccharides, such as fucoidan, alginic acid, and laminarin, in the size-exclusion chromatography stationary phase. In addition, the molecular weights and retention factor of three polysaccharides were calculated from the dextran standard curve. The largest retention factor was 4.26 using the size-exclusion chromatography columns (5?kDa dextran). The molecular weights of fucoidan, alginic acid, and laminarin were determined to be 250, 200, and 5 to 64?kDa, respectively. 相似文献
248.
Risk measures are of considerable current interest. Among other uses, they allow an insurer to calculate a risk-loaded premium for a random loss. However, the premium principle in use by the insurer may be, at least in part, based on considerations other than risk. It is then important to quantify the degree to which the premium compensates the insurer for the risk associated with the loss. This can be done by choosing a suitable risk measure and solving for the parameter that leads to the insurer’s premium. When the loss distribution is unknown, this becomes a statistical estimation problem.In this paper, we investigate the nonparametric estimation of the parameter associated with a distortion-based risk measure. It is assumed that the premium principle is known, but no information is assumed about the loss distribution, and therefore empirical estimators are used. We explore the asymptotic properties of the resulting estimator of the risk measure parameter in general and for three well-known risk measures in particular: the proportional hazards transform, the Wang transform, and the conditional tail expectation. 相似文献
249.
250.
Shou Ri Sheng Ming Gang Hu Qin Xin Xiao Ling Liu Xian Huang 《中国化学快报》2007,18(4):377-379
Reaction of polystyrene-supported lithium selenide with 3-acetoxy-2-methylene-alkanoates afforded the corresponding allyl selenide resins and subsequent cleavage from the polymer by treating with methyl iodide to furnish(Z)-allyl iodides in good yields and high purities.The polymeric selenium reagent can be regenerated and reused.So it is a environmentally benign reagent. 相似文献