Optical absorption and electron energy loss spectra of carbon and boron nitride nanotubes: a first-principles approach

We present results for the optical absorption spectra of small-diameter single-walled carbon and boron nitride nanotubes obtained by ab initio calculations in the framework of time-dependent density-functional theory. We compare the results with those obtained for the corresponding layered structures, i.e. the graphene and hexagonal boron nitride sheets. In particular, we focus on the role of depolarization effects, anisotropies, and interactions in the excited states. We show that the random phase approximation reproduces well the main features of the spectra when crystal local field effects are correctly included, and discuss to what extent the calculations can be further simplified by extrapolating results obtained for the layered systems to results expected for the tubes. The present results are relevant for the interpretation of data obtained by recent experimental tools for nanotube characterization, such as optical and fluorescence spectroscopies, as well as polarized resonant Raman scattering spectroscopy. We also address electron energy loss spectra in the small-q momentum-transfer limit. In this case, the interlayer and intertube interactions play an enhanced role with respect to optical spectroscopy. PACS 71.45.Gm; 77.22.Ej; 78.20.Bh; 78.67.Ch     

Na5, Na6和Na7团簇光学吸收谱的理论研究

以含时密度泛函理论为基础,结合从头赝势方法运用含时局域密度近似计算了Na5,Na6和Na7团簇的动力学极化强度,并通过傅里叶变换得出了团簇的光学吸收谱.研究表明,计算结果可以较好地再现实验谱,而且Na6和Na7团簇的结果和组态相互作用的结果符合较好.二维结构和三维结构的Na6团簇的计算结果表明,只有二维结构可出现低于2 eV以下的峰,而且二维结构光谱的计算结果与实验结果符合较好.     

Atomic structure of icosahedral B4C boron carbide from a first principles analysis of NMR spectra

Density functional theory is demonstrated to reproduce the 13C and 11B NMR chemical shifts of icosahedral boron carbides with sufficient accuracy to extract previously unresolved structural information from experimental NMR spectra. B4C can be viewed as an arrangement of 3-atom linear chains and 12-atom icosahedra. According to our results, all the chains have a CBC structure. Most of the icosahedra have a B11C structure with the C atom placed in a polar site, and a few percent have a B (12) structure or a B10C2 structure with the two C atoms placed in two antipodal polar sites.     

Magnetic states and exchange interaction in hexagonal MnFeAs: A first principles study

The electronic structure and magnetic states for hexagonal-MnFeAs have been studied by a first-principles density functional theory (DFT) calculation. The ground state is ferromagnetic and the calculated magnetic moments for Fe and Mn are 1.1 and 3.1μ_B, respectively, leading to a total magnetization of 4.1μ_B per formula unit due to the small negative moments of As atoms. The exchange interaction between Fe and Mn layers () is positive and tends to form the ferromagnetic ordering. On the other hand, the exchange interaction at the Fe-As₁ layer () is negative while that at the Mn-As₂ layer () is positive. The field induced first order magnetic transition at T_C is related to the competed exchange interaction in the compound.     

高压下LiYF4晶体光学性质的第一性原理研究

本文采用第一性原理方法,在100 GPa的压力范围内, 计算了LiYF4理想晶体和含空位点缺陷晶体的光学性质.吸收谱数据表明,在100 GPa范围内,压力和相变因素的存在不会改变LiYF4晶体在250-1000 nm的波段内没有光吸收的事实. 氟、钇空位点缺陷的出现会使得LiYF4的吸收边蓝移,而锂空位点缺陷将导致它的吸收边微弱红移(但在250-1000 nm的波段内它仍不具有光吸收行为).波长在532 nm处的折射率数据显示, 在LiYF4的三个结构相区,其折射率均随压力的增加而增大. LiYF4从白钨矿结构到褐钇铌矿结构的相变会使得其折射率略微增加,但从褐钇铌矿结构到类黑钨矿结构的相变将导致其折射率显著降低. 同时,空位缺陷的存在将引起LiYF4的折射率明显增大. 分析指明,LiYF4有成为冲击窗口材料的可能. 本文所获得的信息对未来的实验研究有参考作用.     

Lu_2O_3晶体在高压下的光学性质第一性原理研究

本文采用第一性原理方法,计算了Lu_2O_3(氧化镥)的理想晶体和含氧、镥空位点缺陷晶体在100 GPa压力范围内的光吸收谱和折射率性质.结果表明:在100 GPa范围内, Lu_2O_3理想晶体在可见光及红外光区不具有光吸收现象,空位点缺陷的存在将导致吸收边红移,其中氧空位点缺陷引起的红移行为更显著,但这些吸收边仍未进入可见光区的高波段. 532 nm处的折射率数据表明,在立方结构和单斜结构相区, Lu_2O_3晶体的折射率随压力的增加而增大,高压结构相变以及氧、镥空位的存在也会使得折射率增大.结合温度效应分析推测, Lu_2O_3晶体在近红外区有可能透明.     

Configuration interaction of Er3+ with a charge transfer configuration in the elpasolite compound Cs2NaErCl6

The discrepancies of the crystal-field analysis in Cs₂NaErCl₆ are lowered by letting the 4f¹¹ configuration interact with a charge transfer configuration enclosing a p electron. 75 levels with a total degeneracy equal to 130 were fitted with a mean deviation of 10.5 cm^?1, compared with 21.4 cm^?1 in the standard model. It is found that: (i) the p electrons involved in the process originate most probably from the six Cl^? ligands; (ii) 14% of the 3p⁶ orbitals are projected onto the central ion coordinate system as an unique orbital with p angular character; (iii) this projected orbital can be coupled with 4fⁿ configurations and handled by tensorial methods; and (iv) the most probable mechanism responsible for the improvement of the crystal-field analysis is a 4f¹¹ 3p⁶-4f¹²3p⁵ interaction. The first configuration represents Er³⁺ surrounded by six Cl^? ligands, whereas in the second one, the rare-earth ion and the ligands have gained and lost one electron respectively. The crystal-field analysis enables the estimation of the location of the charge transfer band.     

Na5, Na6和Na7团簇光学吸收谱的理论研究

以含时密度泛函理论为基础，结合从头赝势方法运用含时局域密度近似计算了Na₅，Na₆和Na₇团簇的动力学极化强度，并通过傅里叶变换得出了团簇的光学吸收谱.研究表明，计算结果可以较好地再现实验谱，而且Na₆和Na₇团簇的结果和组态相互作用的结果符合较好.二维结构和三维结构的Na₆团簇的计算结果表明，只有二维结构可出现低于2 eV以下的峰，而且二维结构光谱的计算结果与实验结果符合较好. 关键词： 光吸收谱 Na团簇 含时局域密度近似     

Optical identification and electronic configuration of tungsten in 4H- and 6H-SiC

Several optically observed deep level defects in SiC are still unidentified and little is published on their behavior. One of the commonly observed deep level defects in semi-insulating SiC is UD-1.     

F+ laser performance and interaction of Cu,Ag and Au at the reduced oxygen coordination of MgO surface: first principles calculations

F⁺ laser performance and interaction of the title group IB transition metals at the reduced oxygen coordination of MgO surface were investigated using the TD and DFT methods of ab initio molecular electronic structure calculations. The considered ion clusters were embedded in simulated Coulomb fields that closely approximate the Madelung fields of the host surfaces and the nearest neighbor ions to the F⁺ site were allowed to relax to equilibrium in each case. The F⁺ laser performance fades quickly as the reduced oxygen coordination decreases from 5 to 4 to 3. The relaxed excited states (RESs) of the defect containing surfaces are compact and deep below the conduction bands of the perfect MgO surface. The probability of orientational destruction of the center in laser experiment is expected to follow the order flat>corner>edge. The excited state at the edge has higher energy than that at the flat or at the corner. F⁺ is easily formed at the lower oxygen coordination and the disappearance of anisotropy and 2p splitting observed in absorption of F⁺ at the surface follow the order corner>flat>edge. The Glasner–Tompkins relation is generalized to include the F⁺ bands at the reduced oxygen coordination of a metal oxide surface. As far as the adsorbate–substrate interactions are concerned, the F⁺ center enhances the adsorptivity of Ag, Cu and Au by ca. 1.91–3.33 eV and changes the nature of adsorption from physical adsorption to chemical adsorption. The adsorption energies follow the order Cu>Au>Ag and are explainable in terms of electrostatic potential curves, energy gaps and spin pairing. Cu and Ag act as electron donors while Au acts as electron acceptor and the MgO surface cannot be made semiconducting by F⁺ imperfection.     

Optical absorption and emission from irradiated RbMgF3:Eu2+ and KMgF3:Eu2+

The optical properties of irradiated RbMgF₃:Eu²⁺ and KMgF₃:Eu²⁺ have been investigated. Previous research has shown that Eu²⁺ ions in unirradiated RbMgF₃ give rise to broad band absorption around 250 nm and sharp intense line emission at 360 nm. When this material is irradiated little or no change occurs in the 250 nm absorption, but the lifetime of the Eu²⁺ 360 nm transition is reduced. In addition, new emission is observed at 680 nm. In the case of irradiated KMgF₃:Eu²⁺ two new emission bands are observed at 600 and 800 nm. All of these transitions have short lifetimes and are not due to Eu³⁺ ions.     

Structural and electronic properties of Snn-1Pb and Pbn-1Sn clusters: a theoretical investigation through first principles calculations

Here we report a systematic theoretical study of the structure and electronic properties of Sn_n-1Pb and Pb_n-1Sn (n = 2-13) clusters and compare these results with pure Sn_n and Pb_n to understand the influence of the dopant elements. The calculations were carried out using the density functional theory with generalized gradient approximation for the exchange-correlation potential. Extensive search based on large number of initial configurations has been carried out to locate the stable isomers of Sn_n-1Pb and Pb_n-1Sn (n = 2-13) clusters. The relative stability of Sn_n-1Pb and Pb_n-1Sn (n = 2-13) clusters is analyzed based on the calculated binding energies and second difference in energy. The stability analysis of these clusters suggests that, while the substitution of Sn by Pb lowers the stability of Sn_n clusters, presence of Sn enhances the stability of the Pb_n clusters. The results suggest that while for Sn_n-1Pb, n=4, 7, 10, 12 clusters are more stable than their respective neighbors, Pb_n-1Sn clusters with n = 4, 7 and 9 are found to be more stable. Based on the fragmentation pattern it is seen that for Sn_n-1Pb and Pb_n-1Sn clusters favor monomer evaporation of the Pb atom up to n =11 and n =12, respectively. Unlike this trend, the Sn₁₁Pb undergoes fission type fragment into Sn₅Pb and Sn₆ clusters. A comparison between our theoretical results and surface induced dissociation experiment shows good agreement, which gives confidence on the prediction of the ground state geometries.     

Photoluminescence and Optical Absorption of Cs2NaScF6:Cr3+

The main objective of this paper is the characterization of the spectroscopic properties of new materials that are prospective laser media. This approach allows for the comparison of the properties of the Cr³⁺ in different environments. Here, we have studied the photoluminescence and optical absorption of Cs₂NaScF₆:Cr³⁺ single crystals. On the basis of near-infrared luminescence measurements at 2, 77, and 300 K the observed lines originated from the Cr³⁺-centres were associated with the transition and the lifetimes were obtained. In spite of the quenching observed as a function of temperature at least 10% of the 2 K emission intensity for Cs₂NaScF₆ doped with 1% of Cr³⁺ remains at room temperature. Besides, the 2 K emission broad band could be well described in terms of normal modes of the octahedral complex [CrF₆]³⁻, and the Racah and crystal-field parameters calculated.