密度泛函理论研究CO与Nin（n=1—6）团簇的相互作用

采用密度泛函理论对CO吸附在镍团簇表面进行了系统研究.结果表明,Ni_nCO团簇的最低能量结构是在Ni_n团簇最低能量结构的基础上吸附CO生长而成,CO的吸附没有改变Ni_n团簇的结构;CO分子在Ni_n团簇表面发生的是非解离性吸附,与优化的CO键长（0.1138?nm）相比,吸附后C—O键长变长（0.1180—0.1214?nm）,表明吸附后C—O键被削弱,CO分子被活化.自然键轨道分析表明,CO分子只与最近邻的Ni原子发生相互作用;CO分子与Ni原子相互作用的本质是CO分子内的杂化轨道与Ni原子3d, 4s, 4p轨道相互作用的结果. 关键词： nCO团簇')" href="#">Ni_nCO团簇 n团簇')" href="#">Ni_n团簇 平衡结构 电子性质     

Theoretical study of CO and Pb adsorption on the Ni(1 1 1) and Ni3Al(1 1 1) surfaces

Adsorption of CO molecules and Pb atoms on the Ni(1 1 1) and Ni₃Al(1 1 1) substrates is studied theoretically within an ab initio density-functional-theory approach. Stable adsorption sites and the corresponding adsorption energies are first determined for stoichiometric surfaces. The three-fold hollow sites (fcc for Pb and hcp for CO) are found most favourable on both substrates. Next, the effect of surface alloying by a substitution of selected topmost substrate atoms by Pb or Ni atoms on the adsorption characteristics is investigated. When the surface Al atoms of the Ni₃Al(1 1 1) substrate are replaced by Ni atoms, the Pb and CO adsorption energies approach those for a pure Ni(1 1 1) substrate. The Pb alloying has a more substantial effect. On the Ni₃Al(1 1 1) substrate, it reduces considerably adsorption energy of CO. On the Ni(1 1 1) substrate, CO binding strengthens slightly upon the formation of the Ni(1 1 1)p(2×2)-Pb surface alloy, whereas it weakens drastically when the Ni(1 1 1)-Pb surface alloy is formed.     

Neutral currents in alternative U3 (W)-gauge models of weak and electromagnetic interactions

Two alternative U₃(W)-gauge models are presented. Both agree with the recent Abbott-Barnett fits to the neutrino-nucleon neutral-current data, and with the SLAC measurement of the asymmetry parameter for longitudinally polarised electrondeuteron inelastic scattering. Results for are also found in agreement with the latest measurements. The models differ in the parameterQ _W(Z, N) characterising parity-violation in heavy atoms for which, however, the experimental situation is still unclear.     

The structure and stability of Si@Al<Subscript>12</Subscript>H<Subscript>n</Subscript> (n=1−14) clusters

Density functional theory has been employed in order to investigate the structure and stability of Si@Al₁₂H_n () clusters. Hydrogenated Si@Al₁₂ clusters exhibit pronounced stability for even numbers of H atoms. Large binding energy, HOMO-LUMO gaps and increased ionization potentials imply that these clusters should be physically and chemically stable. The analysis of the charge density of the HOMO plot illustrates that a pair of hydrogen atoms prefer to occupy opposing on-top sites for clusters with an even n number. Studies of deformation charge density plots demonstrate that significant charge transfer occurs from the Si@Al₁₂ to the H atoms.     

Si-nanostructures formation in amorphous silicon nitride SiNx:H deposited by remote PECVD

The formation of silicon nanoclusters embedded in amorphous silicon nitride (SiN_x:H) can be of great interest for optoelectronic devices such as solar cells. Here amorphous SiN_x:H layers have been deposited by remote microwave-assisted chemical vapor deposition at 300 °C substrate temperature and with different ammonia [NH₃]/silane [SiH₄] gas flow ratios (R=0.5−5). Post-thermal annealing was carried out at 700 °C during 30 min to form the silicon nanoclusters. The composition of the layers was determined by Rutherford back scattering (RBS) and elastic recoil detection analysis (ERDA). Fourier transform infrared spectroscopy (FTIR) showed that the densities of SiH (2160 cm⁻¹) and NH (3330 cm⁻¹) molecules are reduced after thermal annealing for SiN:H films deposited at flow gas ratio R>1.5. Breaking the SiH bonding provide Si atoms in excess in the bulk of the layer, which can nucleate and form Si nanostructures. The analysis of the photoluminescence (PL) spectra for different stoichiometric layers showed a strong dependence of the peak characteristics (position, intensity, etc.) on the gas flow ratio. On the other hand, transmission electron microscopy (TEM) analysis proves the presence of silicon nanoclusters embedded in the films deposited at a gas flow ratio of R=2 and annealed at 700 °C (30 min).     

Odd autoionizing 2p<Subscript>1/2</Subscript><Superscript>5</Superscript>n(s<Superscript>′</Superscript>/d<Superscript>′</Superscript>) resonances of Ne excited from the 2p<Superscript>5</Superscript>3p[K]<Subscript>J</Subscript> states

Absolute photoionization cross sections for Ne atoms in the excited levels (Paschen notation ) were calculated at near threshold energies within the configuration interaction Pauli-Fock approach including core polarization. The computed spectra and the lineshape parameters of the odd parity 2p_1/2 ⁵ns^′/d^′ autoionizing resonances are found to be in good agreement with high resolution laser spectroscopic results. Guided by the theoretical results, improved analyses of the measured spectra by superimposed Fano-type profiles were achieved. Theoretical predictions are presented for resonances which have not yet been studied experimentally. In addition, we report the absolute partial photoionization cross sections for the ²P_3/2 and ²P_1/2 channel at photoelectron energies up to 7 eV. Except for the highest lying 2p₁(¹S₀) level, these cross sections monotonically decrease with energy (as reported earlier in single-electron calculations for the Ne(2p⁵3p) configuration) with branching ratios which essentially reflect the core composition of the 2p_x levels. For the 2p₁ level the resonance structure and the partial cross sections are strongly influenced by a Cooper-Seaton minimum in the d_3/2 ^′ channel,located just above the ²P_1/2 ionization limit.     

Atomic squeezing in assembly of two two-level atoms interacting with a single mode coherent radiation

Saito and Ueda [Phys. Rev. A 59, 3959 (1999)] studied atomic and radiation squeezing in interaction of a single mode coherent state of radiation with two excited two-level atoms, using the Jaynes Cummings Hamiltonian. They considered α real and studied squeezing of the Dicke operator S_x using the Kitagawa-Ueda criterion for squeezing and coupling times less than or nearly equal to . We obtain results to all orders in coupling time for atoms, which are initially in (i) fully excited, (ii) superradiant or in (iii) ground states and obtain more general results. We use our recently reported criterion for atomic squeezing, of which the Kitagawa-Ueda criterion is a special case, and obtain a much stronger (nearly 95%) atomic squeezing than that (nearly 1.1%) reported by Saito and Ueda.     

四元Heusler合金NiMnFeGa中Fe原子的磁性贡献

用熔炼和甩带的方法制备了组分为Ni₅₀Fe_xMn_25-xGa ₂₅(x=0—25) 的系列样品.x射线衍射实验结果表明，当Fe取代Mn的含量x＜17时，用熔炼和甩带的方法均 能合成高度有序的L2₁结构的Heusler相.而当x＞17时，普通熔炼方法只能得到 低有序度的 γ相，只有采用甩带急冷的方法才能获得高度有序的纯L2₁结构的化合物.根据 交流磁化率 和分子磁矩的测试结果，初步分析了Fe原子对化合物磁性的贡献，认为Fe原子占据了Mn原子 的位置后，具有高于一般含铁合金的原子磁矩，可达2.55—3.55μ_B. 关键词： Heusler合金 50Fe_xMn_25-xGa_{25')" href="#">Ni50FexMn25-xGa25}     

Preliminary Results for Capture of Negative Muons and Antiprotons by Noble-Gas Atoms

Cross sections for capture of negative muons (μ⁻) and antiprotons ( ) by helium, neon, argon, krypton, and xenon atoms (incomplete for the two heaviest noble-gas atoms) are calculated using the fermion molecular dynamics (FMD) method. These cross sections are used to estimate the capture ratios in mixtures, but these ratios are not precise since the total energy-loss cross sections have not yet been determined. This revised version was published online in August 2006 with corrections to the Cover Date.     

第一性原理模拟U在Gd2Zr2O7烧绿石中的固溶

采用第一性原理密度泛函理论模拟U在Gd₂Zr₂O₇烧绿石中的固溶,在低浓度U掺杂时,Gd₂Zr₂O₇烧绿石保持烧绿石结构;随着U掺杂浓度增加,Gd₂(Zr_{2-y}U_y)O₇和(Gd_{2-y}U_y)Zr₂O₇体系的晶格常数发生线性变化．计算结果表明,由于总能较低,U原子更偏向于替代无序换位后Gd₂Zr₂O₇晶格中B位的Gd原子．     

Interface Energies for Carbide Precipitates in TiAl

Carbon has been extensively investigated as an additive to TiAl alloys to improve high-temperature mechanical properties. The ternary-carbide-precipitate phases that have received most attention are Ti₃AlC (cubic perovskite), and Ti₂AlC (hexagonal). The perovskite (P) precipitates in the form of needles along the 001 direction of -TiAl, and the hexagonal phase (H) primarily forms platelets perpendicular to 111 directions. Using the first-principles density functional theory code VASP, calculations of host-precipitate interface energies were performed for these two carbides. An atomic-scale formulation of the interface energy is adopted. Calculations for the coherent interfaces are performed to determine the preferred terminations and translation states. An approximate correction to the interface energy for the effect of misfit is applied to estimate the energy of incoherent interfaces. The relative interface properties of the P-type and H-type carbides are determined by, in addition to the misfit strains, (i) the stronger bonding of the P-type than the H-type precipitate with the host at the interface, and (ii) the more pronounced variations of the interatomic potentials as a function of parallel translation state across non-close-packed (100) P layers than those across close-packed (0001) H layers.     

K-forbidden allowed $ \beta$ transitions in heavy nuclei

The role of the band quantum number K in influencing the character of allowed transitions in heavy deformed nuclei is examined. The conditions for the occurrence of K -forbidden decays in this region are explored. Specific cases of “allowed” decays proceeding via K = 2 to K = 6 channels are presented to illustrate the phenomenon. The listed K = 2 transitions, which by themselves contribute over 10% of all the presently known allowed transitions for A 228 nuclei, are seen to have an average , which is clearly outside the normal range for allowed transitions. It is concluded that, wherever the -connected states can be confidently labelled using the quantum numbers, the K -forbiddenness is in general as significant as that involving the other two (spin and parity) quantum numbers.     

Analytical electron microscopy study of diffusion-bonded multiphase system

This study presents diffusion experiments of NiAl coupled with pure Ni (Ni/NiAl). The couple produces the Ni₃Al-based intermetallic phase ( phase) as an intermediate layer at the interface during diffusion annealing. Analytical electron microscopy is used to examine microstructural features, crystallographic orientation and compositional variations across the interface. Interdiffusivities are measured from the compositional variations. It is shown that the growing behavior of the phase changes between higher and lower annealing temperatures. The growth kinetics of the phase is also discussed.     

Photoproduction of $ \eta$ -mesons off neutrons from a deuteron target

A formalism is developed for the partial-wave analysis of data on meson photoproduction off deuterons and applied to photoproduction of and mesons. Different interpretations of a dip-bump structure of the photoproduction cross-section in the 1670MeV region are presented and discussed. Helicity amplitudes for two low-mass S₁₁ states are determined.     

Metal-like to insulator transition in Lu3Fe5O12

Polycrystalline Lu₃Fe₅O₁₂, prepared using the solid state reaction method, has Fe in the mixed valence state as inferred from the X-ray photoelectron spectroscopy. A spectral change in the impedance plot at 343 K is attributed to metal-like to insulator transition (MIT), which is analyzed in terms of localized and delocalized _eg$e_g$ electrons. The change in the slope at 343 K in the DC conductivity plot also proves the MIT. The dependence of Z^′${\mathrm{Z}}^{\prime }$ on temperature and Z^″${\mathrm{Z}}^{″}$ on frequency clearly substantiates the presence of localized electrons up to 343 K and delocalized electrons above 343 K.     

Segmental relaxation in semicrystalline polymers: A mean-field model for the distribution of relaxation times in confined regimes

The effect of confinement in the segmental relaxation of polymers is considered. On the basis of a thermodynamic model we discuss the emerging relevance of the fast degrees of freedom in stimulating the much slower segmental relaxation, as an effect of the constraints at the walls of the amorphous regions. In the case that confinement is due to the presence of crystalline domains, a quasi-Poissonian distribution of local constraining conditions is derived as a result of thermodynamic equilibrium. This implies that the average free-energy barrier F for conformational rearrangement is of the same order of the dispersion of the barrier heights, ( F) , around F . As an example, we apply the results to the analysis of the -relaxation as observed by dielectric broad-band spectroscopy in semicrystalline poly(ethylene terephthalate) cold-crystallized from either an isotropic or an oriented glass. It is found that in the latter case the regions of cooperative rearrangement are significantly larger than in the former.     

Transport properties of CsHSO4 investigated by impedance spectroscopy and nuclear magnetic resonance

Transport properties of the superprotonic conductor, CsHSO₄, have been investigated by impedance spectroscopy and nuclear magnetic resonance (NMR). It has been found that both, conductivity (σ) and NMR diffusion (D _NMR) are practically isotropic in the high-conductive (superprotonic) phase (above 414 K). The NMR diffusion coefficient, D _NMR , increases rapidly and discontinuously at the melting point (~490 K). The temperature change of D _NMR in the superprotonic phase is characterized by a smaller activation energy compared to that in the liquid state. The values calculated from the Nernst-Einstein relation practically coincide with D _NMR in the superprotonic phase, i.e., the Haven ratio is close to unity. This indicates that in this phase the proton motion is rather uncorrelated.