磁场剪切对离子温度梯度模带状流产生的影响

对离子温度梯度模湍流非线性流体方程进行了解耦处理,得到包含磁场剪切效应的带状流与漂移波相互作用的非线性动力学方程.采用调制不稳定性的四波相互作用模型,研究了磁场剪切对带状流产生的影响.研究表明,在k_//值较小的范围内,当|k_//|增加时,带状流的增长率也呈增加的趋势. 关键词： 托卡马克等离子体 离子温度梯度模湍流 带状流 磁场剪切     

Calculation of the wave functions of the ground and weakly excited states of helium II

The wave functions of the ground (Ψ₀) and the first excited (Ψ_k) states of He II in the second-order approximation, i.e., up to the first two corrections to the corresponding solutions for a weakly nonideal Bose gas, are determined by the collective variable method, which was proposed by Bogolyubov and Zubarev and developed in the studies by Yukhnovskii and Vakarchuk. The functions Ψ₀ and Ψ_k = ψ_kΨ₀ are determined as the eigenfunctions of the N-particle Schrödinger equation from a system of coupled equations for Ψ₀, Ψ_k, and the quasiparticle spectrum E(k) of helium II. The results consist in the following: (1) these equations are solved numerically for a higher order approximation compared with those investigated earlier (the first-order approximation), and (2) Ψ₀ and ψ_k are derived from a model potential of interaction between He⁴ atoms (rather than from the structure factor as earlier) in which the potential barrier is joined with the attractive potential found from experiment. The height V ₀ of the potential barrier is a free parameter. Except for V ₀, the model does not have any free parameters or functions. The calculated values of the structure factor, the ground-state energy E ₀, and the quasiparticle spectrum E(k) of He II are in agreement with the experimental values for V ₀ ≈ 100 K. The second-order correction to the logarithm of Ψ₀ significantly affects the value of E ₀ and provides the asymptotics E(k → 0) = ck, while the second-order correction to ψ_k slightly affects the E(k). The second-order corrections to Ψ₀ and ψ_k have a smaller effect on the results compared with the first-order corrections, whereby the theory is in agreement with experiment; therefore, one may assume that the truncated Ψ₀ and ψ_k well describe the microstructure of He II. Thus, the series for Ψ₀ and Ψ_k can be truncated in spite of the fact that the expansion parameter is not very small (～1/2).     

Optical anisotropy of orthorhombic InS

The optical anisotropy of InS single crystals in the range of photon energy from 1.8 to 3.5 eV has been studied by absorption, electroreflectance and wavelength-derivative reflectance measurements. These systematic optical measurements for the polarizations, E//a and E//b, have revealed that the transition at the fundamental absorption edge of InS is allowed for E//b, and there exist three distinct doublet transitions having different selection rules in the photon energy region from 2 to 3.5 eV; Bo and B^'₀doublet allowed only for E//b, A₀ and A^'₀ allowed only for E//a, and E₁ and E^'₁ allowed for both polarizations. The observed results are discussed based on the anisotropic nature of two chemical bonds in InS, cation-cation and cation-anion.     

A DFT study the solvent effects of H2 adsorption on Cu(h k l) surface

Density functional theory (DFT) combined with conductor-like solvent model (COSMO) have been performed to study the solvent effects of H₂ adsorption on Cu(h k l) surface. The result shows H₂ can not be parallel adsorbed on Cu(h k l) surface in gas phase and only vertical adsorbed. At this moment, the binding energies are small and H₂ orientation with respect to Cu(h k l) surfaces is not a determining parameter. In liquid paraffin, when H₂ adsorbs vertically on Cu(h k l) surface, solvent effects not only influences the adsorptive stability, but also improves the ability of H₂ activation; When H₂ vertical adsorption on Cu(h k l) surface at 1/4 and 1/2 coverage, H-H bond is broken by solvent effects. However, no stable structures at 3/4 and 1 ML coverage are found, indicating that it is impossible to get H₂ parallel adsorption on Cu(h k l) surfaces at 3/4 and 1 ML coverages due to the repulsion between adsorbed H₂ molecules.     

Co-energy of surface and grain boundary in Ag film

A model is proposed for calculating the co-energy of surface and grain boundary (GB) by the modified analytical embedded atom method (MAEAM). As example, the energy densities E_d of the two adjacent grains are obtained when their (0 0 1) twist GB meets the free surface (h₁ k₁ 0)/(h₂ k₂ 0) of Ag film. The period along the boundary line on the surface is found and the energy density is calculated for the situations either with or without periodicity. The results show that, the energy value achieved via this model can be stable even for most grains with boundary line smaller than 100 nm. Among the grains with (h k 0) surface, (1 0 0) grains should be favored and grow fastest when they meet (1 1 0) grains.     

Exciton spatial dispersion determined through the two-photon Raman scattering via excitonic molecule state at large wave vectors in CuCl

With simultaneous excitation of two dye laser beams the Raman scattering is studied in which the intermediate state is the excitonic molecule and the final states are the longitudinal and transverse excitons. The L-T splitting shows a large k-dependence. The effective masses are determined to be (2.3 ± 0.1)m₀ and (3.1 ± 0.1)m₀ for the transverse and longitudinal excitons, respectively.     

Time-resolved optical spectroscopy of CsI(Tl) crystals by pulsed electron beam irradiation

Properties of the color and emission centers induced with an electron pulse beam at temperature within 80-300 K have been studied in CsI(Tl) crystals. It has been established by optical spectrometry with time resolution that initial color centers in this crystal are only Tl⁰ and V_k centers, which spontaneously recombine emitting visible light at 2.25 and 2.55 eV. It has been shown that the emission decay kinetics at 80 K include two fast exponential components with decay constants 3 and 14 μs as well as slow hyperbolic component with the power index depending on the wavelength of the emitting light. The temperature effect on the emission kinetics has been studied and it has been directly proved that the emission rise stage at the temperature above 170 K is caused by the recombination of electrons, which are thermally released from single Tl⁰ centers, with V_kA centers. The origin of scintillations in CsI(Tl) crystal is discussed in terms of the tunnel electron transitions from ground state of Tl⁰ centers to ground state of V_k centers at different distances from each other.     

Neutron scattering study of the metal-semiconductor phase transition in K2Pt(CN)4Br0.3 ·3D2O under pressure

The elastic scattering associated with the 2k_F instability in the one-dimensional (1-d) conductor K₂Pt(CN)₄Br_0.3 · 3D₂O (KCP) has been studied at the low temperature super lattice point (0.5, 0.5, 3.7) as a function of temperature for different pressures. Within the investigated region between 0 and 11 kbar, we find that the value of 2k_F remains constant at $\text{1.703 c}{}^1$, although the absolute value of 2k_F changes by 1.5%. At the same time the Peierls gap Δ(0) and the interchain correlation length ξ_⊥ (0) decrease. We conclude that the 1-d character of KCP is enhanced under pressure.     

Ab initio chemical kinetics for the reactions of HNCN with O(P) and O2

The kinetics and mechanisms of the reactions of cyanomidyl radical (HNCN) with oxygen atoms and molecules have been investigated by ab initio calculations with rate constant prediction. The doublet and quartet state potential energy surfaces (PESs) of the two reactions have been calculated by single-point calculations at the CCSD(T)/6-311+G(3df, 2p) level based on geometries optimized at the CCSD/6-311++G(d, p) level. The rate constants for various product channels of the two reactions in the temperature range of 300-3000 K are predicted by variational transition state and RRKM theories. The predicted total rate constants of the O(³P) + HNCN reaction at 760 Torr Ar pressure can be represented by the expressions k_total (O + HNCN) = 3.12 × 10⁻¹⁰ × T^−0.05 exp (−37/T) cm³ molecule⁻¹ s⁻¹ at T = 300-3000 K. The branching ratios of primary channels of the O(³P) + HNCN are predicted: k₁ for producing the NO + CNH accounts for 0.72-0.64, k₂ + k₉ for producing the ³NH + NCO accounts for 0.27-0.32, and k₆ for producing the CN + HNO accounts for 0.01-0.07 in the temperature range studied. Meanwhile, the predicted total rate constants of the O₂ + HNCN reaction at 760 Torr Ar pressure can be represented by the expression, k_total(O₂ + HNCN) = 2.10 × 10⁻¹⁶ × T^1.28exp (−12200/T) cm³ molecule⁻¹ s⁻¹ at T = 300-3000 K. The predicted branching ratio for k₁₁ + k₁₃ producing HO₂ + ³NCN as the primary products accounts for 0.98-1.00 in the temperature range studied.     

Waves in a superlattice with anisotropic inhomogeneities

Dependences of the dispersion laws and damping of waves in an initially sinusoidal superlattice on inhomogeneities with anisotropic correlation properties are studied for the first time. The period of the superlattice is modulated by the random function described by the anisotropic correlation function K_?(r) that has different correlation radii, k _∥ ^?1 and k _⊥ ^?1 , along the axis of the superlattice z and in the plane xy, respectively. The anisotropy of the correlation is characterized by the parameter λ=1?k_⊥/k_∥ that can change from λ=0 to λ=1 when the correlation wave number k⊥ changes from k_⊥=k_∥ (isotropic 3D inhomogeneities) to k_⊥=0 (1D inhomogeneities). The correlation function of the superlattice K(r) is developed. Its decreasing part goes to the asymptote L that divides the correlation volume into two parts, characterized by finite and infinite correlation radii. The dependences of the width of the gap in the spectrum at the boundary of the Brillouin zone δν and the damping of waves ξ on the value of λ are studied. It is shown that decreasing L leads to the decrease of δν, and increase of ξ, with the increase of λ.     

Comparaison de la méthode C.N.D.O. et de modèles analytiques pour la détermination de la polarisabilité de petits amas linéaires

We compare the values of the polarisabilities F(k) of small aggregates obtained by using the C.N.D.O. method to those given by analytical models. The studied clusters are the hypothetical molecules H₇₆ and H₇₈ where the atoms are equally spaced and arranged along a linear closed chain. Our conclusion is that there is a good agreement between C.N.D.O. and RPA results for the absolute value and for the variation with k of F(k). The difference which remains between the 2 curves is mainly due to the neglect of exchange correlations in RPA. This point is checked by taking into account exchange terms in the analytical approach. Moreover we also show that the C.N.D.O. curve presents some specific particularities due to degeneracies in the level spectra. These effects do not alter the forementioned agreement because they are limited to a few values of k (Δk?5) and localized near k = 38 for H₇₆ and k = 0 for H₇₈. In the first case they are due to the spontaneous formation of a charge density wave with wave vector k = 38, in the second case to a depolarization term which would not appear if, for example, the transitions k = 1 and k = ?1 were not exactly degenerated. We extend our analytical study to the n = 1014 case and show how the F(k) values behave when N → ∞.     

Perpendicular anisotropy and magneto-optical Kerr effect of (Ni/Pd) multilayers

Nickel film, with total thickness t_Ni in the range 1000-2000 Å, is known to exhibit perpendicular magnetic anisotropy (PMA), if the film has been deposited at room temperature. This phenomenon is due to the magneto-elastic (ME) effect. The same is also true for the (Ni/Pd)_n multilayers, where n is the period (n≥3). In this paper, we have made two kinds of multilayers: one, which does not have a Pd cap layer, belongs to the A-group, and the other, which has, belongs to the B-group. The polar Kerr rotation θ_k, the polar Kerr ellipticity ε_k, and the figure of merit (θ_k)²R, where R is the reflectance, were measured for the two wavelengths, i.e. λ=633 and 442 nm, respectively. The effective PMA energy K_⊥ was measured from the vibrating sample magnetometer. It was found that the most favorable multilayer for the magneto-optical (MO) application exists among the A-group samples: i.e. the t_Ni=1300 Å, t_Pd=50 Å (seed layer), and n=1 sample. We obtained θ_k=−9.76 min, ε_k=−9.13 min, (θ_k)²R=1.51 (rad)² at λ=442 nm, and K_⊥=3.21×10⁶ erg/cc for this optimal multilayer. Finally, the effects of the Pd seed layer on PMA and MO are also studied.     

Effect of thermal process on magnetic anisotropy in FeCoB soft underlayer

Relationship between magnetic anisotropy field H_k and thermal processes during the preparation has been studied for FeCoB thin films. The FeCoB films deposited on the glass substrates by facing targets sputtering successfully showed strong magnetic anisotropy when the substrate was heated at the substrate temperature T_s above 100 °C. Additionally, the lattice spacing of FeCo(1 1 0) in the perpendicular direction was found to decrease depending on the substrate temperature T_s. Among various temperature histories, the heating processes with a phase of increasing T_s revealed the further improvement of H_k. Meanwhile, high H_k in the films disappears after the post-deposition annealing at the temperature above 400 °C.     

Short-range order analysis and some physical properties of InxSe1−x glasses

Bulk In_xSe_1−x (with x=5-25 at%) glasses were prepared using the melt-quench technique. Short range order(SRO) was examined by the X-ray diffraction using Cu(k_α) radiation in the wave vector interval 0.28≤k≤6.5 A⁰⁻¹.The SRO parameters have been obtained from the radial distribution function. The inter-atomic distance obtained from the first and second peak are r₁=0.263 and r₂=0.460 nm, which is equivalent In-Se and Se-Se bond length. The fundamental structural unit for the studied glasses is In₂Se₃ pyramid. Using the differential scanning calorimetry (DSC), the crystallization mechanism of In_xSe_1−x chalcogenide glass has been studied. The glass transition activation energy (E_g) is 289±0.3 kj/mol.There is a correlation amongst the glass forming ability, bond strength and the number of lone pair electrons. The utility of the Gibbs-Di Marzio relation was achieved by estimating T_g theoretically.     

Spin-wave interaction in two-dimensional ferromagnets with dipolar forces

We discuss the spin-wave interaction in two-dimensional (2D) Heisenberg ferromagnet (FM) with dipolar forces at T_C?T?0 using 1/S expansion. A comprehensive analysis is carried out of the first 1/S corrections to the spin-wave spectrum. In particular, we obtain that the spin-wave interaction leads to the gap in the spectrum ε_k renormalizing greatly the bare gapless spectrum at small momenta k. Expressions for the spin-wave damping Γ_k are derived self-consistently and it is concluded that magnons are well-defined quasi-particles in both quantum and classical 2D FMs at small T. We observe thermal enhancement of both Γ_k and Γ_k/ε_k at small momenta. In particular, a peak appears in Γ_k and Γ_k/ε_k at small k and at any given direction of k. If S∼1 the height of the peak in Γ_k/ε_k is not larger than a value proportional to T/D?1, where D is the spin-wave stiffness. In the case of large spins S?1 the peak in Γ_k/ε_k cannot be greater than that of the classical 2D FM found at k=0 whose height is small only numerically: Γ₀/ε₀≈0.16 for the simple square lattice.     

Magnetic structure of TmCu2Ge2

TmCu₂Ge₂ compound crystallizes in the tetragonal ThCr₂Si₂-type crystal structure. The neutron diffraction reveals the presence of an incommensurate antiferromagnetic order below T_N=2.5 K. The Tm magnetic moment of 5.0(1) μ_B at 0.47 K is parallel to the c-axis. The order is described by the propagation vector k=[k_x, k_x, 0], where k_x=0.117(3). The increase of the values of the components k_x near the Néel temperature is observed.     

Mössbauer study of CoFeRhO4

CoFeRhO₄ has been studied by Mössbauer spectroscopy and X-ray diffraction. The crystal is found to have a cubic spinel structure with the lattice constant a₀=8.451±0.005 Å. The iron ions are in ferric states. The temperature dependence of the magnetic hyperfine field is analyzed by the Néel theory of ferrimagnetism. The intersublattice superexchange interaction is antiferromagnetic and strong with a strength of J_AB=−12.39k_B while the intrasublattice superexchange interactions are weak with strengths of J_AA=−4.96k_B and J_BB=6.20k_B. As the temperature increases toward the Néel temperature T_N, a systematic line broadening effect in the Mössbauer spectrum is observed and interpreted to originate from different temperature dependences of the magnetic hyperfine fields at various iron sites.     

Energy calculation for symmetrical tilt grain boundaries in iron

An atomic study of [0 0 1] symmetrical tilt grain boundary (STGB) in iron has been made with modified analytical embedded atom method (MAEAM). The energies of two rigid-body crystals joined together directly are unrealistically high due to very short distance between atoms near grain boundary (GB) plane in either crystal. For each of 27 (h k 0) GB planes, a relative slide between grains could result in a decrease in GB energy and a minimum value could be obtained at specific translation distance L_min/L_(h k 0). Three lowest minimum-energies are corresponding to (3 1 0), (5 3 0) and (5 1 0) boundary successively, from minimization of GB energy, these boundaries should be preferable in (h k 0) boundaries. In addition, the minimum energy increases with increasing ∑, but decreases with increasing interplanar spacing.     

Influence of exchange interaction on Γ8 → Γ8 magnetoabsorption in Hg1−kMnkTe

Far infrared (18–337 μm) magnetotransmission of thin samples of Hg_1?kMn_kTe (0?k?0.02) at liquid helium temperature was measured. The strong influence of the exchange interaction on the positions of magnetotransmission minima was observed. The results were explained within the framework of Luttinger model (D₀ + D₁) with additional Heisenberg type Hamiltonian for free carriers-Mn ions exchange interaction.