里兹方法在光波导理论中的应用

本文讨论了在三层平板介质波导中TE波的传播问题.三层平板的芯区由Kerr介质构成,其折射率平方(n²)的剖面分布随x作抛物分布变化.本文将应用变分法中的里兹方法,求出亥姆霍兹方程的第一级近似的解析解.     

梯形截面多量子阱脊形波导模式特性

本文根据多量子阱双折射率的性质,运用有效折射率法和WKB法对具有梯形截面多量子阱脊形波导的模式特性作了分析,导出了E_mn^x模和E_mn^y模的本征值方程.     

含QCD修正的p-A碰撞Drell-Yan过程的核效应

在微扰QCD α_s级近似下,采用双重Q²重标度模型,计算了p-A碰撞Drell-Yan截面与p-N碰撞Drell-Yan截面之比,即核效应函数R_QCD(x_A,Q²).计算结果与没有QCD修正的R(x_A,Q²)值比较,在0.03≤x_A≤0.3之间都有不同程度的压低,与实验符合的情况有所改善.说明对核Drell-Yan过程核效应的研究,QCD修正是有一定意义的,并且在计入这种修正后,双重Q²重标度模型仍然是有效的.     

矩形截面多量子阱脊形波导模式特性

本文根据多量子阱双折射率的性质,运用有效折射率法分析了矩形截面多量子阱脊形波导E_mn^x模和模E_mn^y的模式特性。     

p–N(A)Drell–Yan过程K因子的非常数性和核环境影响

在微扰QCD α_s 阶(NLO)近似和核(A)的部分子分布函数采用双重Q²重标度模型(DQ²RM)下,讨论了p-N(A)Drell-Yan过程中K因子的变化.结果表明K因子不能近似当作常数.它随靶核子中部分子相关的运动学变量x_T(A)显著变化.在ISR和SppS能区,当2×10^－3≤x_T(A)<1时,K因子随x_T(A)减小而显著减小;而x_T(A)<2×10^－3 时随x_T(A)继续减小又显著增加.K因子对核(A)也有一定依赖性,这也是在x_T(A)小时较为明显.这种认识对于在超高能对撞区利用Drell-Yan过程(或其类似过程)精确检验某些模型或物理量都是重要的.     

Tb1－xDyxFe2－y-Fe掺杂BaTiO3多层膜中的磁电耦合

用溶胶-凝胶法制备了Fe掺杂BaTiO₃粉体，在1350℃下烧结成圆片状多晶样品，并与Tb_1-xDy_xFe_2-y胶合成磁电(ME)双层膜或三层膜.实验分析表明Fe：BaTiO₃依然是四方相钙钛矿结构，但是居里温度及相变潜热均略低于纯净BaTiO₃.研究了Tb_1-xDy_xFe_2-y-Fe∶BaTiO₃双层膜和Tb_1-xDy_xFe_2-y-Fe∶BaTiO₃-Tb_1-xDy_xFe_2-y三层膜的ME效应.在2.8×10⁴A/m的磁场下，两者的横向ME电压系数均达其峰值，分别为6.225和26.25mV·(A·m^-1)^-1·cm^-1.并且，用掺杂BaTiO₃制备的双层膜和三层膜的横向ME电压系数均为相同条件下用纯净BaTiO₃制备的双层膜和三层膜的横向ME电压系数的1.5倍.另外由于不含铅，锆等有害物质，符合环保要求，因此采用掺杂BaTiO₃制备的磁电效应器件具有深入研究和应用价值.     

类氖等电子系列离子基态的双电子复合速率系数研究

使用全相对论组态相互作用方法, 能级-能级细致计算了0.1 E_I ≤ kT_e≤ 10 E_I (E_I是类钠离子基态的第一电离能) 温度范围内类氖离子基态双电子复合(DR)速率系数, 双激发自电离能级考虑了(2s2p)⁷3ln'l', (2s2p)⁷4l4l' 以及(2s2p)⁷4l5l'组态. 对于(2s2p)⁷3ln'l'双激发自电离组态, 轨道角量子数l' >8 的(2s2p)⁷3ln'l'双激发自电离态对双电子复合速率系数的贡献可以忽略不计; (2s2p)⁷3ln'l'双激发自电离组态的高里德堡态对双电子复合速率系数的贡献满足 n'^-3组态-组态外推法, 并且核电荷数越大, 趋于n'^-3标度的n'值越小; 对细致能级计算得到的类氖离子基态的总DR速率系数进行了拟合, 得到类氖离子基态的总DR速率系数随核电荷数 Z和电子温度变化的经验公式, 该拟合公式与细致计算结果的偏差在2%以内, 能较准确的计算任意核电荷数Z的类氖离子在0.1E_I ≤ kT_e ≤ 10E_I电子温度范围的DR速率系数. Burgess-Merts(BM)近似公式不适用于估算低温(kT_e<0.3 E_I)类氖离子的DR速率系数, 在高温(kT_e>2E_I)时, 类氖离子的DR速率系数可以用BM近似公式表示.     

Nd掺杂对Bi4Ti3O12铁电薄膜的微结构和铁电性能的影响

利用溶胶-凝胶法在Pt/Ti/SiO₂/Si(100)衬底上制备了Nd掺杂Bi₄Ti₃O₁₂(Bi_4-xNd_xTi₃O₁₂, x=0.00,0.30,0.45,0.75,0.85,1.00,1.50)铁电薄膜样品.研究了Nd掺杂对Bi₄Ti₃O₁₂薄膜的微结构和铁电性能的影响.研究结果表明:Nd掺杂未改变Bi₄Ti₃O₁₂薄膜的基本晶体结构.在掺杂量x<0.45时,Nd³⁺只取代类钙钛矿层中的A位Bi³⁺.当x=0.45时,样品剩余极化强度达最大值,在270kV·cm^-1的电场下为32.7μC·cm^-2.掺杂量进一步增加时,结构无序度开始明显增大,Nd³⁺开始进入(Bi₂O₂)²⁺层,削弱其绝缘层和空间电荷库的作用,导致材料剩余极化逐渐下降.当掺杂量x达到1.50时,掺杂离子最终破坏(Bi₂O₂)²⁺层的结构,材料发生铁电-顺电相变.     

CALCULATION OF THE HEAT OF FORMATION OF TERNARY COMPOUNDS: P-Ga-As, N-Ga-As, N-Ga-P

Miedema method, which was proposed to calculate the heat of formation of binary alloy system, is extended for calculating the heat of formation of ternary system by using the Toop's model. The hypothetical metallic allotropes correspounding to the properties of nonmetallic elements are introduced. In our calculation, the structure-dependent energy contributions have not been taken into account. The heat of formation of GaAs_1-x P_x, GaAs_1-xN_x, and GaP_1-xN_x are obtained by projecting the heat of formation of ternary systems, P-Ga-As, N-Ga-As, and N-Ga-P onto the plane of the concentration of Ga, c₂ = 0.5, respectively. The possibility of existence of ordered ternary compound was predicted. The dependence of direct band gap, E^Γ, or indirect one, E^X, for GaP_1-xN_x and GaAs_1-xN_x, on x, the concentration of N, is obtained by using the dielectric theory of electronegativity.     

HD+离子在两个超短脉冲激光场作用下的解离几率分布

本文报告HD⁺离子在波长λ=306.7nm的三倍频超短脉冲弱光场E₃和不同延时的二倍频超短脉冲强光场E₂作用下解离几率分布的定量计算结果。E₃和E₂的脉冲宽度均为10fs.E₃先行,E₂延时取0、4、8、12、16、20、24、28fs.计算结果用三维图表出。图中看到,沿相对动量坐标有三个峰,分别位于动量2.1×10^-18、4.4×10^-18、5.8×10^-18g·cm/s处,依次称为1号峰、2号峰、3号峰,位置各是λ=306.7nm的单光子能量、双光子能量、三光子能量消耗于解离能之后离子碎片具有相对动量。据分析,1号峰来源于E₂场引起的受激发射作用。     

Mutual Information between Order Book Layers

The order book is a list of all current buy or sell orders for a given financial security. The rise of electronic stock exchanges introduced a debate about the relevance of the information it encapsulates of the activity of traders. Here, we approach this topic from a theoretical perspective, estimating the amount of mutual information between order book layers, i.e., different buy/sell layers, which are aggregated by buy/sell orders. We show that (i) layers are not independent (in the sense that the mutual information is statistically larger than zero), (ii) the mutual information between layers is small (compared to the joint entropy), and (iii) the mutual information between layers increases when comparing the uppermost layers to the deepest layers analyzed (i.e., further away from the market price). Our findings, and our method for estimating mutual information, are relevant to developing trading strategies that attempt to utilize the information content of the limit order book.     

Enhanced superconductivity in superlattices of high-Tc cuprates

The electronic properties of multilayers of strongly correlated models for cuprate superconductors are investigated using cluster dynamical mean-field techniques. We focus on combinations of underdoped and overdoped layers and find that the superconducting order parameter in the overdoped layers is enhanced by the proximity effect of the strong pairing scale originating from the underdoped layers. The enhanced order parameter can even exceed the maximum value in uniform systems. This behavior is well reproduced in slave-boson mean-field calculations which also find higher transition temperatures than in the uniform system.     

Coherent Fan Magnetic Structure in Dy/Gd Superlattices

The magnetic structure of pseudo-single-crystal (0001) Dy/Gd superlattices, as well as its modification with the temperature, is determined by the combined application of SQUID magnetometry, circular magnetic dichroism, and polarized neutron reflectometry. It is established that a fan magnetic order is formed in the Dy layers in relatively low magnetic fields at temperatures below 170 K. Then, this order propagates coherently throughout the superlattice. The magnetic moments in the Dy layers lie in the basal plane, and in the Gd layers, they are oriented mainly along the hexagonal c axis.     

Determination of the outward relaxation of cleaved strained InAs structures by scanning tunneling microscopy

When a semi-conductor structure containing strained layers such as quantum wells (QWs) or quantum dot layers is cleaved, the surface will relax outward in order to release built-in strain. This outward relaxation is directly linked to the composition of the strained layers, and can thus provide accurate information about the local composition of these layers. By using cross-sectional scanning tunneling microscopy (X-STM) it is possible to measure this outward relaxation. The measured height profiles, however, are also dependent on the chemical composition of the measured surface, resulting in an extra height contrast in the images. In order to analyze only the outward relaxation, it is necessary to suppress this latter chemical component in the STM measurements. This can be achieved by choosing the proper tunnel conditions.     

磁性薄膜热动力学性质的变分累积展开研究

采用变分累积展开方法，研究了立方格点上磁性薄膜的热动力学性质.计算自发磁化强度、 内能和热容到了三级累积展开，并对每一级给出了这些物理量对薄膜原子层数的依赖关系. 虽然变分累积展开的收敛性还没有严格证明，但计算结果显示这些物理量的变分累积展开收 敛很快.三级计算结果足已表明：对低于某一临界厚度的薄膜，自发磁化强度随原子层数的 减少而减小；不论在临界温度以下，还是在临界温度以上，每单位格点的内能都随原子层数 的减少而增大；每单位格点的热容在临界温度以下随原子层数的减少而增大，但在临界温度 以上随原子层数的减 关键词： 磁性薄膜 热动力学性质 变分累积展开     

Free energy studies of freezing in slit pores: an order-parameter approach using Monte Carlo simulation

We report a molecular simulation study of freezing transitions for simple fluids in narrow slit pores. A major stumbling block in previous studies of freezing in pores has been the lack of any method for calculating the free energy difference between the confined solid and liquid phases. Conventional thermodynamic integration methods often fail for confined systems, due to the difficulty in choosing a suitable path of integration. We use a different approach that involves calculating the Landau free energy as a function of a suitable order parameter, using the grand canonical Monte Carlo simulation method. The grand free energy for each phase can then be obtained by one-dimensional integration of the Landau free energy over the order parameter. These calculations are carried out for two types of wall—fluid interaction, a hard wall and a strongly attractive wall modelled on carbon. The grand free energy results for both cases clearly indicate a first order fluid to solid transition. In the case of the attractive carbon wall, there are three phases. Phase A corresponds to all layers having a liquid-like structure; phase B corresponds to the contact layers (the layers adjacent to the two pore walls) being frozen and the rest of the layers being fluid-like; phase C corresponds to all the layers being frozen. Our results for the angular structure function in the individual molecular layers show strong evidence of a transition from a two-dimensional liquid phase to a hexatic phase. This is followed by a transition from the hexatic to a crystal phase.     

Magnetic fluctuations in coupled inequivalent Hubbard layers as a model for

We investigate, within the fluctuation-exchange approximation, a correlated-electron model for represented by two inequivalent Hubbard layers coupled by an interlayer hopping . An energy offset is introduced in order to produce a different charge carrier concentration in the two layers. We compare several single-particle and magnetic excitations, namely, the single particle scattering rate, the spectral function and the spin lattice as well as spin-spin relaxation times in the two layers as a function of . We show that the induced interlayer magnetic coupling produces a tendency to “equalization” of the magnetic properties in the two layers whereby antiferromagnetic fluctuations are suppressed in the less doped layer and enhanced in the heavily doped one.The strong antiferromagnetic bilayer coupling causes the charge carriers in the plane with larger doping concentration to behave similar to those of the underdoped layer, they are coupled to. This effect grows for decreasing temperature. For high temperatures or if both layers are optimally or overdoped, i.e. when the antiferromagnetic correlation length becomes of the order or smaller than one lattice site the charge carrier and magnetic dynamics of the two layers is disconnected and the equalization effect disappears. These results are in good agreement with NMR experiments on by Stern et al. [Phys. Rev B 51, 15478 (1995)]. We also compare the results with calculations on bilayer systems with equivalent layers as models for the constituent compounds and . Received: 28 August 1998     

On the theory of giant magnetoresistance (longitudinal current)

The problem of conductivity of a multilayer sample (alternating magnetic and nonmagnetic metal layers) with a current parallel to its surfaces is solved using a set of kinetic equations. Conditions at which the conductivity variations associated with changes in the type of magnetization alignment in adjacent layers reach the values on the order of initial conductivity are considered. Parameters governing the giant magnetoresistance effect and the relations between them are determined.     

Ab initio calculations of magnetic structure and lattice dynamics of Fe/Pt multilayers

The magnetization distribution, its energetic characterization by the interlayer coupling constants and lattice dynamics of (001)-oriented Fe/Pt multilayers are investigated using density functional theory combined with the direct method to determine phonon frequencies. It is found that ferromagnetic order between consecutive Fe layers is favoured, with the enhanced magnetic moments at the interface. The bilinear and biquadratic coupling coefficients between Fe layers are shown to saturate fast with increasing thickness of nonmagnetic Pt layers which separate them. The phonon calculations demonstrate a rather strong dependence of partial iron phonon densities of states on the actual position of Fe monolayer in the multilayer structure.     

Vibration and damping analysis of multilayered rectangular plates with constrained viscoelastic layers

Governing equations of motion for vibrations of a general multilayered plate consisting of an arbitrary number of alternate stiff and soft layers of orthotropic materials are derived by using variational principles. Extension, bending and in-plane shear deformations in stiff layers and only transverse shear deformations in soft layers are considered as in conventional sandwich structural analysis. In addition to transverse inertia, longitudinal translatory and rotary inertias are included, as such analysis gives higher order modes of vibration and leads to accurate results for relatively thick plates. Vibration and damping analysis of rectangular simply supported plates consisting of alternate elastic and viscoelastic layers is carried out by taking a series solution and applying the correspondence principle of linear viscoelasticity. The damping effectiveness, in term of the system loss factor, for all families of modes for three-, five- and seven-layered plates is evaluated and its variations with geometrical and material property parameters are investigated.