磁性单层膜磁学性质的Monte Carlo模拟

采用类Ising模型,利用Monte Carlo方法研究了磁性单层膜中退磁性偶极作用和铁磁性交换作用对系统磁学性质的影响.结果显示,随着偶极相互作用的增加,系统在低温下的磁化出现平台现象,此时磁化曲线可分为2个阶段,在低外场下,温度升高,系统易磁化,在高外场下则反之.这种新奇的磁化行为导致系统的磁熵变在低温低外场下出现大于零的反常行为.在模拟过程中,对长程力作用采用了比较精确的处理方法.     

Direct measurement of the oscillation frequency in an optical-tweezers trap by parametric excitation

We demonstrate a novel technique for direct measurement of the oscillation frequency in an optical-tweezers trap. The technique uses the phenomenon of parametric resonance in an oscillator when the stiffness of the trapping potential is modulated. The trapped particle is a strongly damped oscillator; hence, the signature of parametric resonance is not an increase in the amplitude but an increase in the size of Brownian fluctuations. The trap frequency is measured with an accuracy of 0.1%, which is better than previous techniques and thus opens up new possibilities in experiments with optical tweezers.     

Comparison of nitride-based dual-wavelength light- emitting diodes with an InAlN electron-blocking layer and with p-type doped barriers

The advantages of nitride-based dual-wavelength light-emitting diodes (LEDs) with an InAlN electron blocking layer (EBL) are studied. The emission spectra,carrier concentration in the quantum wells (QWs),energy band and internal quantum efficiency (IQE) are investigated. The simulation results indicate that an LED with an InAlN EBL performs better over a conventional LED with an AlGaN EBL and an LED with p-type-doped QW barriers. All of the advantages are due to the enhancement of carrier confinement and the lower electron leakage current. The simulation results also show that the efficiency droop is markedly improved and the luminous intensity is greatly enhanced when an InAlN EBL is used.     

The surface energy of an electron gas in model crystals

The surface energy of an electron gas in a crystal is considered. The results obtained for a quadratic spectrum are generalized to an arbitrary energy spectrum in certain crystal models. The surface energy of an electron gas with a quadratic spectrum is found for a sample with a rough boundary when the height of irregularities is small compared with the electron wavelength.     

铋基掺铒光纤放大器中放大自发辐射处理研究

分别设计了一个带光隔离器和一个带光环形器的复合型宽带铋基掺铒光纤放大器(Bi-EDFA),理论上研究了Bi-EDFA中对于后向传输放大自发辐射(ASE)的处理对放大器性能的影响。研究表明,在C+L波段多波长信号同时输入情形下,通过在铋基掺铒光纤中插入一个光隔离器抑制后向传输的ASE,可以提高放大器各波长信号增益,降低噪声系数。利用一个光环形器引导后向传输的ASE去泵浦一段未抽运的低掺杂铋基掺铒光纤,可以显著地提高放大器长波段信号增益,而噪声系数只是略有增加,理论研究与报道的实验结果达到了比较好的一致。研究结果对于新型宽带Bi-EDFA的设计和应用具有一定实际指导意义。     

Propagation in an elastic wedge using the virtual source technique

The virtual source technique, which is based on the boundary integral method, provides the means to impose boundary conditions on arbitrarily shaped boundaries by replacing them by a collection of sources whose amplitudes are determined from the boundary conditions. In this paper the virtual source technique is used to model propagation of waves in a range-dependent ocean overlying an elastic bottom with arbitrarily shaped ocean-bottom interface. The method is applied to propagation in an elastic Pekeris waveguide, an acoustic wedge, and an elastic wedge. In the case of propagation in an elastic Pekeris waveguide, the results agree very well with those obtained from the wavenumber integral technique, as they do with the solution of the parabolic equation (PE) technique in the case of propagation in an acoustic wedge. The results for propagation in an elastic wedge qualitatively agree with those obtained from an elastic PE solution.     

Effects of substrate temperature on the structure and mechanical properties of (TiVCrZrHf)N coatings

The present paper reports the influence of growth conditions on the characteristics of (TiVCrZrHf)N films prepared by rf reactive magnetron sputtering at various substrate temperatures. The nitrogen content is observed to decrease with increasing substrate temperature. The X-ray diffraction results indicate that all (TiVCrZrHf)N films are simple face centered cubic (FCC) structures. Initially, there is an obvious decrease followed by an increase in grain size with the increase in substrate temperature. The lower part of the microstructure has an amorphous structure. A nano grain structure (size ∼1 nm) with a random orientation is also observed above the amorphous structure. The fully dense columnar structure with an fcc crystal phase then starts to develop. Extreme hardness of around 48 GPa is obtained in the present alloy design.     

A spherical acoustic source in an ideal waveguide

The acoustic field of a spherical source in an ideal waveguide is considered with allowance for the diffraction by the source. The consideration uses the previously obtained results on the diffraction by a spherical source in a halfspace with ideal boundaries. The resultant field is shown to be representable as an infinite sum of the fields of some equivalent sources. The errors that appear when the number of these sources is limited are estimated. The field produced by the sphere in an ideal waveguide is calculated with and without allowance made for the scattering.     

Waves in an active medium guided by a reactive surface with reactance of gain modulation

The characteristics of surface waves (i) in a passive medium supported by a passive surface with reactance modulation and an active surface with the modulation of either the reactance or the negative resistance, and (ii) in an active medium supported by a passive surface with a reactance modulation are investigated with emphasis on the band regions. The periodic variation is sinusoidal and is in the propagation direction. For a passive medium terminated by a passive surface with a reactance modulation, there are stop bands in frequency for the surface waves and the structure of the first two stop bands is analyzed. For a passive medium terminated by an active surface and for an active medium terminated by a reactance-modulated passive surface, the characteristics of the absolute instabilities occurring in the first-order band are examined. The nature of the convective instability taking place in the second-order band for an active medium terminated by a reactance-modulated surface is discussed. Analytical expressions for the frequency shift of the second-order band are deduced. The Floquet theory is used to obtain the exact dispersion relation in the form of a continued fraction which is analyzed by singular expansions.     

中国工程物理研究院FEL-THz束线从头至尾模拟设计(英文)

中国工程物理研究院正在开展自由电子激光太赫兹源(FEL-THz)的研究。介绍了中物院FEL-THz束线的设计,通过PARMELA和TRANSPORT等程序优化了束线参数,并通过IMPACT-T和ELEGANT程序核对了模拟结果。优化结果表明,电子束在摇摆器入口处能量7～8MeV、平均电流5mA、横向归一化发射度小于10mm·mrad,满足设计要求。     

Scattering of waves by an aperture in an impedance screen

The transmission and diffraction of waves by an aperture in an impedance screen is investigated. The Senior's solution of the scattering problem of waves by an impedance half-plane is transformed into a physical optics type integral. The obtained method is applied to the transmission problem of waves by a slit in an impedance screen. The results are compared with the aperture problem in black and conducting screens numerically.     

A sector finite element for dynamic analysis of thick plates

A 24 degree of freedom sector finite element is developed for the static and dynamic analysis of thick circular plates. The element formulation is based on Reissner's thick plate theory. The convergence characteristic of the elements is first studied in a static example of an unsymmetrically loaded annular plate. The obvious advantageous effect of including the twist derivatives of deflection as degrees of freedom is shown. The elements are then used to analyze the natural frequencies of an annular plate with various ratios of inner to outer radius. The results are in good agreement with an alternative solution in which thick plate theory is used. The versatility of this finite element is finally demonstrated by performing free vibration analysis of an example of clamped sector plates with various thicknesses and different sectorial angles.     

Large scale data acquisition of simultaneous MRI and speech

We describe an arrangement for simultaneous recording of speech and vocal tract geometry in patients undergoing surgery involving this area. Experimental design is considered from an articulatory phonetic point of view. The speech signals are recorded with an acoustic-electrical arrangement. The vocal tract is simultaneously imaged with MRI. A MATLAB-based system controls the timing of speech recording and MR image acquisition. The speech signals are cleaned from acoustic MRI noise by an adaptive signal processing algorithm. Finally, a vowel data set from pilot experiments is qualitatively compared both with validation data from the anechoic chamber and with Helmholtz resonances of the vocal tract volume, obtained using FEM.     

Effect of an intermediate layer with space-dependent permittivity on the ground state energy of an electron in a spherical complex nanoheterosystem

The effect of the interfaces of a multilayer spherical microcrystal on a charged particle is investigated. The case is considered where an intermediate layer with space-dependent permittivity exists near the interfaces. The dependence of the potential energy of the charged particle on distance is established by the method of the classical Green’s functions. For the example of an HgS/CdS spherical structure, the energy of the ground and excited states of an electron is calculated both in the presence of an intermediate layer with space-dependent permittivity and in its absence.     

Moment coupling in the interaction of atoms and their ions with a 3<Emphasis Type="Italic">d</Emphasis>-electron shell

The moment coupling of an interacting ion and an atom with a 3d-electron shell is analyzed for the ground state of identical atoms and ions where resonant charge exchange proceeds with transition of a 4s-electron. The interaction of the ion charge with the atom quadrupole moment is important for this system along with the exchange interactions and spin-orbit interactions inside an isolated atom and an ion. The quadrupole moment for 3d-atoms in the ground states is evaluated. The hierarchy of interactions in a molecular ion is analyzed depending on ion-atom distances and is compared with the standard Hund scheme. The resonant charge exchange proceeds effectively at separations corresponding to an intermediate case between cases “a” and “c” of the Hund coupling scheme.     

非常偏振光在单轴晶体表面的反射-透射研究

为分析非常偏振光在晶体表面的能量损耗,给出一种求解反射率和透射率的方法,即根据电磁场的边值关系以及晶体的双折射和双反射现象,求解晶体光轴在入射面内时,非常偏振光从各向同性介质入射到晶体和从晶体出射到各向同性介质两种情况的反射率和透射率的方法,并给出反射率和透射率的解析解,同时得到对于晶体光轴在入射面内的情况,光轴的取向对非常偏振光反射率的大小几乎没有影响,但对产生全反射的临界角θc影响较大的结论。实验上用LiNbO3晶体验证了非常偏振光从各向同性介质入射到晶体时的反射率公式。求解方法简单实用,所给的表达式具有一般性,可以直接使用。     

厄米-高斯光束在内含硬边光阑光学系统中的传输

把硬边光阑从复杂光学系统中“移”出来,即把任意一个含有硬边光阑的复杂光学系统体现到含有入射光瞳和出射光瞳的光学系统中，使硬边光阑的作用转化为出射光瞳的作用，以便求出观察面上的解析解。对入射面上的二维厄米 高斯光束，利用柯林斯公式和入射光瞳与出射光瞳的物像关系，可得到所要求观察面的场分布和解析结果，通过数值模拟验证了其正确性。通过分析计算得到如下结论：对于任意一个含有硬边光阑的复杂光学系统，都可以把光阑的作用从系统中“移”出来，把其看作是出射光瞳的作用，从而可以较为简便地求出观察面上的场分布。     

Laser induced thermal profiles in thermally and optically thin films

The temperature field generated by the weak absorption of a gaussian laser beam in an optically and thermally thin film bounded by two transparent plates is discussed. An analytical solution of the problem is presented together with an algorithm for the numerical integration. The influence of the finite thermal conductivity of the plates is shown in an example.     

碘激光的大气衰减特性

碘激光波长处在大气微窗口，水汽分子的吸收是碘激光在大气中传输时衰减的主要原因。利用HITRAN数据库，计算了碘激光波长模式的大气衰减情况。利用高分辨串光谱实验系统，获得了1．315μm附近的谱线参量。在我国，由南向北，由夏到冬，水汽浓度逐渐减少，水汽分子的吸收率递减。碘激光有6条超精细谱线，单一最强线频率的碘激光的大气分子吸收比多谱线的碘激光小，更有利于大气传输。在中纬度夏季，单一最强线垂直向上全程的大气分子吸收率与多谱线碘激光水平传输1km相当，为9．0％。碘激光垂直向上传输比海平面水平传输受大气影响小。     

无障板圆形活塞换能器的辐射阻抗*

辐射阻抗是换能器的最重要辐射声场特性参数之一,影响着换能器的谐振频率、频带宽度、效率以及辐射声功率等。经典声学理论用一阶贝塞尔函数和斯特鲁夫函数给出了圆形活塞换能器辐射阻抗的数学表达式,但仅适用于无限大障板条件。该文用ANSYS有限元软件计算了无障板圆形活塞换能器的辐射阻抗,给出了数值解结果,通过与无限大障板条件下的理论结果相比较,显示了二者之间的偏离情况,尤其是在低频时差异显著。为了借助MATLAB工具模拟无障板圆形活塞换能器的辐射阻抗,并给出较准确的数学表达,选择了一系列初等函数以及它们的复合函数进行曲线拟合,最终给出了辐射阻和辐射抗关于ka的函数表达式,在ka值从0.08到20范围内与有限元计算结果很好吻合。