Analytical study of the layered XY model with VCE method

Motivated by experiments on liquid-crystal films, we study the development of specific heat anomaly of finite layer system. With the VCE method, we introduce the strong surface interaction into the layered XY model and get the results of the forth-order analytical expansion. The results show that when the strong surface interaction becomes strong enough, the order trend defeats the quantum noise and the specific heat peak moves abnormally to the high temperature with the number of layers decreasing.

Analytical study of the antiferromagnetic Heisenberg films

With the variational cumulant expansion (VCE) method, the thermodynamic behaviors of S = 1/2 antiferromagnetic Heisenberg films in simple cubic lattices are studied analytically. From the analytic properties of the free energy, in principle we are able to calculate analytically the critical temperatures T-c(L) and the thermodynamic functions, to any order cumulant as the functions of the number of L (the hyperlayers in the hyperfilm). Explicit expressions for T-c(L) up to the fourth order are given. A comparison with the existing results for 3-dimensional system is given. The effective range of the interaction is obtained from numerical results.

Evaluation of thermal radiation dependent performance of GaSb thermophotovoltaic cell based on an analytical absorption coefficient model

Applying the model dielectric function method, we have expressed the absorption coefficient of GaSb analytically at room temperature relating to the contribution of various critical points of its electronic band structure. The calculated absorption spectrum shows good agreement with the reported experimental data obtained by spectral ellipsometry on nominally undoped sample. Based on this analytical absorption spectrum, we have qualitatively evaluated the response of active absorbing layer structure and its photoelectric conversion properties of GaSb thermophotovoltaic device on the perturbation of external thermal radiation induced by the varying radiator temperature or emissivity. Our calculation has demonstrated that desirable thickness to achieve the maximum conversion efficiency should be decreased with the increment of radiator temperature and the performance degradation brought by any structure deviation from its optimal one would be stronger meanwhile. For the popular radiator temperature, no more than 1500 K in a real solar thermophotovoltaic system, and typical doping profile in GaSb cell, a reasonable absorbing layer structure parameter should be controlled within 100-300 nm for the emitter while 3000-5000 nm for the base.

Evaluating 0.53 eVGaInAsSbthermophotovoltaic diode based on an analytical absorption model

Radiant heat conversion performance dominated by the active layer of Ga0.84In0.16As0.14Sb0.86 diode has been systematically investigated based on an analytic absorption spectrum, which is suggested here by numerically fitting the limited experimental data. For the concerned diode configuration, our calculation demonstrates that the optimal base doping is 3-4 x 10(17) cm(-3), which is less sensitive to the variation of the external radiation spectrum. Given the scarcity of the alloy elements, an economical device configuration of the 0.2-0.6 mu m emitter and the 4-6 mu m base would be particularly acceptable because the corresponding conversion efficiency cannot exhibit discouraging degradation in comparison to the one for the optimal structure, the thickness of which may be up to 10 mu m. More importantly, the method we suggested here to calculate alloy absorption can be easily transferred to other composition, thus bringing great convenience for design or optimization of the optoelectronic device formed by these alloys.

A three-dimensional analytical solution of the microheater temperature before bubble nucleation

A three-dimensional analytical solution of the microheater temperature based on heat diffusion equation is developed and compared with experimental results. Dimensionless parameters are introduced to analyze the temperature rise time and the distribution under steady state. To study the microheater temperatures before bubble nucleation, a set of working fluids and microheaters are considered. It is shown that the dimensionless time xi(-)(0) required for the temperature rise from room to 95% of the steady state temperature is about 75, not dependent on working fluids and microheaters. Heat transfer to the surrounding liquid is mainly caused by conduction, not by convection and radiation mechanisms. The microheater length affects the surface temperature uniformity, while its width influences the steady temperatures significantly, yielding the transition from heterogeneous to homogeneous nucleation mechanism from square microheaters to narrow line microheaters. 

Detection of indium segregation effects in InGaAs/GaAs quantum wells using reflectance-difference spectrometry

The influence of the Indium segregation on the interface asymmetry in InGaAs/GaAs quantum wells have been studied by reflectance-difference spectroscopy (RDS). It is found that the anisotropy of the 2H1E (2HH --> 1E) transition is very sensitive to the degree of the interface asymmetry. Calculations taking into account indium segregation yield good agreement with the observed anisotropy structures. It demonstrates that the anisotropy intensity ratio of the 1L1E (1LH --> 1E) and 2H1E transitions measured by RDS can be used to characterize the interface asymmetry. (C) 2002 Elsevier Science B.V. All rights reserved.

Approximate analytical optical transfer function for a wavefront-coded imaging system with two-dimensional rectangularly separable phase masks

Wavefront coding can be used to extend the depth of field of incoherent imaging systems and is a powerful system-level technique. In order to assess the performance of a wavefront-coded imaging system, defocused optical transfer function (OTF) is the metric frequently used. Unfortunately, to the best of our knowledge, among all types of phase masks, it is usually difficult to obtain the analytical OTF except the cubic one. Although numerical computation seems good enough for performance evaluation, the approximate analytical OTF is still indispensable because it can reflect the relationship between mask parameters and system frequency response in a clearer way. Thus, a method is proposed to derive the approximate analytical OTF for two-dimensional rectangularly separable phase masks. The analytical results are well consistent with the direct numerical computations, but the proposed method can be accepted only from engineering point of view and needs rigorous proof in future. (c) 2010 Society of Photo-Optical Instrumentation Engineers. [DOI: 10.1117/1.3485759]

Analytical method of predicating the instabilities of a micro arch-shaped beam under electrostatic loading

An arch-shaped beam with different configurations under electrostatic loading experiences either the direct pull-in instability or the snap-through first and then the pull-in instability. When the pull-in instability occurs, the system collides with the electrode and adheres to it, which usually causes the system failure. When the snap-through instability occurs, the system experiences a discontinuous displacement to flip over without colliding with the electrode. The snap-through instability is an ideal actuation mechanism because of the following reasons: (1) after snap-through the system regains the stability and capability of withstanding further loading; (2) the system flips back when the loading is reduced, i.e. the system can be used repetitively; and (3) when approaching snap-through instability the system effective stiffness reduces toward zero, which leads to a fast flipping-over response. To differentiate these two types of instability responses for an arch-shaped beam is vital for the actuator design. For an arch-shaped beam under electrostatic loading, the nonlinear terms of the mid-plane stretching and the electrostatic loading make the analytical solution extremely difficult if not impossible and the related numerical solution is rather complex. Using the one mode expansion approximation and the truncation of the higher-order terms of the Taylor series, we present an analytical solution here. However, the one mode approximation and the truncation error of the Taylor series can cause serious error in the solution. Therefore, an error-compensating mechanism is also proposed. The analytical results are compared with both the experimental data and the numerical multi-mode analysis. The analytical method presented here offers a simple yet efficient solution approach by retaining good accuracy to analyze the instability of an arch-shaped beam under electrostatic loading.