DISTRIBUTION FUNCTION OF A DRIFTING ELECTRON-GAS FOR GENERAL ENERGY-BAND STRUCTURES

The usual application of the Lei-Ting balance equation method for treating electron transport problems makes use of a Fermi distribution function for the electron motion relative to the center of mass. It is pointed out that this presumes the existence of a moving frame of reference that is dynamically equivalent to the rest frame of reference, and this is only true for electrons with a constant effective mass. The method is thus inapplicable to problems where electrons governed by a general energy-band dispersion E(k) are important (such as in miniband conduction). It is demonstrated that this difficulty can be overcome by introducing a distribution function for a drifting electron gas by maximizing the entropy subject to a prescribed average drift velocity. The distribution function reduces directly to the usual Fermi distribution for electron motion relative to the center of mass in the special case of E(k)=($) over bar h(2)\k\(2)/2m*. This maximum entropy treatment of a drifting electron gas provides a physically more direct as well as a more general basis for the application of the balance equation method.

SEMICLASSICAL AND ENVELOPE-FUNCTION TREATMENT OF MAGNETIC LEVELS IN SUPERLATTICES UNDER AN INPLANE MAGNETIC-FIELD

The theoretical treatment of magnetic levels formed in the minibands of superlattices under an in-plane magnetic field is discussed. It is found that the results of semiclassical and envelope-function treatments based on miniband structures are in good agreement with the results calculated strictly by the quantum-mechanical method, so long as the critical parameter 2hc/eBL2 is larger than 1. The wave functions obtained are in the nature of superlattice envelope functions, which are over and above the usual effective-mass envelope functions for bulk materials.

2-DELTA-FUNCTION GENERALIZED PLASMA POLE MODEL FOR 1ST PRINCIPLE CALCULATIONS OF QUASI-PARTICLE ENERGIES IN MANY-ELECTRON SYSTEMS

To evaluate the dynamical effects of the screened interaction in the calculations of quasiparticle energies in many-electron systems a two-delta-function generalized plasma pole model (GPP) is introduced to simulate the dynamical dielectric function. The usual single delta-function GPP model has the drawback of over simplifications and for the crystals without the center of symmetry is inappropriate to describe the finite frequency behavior for dielectric function matrices. The discrete frequency summation method requires too much computation to achieve converged results since ab initio calculations of dielectric function matrices are to be carried out for many different frequencies. The two-delta GPP model is an optimization of the two approaches. We analyze the two-delta GPP model and propose a method to determine from the first principle calculations the amplitudes and effective frequencies of these delta-functions. Analytical solutions are found for the second order equations for the parameter matrices entering the model. This enables realistic applications of the method to the first principle quasiparticle calculations and makes the calculations truly adjustable parameter free.

Green's function of a two-dimensional interacting electron gas in a perpendicular magnetic field

It is rigorously proved that the Green's function of a uniform two-dimensional interacting electron gas in a perpendicular magnetic field is diagonal with respect to single-particle states in the Landau gauge. The implication of this theorem is briefly discussed.

Using photoluminescence as optimization criterion to achieve high-quality InGaAs/AlGaAs pHEMT structure

The single delta -doped InGaAs/AlGaAs pseudomorphic HEMT structure materials were grown by molecular beam epitaxy. The photoluminescence spectra of the materials were studied. There are two peaks in the photoluminescence spectra of the materials, corresponding to two sub energy levels of InGaAs quantum well. The ratio of the two peak's intensity was used as criterion to optimize the layer structures of the materials. The material with optimized layer ;tructures exhibits the 77 It mobility and two-dimensional electron gas density of 16 500 cm(2)/Vs and 2.58 x 10(12) cm(-2) respectively, and the 300 K mobility and two-dimensional electron gas density of 6800 cm(2)/Vs and 2.55 x 10(12) cm(-2) respectively. The pseudomorphic HEMT devices with gate length of 0.2 mum were fabricated using this material. The maximum transconductance of 650 mS/mm and the cut-off frequency of 81 GHz were achieved. (C) 2001 Elsevier Science B.V. All rights reserved.

Optimized logarithmic phase masks used to generate defocus invariant modulation transfer function for wavefront coding system

In a previous Letter [Opt. Lett. 33, 1171 (2008)], we proposed an improved logarithmic phase mask by making modifications to the original one designed by Sherif. However, further studies in another paper [Appl. Opt. 49, 229 (2010)] show that even when the Sherif mask and the improved one are optimized, their corresponding defocused modulation transfer functions (MTFs) are still not stable with respect to focus errors. So, by further modifying their phase profiles, we design another two logarithmic phase masks that exhibit more stable defocused MTF. However, with the defocus-induced phase effect considered, we find that the performance of the two masks proposed in this Letter is better than the Sherif mask, but worse than our previously proposed phase mask, according to the Hilbert space angle. (C) 2010 Optical Society of America

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]

A modified criterion for shear band formation in bulk metallic glass under complex stress states

A new criterion for shear band formation in metallic glasses is proposed based on the shear plane criterion proposed by Packard and Schuh [1]. This modified shear plane (MSP) criterion suggests that a shear band is not initiated randomly throughout the entire material under stress but is initiated at the physical boundaries or defects and at locations where the highest normal stress modified maximum shear stress occurs. Moreover, the same as in the shear plan criterion, the shear stress all over the shear band should exceed the shear yield strength of the material. For a complete shear band to form, both requirements need to be fulfilled. The shear yield strength of the material is represented by the shear stress of the point at which the shear band stops. The new criterion agrees very well with experimental results in both the determination of the shear yield strength and the shear band path. (C) 2010 Elsevier B.V. All rights reserved.

Criterion of aerodynamic performance of large-scale offshore horizontal axis wind turbines

With the background of offshore wind energy projects, this paper studies aerodynamic performance and geometric characteristics of large capacity wind turbine rotors ( 1 to 10 MW), and the main characteristic parameters such as the rated wind speed, blade tip speed, and rotor solidity. We show that the essential criterion of a high-performance wind turbine is a highest possible annual usable energy pattern factor and a smallest possible dimension, capturing the maximum wind energy and producing the maximum annual power. The influence of the above-mentioned three parameters on the pattern factor and rotor geometry of wind turbine operated in China's offshore meteorological environment is investigated. The variation patterns of aerodynamic and geometric parameters are obtained, analyzed, and compared with each other. The present method for aerodynamic analysis and its results can form a basis for evaluating aerodynamic performance of large-scale offshore wind turbine rotors.