Abnormal ferroelectric response and enhancement of piezoelectricity of PbTiO3 under uniaxial compression

The ferroelectricity of rhombohedral PbTiO3 under uniaxial compression is investigated from first-principles study. Upon compression, the ferroelectricity decreases until a critical stress of -29 GPa and then increases with a further increase of the magnitude of the uniaxial compressive stress. We also find that uniaxial compression could enhance piezoelectricity and that the maximum piezoelectric coefficient d(33) occurs at sigma(33)=-49 GPa, which supports the experimentally observed piezoelectric behavior in rhombohedral Pb(Mg1/3Nb2/3O3)-0.32PbTiO(3) [Q. Wan, C. Chen, and Y. P. Shen, J. Appl. Phys. 98, 024103 (2005)]. Our calculated results show that the Pb, Ti, and O atoms have different contributions to the total polarization with increasing the magnitude of uniaxial compressive stress, and that when -sigma(33)>55 GPa, the Ti atoms no longer have contributions to the polarization, which leads to the changes of ferroelectricity and piezoelectricity. (C) 2008 American Institute of Physics.

Studies of tetragonal PbTiO3 subjected to uniaxial stress along the c-axis

Tetragonal PbTiO3 under uniaxial stress along the c-axis is investigated from first-principles. The structural parameters, polarization, and squares of the lowest optical phonon frequencies for E(1TO) and A(1)(1TO) modes at Gamma show abrupt changes near a stress sigma(c) of 1.04 GPa, which is related to the dramatic change of elastic constant c(33) resulting from the uniaxial stress applied along the c-axis. We also find that the uniaxial compressive stress could enhance the piezoelectric stress coefficients, whereas the uniaxial tensile stress could enhance the piezoelectric strain coefficients. It is also found that when the magnitude of uniaxial compressive stress sigma(33) is greater than 12 GPa, PbTiO3 is transformed to the paraelectric tetragonal phase.

Spin Hall effect on the kagome lattice with Rashba spin-orbit interaction

We study the spin Hall effect in the kagome lattice with Rashba spin-orbit coupling. The conserved spin Hall conductance sigma(s)(xy) (see text) and its two components, i.e., the conventional term sigma(s0)(xy) and the spin-torque-dipole term sigma(s tau)(xy), are numerically calculated, which show a series of plateaus as a function of the electron Fermi energy epsilon(F). A consistent two-band analysis, as well as a Berry-phase interpretation, is also given. We show that these plateaus are a consequence of various Fermi-surface topologies when tuning epsilon(F). In particular, we predict that compared to the case with the Fermi surface encircling the Gamma point in the Brillouin zone, the amplitude of the spin Hall conductance with the Fermi surface encircling the K points is twice enhanced, which makes it highly meaningful in the future to systematically carry out studies of the K-valley spintronics.

Characterization of polymorphous silicon thin film and solar cells

Polymorphous silicon (pm-Si:H) films have been prepared by a new regime of plasma enhanced chemical vapour deposition in the region adjacent of phase transition from amorphous to microcrystalline state. Comparing to the conventional amorphous silicon (a-Si:H), the pm-Si:H has higher photoconductivity (sigma(ph)), better stability, and a broader light spectral response range in the longer wavelength range. It can be found from Raman spectra that there is a notable improvement in the medium range order. There are a blue shift for the stretching mode of IR spectra and a red shift for the wagging mode. The shifts are attributed to the variation of the microstructure. By using pm-Si:H film as intrinsic layer, a p-i-n junction solar cell was prepared with the initial efficiency of 8.51% and a stabilized efficiency of 8.01% (AM1.5, 100mw/cm(2)) at room temperature (T-R).

Electronic structure of Mn-doped ZnO quantum wires: A mean-field theory study

Based on the effective-mass model and the mean-field approximation, we investigate the energy levels of the electron and hole states of the Mn-doped ZnO quantum wires (x=0.0018) in the presence of the external magnetic field. It is found that either twofold degenerated electron or fourfold degenerated hole states split in the field. The splitting energy is about 100 times larger than those of undoped cases. There is a dark exciton effect when the radius R is smaller than 16.6 nm, and it is independent of the effective doped Mn concentration. The lowest state transitions split into six Zeeman components in the magnetic field, four sigma(+/-) and two pi polarized Zeeman components, their splittings depend on the Mn-doped concentration, and the order of pi and sigma(+/-) polarized Zeeman components is reversed for thin quantum wires (R < 2.3 nm) due to the quantum confinement effect.

Silicon thin films prepared in the transition region and their use in solar cells

Diphasic silicon films (nc-Si/a-Si:H) have been prepared by a new regime of plasma enhanced chemical vapour deposition in the region adjacent of phase transition from amorphous to microcrystalline state. Comparing to the conventional amorphous silicon (a-Si:H), the nc-Si/a-Si:H has higher photoconductivity (sigma(ph)), better stability, and a broader light spectral response range in the longer wavelength range. It can be found from Raman spectra that there is a notable improvement in the medium range order. The blue shift for the stretching mode and red shift for the wagging mode in the IR spectra also show the variation of the microstructure. By using this kind of film as intrinsic layer, a p-i-n junction solar cell was prepared with the initial efficiency of 8.51 % and a stabilized efficiency of 8.01% (AM 1.5, 100 mw/cm(2)) at room temperature. (c) 2006 Published by Elsevier B.V.

Structual and optoelectronic properties of polycrystalline silicon thin films prepared by hot-wire chemical vapor deposition at low temperatures

Polycrystalline silicon thin films were prepared by hot-wire chemical vapor deposition ( HWCVD) on glass at 250 degreesC with W or Ta wire as the catalyzers. The structual and optoelectronic properties as functions of the filament temperature, deposition pressure and the filament-substrate distance were studied, and the optimized polycrystalline silicon thin films were obtained with X-c > 90 % ( X-c denotes the crystalline ratio of the film), crystal grain size about 30-40nm, R-d approximate to 0.8nm/s, sigma(d) about 10(-7) - 10(-6) Omega(-1) cm(-1), Ea(a) approximate to 0.5eV and E-opt less than or equal to 1.3eV.

Magnetic field tuning of the effective g factor in a diluted magnetic semiconductor quantum dot

The spin interaction and the effective g factor of a magnetic exciton (ME) are investigated theoretically in a diluted magnetic semiconductor (DMS) quantum dot (QD), including the Coulomb interaction and the sp-d exchange interaction. At low magnetic field, the ME energy decreases rapidly with increasing magnetic field and saturates at high magnetic field for high Mn concentration. The ground state of the ME exhibits an interesting crossing behavior between sigma(+)-ME and sigma(-)-ME for low Mn concentration. The g(ex) factor of the ME in a DMS QD displays a monotonic decrease with increasing magnetic field and can be tuned to zero by an external magnetic field. (C) 2003 American Institute of Physics.

Binding energy of excitons bound to neutral donors in two-dimensional semiconductors

The binding energies of excitons bound to neutral donors in two-dimensional (2D) semiconductors within the spherical-effective-mass approximation, which are nondegenerate energy bands, have been calculated by a variational method for a relevant range of the effective electron-to-hole mass ratio sigma. The ratio of the binding energy of a 2D exciton bound to a neutral donor to that of a 2D neutral donor is found to be from 0.58 to 0.10. In the limit of vanishing sigma and large sigma, the results agree fairly well with previous experimental results. The results of this approach are compared with those of earlier theories.

Molecular beam epitaxial growth of GaN on 3c-SiC/Si(111) substrates using a thick AIN buffer layer

Hexagonal GaN films (similar to 3 mu m) were grown on 3c-SiC/Si(111) and carbonized Si(111) substrates using a thick AlN buffer Cracks are observed on the surface of the GaN film grown on the carbonized Si(111), while no cracks are visible on the 3c-SiC/Si(111). XRD exhibits polycrystalline nature of the GaN film grown on the carbonized Si(111) due to poorer crystalline quality of this substrate. Raman spectra reveal that all GaN layers are under tensile stress, and the GaN layer grown on 3c-SiC/Si(111) shows a very low stress value of sigma(xx) = 0.65 Gpa. In low-temperature Photoluminescence spectra the remarkable donor-acceptor-pair recombination and yellow band can be attributed to the incorporation of Si impurities from the decomposition of SiC.

Silicon thin films prepared in the transition region and their use in solar cells

Diphasic silicon films (nc-Si/a-Si:H) have been prepared by a new regime of plasma enhanced chemical vapour deposition in the region adjacent of phase transition from amorphous to microcrystalline state. Comparing to the conventional amorphous silicon (a-Si:H), the nc-Si/a-Si:H has higher photoconductivity (sigma(ph)), better stability, and a broader light spectral response range in the longer wavelength range. It can be found from Raman spectra that there is a notable improvement in the medium range order. The blue shift for the stretching mode and red shift for the wagging mode in the IR spectra also show the variation of the microstructure. By using this kind of film as intrinsic layer, a p-i-n junction solar cell was prepared with the initial efficiency of 8.51 % and a stabilized efficiency of 8.01% (AM 1.5, 100 mw/cm(2)) at room temperature. (c) 2006 Published by Elsevier B.V.

Electron concentration dependence of exciton localization and freeze-out at local potential fluctuations in InN films

InN films with electron concentration ranging from n similar to 10(17) to 10(20) cm(-3) grown by metal-organic chemical vapor deposition (MOCVD) and molecular beam epitaxy (MBE) were investigated by variable-temperature photoluminescence and absorption measurements. The energy positions of absorption edge as well as photoluminescence peak of these InN samples with electron concentration above 10(18) cm(-3) show a distinct S-shape temperature dependence. With a model of potential fluctuations caused by electron-impurity interactions, the behavior can be quantitatively explained in terms of exciton freeze-out in local potential minima at sufficiently low temperatures, followed by thermal redistribution of the localized excitons when the band gap shrinks with increasing temperature. The exciton localization energy sigma (loc) is found to follow the n (5/12) power relation, which testifies to the observed strong localization effects in InN with high electron concentrations.

零知识证明的两种变体与非延展性质的研究

零知识证明已经成为密码协议设计中一个非常重要的工具，它的发展也对计算复杂性的发展有着极为深远的影响。本文研究了零知识证明的两种变体和零知识证明的非延展性质。 文中所研究的零知识证明的两种变体是指Pass在EUROCRYPT'03上提出来的$n^{poly(logn)}$-模拟的证明和他在CRYPTO'03上提出来的公共参考串模型下的可否认零知识。对这两种变体，得到了如下结果： （1）基于抵抗亚指数线路的单向置换的存在性，构造了5-轮的高效并发的$n^{poly(logn)}$-模拟的知识论证系统。该方案使用了普通的$\Sigma$-协议作为工具。在EUROCRYPT'03中，使用了特殊的诚实验证者完全零知识（我们称之为完全$\Sigma$-协议）来构造4-轮$n^{poly(logn)}$-完全模拟的知识论证系统。这个构造是基于无爪函数的存在性的，并且它在并发合成下的封闭性紧紧依赖于$\Sigma$-协议的完全模拟性质。在EUROCRYPT'03的方案中，若使用普通的$\Sigma$-协议代替完全$\Sigma$-协议作为构造模块，得到的协议在并发合成下未必封闭。 （2）给出了公共参考串模型下的可否认零知识的一个正面结果，即：从$\Sigma$-协议到公共参考串模型下的可否认零知识的高效转化。由Pass给出的关于非平凡语言的公共参考串模型下的可否认零知识的轮数下界可知，该 转化取得了最优的轮效率。另外，转化前后增加的通信复杂度比较低。 关于非延展零知识，我们的工作主要是对现有的非延展的非交互零知识证明方案进行分析和简化。另外，引入了实例依赖的可验证随机函数（简写为InstD-VRF）这一新工具来构造非延展的非交互零知识协议。 得到的结果如下： （1）利用签名方案构造非延展的非交互零知识协议： 把Garay等人在EUROCRYPT'03中给出的公共参考串模型下的3-轮并发非延展零知识论证转化成了健壮的非交互零知识论证系统。一方面，与Garay的方案相比，在相同的模型下，提高了轮效率。另一方面，和现有的健壮非交互零知识方案相比，新方案中证明的长度比较短，并且归约计算的速度比较快。 （2）利用“隐藏地选取不可拷贝的集合”技术构造非延展的非交互零知识协议：对De Santis等给出的基于“隐藏地选取不可拷贝的集合”（hidden unduplicatable set selection）技术的方案做了如下简化：去掉了公共参考串中的冗余部分，缩短了公共参考串中部分随机串的长度，同时简化了两个子协议中所要证明的定理。这使得证明过程中的归约效率有所提高，同时缩短了证明的长度。 （3）利用InstD-VRF构造模非延展的非交互零知识证明：以InstD-VRF为工具，构造了比较简单的非延展的非交互零知识证明。实际上，由于InstD-VRF的构造方法有很多种，这个方案可以看成是构造非延展的非交互零知识证明的一个框架。在某种程度上，利用个框架，可以把构造高效的非延展的非交互零知识证明问题归结为构造高效的InstD-VRF的问题。

6Sigma在系统集成过程中的应用

6Sigma是实现过程控制和持续改进的有效方法,本文针对复杂系统集成过程管理存在的问题,分析了6Sigma在系统集成过程管理中应用的可行性,提出了一种基于6Sigma的系统集成过程模型.该模型具有信息及时反馈、过程持续改进、提高质量和降低成本的特点,在项目中得到应用并取得明显效果.

Steady-state bright-dark vector spatial solitons in biased photorefractive-photovoltaic crystals

We show that bright-dark vector solitons are possible in biased photorefractive-photovoltaic crystals under steady-state conditions, which result from both the bulk photovoltaic effect and the spatially nonuniform screening of the external bias field. The analytical solutions of these vector solitons can be obtained in the case of \sigma\ much less than 1, where sigma is the parameter controlling the intensities of the two optical beams. In the limit of -1 < sigma much less than 1, these vector solitons can also be determined by use of simple numerical integration procedures. When the bulk photovoltaic effect is neglectable, these vector solitons are bright-dark vector screening solitons studied previously in the \sigma\ much less than 1 regime, and predict bright-dark vector screening solitons in the -1 < sigma less than or equal to 1 regime. When the external bias field is absent, these vector solitons predict bright-dark vector photovoltaic solitons in closed and open circuits. (C) 2002 Elsevier Science B.V. All rights reserved.