Intrinsically limited critical temperatures of highly doped Ga1-xMnxAs thin films

Ga1-xMnxAs films with exceptionally high saturation magnetizations of approximate to 100 emu/cm(3) corresponding to effective Mn concentrations of x(eff)approximate to 0.10 still have a Curie temperature T-C smaller than 195 K contradicting mean-field predictions. The analysis of the critical exponent beta of the remnant magnetization-beta = 0.407(5)-in the framework of the models for disordered/amorphous ferromagnets suggests that this limit on T-C is intrinsic and due to the short range of the ferromagnetic interactions resulting from the small mean-free path of the holes. This result questions the perspective of room-temperature ferromagnetism in highly doped GaMnAs.

Strong resonant intersubband magnetopolaron effect in heavily modulation-doped GaAs/AlGaAs single quantum wells at high magnetic fields

Electron cyclotron resonance (CR) has been studied in magnetic fields up to 32 T in two heavily modulation-delta-doped GaAs/Al0.3Ga0.7As single quantum well samples. Little effect on electron CR is observed in either sample in the region of resonance with the GaAs LO phonons. However, above the LO-phonon frequency energy E-LO at B > 27 T, electron CR exhibits a strong avoided-level-crossing splitting for both samples at energies close to E-LO + (E-2 - E-1), where E-2, and E-1 are the energies of the bottoms of the second and the first subbands, respectively. The energy separation between the two branches is large, reaching a minimum of about 40 cm(-1) around 30.5 T for both samples. This splitting is due to a three-level resonance between the second LI, of the first electron subband and the lowest LL of the second subband plus an LO phonon. The large splitting in the presence: of high electron densities is due to the absence of occupation (Pauli-principle) effects in the final states and weak screening for this three-level process. (C) 2000 Published by Elsevier Science B.V. All rights reserved.

The growth of SiC on Si substrates with C2H4 and Si2H6

SiC was grown on Si (100) substrates oriented and off-oriented by 2-5 degrees towards [011] with simultaneous supply of C2H4 and S2H6 at 1050 degrees C. SiC formed during removal of oxide could be removed at 1150 degrees C. Twinned growth occurred on both oriented and off-oriented substrates during carbonization, but fewer twins formed on the off-oriented substrate than that on the oriented substrate. In SiC growth process, twinned growth continued on the off-oriented substrate whereas twinned growth stopped and single crystal SiC with double-domain (2 x 1) superstructure formed on the oriented substrate. SiC single crystal could grow on a carbonized twinned buffer layer. Obvious SiC LO and TO phonon modes were observed with Raman spectroscopy in the epilayer grown on the oriented substrate. The surface of the epilayer grown on the oriented substrate was smooth, while there was a high density of islands on the epilayer grown on the off-oriented substrate. The film grown on the oriented substrate is superior than that grown on the off-oriented substrate. (C) 1999 Elsevier Science B.V. All rights reserved.

Growth and characterization of GaN on LiGaO2

The nearly lattice-matched (0 0 1)LiGaO2 substrates have been used for the growth of GaN by LP-MOVPE, GaN epilayers have been grown on both domains at very low input partial pressure of hydrogen and relatively low growth temperature. The differences in the growth rate, crystal and optical qualities of hexagonal GaN epilayers grown on LiGaO2 substrate with two polar domains are investigated. LiGaO2 single crystal with a single domain structure and an adequate surface plane is a promising substrate for the growth of high quality of hexagonal GaN thin films. (C) 1998 Published by Elsevier Science B.V. All rights reserved.

二维条码和数字签名技术及其在毕业证书中的应用

由于二维条码技术的突出优点,在各领域应用越来越广泛.二维条码技术和数字签名技术相结合可以有效解决各类证书数字化带来的一些问题,可以避免签名伪造和内容篡改.文章介绍了这些技术在毕业证书数字化中的应用,提出了解决方案并进行了实现.类似地他们也可以推广应用到其他各类证卡系统中.

Web应用服务器自适应配置框架的设计与实现

以Web应用服务器为代表的分布式组件中间件系统(如EJB，CORBA，.NET)已发展为Web计算环境中的主要基础软件。中间件系统通过屏蔽底层平台的异构性，提供大量应用所需要的服务(如事务、安全等)，极大地简化了大规模复杂分布式系统的开发；另外，通过定义良好的组件模型，大量COTS组件能部署到任何与标准兼容的中间件平台实现上，提高了软件复用的程度。 中间件在支持应用的功能性需求方面虽然取得了较好的效果，然而在非功能性支持方面，中间件尚处于“尽力而为”的阶段，缺乏相应的服务质量保障机制，难以满足复杂多变的计算环境的要求。性能是应用系统一种非常关键的非功能特征，基于组件的应用，其性能不但受到应用设计的影响，同时受到应用所部署的中间件系统的影响，而这种影响很大程度上是中间件资源参数配置引起的（下文中如无明确说明，资源配置简称为配置）。目前大部分中间件系统只支持静态配置方式，必须通过反复地试运行来确定手工配置的参数是否能够满足应用的性能需求，该方式效率低下，而且对管理人员的要求很高；同时，对于诸如e-commerce之类的计算环境，负载始终处于高动态变化之中，静态配置方式也难以适应这种负载变化。针对上述问题，本文以EJB中间件为目标平台，提出了一种基于性能模型的自适应配置框架，能够在系统负载变化的情况下，自适应地调整中间件配置参数，更好地满足应用的性能需求。 首先，本文研究了自适应配置框架的总体架构。该框架的核心是一个基于分层排队网络的性能模型，它能够预测在给定中间件配置和负载下的性能度量。在配置决定过程中，性能模型用于评估不同候选配置，指导搜索最优的配置，从而提高性能保障的准确性和有效性。 其次，本文研究了基于分层排队网络模型的EJB性能建模技术。通过分析不同类型组件容器的请求处理行为，我们为不同类型组件建立性能模板。通过基于模型分解/组合的建模方法和模版实例化构建整个EJB应用的完整性能模型。 最后，本文描述了自适应配置框架在OnceAS2.0应用服务器上的原型实现，以及相关实验对该框架有效性的验证。

垄上线源滴灌湿润体特性研究

在米脂县孟岔试验基地进行大田试验,研究了同一灌水量、不同流量条件下,垄上线源滴灌湿润体特征值的变化规律及滴灌结束后的水分再分布规律。试验结果表明:在线源交汇入渗情况下,滴头处和交汇面处垂直入渗距离与入渗时间均满足良好的幂函数关系。在灌水量为9L的情况下,湿润体沿滴灌管布置方向剖面上的土壤含水率等值线大致以15cm深度为分界线,上层呈现以滴头为中心的椭圆状,而下层则相对平缓。滴灌停止后24h时的湿润体较为稳定,其特征值可以作为线源滴灌灌水量设计的依据,同一灌水量条件下,滴头流量对滴灌停止后24h的湿润体特征值影响并不大。

干旱胁迫下蚕豆叶片抗氧化酶活性的变化

抗氧化防御系统在植物抗旱中发挥着极大的作用。本试验采用PEG胁迫和叶片35℃烘干处理的方法,研究了模拟干旱胁迫条件下蚕豆叶片SOD、POD、CAT活性变化。结果表明,三种抗氧化酶随胁迫时间的延长活性升高;随胁迫程度增加SOD活性活性降低,POD和CAT活性变化规律性不明显。总体而言,SOD活性对胁迫的耐受性更强。

不同基因型小麦麦茬对杂草的化感抑制作用

采用改进的田间抑制圈生物测定方法,分析了10种不同基因型小麦材料麦茬对杂草生长的影响。结果表明,不同基因型之间以及同一基因型的不同材料之间对杂草的影响均呈现显著差异,说明化感作用基因在稳定遗传的同时存在一定的变异。以现代品种“丰产3号”麦茬地杂草的生物量为对照,其他小麦品种麦茬对杂草均有一定的抑制作用;对10个小麦品种麦茬地杂草的生物量与残茬滞留时间(天)逐步回归分析发现,杂草生长与时间存在明显的正相关关系,其中小麦残茬抑制杂草生长的化感作用有效天数为3～29天,而后杂草生物量逐渐增大,最后达到稳定;染色体组型为 AA、AABB 及 AABBDD 的麦茬化感抑制作用均随残留时间的变化而逐渐减弱,抑制杂草的平均有效期分别为21、24和25天,说明小麦在从二倍体到四倍体到六倍体的染色体组变异与倍体变化的长期进化过程中,小麦的遗传可塑性可能有助于化感作用的增强。