Growth and Characterization of A1GaN/A1N/GaN HEMT Structures with a Compositionally Step-Graded A1GaN Barrier Layer

A new A1GaN/A1N/GaN high electron mobility transistor （HEMT） structure using a compositionally step-graded A1GaN barrier layer is grown on sapphire by metalorganic chemical vapour deposition （MOCVD）. The structure demonstrates significant enhancement of two-dimensional electron gas （2DEG） mobility and smooth surface morphology compared with the conventional HEMT structure with high A1 composition A1GaN barrier. The high 2DEG mobility of 1806 cm2/Vs at room temperature and low rms surface roughness of 0.220 nm for a scan area of 5μm×5 μm are attributed to the improvement of interracial and crystal quality by employing the stepgraded barrier to accommodate the large lattice mismatch stress. The 2DEG sheet density is independent of the measurement temperature, showing the excellent 2DEG confinement of the step-graded structure. A low average sheet resistance of 314.5Ω/square, with a good resistance uniformity of 0.68%, is also obtained across the 50 mm epilayer wafer. HEMT devices are successfully fabricated using this material structure, which exhibits a maximum extrinsic transconductance of 218 mS/ram and a maximum drain current density of 800 mA/mm.     

二极管激光侧面泵浦腔内倍频Nd：YAG板条激光器研究

用准连续６０Ｗ的二极管激光列阵侧面泵浦“之”字形Ｎｄ：ＹＡＧ板条激光器，当泵浦功率为４５Ｗ，脉宽为４００μｓ时，得到３．５ｍＪ的激光输出。用ＫＤ＊Ｐ电光开关调Ｑ，得到１８ｎｓ（ＦＷＨＭ）、２ｍＪ的脉冲激光输出，用ＫＴＰ晶体作腔内倍频，得到１５ｎｓ、０．８５ｍＪ的二次谐波激光输出。在腔内无调Ｑ元件且腔长小于５ｃｍ时，于近阈值处得到单频绿光输出，而当泵浦功率稍大，腔内有数个纵模振荡时，观察到激光的反相态，并用数值模拟的结果对此作了解释。     

算子半群生成元的等价豫解特征

本文给出了Ｈｉｌｌｅ－Ｙｏｓｉｄａ条件等价的几个新的算子半群生成元的豫解特征。     

液态五元Zr57Cu20Al10Ni8Ti5合金的微观结构演变与非晶形成机制

利用电磁悬浮无容器处理技术实现了液态五元Zr₅₇Cu₂₀Al₁₀Ni₈Ti₅合金的深过冷与快速凝固,同时通过分子动力学模拟计算揭示了非晶形成的微观机制.实验发现,凝固组织具有明显的核-壳结构特征,核区为非晶相,壳区主要由ZrCu, Zr₂Cu和Zr₈Cu₅晶体相组成.非晶体积分数随合金过冷度的升高逐渐增大,当达到实验最大过冷度300 K (0.26T_L)时,非晶体积分数增至81.3%.由此导出完全非晶凝固所需临界过冷度为334 K. TEM分析显示,过冷度增大并接近临界过冷度时,合金凝固组织中晶体相主要为Zr₈Cu₅相,而ZrCu和Zr₂Cu相的生长被抑制.在达到临界过冷度后,过冷液相的凝固路径由Zr₈Cu₅结晶生长转变为非晶凝固.此外,合金的晶体壳中存在少量的晶间非晶相,而非晶核中...     

资产收益率分形相关性下的Mean-PDCCA组合策略研究

准确地测量资产之间的相关性,是构建有效投资组合模型的前提.文章针对资产收益率存在分形相关性的现实情况,首先通过消除趋势交叉相关分析(DCCA)等方法,构建了分形相关性统计测度,用于测量资产之间的相关性;随后,通过将分形相关性统计测度纳入到收益-风险准则之中,构建了多时间标度前置下的投资组合模型Mean-PDCCA,即分形投资组合模型,并给出了模型的解析解;最后,实证分析发现,在资产收益率存在分形相关性的典型事实约束下,分形投资组合整体上优于经典投资组合,不仅能够提升投资业绩,还具有更好的稳健性,为投资者提供了有效的决策参考.     

表面分形球、柱的光散射

分形和分维是近些年才兴起的一门处理复杂事物的新方法￣［１］。本文主要研究具有表面分形的球、柱的光散射特征。我们首先利用几何光学近似研究了一种表面满足自仿射原理的分形均匀球的散射，其散射特性与分维存在一定的关系。我们还利用微扰法讨论了表面具有沟槽的分形柱的光散射的分形特征。     

(La1-xCex)2/3Ca1/3MnO3体系的输运性质和反常磁特性研究

本文报告了对Ce掺杂锰氧化物(La1-xCex)2/3Ca1/3MnO3 (x=0～1.0)系列样品的输运特性和反常磁特性的研究结果.实验表明,Ce掺杂对Tc有明显的抑制作用,整体上电阻率随Ce掺杂含量增加而上升,在外加磁场时表现出极大的磁电阻效应.磁化强度随温度变化的曲线出现了两个转变,高温处对应于Mn离子磁矩的铁磁金属转变,低温处的转变则对应于Ce离子磁矩自旋有序排列的形成.表明Ce掺杂引起样品中铁磁双交换作用和反铁磁超交换作用之间的竞争,Ce离子与Mn离子有很强的相互作用.随Ce掺杂含量的增加,铁磁有序转变温度下降,而反铁磁有序转变温度则向高温处移动,铁磁区域明显减小.     

2-(2-喹啉偶氮)-1,5-苯二酚分光光度法测定烟草样品中的钙

在 p H为 8.0的柠檬酸钠 -氢氧化钠缓冲介质中 ,Triton X- 10 0存在下 ,2 - (2喹啉偶氮 ) - 1,5苯二酚(QADHB)与钙反应生成 1∶ 1稳定配合物 ,λmax=5 5 5 nm,ε=4 .18× 10 4L· mol-1· cm-1。钙含量在 0 .1—2 5μg/ 2 5 m L的范围内符合比耳定律 ,方法用于烟草中钙含量的测定 ,结果令人满意。     

New bifurcations of basin boundaries involving Wada and a smooth Wada basin boundary

This paper demonstrates and analyses double heteroclinic tangency in a three-well potential model, which can produce three new types of bifurcations of basin boundaries including from smooth to Wada basin boundaries, from fractal to Wada basin boundaries in which no changes of accessible periodic orbits happen, and from Wada to Wada basin boundaries. In a model of mechanical oscillator, it shows that a Wada basin boundary can be smooth.     

An alternating periodic－chaotic ISI sequence of HH neuron under external sinusoidal stimulus

A study of Hodgkin－Huxley (HH) neuron under external sinusoidal excited stimulus is presented in this paper. As is well known, the stimulus frequency is to be considered as a bifurcate parameter, and numerous phenomena, such as synchronization, period, and chaos appear alternatively with the changing of the stimulus frequency. For the stimulus frequency less than 2f_B (f_B being the base frequency in this paper), the simulation results demonstrate that the single HH neuron could completely convey the sinusoidal signal in anti-phase into interspike interval (ISI) sequences. We also report, perhaps for the first time, another kind of phenomenon, the beat phenomenon, which exists in the phase dynamics of the ISI sequences of the HH neuron stimulated by a sinusoidal current. It is shown furthermore that intermittent transition results in the general route to chaos.