Triterpenoid Constituents of Huperzia miyoshiana

Thirteen triterpenoids, including three new ones, miyoshianois A (1), B (2) and C (3), were isolated from Huperzia miyoshiana. The structures of these new compounds were established as 3-O-dihydroferuloyltohogenol (1), 16-oxo-3β,21β-dihydrox-y-serrat-14-en-24-ferulate (2) and 16-oxo-3a, 21β-dihydroxy-serrat-14-en-24-ferulate (3), respectively, on the basis of their spectroscopic analysis.     

Defect Structures in 4H-SiC Irradiated with Highly-energetic ^20Ne^4＋ and ^120Xe^26＋ Ions

The study of damage evolution in silicon carbide bombarded with energetic helium ions is important for the use of this material in future fusion reactors. Heavier inert gas atoms like Ne and Xe have similar behavior of diffusion and clustering with helium, and the comparison of damage accumulation behavior between energetic helium and heavier inert gas ions can reveal important aspects of underlying mechanisms. As an extension of our     

有机化合物电子光谱中的助色基及其作用机理探讨(Ⅰ)

本文就电子光谱中助色基的作用进行了讨论 ,认为不能一概地认为助色基能使与之相连的生色基最大吸收波长统统发生红移 ,而应具体问题具体分析 :对于n→π 来说 ,一方面助色基的p轨道和生色基的最低π 反键空轨道相互作用 ,使π 反键轨道能量上升 ,另一方面助色基的p轨道和生色基的n轨道正交 ,n轨道能量基本保持不变 ,因此n→π 跃迁能增加 ,吸收波长发生蓝移。对于π→π 跃迁来说 ,助色基的p轨道和生色基的最高π成键轨道相互作用生成的新的最高π成键轨道 ,能量虽然有所升高 ,但升高的程度与π 反键轨道升高的程度相比 ,在一些化合物中π轨道升高程度较大 ,因而π→π 跃迁能减小 ,吸收波长发生红移 ;在另一些化合物中π轨道升高程度较小 ,因而π→π 跃迁能增加 ,吸收波长发生蓝移。     

直接求线性规划可行基的一种方法

本文给出直接求线性规划问题基可行解的一种简易方法,该方法既避免了引入人工变量,减少存储,一般又能较快地得到一个较好的基可行解.     

Time-Resolved Femtosecond Degenerate Four-Wave Mixing in LiNbO3：Fe,Mg Crystal

Forward degenerate four-wave mixing （DFWM） processes are investigated with a femtosecond pulsed laser in lithium niobate crystal doubly-doped with magnesium and iron （LiNbO3：Fe, Mg）. The pulse energy dependence reveals a pure third-order nonlinear response, and the third-order nonlinear susceptibility x^（3） in the material is evaluated to be 4.96 × 10^-13 esu. The time-resolved DFWM process shows a response time of x^（3） shorter than 100fs, which is due to the nonresonant electronic nonlinearities. Our results indicate that LiNbO3 crystals have potentials for ultrafast real-time optical processing systems, which require a large and fast x^（3） optical nonlinearity.     

反铁磁铜氧化合物中的磁致形状记忆

La2-xSrxCuO4(以下称LSCO)是最早发现的高温超导体之一.当x≈0.15时,它具有最高的超导转变温度,Tc=33K.对于x<0.02的轻度掺杂,材料的低温相是反铁磁绝缘体.在室温以上时,层状化合物LSCO晶格具有四方对称性;当冷却样品通过特征温度To,将发生从四方到正交的结构相变.此时,晶体发展出一种被称为孪晶的畴结构,被畴壁分开的相邻区域具有不同的晶轴取向.正交结构的晶格常数a=05339nm,b=0.5422nm,两者相差约1%.为了探求高温超导机理,LSCO曾被广泛深入地研究.由于在超导LSCO中根本不存在41meV反铁磁自…     

金属铀各相电子结构与原子力

锕系元素不仅存在σ，π，δ对称，还出现了φ轨道，而且相对论效应十分明显，作用机理比较复杂，而全电子计算由于对计算机硬件的要求十分苛刻，因而尚难以开展。因而国际上除了做了一些铀、钚化合物的物性研究外，几乎难以找到关于铀钚腐蚀机理的第一原理研究的报道。而用第一原理研究对于一些非镧、锕系元素，如硅、铜、钯等这种研究工作已经开展得很普遍，而且已经用于指导催化和腐蚀研究的实践工作。在LLUL钚科学在挑战专辑中，对于钚的基态性质，采用局域密度近似（简称LDA）和综合梯度近似方法简称GGA）进行了研究，     

Experimental and Theoretical Evidence for the Existence of Broad Forbidden Gaps in the Three—Component Composite

The influence of localized resonance on the properties of band-gaps of the two-dimensional periodic composite are analysed numerically and experimentally by comparing the two-component phononic crystal with the three-component one. The three-component composite which exhibits localized resonance in the elastically soft coating ring is composed of a square array of coated cylinders embedded in an epoxy resin. The coated cylinders consist of a steel inner core and a rubber coating, which has much smaller wave velocity and mass density than the matrixand tile inner material. Tile measured transmission power of tile three-component composite drops to the noise level in a broad ultrasonic frequency interval in contrast to the binary composite, which is in good agreement with the calculation using the finite element method.     

Y（Fe，Si）12化合物结构与磁性的第一原理计算

R（Fe，Si）12（R=Y，Nd）型稀土永磁材料具用重要的实际应用价值，然而对于V（Fe，Si）12化合物微观机理的理论计算工作至今尚未开展。采用新近发展的全势能线性缀加平面波（（L）APW）＋局域轨道（10）和广义梯度近似（GGA）密度泛函方法对其结构与磁性进行了研究。     

Time—Resolved Photoluminescence Studies of AlInGaN Alloys

We study the two samples of AlInGaN,i.e.,1-μm Gan grown at 1030℃ on the buffer and followed by a 0.6μm-thick epilayer of AlInGaN under the low pressure of 76 Torr and the AlInGaN layer deposited diectly on the buffer layer without the high-temperature GaN layer,by temperature-dependent photoluminescence(PL) spectroscopy and picosecond time-resolved photoluminescence(TRPL) spectroscopy.The TRPL signals of both the samples were fitted well as a stretched exponential decay at all temperatures,indicating significant disorder in the material.We attribute the disorder to nanoscale quantum dots or discs of high indium concentration.Temperature dependence of dispersive exponent β shows that the stretched exponential decay of the two samples comes from dfferent mechanisms.The different depths of the localization potential account for the difference,which is illustrated by the results of temperature dependence of radiative recombination lifetime and PL peak energy.