Activation energy and hall-coefficient measurements in lightly dopedn-InP in the thermal freeze-out region

Summary The direct-current resistivity, β, and Hall coefficient,R _H, of lightly dopedn-type InP samples were measured at temperatures (T) down to 12K and magnetic fields up to 4.8 kG. A sharp exponential increase in β, asT was decreased, was observed for temperatures below 80 K. The Hall coefficient showed a similar trend,i.e. R _H increased sharply asT was reduced below 80 K. This is attributed to the freeze-out of conduction electrons onto their donor sites. The donor activation energy,E _d, calculated from the temperature dependence of the resistivity, was less than the theoretical prediction. An enhanced dielectric constant would be a possible candidate for such behaviour. The initial decrease in β (asT is reduced) recorded in the higher-temperature region is due to impurity scattering probably combined with lattice scattering. The authors of this paper have agreed to not receive the proofs for correction.     

Resonant response of a thermalized ensemble of nonlinear oscillators

The analysis is carried out of the response of the center of gravity (dipole moment) of the distribution of noninteracting thermalized nonlinear oscillators to a sinusoidal driving force. Heat bath coupling is modeled by damping and noise. The driving is weak, but the frequency is resonant, so that there is a nonlinear resonance in the phase space. The response has a linear part that can be obtained from the perturbation analysis and a small nonlinear correction that is specific for the resonant structure.     

铥原子激发能级超精细结构常数的测定

用铥空心阴极灯,采用光电流光谱技术测得铥原子21条一级跃迁谱线,确定了相应跃迁上能级的超精细结构常数,其中7个能级的参数为首次发表,运用同一空心阴极灯,采用双色双光子级联共振技术,由荧光法首次测得4个第二激发能级的超精细结构常数.     

薁，苯并环丁二烯和戊搭烯的拓扑共振能

为了阐明芳香性及磁性原因，用Ａｉｈａｒａ提出的ＴＲＥ概念计算了　，苯并环丁二烯和戊搭烯等烃的共振能，环共振能和环电流．计算结果表明　因有正共振能具有芳香性并产生反磁性环电流，而苯并环丁二烯和戊塔烯因有负共振能具有反芳香性并产生顺磁性环电流．     

低密度聚乙烯光引发交联机理——Ⅱ.BNB自旋捕捉ESR研究

在光引发剂二苯甲酮(BP)存在下,紫外光辐照低密度聚乙烯(LDPE)所形成的自由基中间体已被自旋捕捉剂2,4,6-三特丁基亚硝基苯(BNB)所捕捉,其自旋加合物自由基已为电子自旋共振(ESR)所特征。现已检测和鉴定到二种自旋加合物:一种是叔碳自由基;另一种是仲碳自由基。它们分别是由BP的激发三重态从LDPE链的支化点和亚甲基团上夺氢所形成的自由基中间体同自旋捕捉剂BNB反应生成的。上述的证据表明:LDPE的光引发交联点主要发生在叔碳和仲碳原子上,且H-型交联点占主导地位。     

Echo planar imaging of perfluorocarbons

Emulsions of perfluorotributylamine (FTBA) and perflubron were evaluated for their utility in ¹⁹F echo planar imaging. Fluorine images of the emulsions were obtained in a phantom and two mice that had been predosed. Both agents, but particularly perflubron, show potential for fluorine echo planar studies because of the long spin-spin relaxation times of the CF₃ resonances. High resolution thin slice images obtained in as little as 26.6 ms are presented.     

Superparamagnetic iron oxide particles and positive enhancement for myocardial perfusion studies assessed by subsecond T1-weighted MRI

Superparamagnetic iron oxide particles (SPIOs) are usually referred to as T₂ MR contrast agents, reducing signal intensity (SI) on T₂-weighted MR images (negative enhancement). This study reports the original use of SPIOs as T₁-enhancing contrast agents, primarily assessed in vitro, and then applied to an in vivo investigation of a myocardial perfusion defect. Using a strongly T₁-weighted subsecond MR sequence with SPIOs intravenous (IV) bolus injection, MR imaging of myocardial vascularization after reperfusion was performed, on a dog model of coronary occlusion followed by reperfusion. Immediately after the intravenous bolus injection of 20 μmol/kg of SPIOs, a positive signal intensity enhancement was observed respectively, in the right and left ventricular cavity and in the nonischemic left myocardium. Moreover, compared to normal myocardium, the remaining ischemic myocardial region (anterior wall of the left ventricle) appeared as a lower and delayed SI enhancing area (cold spot). Mean peak SIE in the nonischemic myocardium (posterior wall) was significantly higher than in the ischemic myocardium (anterior wall) (110 ± 23% vs. 74 ± 22%, Mann-Whitney test < 1%, n₁ = 6, n₂ − n₁ = 0, U > 2). In conclusion, the T₁ effect of SPIOs at low dose, during their first intravascular distribution, suggests their potential use as positive markers to investigate the regional myocardial blood flow and some perfusion defects such as the “no-reflow phenomenon”.     

Chaotic resonance: A simulation

Stochastic resonance is a statistical phenomenon that has been observed in periodically modulated, noise-driven, bistable systems. The characteristic signatures of the effect include an increase in the signal-to-noise of the output as noise is added to the system, and exponentially decreasing peaks in the probability density as a function of residence times in one state. Presented are the results of a numerical simulation where these same signatures were observed by adding achaotic driving term instead of a white noise term. Although the probability distributions of the noise and chaos inputs were significantly different, the stochastic and chaotic resonances were equal within the experimental error.     

Effect of local structure on electron paramagnetic resonance spectra for trigonal [Cr(H2O)6]3+coordination complex in the sulfate alums series: a ligand field theory study

A simple theoretical method is introduced for studying the interrelation between electronic and molecular structures.By diagonalizing the 120 × 120 complete energy matrices,the relationships between zero-field splitting(ZFS) parameter D and local distortion parameter △θ for Cr 3+ ions doped,separately,in α-and β-alums are investigated.Our results indicate that there exists an approximately linear relationship between D and △θ in a temperature range 4.2-297 K and the signs of D and △θ are opposite to each other.Moreover,in order to understand the contribution of spin-orbit coupling coefficient ζ to ZFS parameter D,the relation between D and ζ is also discussed.     

核磁共振成像一维空间编码教学实验

利用梯度磁场实现检测信号的空间编码,是核磁共振成像(MRI)的关键技术.本文采用ccc系列样品将二维问题简化为一维,使用超小型教学用核磁共振成像仪进行了一维空间编码实验研究,并对实验过程及实验结果进行了计算模拟和分析.