Structural and magnetic properties of hydrides R3Fe29-xVxHy (R = Y, Ce, Nd, Sm, Gd, Tb, and Dy)

A systematic investigation of crystallographic and intrinsic magnetic properties of the hydrides R3Fe29 - xVxHy (R = Y, Ce, Nd, Sm, Gd, Tb, and Dy) has been performed in this work. The lattice constants a, b, and c and the unit cell volume of R3Fe29 - xVxHy decrease with increasing rare-earth atomic number from Nd to Dy, except for Ce, reflecting the lanthanide contraction. Hydrogenation results in regular anisotropic expansions along the a-, b-, and c-axes in this series of hydrides. Abnormal crystallographic and magnetic properties of Ce3Fe27.5V1.5H6.5, like Ce3Fe27.5V1.5, suggest that the Ce ion is non-triply ionized. Hydrogenation leads to the increase in both Curie temperature for all the compounds and in the saturation magnetization at 4.2 K and RT for R3Fe29 - xVx with R = Y, Ce, Nd, Sm, Gd, and Dy, except for Tb. Hydrogenation also leads to a decrease in the anisotropy field at 4.2 K and RT for R3Fe29 - xVx with R = Y, Ce, Nd, Gd, Tb, and Dy, except for Sm. The Ce3Fe27.5V1.5 and Gd3Fe28.4V0.6 show the larger storage of hydrogen with y = 6.5 and 6.9 in these hydrides. (C) 1998 Elsevier Science B.V. All rights reserved.

Intrawell and interwell transfer of excitons in growth-interrupted quantum wells

Intrawell and interwell transfers of excitons are observed by a temperature-dependent continuous-wave photoluminescence study of growth-interrupted single quantum wells. The intrawell transfer among the interface localization areas suggests a thermodynamic equilibrium between energy relaxation via LO-phonon emission and thermal population via phonon absorption. Thermal population is dominant in wider wells while relaxation is clearly observable in a four-monolayer narrow well at low temperatures. Interwell transfer of excitons also occurs between two narrow wells. (C) 1998 Academic Press.

Crystallographic and magnetic properties of the hydrides R3Fe29-xCrxHy (R = Y, Ce, Nd, Sm, Gd, Tb, and Dy)

A systematic study of the structural and intrinsic magnetic properties of the hydrides R3Fe29-xCrxHy (R = Y, Ce, Nd, Sm, Gd, Tb, and Dy) has been performed. Hydrogenation lends to a relative volume expansion of the unit cell and a decrease in x-ray density for each compound. Anisotropic expansions mainly along the n- and b-axes rather than along the c-axis for all of the compounds upon hydrogenation are observed. The lattice constants and the unit-cell volume of R3Fe29-xCrx and R3Fe29-xCrxHy decrease with increasing R atomic number from Nd to Dy, except for Ce, reflecting the lanthanide contraction. Hydrogenation results in an increase in the Curie temperature and a corresponding increase in the saturation magnetization at room temperature for each compound. After hydrogenation a decrease of 0.34 mu(B)/Fe in the average Fe atomic magnetic moment and a slight increase in the anisotropy field for Y3Fe27.2Cr1.8 are achieved at 4.2 K. First-order magnetization processes (FOMP) occur in magnetic fields of around 1.5 T and 4.0 T at 4.2 K for Nd3Fe24.5Cr4.5H5.0 and TD3Fe27.0Cr2.0H2.8, and around 1.4 T at room temperature for Gd3Fe28.0Cr1.0H4.2. The abnormal crystallographic and magnetic properties of Ce3Fe25.0Cr4.0 and Ce3Fe25.0Cr4.0H5.4 suggest that the Ce ion non-triply ionized.

Influence of A-type Zeolite on Methane Hydrate Formation

The porous medium has an important effect on hydrate formation. In this paper, the formation process and the gas storage capacity of the methane hydrate were investigated with A-type zeolite and Sodium Dodecyl Sulfate (SDS) existing in the system. The results show that A-type zeolite can influence methane hydrate formation. At the temperature of 273.5 K and pressure of 8.3 MPa, the distilled water with A-type zeolite can form methane hydrate with gaseous methane in 12 hours. The formation process of the system with A-type zeolite was quite steady and the amount of A-type zeolite can influence the gas storage capacity significantly. The adding of A-type zeolite with 0.067 g.(g water)(-1) into 2 x 10(-3) g.g(-1) SDS-water solution can increase the gas storage capacity, and the maximum increase rate was 31%. Simultaneously the promotion effect on hydrate formation of 3A-type zeolite is much more obvious than that of 5A-type zeolite when the water adding amounts are 0.033 g.g(-1) and 0.067 g.g(-1) at the experimental conditions.

Influence of 5A-Type Zeolite Powder on Tetrahydrofuran Hydrate Formation and Dissociation Process

Visual observations of tetrahydrofuran (THF) hydrate formation and dissociation processes with 5A-type zeolite powder were made at normal atmospheric conditions and below zero temperature by microscope. Results indicate that 5A-type zeolite powder can promote THF hydrate growth. At the same time, in the presence of 5A-type zeolite, agglomerated crystals and vein-like crystals of THF hydrate were also formed. SA-type zeolite powder increases the crystallization temperature and decreases the dissociation temperature. The particle size distribution of 5A-type zeolite powder influences THF hydrate formation and its dissociation characteristics significantly.

苯乙炔类分子导线的合成、组装及电子传输性能

分子导线作为未来分子电子器件的重要组成部分，其合成，组装及电子传输性能研究是当今化学、物理、生物和微电子工程等领域里一个非常热门的研究课题。本论文在齐聚苯乙炔及齐聚苯乙炔一唾吩乙炔分子导线的合成、组装及电子传输性能研究方面进行了一些工作，主要成果有以下几个方面：一、官能化短链分子导线的合成与表征比较系统地合成不同端基，不同分子长度和不同主链结构的乙酞琉基官能化的齐聚苯乙炔类分子导线，以便比较系统地研究各种因素对这类分子导线的自组装及电子传输性能的影响。对所有合成的官能化分子导线进行了红外光谱、核磁共振氢谱和质谱表征以确定其结构。二、长链分子导线的合成与表征用溶液和固定相快速合成方法合成了一系列苯乙炔齐聚物及苯乙炔一（蜜份乙炔交替共聚齐聚物：1）采用简便的路线，用溶液和固定相方法快速合成出十二烷氧基取代的苯乙炔齐聚物，最一长达到了八聚体。（2）采用一条最简便的路线，用固定相方法快速合成了异丙基取代的苯乙炔齐聚物，最长达到了八聚体。（3）用溶液和固定相方法首次合成了苯乙炔一唾吩乙炔交替共聚齐聚物。（4）用一种新颖的“现场去保护／偶联”二倍速方案快速合成出十二烷氧基取代的苯乙炔齐聚物，最长达到了八聚体。该方案最大的优点在于无需分离出对空气敏感的芳香端炔化合物，从而简化了实验操作以及提高了产物的纯度。对所有合成的齐聚物进行了红外光谱、核磁共振氢谱、核磁共振碳谱和激光质谱表征以确定其结构。三、官能化分子导线的组装及电子传输性能研究（l）用STM和CP-AFM研究了合成的官能化分子导线在金基底的自组装行为，发现形成的自组装单层缺陷很少，而且自组装单层非常均一。（2）用电化学和导电原子力显微镜技术研究了上述官能化齐聚苯乙炔分子导线的电子传输性能，发现界面接触和分子长度对分子导线的电子传导能力有很大的影响，而链结构的影响则相对要小些。此外，我们还发现齐聚苯乙炔体系的电子传输衰减系数β值仅为0.19A-1，说明它是一类性能优异的分子导线侯选物。（3）通过量子化学计算，我们对实验结果进行了初步解释。

盐和有机物对氨基酸热力学性质的影响-NaCl氨基酸-葡萄糖-水体系

在278.15-318.15范围内，本文测定了以下四个无液接电池的电动势：Pt,H_2(g, 1 atm)|HCl(m)、X Mass% Glucose-H_2O|Ag-AgCl (A) Pt,H_2 (g, 1 atm)|G(m_1), HGCl (m_2), X Mass% Glucose-H_2O|Ag-AgCl (B) Pt,H_2 (g, 1 atm) |HCl (m), NaCl (M-m), X Mass% Glucose-H_2O|Ag-AgCl (C) Pt,H_2 (g, 1 atm)|G(m_1), HGCl (m_2), NaCl (M-m), X Mass% Glucose-H_2O|Ag-AgCl (D) 其中G为中性甘氨酸，NH_3CH_2COO~-, HGClm为甘氨酸的盐酸盐，为相应电解质的质量摩尔浓度，X为葡萄糖在葡萄糖-水混合溶剂中的质量百分数，M为恒定的离子强度且M = 1.0mol/kg。并且测定了Glucose-H_2O的密度和介电常数。利用传统的D-H公式外推法和基于Pitzer理论的多项式逼近法分别确定了电池的标准电动势E°_3、E°_3，以及甘氨酸的一级热力学解离1.0mol/kg）-葡萄糖-水-HCl的HCl无限稀释溶液为参考态，并将两种方法得到的结果作了比较。甘氨酸的一级热力学解离常数符合Harned-Robinson方程：pK = A_1/T + A_2 + A+3 T 本文讨论了盐和有机物对pK_1的影响，并根据前人及我们的工作，指出在极性质子溶剂中和在极性非质子溶剂中的pK_1对1/D作图，分别得到直线和曲线。同时讨论了混合溶剂中甘氨酸的一级解离过程的各个热力学量ΔG°，ΔH°，ΔS°，ΔCp°，并讨论HCl的迁移性质和有机物葡萄糖、盐对它们的影响。最后将不同混合溶剂中甘氨酸解离过程的迁移能，迁移熵等作了比较。

WELL-WIDTH DEPENDENCE OF THE EXCITON LIFETIME IN GAAS/ALGAAS QUANTUM-WELLS

The dependence of the excitonic lifetime on the well width has been studied in conventional GaAs/AlGaAs quantum wells. Two clearly different variations of the measured excitonic lifetime have been observed. For wide well widths, we find a nearly linear decrease of the lifetime with decreasing well width. However, when the well is further decreased, a saturation and even increase of the lifetime with decreasing well width are observed. The experimental data are compared with the theory of radiative excitonic recombination, and show that well width dependence of the measured photoluminescence lifetime can be attributed mainly to the change of the excitonic effective volume and the overlap integral as well.

HIGH-RESOLUTION PHOTOLUMINESCENCE STUDIES OF (211) CDTE GROWN ON (211)B GAAS SUBSTRATE

Sharp and rich photoluminescence lines accociated with free exciton (FE), excitons bound to neutral acceptors (A0X) and donors (D0X) in molecular beam epitaxially (MBE) grown (211) CdTe/(211)B GaAs have been reported for the first time. The results show that the (211) CdTe/(211)B GaAs grown under optimized conditions could have as high a crystal perfection as those grown on lattice-matched substrates.

SIMULATION OF LATERAL CONFINEMENT IN VERY NARROW CHANNELS

A theoretical study is presented of the lateral confinement potential (CP) in the very narrow mesa channels fabricated in the conventional two-dimensional (2D) electron gas in GaAs-AlxGa1-xAs heterostructures. The ID electronic structures are calculated in the framework of the confinement potential: V(x) = m* omega0(2)x2/2 for Absolute value of x

PHOTOVOLTAIC EFFECT AND ITS POLARITY IN SI DOPING SUPERLATTICES

We have studied the photovoltaic effects in Si doping superlattices (nipi) under different excitation conditions with and without additional cw optical biasing using a He-Ne laser. On the basis of the photovoltaic theory of carrier spatial separation in superlattices, we propose the concept of spatial fixity of the photovoltage polarity in type-II superlattices and examine the experimental results. The photovoltaic effect in Si nipi is found mainly from the direct transitions related with shallow impurities in real space, not the electron-hole band-to-band process as in GaAs nipi.

WELL WIDTH DEPENDENCE OF THE EXCITON LIFETIME IN NARROW GAAS/GAALAS QUANTUM-WELLS

We have studied the radiative excitonic lifetime as a function of the well width in GaAs/GaAlAs quantum wells. An increasing lifetime with decreasing well width has been observed in very narrow and high quality GaAs/GaAlAs quantum wells, and attributed to the reduced overlap of the electron and hole wave functions and the increase of the exciton effective volume. This is the first observation of its kind in the conventional GaAs/GaAlAs quantum wells.

LO-PHONON-ASSISTED TUNNELING IN ASYMMETRIC DOUBLE-WELL STRUCTURES WITH THICK BARRIERS

Nonresonant electron tunneling between asymmetric double quantum wells in AlxGa1-xAs/GaAs systems has been investigated by using steady-state and time-resolved photoluminescence spectra. Experimental evidence of LO-phonon-assisted tunneling through thick barriers has been obtained by enhancing excitation power densities or applying electric fields perpendicular to the well plane. LO-phonon-assisted tunneling times have also been estimated from the variation of the decay time of the narrow-well photoluminescence with applied electric fields. Our findings suggest that LO phonons in the barriers play an important role in the tunneling transfer.

Optical characterization of InAs monolayer quantum structures grown on (311)A, (311)B, and (100)GaAs substrates

Low-temperature photoluminescence and excitation spectra from InAs monolayer quantum structures, grown on (311)A, (311)B, and (100) GaAs substrates, are investigated, The structures were grown simultaneously by conventional molecular-beam epitaxy (MBE), The experimental results show that the quality of InAs monolayer on (311)B GaAs substrate is obviously better in crystal quality than those on the two other oriented GaAs substrates. In addition, the transition peaks of the InAs layer grown on (311) GaAs substrates shift to higher energy with respect to that from the InAs layer grown on (100) GaAs substrate.

Optical study of heterointerface configuration in narrow GaAs/AlGaAs single quantum wells prepared with growth interruption

Photoluminescence and time-resolved photoluminescence were used to study the heterointerface configuration in GaAs/AlGaAs quantum wells grown by molecular-beam epitaxy with growth interruption. Photoluminescence spectra of the growth-interrupted sample are characterized by multiplet structures, with energy separation corresponding to a 0.8 monolayer difference in well width, rather than 1 monolayer as expected from the ''atomically smooth island'' picture. By analyzing the thermal transfer process of the photogenerated carriers and luminescence decay process, we further exploit the exciton localization at the interface microroughness superimposed on the extended growth islands. The lateral size of the microroughness in our sample was estimated to be 5 nm, less than the exciton diameter of 15 nm. Our results strongly support the bimodal roughness model proposed by Warwick et al. [Appl. Phys. Lett. 56, 2666 (1990)]. (C) 1996 American Institute of Physics.