简单金属在流体静负载下的力学稳定性与理论拉伸强度

本文根据经典力学的稳定性条件,用膺势方法导出了立方晶体在流体静负载下的经典稳定性判据:B(P)>0,C′(P)>0和C(P)>0。然后计算了七种简单金属(Li,Na,K,Rb,Cs,Al和Pb)在零温和流体静负载下的稳定性范围与理论拉伸强度。

用赝势方法计算简单金属的弹性常数

本文利用单参数Heine-Abarenkov模型势及Hubbard-Sham介电函数计算了七种简单金属(Li,Na,K,Rb,Cs,Al和Pb)的二阶与三阶弹性常数,两个可调参数用零温零压下晶体的弹性常数C_(44)与晶格常数的实验值来确定,计算结果与实验值或其他作者的理论计算值符合得比较好,尤其是Al的三阶弹性常数,本文的计算结果比其他作者的计算结果更接近于实验值。

基于第一原理的固体变形、强度及外延成长研究

本文基于第一原理的能量计算和稳定性理论，研究了双原子组分材料β-SiC在各种载荷方式下的力学性质：弹性行为、应力－应变关系、稳定性和强度。由于沿[111]方向加载时四个Si-C键是不等价的，所以有相对内位移出现。本文的分析揭示了如下事实：当载荷较小时，内位移的影响不明显；当载荷较大时，影响则越来越显著；裂纹在{111}shuffle面上成核，并最终导致材料以解理的形式破坏。在沿[001]单轴拉伸的情况下，spinodal失稳（即体破坏）和Born失稳被同时触发，并体现了一种级联破坏的模式。基于同样的理论和方法，进一步研究了铝在各种加载方式下的响应及分叉行为。得到了铝的沿[001]和[111]方向单轴加载和单轴应变、及沿[010]和[001]方向的双轴比例加载情况下的、完整的能量－应变曲线和应力－应变曲线。详细地分析了沿[001]方向的单轴加载情况下的稳定性及分叉行为。研究结果表明，除了自然的面心立方结构外，所有其它的、应力自由的立方结构都是不稳定的。对于铝而言，稳定的面心立方结构不能从沿任何等价的[001]和[111]方向的单轴压缩的方法而得到。本文的结果丰富了现存的第一原理数据库。基于第一原理的能量计算，分析了碱金属K、Rb和Cs在如下四个结晶面上外延成长的行为，即{001}、{110}、{111}和{201}。发现除了在{110}面以外，在其它的三个结晶面上都发现有亚稳态存在。当外延应变为拉伸状态时，在{001}和{111}面上存在着亚稳态，其结构为b.c.c.结构。当外延应变为压缩状态时，在{201}面存在一个亚稳态，是一个超结构，。亚稳态的存在显著地影响着材料外延成长的软化行为。基于正交变形路径，并考虑了温度的影响以后，解释了实验上观察到的碱金属从h.c.p.结构转变到b.c.c.结构的现象。本文第五章归纳整理在赝势平面波框架下的总能、力和应力在实空间和动量空间的解析表达式，以方便参阅和使用。

Phase behavior of complex superprotonic solid acids

Superprotonic phase transitions and thermal behaviors of three complex solid acid systems are presented, namely Rb3H(SO4)2-RbHSO4 system, Rb3H(SeO4)2-Cs3H(SeO4)2 solid solution system, and Cs6(H2SO4)3(H1.5PO4)4. These material systems present a rich set of phase transition characteristics that set them apart from other, simpler solid acids. A.C. impedance spectroscopy, high-temperature X-ray powder diffraction, and thermal analysis, as well as other characterization techniques, were employed to investigate the phase behavior of these systems.

Rb3H(SO4)2 is an atypical member of the M3H(XO4)2 class of compounds (M = alkali metal or NH4+ and X = S or Se) in that a transition to a high-conductivity state involves disproportionation into two phases rather than a simple polymorphic transition [1]. In the present work, investigations of the Rb3H(SO4)2-RbHSO4 system have revealed the disproportionation products to be Rb2SO4 and the previously unknown compound Rb5H3(SO4)4. The new compound becomes stable at a temperature between 25 and 140 °C and is isostructural to a recently reported trigonal phase with space group P3̅m of Cs5H3(SO4)4 [2]. At 185 °C the compound undergoes an apparently polymorphic transformation with a heat of transition of 23.8 kJ/mol and a slight additional increase in conductivity.

The compounds Rb3H(SeO4)2 and Cs3H(SeO4)2, though not isomorphous at ambient temperatures, are quintessential examples of superprotonic materials. Both adopt monoclinic structures at ambient temperatures and ultimately transform to a trigonal (R3̅m) superprotonic structure at slightly elevated temperatures, 178 and 183 °C, respectively. The compounds are completely miscible above the superprotonic transition and show extensive solubility below it. Beyond a careful determination of the phase boundaries, we find a remarkable 40-fold increase in the superprotonic conductivity in intermediate compositions rich in Rb as compared to either end-member.

The compound Cs6(H2SO4)3(H1.5PO4)4 is unusual amongst solid acid compounds in that it has a complex cubic structure at ambient temperature and apparently transforms to a simpler cubic structure of the CsCl-type (isostructural with CsH2PO4) at its transition temperature of 100-120 °C [3]. Here it is found that, depending on the level of humidification, the superprotonic transition of this material is superimposed with a decomposition reaction, which involves both exsolution of (liquid) acid and loss of H2O. This reaction can be suppressed by application of sufficiently high humidity, in which case Cs6(H2SO4)3(H1.5PO4)4 undergoes a true superprotonic transition. It is proposed that, under conditions of low humidity, the decomposition/dehydration reaction transforms the compound to Cs6(H2-0.5xSO4)3(H1.5PO4)4-x, also of the CsCl structure type at the temperatures of interest, but with a smaller unit cell. With increasing temperature, the decomposition/dehydration proceeds to greater and greater extent and unit cell of the solid phase decreases. This is identified to be the source of the apparent negative thermal expansion behavior.

References

[1] L.A. Cowan, R.M. Morcos, N. Hatada, A. Navrotsky, S.M. Haile, Solid State Ionics 179 (2008) (9-10) 305.

[2] M. Sakashita, H. Fujihisa, K.I. Suzuki, S. Hayashi, K. Honda, Solid State Ionics 178 (2007) (21-22) 1262.

[3] C.R.I. Chisholm, Superprotonic Phase Transitions in Solid Acids: Parameters affecting the presence and stability of superprotonic transitions in the MHnXO4 family of compounds (X=S, Se, P, As; M=Li, Na, K, NH4, Rb, Cs), Materials Science, California Institute of Technology, Pasadena, California (2003).

Composição florística e estrutura da comunidade arbustivo-arbórea de um trecho de floresta submontana na vertente sudeste do Parque Estadual da Ilha Grande, Angra dos Reis / RJ

O estudo objetivou avaliar a composição florística e estrutural dos componentes arbustivo-arbóreo da Floresta Ombrófila Densa submontana em diferentes estágios de regeneração natural, na vertente sudeste do Parque Estadual da Ilha Grande/RJ. Para o inventário florístico foram realizadas coletas assistemáticas em diferentes trechos nessa vertente. A complementação da lista de espécies foi feita a partir, da consulta às exsicatas dos herbários do Rio de Janeiro (FCAB, GUA, HB, HRJ, R, RB, RBR, RFA, RFFP e RUSU) e do inventário fitossociológico. Foi verificado o status de conservação das espécies inventariadas para a Flora Brasileira. Para o inventário fitossociológico foram estabelecidas 34 parcelas amostrais, totalizando 1,02 ha de área amostrada. Todos os indivíduos arbustivo-arbóreos com DAP ≥ 5 cm foram registrados e, após identificação, foram depositados no Herbário da Universidade do Estado do Rio de Janeiro (HRJ). O pacote estatístico FITOPAC 2.1. foi utilizado para a análise dos dados. A similaridade entre o remanescente investigado neste estudo e as outras quatorze áreas distintas do Rio de Janeiro, da própria Ilha Grande ou não, foi avaliada, utilizando-se o coeficiente de Similaridade de Sorensen; pelo critério de agrupamento por ligação média não ponderada (UPGMA) e pelo método de autorreamostragem para a estrutura de grupos; utilizados os programas PAST v1.34 e Multiv 2.4. A partir do levantamento em herbários e dos inventários florístico e fitossociológico realizados neste trabalho, foram analisados 3.470 registros, sendo 1.778 do levantamento de herbários, 1.536 do levantamento fitossociológico e 156 do inventário florístico. Esses registros corresponderam a 606 espécies ou morfo-espécies de Angiospermas e uma de Pteridófita. Os resultados obtidos revelaram a existência de 22 espécies ameaçadas de extinção para a Flora do Brasil. Dentre, as quais, sete são exclusivas da amostragem fitossociológica: Abarema cochliacarpos (Gomes) Barneby & J.W. Grimes, Chrysophyllum flexuosum Mart., Ficus pulchella Schott ex Spreng., Macrotorus utriculatus Perkins, Myrceugenia myrcioides (Cambess.) O.Berg, Rudgea interrupta Benth e Urbanodendron bahiense (Meisn.) Rohwer. No estudo fitossociológico, inventariou-se 1.536 indivíduos de 217 espécies, subordinadas a 53 famílias. O índice de diversidade de Shannon (H) calculado foi de 4,702 nats/ind e equabilidade (J) de 0,874. As 10 famílias com maior riqueza foram: Myrtaceae (31 spp.), Rubiaceae (21), Fabaceae (17), Lauraceae (12), Euphorbiaceae (11), Monimiaceae (8), Melastomataceae (7), Sapindaceae (7), Sapotaceae (6) e Annonaceae (6). Os 10 maiores Valores de Importância das espécies foram para Chrysophyllum flexuosum (3,43%), Lamanonia ternata Vell. (3,40%), Hyeronima alchorneoides Allemão (2,83%), Actinostemon verticillatus (Klotzsch) Baill. (2,55%), Psychotria brasiliensis Vell. (2,55%), Eriotheca pentaphylla (Vell.) A. Robyns (2,28%), Guatteria australis A. St.-Hil. (2,12%), Mabea brasiliensis Müll. Arg. (2,04%), Miconia prasina (Sw.) DC. (1,89%) e Rustia formosa (Cham. & Schltdl. ex DC.) Klotzsch (1,82%). Amostraram-se 27% de espécies representadas por apenas um indivíduo. As análises florísticas avaliadas a partir do Índice de Similaridade de Sorensen indicaram como principais variáveis para a formação dos blocos, os diferentes valores de diversidade para as áreas e a distribuição fitogeográfica das espécies. Os resultados obtidos junto aos dados dos grupos ecológicos, para os indivíduos da fitossociologia, indicaram maior percentual de indivíduos secundários tardios amostrados. Conclui-se que a área de estudo é uma floresta secundária em estágio intermediário de regeneração, com grande riqueza de espécies, muitas das quais de relevante importância ecológica.

Temperature dependence of dark lines in coherent population trapping

We measure the signal amplitude and linewidth of a dark line in coherent population trapping in the Rb vapour cell filled with mixed buffer gas N-2 and Ar as a function of cell temperature. We find that the dark line signal amplitude increases with temperature up to a maximum at 49 degrees C and then drops at higher temperatures due to quenching effects of N-2. The linewidth of the dark line remains basically constant, at 1080 Hz. We also measure the linewidth of the dark line as a function of laser intensity. The linewidth increases linearly with laser intensity. An intrinsic linewidth (FWHM=896 Hz at 3.4 GHz) of the Rb cell is obtained.

激光冷却铷原子喷泉钟的微波谐振腔设计

对自行研制的激光冷却铷原子喷泉钟的微波谐振腔进行了分析和设计，确定了需要的微波谐振腔基本参数。对影响微波谐振腔共振频率的因素进行了分析和研究，得到了共振频率随环境因素的变化规律。这些对调节微波腔共振频率和提高原子钟的准确度有重要意义。还对研制的微波谐振腔进行了测试，结果表明微波谐振腔的性能满足激光冷却铷原子喷泉钟的要求。由测试结果进一步估算了微波谐振腔引起的横向腔相移。

Abnormal phenomenon of ac Stark splitting in magneto-optical traps

We experimentally study the ac Stark splitting in D2 line of cold Rb-87 atoms. The frequency span between the Autler-Townes doublets is obviously larger than that derived from theoretical calculation. Two physical effects, which increase the effective Rabi frequency, contribute to the splitting broadening. First, atoms tend to distribute in strong lield places of a inhomogeneous red-detuned light field. Second, atoms reabsorb scattered light when they are huge in number and high in density.

A radio frequency field guide using field induced adiabatic potential

We study the behaviour of atoms in a field with both static magnetic field and radio frequency (rf) magnetic field. We calculate the adiabatic potential of atoms numerically beyond the usually rotating wave approximation, and it is pointed that there is a great difference between using these two methods. We find the preconditions when RWA is valid. In the extreme of static field almost parallel to rf field, we reach an analytic formula. Finally, we apply this method to Rb-87 and propose a guide based on an rf field on atom chip.

Coherent scattering of light by Bose-Einstein condensation in a tight confinement

We have observed strong scattering of a probe light by dilute Bose-Einstein condensate (BEC) Rb-87 gas in a tight magnetic trap. The scattering light forms fringes at the image plane. It is found that we can infer the real size of the condensation and the number of the atoms by modelling the imaging system. We present a quantitative calculation of light scattering by the condensed atoms. The calculation shows that the experimental results agree well with the prediction of the generalized diffraction theory, and thus we can directly observe the phase transition of BEC in a tight trap.

以慢原子束方式进行原子转移的双磁光阱系统

建立了一套用于玻色．爱因斯坦凝聚实验的铷原子双磁光阱装置．从低速强源中获得慢原子柬，向超高真空磁光阱进行原子转移．低速强源磁光阱与超高真空磁光阱之间可维持3个量级的压强差，超高真空磁光阱的真空度最高可达1×10^-9Pa．慢原子束的束流通量达1×10^9／s．约4×10^8个^17Rb原子被装载到超高真空磁光阱中．还讨论了两种典型情况下磁光阱中装载的最大原子数．

利用原子芯片上Z形磁阱囚禁中性^87Rb原子

利用解析和数值方法计算了Z形磁阱的囚禁势,发现当囚禁中心和芯片表面距离较远时（该距离和Z形线中部导线的一半长度相差不超过一个量级）,势阱的深度不能近似表示成偏置磁场By对应的能量,而要减去囚禁中心的势能高度;而增加By进行磁阱压缩到一定值时,势阱深度反而会下降.此外介绍了原子芯片的制作方法,以及利用原子芯片上Z形磁阱囚禁中性87Rb原子的实验装置和实验过程.最终有2×10^6个^87Rb原子被转移到Z形磁阱中.

Ultra-slow atomic beam generation by velocity selective resonance

We describe a method to generate an ultra-slow atomic beam by velocity selective resonance (VSR). A VSR experiment on a metastable helium beam in a magnetic field is presented and the results show that the transverse velocity of the defected beam can be cooled and precisely controlled to less than the recoil velocity, depending on the magnitude of the magnetic field. We extend this idea to a cold atomic cloud to produce an ultra-slow Rb-87 beam that can be used as a source of an atomic fountain clock or a space clock.

Laser cooling of rubidium atoms from background vapor in diffuse light

In this paper we describe an experiment on laser cooling of Rb-87 atoms directly from a vapor background in diffuse light. Diffuse light is produced in a ceramic integrating sphere by multiple scattering of two laser beams injected through multimode fibers. A probe beam, whose propagation direction is either horizontal or vertical, is used to detect cold atoms. We measured the absorption spectra of the cold atoms by scanning the frequency of the probe beam, and observed both the absorption signal and the time of flight signal after we switched off the cooling light, from which we estimated the temperature and the number of cold atoms. This method is clearly attractive for building a compact cold atom clock.

Nonlinear spectroscopy of cold atoms in diffuse laser light

The nonlinear spectroscopy of cold atoms in the diffuse laser cooling system is studied in this paper. We present the theoretical models of the recoil-induced resonances (RIR) and the electromagnetically-induced absorption (EIA) of cold atoms in diffuse laser light, and show their signals in an experiment of cooling Rb-87 atomic vapor in an integrating sphere. The theoretical results are in good agreement with the experimental ones when the light intensity distribution in the integrating sphere is considered. The differences between nonlinear spectra of cold atoms in the diffuse laser light and in the optical molasses are also discussed. (c) 2009 Optical Society of America