A GaAs polariton light-emitting diode operating near room temperature

The increasing ability to control light-matter interactions at the nanometre scale has improved the performance of semiconductor lasers in the past decade. The ultimate optimization is realized in semiconductor microcavities, in which strong coupling between quantum-well excitons and cavity photons gives rise to hybrid half-light/half-matter polariton quasiparticles. The unique properties of polaritons-such as stimulated scattering, parametric amplification, lasing, condensation and superfluidity-are believed to provide the basis for a new generation of polariton emitters and semiconductor lasers. Until now, polariton lasing and nonlinearities have only been demonstrated in optical experiments, which have shown the potential to reduce lasing thresholds by two orders of magnitude compared to conventional semiconductor lasers. Here we report an experimental realization of an electrically pumped semiconductor polariton light-emitting device, which emits directly from polariton states at a temperature of 235 K. Polariton electroluminescence data reveal characteristic anticrossing between exciton and cavity modes, a clear signature of the strong coupling regime. These findings represent a substantial step towards the realization of ultra-efficient polaritonic devices with unprecedented characteristics.     

中祁连地块东段元古宙基底湟源群沉积构造环境

通过对中祁连地块东段元古宙基底湟源群原岩建造及其地球化学特征的研究,认为：湟源群主要由各种千枚岩、片岩、变粒岩、大理岩、石英岩和斜长角闪岩等组成的中浅变质的表壳岩组合;其原岩为一套由陆源碎屑岩、不纯碳酸盐岩、中基性大山岩等组成的火山岩沉积岩组合;湟源群中变质沉积岩系中沉积韵律、沉积条带构造的广泛发育以及浊积层的存在等,指示了其原岩建造具有复理石或类复理石建造的特征;其中变质杂砂岩的地球化学特征类似于大陆岛弧或安底斯岛弧型杂砂岩的特征;湟源群变质中基性火山岩的地球化学特征类似于岛弧或活动大陆边缘的钙碱性火山岩的特征。经综合分析得出,湟源群沉积构造环境可能为陆缘弧后盆地且靠近大陆一侧。     

On the Construction of Mohr Stress Circle and a Graphic Method in Representing the Stress Components on Different Oblique Planes

The Mohr stress circle for three-dimensional stress is usually determined by equations,which is lack of intuitionistic meanings and difficult to understand. In this paper, the construction of Mohr stress circle is illustrated directly by numerical method. The shortcoming of Mohr stress circle in representing the stress components on different oblique planes for three-dimensional stress is analyzed. A three-dimensional figure is given to describe the variation of normal and shear stress on different oblique planes at one point for the first time.     

世界文化和不可知论的政见——一个致力于跨文化融洽交流的计划

就世界文化课程内容的具体设置,及联合大学与传媒以更好地实现课程积极目标等诸问题进行探讨.通过多层次的论证强调指出人们始终持有相异或相冲突的宗教、哲学以及意识形态,尚没有一种普世真理足以使世间所有人心悦诚服.即便是广泛接受和运用中的科学知识,其发展亦受阻于人类本身的局限而不可确定.尽管科学在前沿问题研究上竞争激烈,也仍然无法回答和消除所有的问题和差异.因此,从全球或泛人类立场观之,人类现有的知识与理解能力具有不确定和不完整的特点,而这样的特点浸透在所有的具体文化信仰之中.认识到共同局限性的人们--尤其是政界的领导人--亟需摒弃极端的文化分离思想,以更宽容理解的心态思考和行动,这样才能避免不同民族及不同国家间的不断冲突和互相伤害.     

聚甲基乙撑碳酸酯的交联(英文)

利用过氧化物过氧化二异丙苯(DCP)和多官能团化合物三烯丙基异氰尿酸酯(TAIC),对聚甲基乙撑碳酸酯(PPC)进行了交联研究。研究发现, DCP和TAIC的用量对PPC的交联均有较大影响, PPC交联后,其凝胶含量、机械性能、弹性模量、玻璃化转变温度均有所增加, 1%的DCP和4%的TAIC用量可得到较理想的交联PPC材料。     

微观相场计算D0_（22）-Ni_3V结构反位缺陷类型及演化

微观相场法研究Ni75AlxV25-x中D022-Ni3V反位缺陷类型及演化规律发现：D022结构中存在两种类型反位缺陷：VNi和NiV,其中NiV占位几率值远大于VNi,是反位缺陷主要类型;增加Al：V比,析出相由D022单相析出逐渐向D022＋L12两相析出转变,反位缺陷VNi与Al：V比无明显响应关系,NiV在第二相L12析出前后呈增大和减小两种截然相反的两种变化趋势;VNi和NiV两种反位缺陷同时具有时间相关性和温度相关性,时间推移,二者均从初始高度反位状态逐渐降低至平衡,温度提高,二者均呈增加趋势;第三组元Al原子同时占据D022结构的α位和β位,且AlNi〉AlV,Al优先占据D022结构α位.     

Competition for antigen presentation in living cells involves exchange of peptides bound by class II MHC molecules

T cells recognize foreign proteins as peptides bound to self molecules encoded by the major histocompatibility complex (MHC). The kinetics of interaction between purified class II MHC molecules and peptides is unusual, in that the rate of association is very slow, but once formed, the complexes are extremely stable. This raises the question of how the antigen-presenting cell provides a sufficient number of free MHC binding sites to ensure T cell immunity. We present results suggesting that an exchange of peptide in MHC binding sites may take place under physiological conditions.     

Three-dimensional X-ray structural microscopy with submicrometre resolution

Advanced materials and processing techniques are based largely on the generation and control of non-homogeneous microstructures, such as precipitates and grain boundaries. X-ray tomography can provide three-dimensional density and chemical distributions of such structures with submicrometre resolution; structural methods exist that give submicrometre resolution in two dimensions; and techniques are available for obtaining grain-centroid positions and grain-average strains in three dimensions. But non-destructive point-to-point three-dimensional structural probes have not hitherto been available for investigations at the critical mesoscopic length scales (tenths to hundreds of micrometres). As a result, investigations of three-dimensional mesoscale phenomena--such as grain growth, deformation, crumpling and strain-gradient effects--rely increasingly on computation and modelling without direct experimental input. Here we describe a three-dimensional X-ray microscopy technique that uses polychromatic synchrotron X-ray microbeams to probe local crystal structure, orientation and strain tensors with submicrometre spatial resolution. We demonstrate the utility of this approach with micrometre-resolution three-dimensional measurements of grain orientations and sizes in polycrystalline aluminium, and with micrometre depth-resolved measurements of elastic strain tensors in cylindrically bent silicon. This technique is applicable to single-crystal, polycrystalline, composite and functionally graded materials.     

Insights into SCF ubiquitin ligases from the structure of the Skp1-Skp2 complex

F-box proteins are members of a large family that regulates the cell cycle, the immune response, signalling cascades and developmental programmes by targeting proteins, such as cyclins, cyclin-dependent kinase inhibitors, IkappaBalpha and beta-catenin, for ubiquitination (reviewed in refs 1-3). F-box proteins are the substrate-recognition components of SCF (Skp1-Cullin-F-box protein) ubiquitin-protein ligases. They bind the SCF constant catalytic core by means of the F-box motif interacting with Skp1, and they bind substrates through their variable protein-protein interaction domains. The large number of F-box proteins is thought to allow ubiquitination of numerous, diverse substrates. Most organisms have several Skp1 family members, but the function of these Skp1 homologues and the rules of recognition between different F-box and Skp1 proteins remain unknown. Here we describe the crystal structure of the human F-box protein Skp2 bound to Skp1. Skp1 recruits the F-box protein through a bipartite interface involving both the F-box and the substrate-recognition domain. The structure raises the possibility that different Skp1 family members evolved to function with different subsets of F-box proteins, and suggests that the F-box protein may not only recruit substrate, but may also position it optimally for the ubiquitination reaction.