Observation of surface and bulk phase transitions in nematic liquid crystals

The behaviour of liquid crystal (LC) molecules near a surface is of both fundamental and technological interest: it gives rise to various surface phase-transition and wetting phenomena, and surface-induced ordering of the LC molecules is integral to the operation of LC displays. Here we report the observation of a pure isotropic-nematic (IN) surface phase transition-clearly separated from the bulk IN transition-in a nematic LC on a substrate. Differences in phase behaviour between surface and bulk are expected, but have hitherto proved difficult to distinguish, owing in part to the close proximity of their transition temperatures. We have overcome these difficulties by using a mixture of nematic LCs: small, surface-induced composition variations lead to complete separation of the surface and bulk transitions, which we then study independently as a function of substrate and applied magnetic field. We find the surface IN transition to be of first order on surfaces with a weak anchoring energy and continuous on surfaces with a strong anchoring. We show that the presence of high magnetic fields does not change the surface IN transition temperature, whereas the bulk IN transition temperature increases with field. We attribute this to the interaction energy between the surface and bulk phases, which is tuned by magnetic-field-induced order in the surface-wetting layer.     

Origin of ferromagnetic exchange interactions in a fullerene-organic compound

Organic ferromagnets, which exhibit exchange interactions between unpaired electrons in pi-orbitals, are rare, and the origin of ferromagnetism in these compounds has so far remained unexplained. Tetrakis(dimethylamino)ethylene-fullerene[60] (TDAE-C60) shows a transition to a ferromagnetic state with fully saturated s = 1/2 molecular spins at the relatively high Curie temperature (for organic materials) of 16 K (ref. 4). It has been suggested that the orientations of the C60 molecules may be important for ferromagnetism in this material, but in the absence of structural data at low temperatures there has been little progress towards understanding these microscopic interactions. Here we report the results of a comparative structural study of two different magnetic forms of TDAE-C60 crystals at low temperatures, correlating the structural properties--in particular, the intermolecular orientations--with the magnetic properties. We find that both ferromagnetism and spin-glass-like ordering are possible in this material, and depend on the orientational state of C60 molecules. This resolves the apparent contradictions posed by different macroscopic measurements, and opens the way to a microscopic understanding of pi-electron ferromagnetic exchange interactions in organic materials.     

Liquid-crystal-mediated self-assembly at nanodroplet interfaces

Technological applications of liquid crystals have generally relied on control of molecular orientation at a surface or an interface. Such control has been achieved through topography, chemistry and the adsorption of monolayers or surfactants. The role of the substrate or interface has been to impart order over visible length scales and to confine the liquid crystal in a device. Here, we report results from a computational study of a liquid-crystal-based system in which the opposite is true: the liquid crystal is used to impart order on the interfacial arrangement of a surfactant. Recent experiments on macroscopic interfaces have hinted that an interfacial coupling between bulk liquid crystal and surfactant can lead to a two-dimensional phase separation of the surfactant at the interface, but have not had the resolution to measure the structure of the resulting phases. To enhance that coupling, we consider the limit of nanodroplets, the interfaces of which are decorated with surfactant molecules that promote local perpendicular orientation of mesogens within the droplet. In the absence of surfactant, mesogens at the interface are all parallel to that interface. As the droplet is cooled, the mesogens undergo a transition from a disordered (isotropic) to an ordered (nematic or smectic) liquid-crystal phase. As this happens, mesogens within the droplet cause a transition of the surfactant at the interface, which forms new ordered nanophases with morphologies dependent on surfactant concentration. Such nanophases are reminiscent of those encountered in block copolymers, and include circular, striped and worm-like patterns.     

Quantum superposition of distinct macroscopic states

In 1935, Schrodinger attempted to demonstrate the limitations of quantum mechanics using a thought experiment in which a cat is put in a quantum superposition of alive and dead states. The idea remained an academic curiosity until the 1980s when it was proposed that, under suitable conditions, a macroscopic object with many microscopic degrees of freedom could behave quantum mechanically, provided that it was sufficiently decoupled from its environment. Although much progress has been made in demonstrating the macroscopic quantum behaviour of various systems such as superconductors, nanoscale magnets, laser-cooled trapped ions, photons in a microwave cavity and C60 molecules, there has been no experimental demonstration of a quantum superposition of truly macroscopically distinct states. Here we present experimental evidence that a superconducting quantum interference device (SQUID) can be put into a superposition of two magnetic-flux states: one corresponding to a few microamperes of current flowing clockwise, the other corresponding to the same amount of current flowing anticlockwise.     

Detection of geometric phases in superconducting nanocircuits

When a quantum-mechanical system undergoes an adiabatic cyclic evolution, it acquires a geometrical phase factor' in addition to the dynamical one; this effect has been demonstrated in a variety of microscopic systems. Advances in nanotechnology should enable the laws of quantum dynamics to be tested at the macroscopic level, by providing controllable artificial two-level systems (for example, in quantum dots and superconducting devices). Here we propose an experimental method to detect geometric phases in a superconducting device. The setup is a Josephson junction nanocircuit consisting of a superconducting electron box. We discuss how interferometry based on geometrical phases may be realized, and show how the effect may be applied to the design of gates for quantum computation.     

煤炭自燃模拟实验装置设计与研制

设计与研制煤炭自燃模拟装置,该装置能够模拟和研究煤炭在常温条件下自燃的发生和发展过程、发生条件及其影响因素、自燃过程煤炭微观和宏观参数变化,研究自燃火源形成及其分布规律。     

Solitons and production of defects in flow-aligning nematic liquid crystals under simple shear flow

The production of defects in flow-aligning nematic liquid crystals under simple shear flow is analyzed by linear stability analysis based on Leslie-Ericksen theory. It is pointed out that the equation of motion of the nematic director under simple shear flow conforms to the driven over-damped sine-Gordon equation and has a soliton solution of amplitude π. It has also been shown that the stationary state with the director uniformly oriented at a Leslie angle is only a metastable state and that the potential, which governs the motion of the director, has infinite numbers of stable stationary states. Therefore, the defects, appearing as a stable solitary solution, can be nucleated from a uniformly aligned flow-aligning type of nematic liquid crystal by shear flow. On the other hand, the bands with long axis parallel to the vorticity axis, appearing as an unstable solution, can be observed as transient patterns at low shear rate and low shear strain value. The theoretical predictions are compared with previous experimental observations.     

单轴压强技术在超导研究中的应用

近年来,随着相关技术的快速发展,单轴应变或单轴压强技术在研究非常规超导体中被广泛地应用.在单轴压强或单轴应变研究中,样品沿着某个晶轴的方向发生应变,其物理性质也会发生相应的改变.通过研究这种改变,就可能获得超导或其他序的重要信息.和静水压相比,单轴压强对物理性质的改变主要和对称性有关,这使得单轴压强和单轴应变的研究特别适用于与对称性相关联的物理性质,例如,与旋转对称性相关的电子向列相.更重要的是,单轴压强技术也能够用于超导电性性质的研究.本文将介绍常用的单轴压强和单轴应变装置和技术,并介绍其在铁基超导体和其他超导体中的一些典型应用.表明单轴压强和单轴应变技术在研究电子向列相、超导序参量和其他一些竞争序方面具有独特的优势,并将随着相关技术的发展在超导及其他强关联电子体系研究中发挥更重要的作用.     

Imaging the vortex-lattice melting process in the presence of disorder

General arguments suggest that first-order phase transitions become less sharp in the presence of weak disorder, while extensive disorder can transform them into second-order transitions; but the atomic level details of this process are not clear. The vortex lattice in superconductors provides a unique system in which to study the first-order transition on an inter-particle scale, as well as over a wide range of particle densities. Here we use a differential magneto-optical technique to obtain direct experimental visualization of the melting process in a disordered superconductor. The images reveal complex behaviour in nucleation, pattern formation, and solid-liquid interface coarsening and pinning. Although the local melting is found to be first-order, a global rounding of the transition is observed; this results from a disorder-induced broad distribution of local melting temperatures, at scales down to the mesoscopic level. We also resolve local hysteretic supercooling of microscopic liquid domains, a non-equilibrium process that occurs only at selected sites where the disorder-modified melting temperature has a local maximum. By revealing the nucleation process, we are able to experimentally evaluate the solid-liquid surface tension, which we find to be extremely small.     

Pixel switching of epitaxial Pd/YHx/CaF2 switchable mirrors

Exposure of rare-earth films to hydrogen can induce a metal-insulator transition, accompanied by pronounced optical changes. This 'switchable mirror' effect has received considerable attention from theoretical, experimental and technological points of view. Most systems use polycrystalline films, but the synthesis of yttrium-based epitaxial switchable mirrors has also been reported. The latter form an extended self-organized ridge network during initial hydrogen loading, which results in the creation of micrometre-sized triangular domains. Here we observe homogeneous and essentially independent optical switching of individual domains in epitaxial switchable mirrors during hydrogen absorption. The optical switching is accompanied by topographical changes as the domains sequentially expand and contract; the ridges block lateral hydrogen diffusion and serve as a microscopic lubricant for the domain oscillations. We observe the correlated changes in topology and optical properties using in situ atomic force and optical microscopy. Single-domain phase switching is not observed in polycrystalline films, which are optically homogeneous. The ability to generate a tunable, dense pattern of switchable pixels is of technological relevance for solid-state displays based on switchable mirrors.     

稠油SAGD开发中受效区的判识方法及应用——以辽河西部凹陷曙一区馆陶油藏为例

辽河稠油开发建设历经四次技术创新,形成了以蒸汽吞吐为主,蒸汽驱、蒸汽辅助重力泄油(SAGD)有效接替,火驱起步探索的开发格局。截止到目前,热采稠油已逾1.65亿吨,为辽河油田千万吨稳产做出重要贡献。SAGD技术作为方式转换核心技术之一,扮演着非常重要的角色。随着SAGD开发的不断深入,受效区的识别以及未受效区即剩余油潜力区的挖掘显得尤为重要。因此,本文以辽河西部凹陷S区馆陶油藏为例,在大量的岩心宏观特征资料、岩心化验分析资料、高密度井网资料以及开发动态监测资料等的基础上,先从岩心宏观特征、微观特征、电性特征和开发动态监测特征等方面多手段定性判别受效区,再从矿物或流体的类型、成分及含量的变化等这些最直接的证据入手,建立受效区判别标准,定量识别受效区,最后依据判别标准识别受效区,确定研究区未受效区的类型主要包含两种：“原生”非优势渗流区和“次生”非优势渗流区。针对不同类型的未受效区即剩余油潜力区制定低物性段射孔、压裂改造和改变注入介质等调控措施,来有效动用蒸汽腔未波及区域的储量,从而对类似区块开发具有重要的指导意义。     

The structures of HsIU and the ATP-dependent protease HsIU-HsIV

The degradation of cytoplasmic proteins is an ATP-dependent process. Substrates are targeted to a single soluble protease, the 26S proteasome, in eukaryotes and to a number of unrelated proteases in prokaryotes. A surprising link emerged with the discovery of the ATP-dependent protease HslVU (heat shock locus VU) in Escherichia coli. Its protease component HslV shares approximately 20% sequence similarity and a conserved fold with 20S proteasome beta-subunits. HslU is a member of the Hsp100 (Clp) family of ATPases. Here we report the crystal structures of free HslU and an 820,000 relative molecular mass complex of HslU and HslV-the first structure of a complete set of components of an ATP-dependent protease. HslV and HslU display sixfold symmetry, ruling out mechanisms of protease activation that require a symmetry mismatch between the two components. Instead, there is conformational flexibility and domain motion in HslU and a localized order-disorder transition in HslV. Individual subunits of HslU contain two globular domains in relative orientations that correlate with nucleotide bound and unbound states. They are surprisingly similar to their counterparts in N-ethylmaleimide-sensitive fusion protein, the prototype of an AAA-ATPase. A third, mostly alpha-helical domain in HslU mediates the contact with HslV and may be the structural equivalent of the amino-terminal domains in proteasomal AAA-ATPases.     

Spontaneous macroscopic magnetization at the superconducting transition temperature of YBa2Cu3O(7-delta)

A noteworthy feature of the high-temperature superconductors is the unconventional symmetry of the superconducting order parameter. Several experiments have established that the order parameter has a four-fold d(x2 - y2) symmetry under rotation of the lattice (the order parameter of conventional superconductors is, in contrast, isotropic). An intriguing and much debated possibility is that, in certain cases, an additional imaginary component might be present, having an isotropic s-wave or d(xy) symmetry. A consequence of a complex order parameter of the form d(x2 - y2) + id(xy) is that it would break both reflection (parity, P) symmetry and time-reversal (T) symmetry, a clear signature of which would be the spontaneous appearance of a macroscopic magnetization at the superconducting transition temperature. Broken T symmetry has been reported, but searches for the effects of combined P and T symmetry breaking have so far yielded null results. Here we report the observation of a weak (approximately 10(-5) gauss) magnetic field that appears spontaneously at the superconducting transition temperature of epitaxial thin films of YBa2Cu3O(7-delta). The magnetic signal originates near the edges of the samples. One interpretation for this observation is that the order parameter carries an intrinsic angular momentum, related to the breaking of P and T symmetries, but other possibilities cannot yet be excluded.     

Bose-Einstein condensation on a microelectronic chip

Although Bose-Einstein condensates of ultracold atoms have been experimentally realizable for several years, their formation and manipulation still impose considerable technical challenges. An all-optical technique that enables faster production of Bose-Einstein condensates was recently reported. Here we demonstrate that the formation of a condensate can be greatly simplified using a microscopic magnetic trap on a chip. We achieve Bose-Einstein condensation inside the single vapour cell of a magneto-optical trap in as little as 700 ms-more than a factor of ten faster than typical experiments, and a factor of three faster than the all-optical technique. A coherent matter wave is emitted normal to the chip surface when the trapped atoms are released into free fall; alternatively, we couple the condensate into an 'atomic conveyor belt', which is used to transport the condensed cloud non-destructively over a macroscopic distance parallel to the chip surface. The possibility of manipulating laser-like coherent matter waves with such an integrated atom-optical system holds promise for applications in interferometry, holography, microscopy, atom lithography and quantum information processing.     

Electronic nematicity above the structural and superconducting transition in BaFe2(As(1-x)P(x))2

Electronic nematicity, a unidirectional self-organized state that breaks the rotational symmetry of the underlying lattice, has been observed in the iron pnictide and copper oxide high-temperature superconductors. Whether nematicity plays an equally important role in these two systems is highly controversial. In iron pnictides, the nematicity has usually been associated with the tetragonal-to-orthorhombic structural transition at temperature T(s). Although recent experiments have provided hints of nematicity, they were performed either in the low-temperature orthorhombic phase or in the tetragonal phase under uniaxial strain, both of which break the 90° rotational C(4) symmetry. Therefore, the question remains open whether the nematicity can exist above T(s) without an external driving force. Here we report magnetic torque measurements of the isovalent-doping system BaFe(2)(As(1-x)P(x))(2), showing that the nematicity develops well above T(s) and, moreover, persists to the non-magnetic superconducting regime, resulting in a phase diagram similar to the pseudogap phase diagram of the copper oxides. By combining these results with synchrotron X-ray measurements, we identify two distinct temperatures-one at T*, signifying a true nematic transition, and the other at T(s) (相似文献   

Extended surface chirality from supramolecular assemblies of adsorbed chiral molecules

The increasing demand of the chemical and pharmaceutical industries for enantiomerically pure compounds has spurred the development of a range of so-called 'chiral technologies' (ref. 1), which aim to exert the ultimate control over a chemical reaction by directing its enantioselectivity. Heterogeneous enantioselective catalysis is particularly attractive because it allows the production and ready separation of large quantities of chiral product while using only small quantities of catalyst. Heterogeneous enantioselectivity is usually induced by adsorbing chiral molecules onto catalytically active surfaces. A mimic of one such catalyst is formed by adsorbing (R,R)-tartaric acid molecules on Cu(110) surfaces: this generates a variety of surface phases, of which only one is potentially catalytically active, and leaves the question of how adsorbed chiral molecules give rise to enantioselectivity. Here we show that the active phase consists of extended supramolecular assemblies of adsorbed (R,R)-tartaric acid, which destroy existing symmetry elements of the underlying metal and directly bestow chirality to the modified surface. The adsorbed assemblies create chiral 'channels' exposing bare metal atoms, and it is these chiral spaces that we believe to be responsible for imparting enantioselectivity, by forcing the orientation of reactant molecules docking onto catalytically active metal sites. Our findings demonstrate that it is possible to sustain a single chiral domain across an extended surface--provided that reflection domains of opposite handedness are removed by a rigid and chiral local adsorption geometry, and that inequivalent rotation domains are removed by successful matching of the rotational symmetry of the adsorbed molecule with that of the underlying metal surface.     

Crystal symmetry and the reversibility of martensitic transformations

Martensitic transformations are diffusionless, solid-to-solid phase transitions, and have been observed in metals, alloys, ceramics and proteins. They are characterized by a rapid change of crystal structure, accompanied by the development of a rich microstructure. Martensitic transformations can be irreversible, as seen in steels upon quenching, or they can be reversible, such as those observed in shape-memory alloys. In the latter case, the microstructures formed on cooling are easily manipulated by loads and disappear upon reheating. Here, using mathematical theory and numerical simulation, we explain these sharp differences in behaviour on the basis of the change in crystal symmetry during the transition. We find that a necessary condition for reversibility is that the symmetry groups of the parent and product phases be included in a common finite symmetry group. In these cases, the energy barrier to lattice-invariant shear is generically higher than that pertaining to the phase change and, consequently, transformations of this type can occur with virtually no plasticity. Irreversibility is inevitable in all other martensitic transformations, where the energy barrier to plastic deformation (via lattice-invariant shears, as in twinning or slip) is no higher than the barrier to the phase change itself. Various experimental observations confirm the importance of the symmetry of the stable states in determining the macroscopic reversibility of martensitic transformations.     

Stochastic simulation of chemical chaos

Conclusion Our stochastic simulations of Willamowski-Rossler indicated that the macroscopic rate equations of chemical kinetics can satisfactorily describe the microscopic reaction processes in the vicinity of nonequilibrium stable steady states and limit cycles. However, in the presence of chaos, the chemical kinetic rate equations cease to be useful even though they still serve to tell when they become invalid. From the viewpoint of stochastic theory, a chemical reaction process can be well described by the Master equation, and the usual macroscopic rate equation is only its first-order approximation when the system is enlarged to infinite^[5]. When the system approaches one of its bifurcation points, the formerly trivial fluctuations can now grow to macroscopic size and make the macroscopic variables lose their meanings. The simulation of a chemical chaos of this report has shown that the time evolution of averaged molecular numbers of direct simulation has nothing in common with trajectories predicted by rate equations. Therefore the first-order approximation of the Master equation is no longer sufficient to describe the true reaction processes when the rate equations display chaotic dynamics: These suggest that a microscopic or mesoscopic approach is necessary at this point. Further investigation toward the microscopic aspect of a reaction when its chemical rate law predicts chaos is always desired.     

Observation of five-fold local symmetry in liquid lead

The local point symmetry of the short-range order in simple monatomic liquids remains a fundamental open question in condensed-matter science. For more than 40 years it has been conjectured that liquids with centrosymmetric interactions may be composed of icosahedral building blocks. But these proposed mobile, randomly orientated structures have remained experimentally inaccessible owing to the unavoidable averaging involved in scattering experiments, which can therefore determine only the isotropic radial distribution function. Here we overcome this limitation by capturing liquid fragments at a solid-liquid interface, and observing the scattering of totally internally reflected (evanescent) X-rays, which are sensitive only to the liquid structure at the interface. Using this method, we observe five-fold local symmetry in liquid lead adjacent to a silicon wall, and obtain an experimental portrait of the icosahedral fragments that are predicted to occur in all close-packed monatomic liquids. By shedding new light on local bond order in disordered structures such as liquids and glasses, these results should lead to a better microscopic understanding of melting, freezing and supercooling.     

A complementarity experiment with an interferometer at the quantum-classical boundary

To illustrate the quantum mechanical principle of complementarity, Bohr described an interferometer with a microscopic slit that records the particle's path. Recoil of the quantum slit causes it to become entangled with the particle, resulting in a kind of Einstein-Podolsky-Rosen pair. As the motion of the slit can be observed, the ambiguity of the particle's trajectory is lifted, suppressing interference effects. In contrast, the state of a sufficiently massive slit does not depend on the particle's path; hence, interference fringes are visible. Although many experiments illustrating various aspects of complementarity have been proposed and realized, none has addressed the quantum-classical limit in the design of the interferometer. Here we report an experimental investigation of complementarity using an interferometer in which the properties of one of the beam-splitting elements can be tuned continuously from being effectively microscopic to macroscopic. Following a recent proposal, we use an atomic double-pulse Ramsey interferometer, in which microwave pulses act as beam-splitters for the quantum states of the atoms. One of the pulses is a coherent field stored in a cavity, comprising a small, adjustable mean photon number. The visibility of the interference fringes in the final atomic state probability increases with this photon number, illustrating the quantum to classical transition.