Direct observation of attosecond light bunching

Temporal probing of a number of fundamental dynamical processes requires intense pulses at femtosecond or even attosecond (1 as = 10(-18) s) timescales. A frequency 'comb' of extreme-ultraviolet odd harmonics can easily be generated in the interaction of subpicosecond laser pulses with rare gases: if the spectral components within this comb possess an appropriate phase relationship to one another, their Fourier synthesis results in an attosecond pulse train. Laser pulses spanning many optical cycles have been used for the production of such light bunching, but in the limit of few-cycle pulses the same process produces isolated attosecond bursts. If these bursts are intense enough to induce a nonlinear process in a target system, they can be used for subfemtosecond pump-probe studies of ultrafast processes. To date, all methods for the quantitative investigation of attosecond light localization and ultrafast dynamics rely on modelling of the cross-correlation process between the extreme-ultraviolet pulses and the fundamental laser field used in their generation. Here we report the direct determination of the temporal characteristics of pulses in the subfemtosecond regime, by measuring the second-order autocorrelation trace of a train of attosecond pulses. The method exhibits distinct capabilities for the characterization and utilization of attosecond pulses for a host of applications in attoscience.     

Direct observation of microtubule dynamics in living cells

The study of cell locomotion is fundamental to such diverse processes as embryonic development, wound healing and metastasis. Since microtubules play a role in establishing the leading lamellum and maintaining cell polarity, it is important to understand their dynamic behaviour. In vitro, subunits exchange with polymer by treadmilling and by dynamic instability. Disassembly events can be complete (catastrophic) or incomplete (tempered). In vivo, microtubules are in dynamic equilibrium with subunits with a half-time for turnover of 4-20 min. Microtubules grow by elongation of their ends and are replaced one by one with turnover being most rapid at the periphery. Although previous results are consistent with dynamic instability, we sought to directly test the mechanism of turnover. Direct observations of fluorescent microtubules in the fibroblast lamellum show that individual microtubules undergo rounds of assembly and disassembly from the same end. Reorganization of the microtubule network occurs by a tempered mode of dynamic instability.     

Probing molecular dynamics with attosecond resolution using correlated wave packet pairs

Spectroscopic measurements with increasingly higher time resolution are generally thought to require increasingly shorter laser pulses, as illustrated by the recent monitoring of the decay of core-excited krypton using attosecond photon pulses. However, an alternative approach to probing ultrafast dynamic processes might be provided by entanglement, which has improved the precision of quantum optical measurements. Here we use this approach to observe the motion of a D2+ vibrational wave packet formed during the multiphoton ionization of D2 over several femtoseconds with a precision of about 200 attoseconds and 0.05 ?ngstr?ms, by exploiting the correlation between the electronic and nuclear wave packets formed during the ionization event. An intense infrared laser field drives the electron wave packet, and electron recollision probes the nuclear motion. Our results show that laser pulse duration need not limit the time resolution of a spectroscopic measurement, provided the process studied involves the formation of correlated wave packets, one of which can be controlled; spatial resolution is likewise not limited to the focal spot size or laser wavelength.     

Direct measurement of antiferromagnetic domain fluctuations

Measurements of magnetic noise emanating from ferromagnets owing to domain motion were first carried out nearly 100 years ago, and have underpinned much science and technology. Antiferromagnets, which carry no net external magnetic dipole moment, yet have a periodic arrangement of the electron spins extending over macroscopic distances, should also display magnetic noise. However, this must be sampled at spatial wavelengths of the order of several interatomic spacings, rather than the macroscopic scales characteristic of ferromagnets. Here we present a direct measurement of the fluctuations in the nanometre-scale superstructure of spin- and charge-density waves associated with antiferromagnetism in elemental chromium. The technique used is X-ray photon correlation spectroscopy, where coherent X-ray diffraction produces a speckle pattern that serves as a 'fingerprint' of a particular magnetic domain configuration. The temporal evolution of the patterns corresponds to domain walls advancing and retreating over micrometre distances. This work demonstrates a useful measurement tool for antiferromagnetic domain wall engineering, but also reveals a fundamental finding about spin dynamics in the simplest antiferromagnet: although the domain wall motion is thermally activated at temperatures above 100 K, it is not so at lower temperatures, and indeed has a rate that saturates at a finite value-consistent with quantum fluctuations-on cooling below 40 K.     

Direct observation of second-order atom tunnelling

Tunnelling of material particles through a classically impenetrable barrier constitutes one of the hallmark effects of quantum physics. When interactions between the particles compete with their mobility through a tunnel junction, intriguing dynamical behaviour can arise because the particles do not tunnel independently. In single-electron or Bloch transistors, for example, the tunnelling of an electron or Cooper pair can be enabled or suppressed by the presence of a second charge carrier due to Coulomb blockade. Here we report direct, time-resolved observations of the correlated tunnelling of two interacting ultracold atoms through a barrier in a double-well potential. For the regime in which the interactions between the atoms are weak and tunnel coupling dominates, individual atoms can tunnel independently, similar to the case of a normal Josephson junction. However, when strong repulsive interactions are present, two atoms located on one side of the barrier cannot separate, but are observed to tunnel together as a pair in a second-order co-tunnelling process. By recording both the atom position and phase coherence over time, we fully characterize the tunnelling process for a single atom as well as the correlated dynamics of a pair of atoms for weak and strong interactions. In addition, we identify a conditional tunnelling regime in which a single atom can only tunnel in the presence of a second particle, acting as a single atom switch. Such second-order tunnelling events, which are the dominating dynamical effect in the strongly interacting regime, have not been previously observed with ultracold atoms. Similar second-order processes form the basis of superexchange interactions between atoms on neighbouring lattice sites of a periodic potential, a central component of proposals for realizing quantum magnetism.     

离散空间直接建模的计算流体力学方法

本文提出了一种发展适合多尺度、多物理流动的CFD格式的全新途径,即直接在离散空间利用物理模型的跨尺度演化解来建立数值格式的新方法.与直接离散偏微分方程的传统做法相比,基于离散空间直接建模构造出的数值方法考虑了网格尺度和物理模型之间的匹配,能做到不同尺度上物理模型的连续过渡,从而实现对多尺度流动的高效模拟.Boltzmann方程的模型演化解为直接建模方法提供了重要的支撑.本文提出了直接建模构造计算流体力学方法的理论基础,并给出了在建立统一气体动理学格式中的成功运用,验证了新方法的可行性和优越性.     

Direct observation of steps in rotation of the bacterial flagellar motor

The bacterial flagellar motor is a rotary molecular machine that rotates the helical filaments that propel many species of swimming bacteria. The rotor is a set of rings up to 45 nm in diameter in the cytoplasmic membrane; the stator contains about ten torque-generating units anchored to the cell wall at the perimeter of the rotor. The free-energy source for the motor is an inward-directed electrochemical gradient of ions across the cytoplasmic membrane, the protonmotive force or sodium-motive force for H+-driven and Na+-driven motors, respectively. Here we demonstrate a stepping motion of a Na+-driven chimaeric flagellar motor in Escherichia coli at low sodium-motive force and with controlled expression of a small number of torque-generating units. We observe 26 steps per revolution, which is consistent with the periodicity of the ring of FliG protein, the proposed site of torque generation on the rotor. Backwards steps despite the absence of the flagellar switching protein CheY indicate a small change in free energy per step, similar to that of a single ion transit.     

Attosecond real-time observation of electron tunnelling in atoms

Atoms exposed to intense light lose one or more electrons and become ions. In strong fields, the process is predicted to occur via tunnelling through the binding potential that is suppressed by the light field near the peaks of its oscillations. Here we report the real-time observation of this most elementary step in strong-field interactions: light-induced electron tunnelling. The process is found to deplete atomic bound states in sharp steps lasting several hundred attoseconds. This suggests a new technique, attosecond tunnelling, for probing short-lived, transient states of atoms or molecules with high temporal resolution. The utility of attosecond tunnelling is demonstrated by capturing multi-electron excitation (shake-up) and relaxation (cascaded Auger decay) processes with subfemtosecond resolution.     

Direct measurement of hole transport dynamics in DNA

Our understanding of oxidative damage to double helical DNA and the design of DNA-based devices for molecular electronics is crucially dependent upon elucidation of the mechanism and dynamics of electron and hole transport in DNA. Electrons and holes can migrate from the locus of formation to trap sites, and such migration can occur through either a single-step "superexchange" mechanism or a multistep charge transport "hopping" mechanism. The rates of single-step charge separation and charge recombination processes are found to decrease rapidly with increasing transfer distances, whereas multistep hole transport processes are only weakly distance dependent. However, the dynamics of hole transport has not yet been directly determined. Here we report spectroscopic measurements of photoinduced electron transfer in synthetic DNA that yield rate constants of approximately 5 x 10(7) s(-1) and 5 x 10(6) s(-1), respectively, for the forward and return hole transport from a single guanine base to a double guanine base step across a single adenine. These rates are faster than processes leading to strand cleavage, such as the reaction of guanine cation radical with water, thus permitting holes to migrate over long distances in DNA. However, they are too slow to compete with charge recombination in contact ion pairs, a process which protects DNA from photochemical damage.     

Direct observation of molecular cooperativity near the glass transition

The increasingly sluggish response of a supercooled liquid as it nears its glass transition (for example, refrigerated honey) is prototypical of glassy dynamics found in proteins, neural networks and superconductors. The notion that molecules rearrange cooperatively has long been postulated to explain diverging relaxation times and broadened (non-exponential) response functions near the glass transition. Recently, cooperativity was observed and analysed in colloid glasses and in simulations of binary liquids well above the glass transition. But nanometre-scale studies of cooperativity at the molecular glass transition are lacking. Important issues to be resolved include the precise form of the cooperativity and its length scale, and whether the broadened response is intrinsic to individual cooperative regions, or arises only from heterogeneity in an ensemble of such regions. Here we describe direct observations of molecular cooperativity near the glass transition in polyvinylacetate (PVAc), using nanometre-scale probing of dielectric fluctuations. Molecular clusters switched spontaneously among two to four distinct configurations, producing random telegraph noise. Our analysis of these noise signals and their power spectra reveals that individual clusters exhibit transient dynamical heterogeneity and non-exponential kinetics.     

Direct observation of the mechanochemical coupling in myosin Va during processive movement

Myosin Va transports intracellular cargoes along actin filaments in cells. This processive, two-headed motor takes multiple 36-nm steps in which the two heads swing forward alternately towards the barbed end of actin driven by ATP hydrolysis. The ability of myosin Va to move processively is a function of its long lever arm, the high duty ratio of its kinetic cycle and the gating of the kinetics between the two heads such that ADP release from the lead head is greatly retarded. Mechanical studies at the multiple- and the single-molecule level suggest that there is tight coupling (that is, one ATP is hydrolysed per power stroke), but this has not been directly demonstrated. We therefore investigated the coordination between the ATPase mechanism of the two heads of myosin Va and directly visualized the binding and dissociation of single fluorescently labelled nucleotide molecules, while simultaneously observing the stepping motion of the fluorescently labelled myosin Va as it moved along an actin filament. Here we show that preferential ADP dissociation from the trail head of mouse myosin Va is followed by ATP binding and a synchronous 36-nm step. Even at low ATP concentrations, the myosin Va molecule retained at least one nucleotide (ADP in the lead head position) when moving. Thus, we directly demonstrate tight coupling between myosin Va movement and the binding and dissociation of nucleotide by simultaneously imaging with near nanometre precision.     

Direct observation of motion of single F-actin filaments in the presence of myosin

Actin is found in almost all kinds of non-muscle cells where it is thought to have an important role in cell motility. A proper understanding of that role will only be possible when reliable in vitro systems are available for investigating the interaction of cellular actin and myosin. A start has been made on several systems, most recently by Sheetz and Spudich who demonstrated unidirectional movement of HMM-coated beads along F-actin cables on arrays of chloroplasts exposed by dissection of a Nitella cell. As an alternative approach, we report here the direct observation by fluorescence microscopy of the movements of single F-actin filaments interacting with soluble myosin fragments energized by Mg2+-ATP.     

Direct observation of catch bonds involving cell-adhesion molecules

Bonds between adhesion molecules are often mechanically stressed. A striking example is the tensile force applied to selectin-ligand bonds, which mediate the tethering and rolling of flowing leukocytes on vascular surfaces. It has been suggested that force could either shorten bond lifetimes, because work done by the force could lower the energy barrier between the bound and free states ('slip'), or prolong bond lifetimes by deforming the molecules such that they lock more tightly ('catch'). Whereas slip bonds have been widely observed, catch bonds have not been demonstrated experimentally. Here, using atomic force microscopy and flow-chamber experiments, we show that increasing force first prolonged and then shortened the lifetimes of P-selectin complexes with P-selectin glycoprotein ligand-1, revealing both catch and slip bond behaviour. Transitions between catch and slip bonds might explain why leukocyte rolling on selectins first increases and then decreases as wall shear stress increases. This dual response to force provides a mechanism for regulating cell adhesion under conditions of variable mechanical stress.     

基于稀疏恢复的直接数据域STAP算法

在机载/星载雷达系统中,空时自适应处理(STAP)可有效抑制杂波并实现动目标检测。基于统计的STAP算法通过平稳的训练样本来估计检测单元内的杂波协方差矩阵,并设计相应的滤波器以提高检测单元的输出信杂比。但训练样本的平稳性在实际快变的杂波环境中无法保证,因而此类算法在实际非均匀杂波环境中性能较差。该文通过挖掘检测单元数据在角度-Doppler域上的稀疏性,提出一种新的直接数据域STAP算法。该算法通过稀疏恢复来获得检测单元的高分辨空时谱估计,有效地避免杂波旁瓣对目标检测的影响,进而实现不经过杂波抑制而直接运动目标检测的目的。同时由于不使用训练样本,可很好地避免训练样本内的非均匀性,该算法在实际非均匀杂波场景中有更广泛的应用前景。     

Direct observation of ligand recognition by T cells

The activation of T cells through interaction of their T-cell receptors with antigenic peptide bound to major histocompatibility complex (MHC) on the surface of antigen presenting cells (APCs) is a crucial step in adaptive immunity. Here we use three-dimensional fluorescence microscopy to visualize individual peptide-I-E(k) class II MHC complexes labelled with the phycobiliprotein phycoerythrin in an effort to characterize T-cell sensitivity and the requirements for forming an immunological synapse in single cells. We show that T cells expressing the CD4 antigen respond with transient calcium signalling to even a single agonist peptide-MHC ligand, and that the organization of molecules in the contact zone of the T cell and APC takes on the characteristics of an immunological synapse when only about ten agonists are present. This sensitivity is highly dependent on CD4, because blocking this molecule with antibodies renders T cells unable to detect less than about 30 ligands.     

Direct observation of the alignment of ferromagnetic spins by antiferromagnetic spins

The arrangement of spins at interfaces in a layered magnetic material often has an important effect on the properties of the material. One example of this is the directional coupling between the spins in an antiferromagnet and those in an adjacent ferromagnet, an effect first discovered in 1956 and referred to as exchange bias. Because of its technological importance for the development of advanced devices such as magnetic read heads and magnetic memory cells, this phenomenon has received much attention. Despite extensive studies, however, exchange bias is still poorly understood, largely due to the lack of techniques capable of providing detailed information about the arrangement of magnetic moments near interfaces. Here we present polarization-dependent X-ray magnetic dichroism spectro-microscopy that reveals the micromagnetic structure on both sides of a ferromagnetic-antiferromagnetic interface. Images of thin ferromagnetic Co films grown on antiferromagnetic LaFeO3 show a direct link between the arrangement of spins in each material. Remanent hysteresis loops, recorded for individual ferromagnetic domains, show a local exchange bias. Our results imply that the alignment of the ferromagnetic spins is determined, domain by domain, by the spin directions in the underlying antiferromagnetic layer.     

Direct observation of the discrete character of intrinsic localized modes in an antiferromagnet

In a strongly nonlinear discrete system, the spatial size of an excitation can become comparable to, and influenced by, the lattice spacing. Such intrinsic localized modes (ILMs)--also called 'discrete breathers' or 'lattice solitons'--are responsible for energy localization in the dynamics of discrete nonlinear lattices. Their energy profiles resemble those of localized modes of defects in a harmonic lattice but, like solitons, they can move (although, unlike solitons, some energy is exchanged during collisions between them). The manipulation of these localized energy 'hotspots' has been achieved in systems as diverse as annular arrays of coupled Josephson junctions, optical waveguide arrays, two-dimensional nonlinear photonic crystals and micromechanical cantilever arrays. There is also some evidence for the existence of localized excitations in atomic lattices, although individual ILMs have yet to be identified. Here we report the observation of countable localized excitations in an antiferromagnetic spin lattice by means of a nonlinear spectroscopic technique. This detection capability permits the properties of individual ILMs to be probed; the disappearance of each ILM registers as a step in the time-dependent signal, with the surprising result that the energy staircase of ILM excitations is uniquely defined.