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Markus Gühr 《Synchrotron Radiation News》2016,29(5):8-12
The molecular ability to selectively and efficiently convert sunlight into other forms of energy like heat, bond change, or charge separation is truly remarkable. The decisive steps in these transformations often happen on a femtosecond timescale and require transitions among different electronic states that violate the Born-Oppenheimer approximation (BOA) [1]. Non-BOA transitions pose challenges to both theory and experiment. From a theoretical point of view, excited state dynamics and nonadiabatic transitions both are difficult problems [2, 3] (see Figure 1(a)). However, the theory on non-BOA dynamics has advanced significantly over the last two decades. Full dynamical simulations for molecules of the size of nucleobases have been possible for a couple of years [4, 5] and allow predictions of experimental observables like photoelectron energy [6] or ion yield [7–9]. The availability of these calculations for isolated molecules has spurred new experimental efforts to develop methods that are sufficiently different from all optical techniques. For determination of transient molecular structure, femtosecond X-ray diffraction [10, 11] and electron diffraction [12] have been implemented on optically excited molecules. 相似文献
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Charles S. Fadley 《Synchrotron Radiation News》2013,26(5):26-31
Traditional angle-resolved photoemission (ARPES) with excitation in the ca. 20 to 150 eV range has clearly evolved to be the technique of choice for studying the electronic structure of surfaces and complex new strongly correlated and magnetic materials. However, it is clear that ARPES with excitation only up to 150 eV or so remains a very surface-sensitive probe, thus necessitating careful in-situ sample treatment, cleaving, or even synthesis to avoid the measurement of surface-associated artifacts. A key measure of this surface sensitivity is the electron inelastic mean free path (IMFP orΛe), which measures the mean depth of electron emission without inelastic scattering, and both experimental [1, 2] and theoretical [3] IMFP studies showing that the only reliable way to increase bulk or buried layer/interface sensitivity for all material types is to go to higher photon energies in the soft X-ray (ca. 0.5–2 keV) or hard X-ray (ca. 2–10 keV) regime. 相似文献
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The thermodynamic behaviour of two-dimensional single-component elastic crystalline solids is developed: the surface Euler's equation, the surface Gibbs equation, the surface Gibbs–Duhem equation, and the conditions to be expected at equilibrium, including the stress-deformation behaviour of the crystal. The analysis recognizes that the surface Helmholtz free energy is an explicit function of the lattice vectors defining the crystalline structure. As an application, we obtain the stress-deformation behaviour of single-wall carbon nanotubes which are composed of a regular two-dimensional array of hexagonal lattices of carbon atoms. Using two potentials, Tersoff [1]–Brenner [2] and Brenner et al. [3] to describe interatomic potentials and hence the specific surface Helmholtz free energy, we compute the surface elastic properties for the single-wall carbon nanotubes. These are compared with the available experimental values. 相似文献
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Proteins are the workhorses of living cells, providing essential functions such as structural support, signal transduction, enzymatic catalysis, transport and storage of small ligands. Atomic-resolution structures obtained with conventional X-ray crystallography show proteins essentially as static. In reality, however, proteins move and their motion is crucial for functioning. Although the structure and dynamics of proteins are intimately related, they are not equally well understood. A very large number of protein structures have been determined, but only a few studies have been able to monitor experimentally the dynamics of proteins in real time. In the last two decades, the availability of short (~100 ps) and intense (~109–1010 photons) X-ray pulses produced by third-generation synchrotrons have allowed the implementation of structural methods like time-resolved X-ray crystallography and time-resolved X-ray solution scattering that have allowed us to monitor protein motion in the nanosecond-to-millisecond timescale [1–4]. Time-resolved X-ray crystallography has been used to monitor processes such as the migration of a ligand from the protein active site to the surrounding solvent [5–7] or tertiary structural changes associated with allosteric transitions [8, 9]. On the other hand, time-resolved X-ray scattering in the so-called wide-angle X-ray scattering (WAXS) region [10] has been used to track conformational changes corresponding to large-amplitude protein motions such as the quaternary R-T transition of human hemoglobin [11–13], the relative motion of bacteriorhodopsin α-helices following retinal isomerization [14], or the open-to-close transition in bacterial phytochromes [15]. 相似文献
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Although nineteen years have passed since the discovery of high temperature cuprate superconductivity 1, there is still no consensus on its physical origin. This is in large part because of a lack of understanding of the state of matter out of which the superconductivity arises. In optimally and underdoped materials, this state exhibits a pseudogap at temperatures large compared to the superconducting transition temperature 2, 3. Although discovered only three years after the pioneering work of Bednorz and Müller, the physical origin of this pseudogap behavior and whether it constitutes a distinct phase of matter is still shrouded in mystery. In the summer of 2004, a band of physicists gathered for five weeks at the Aspen Center for Physics to discuss the pseudogap. In this perspective, we would like to summarize some of the results presented there and discuss the importance of the pseudogap phase in the context of strongly correlated electron systems.
The pseudogap: friend or foe of high T c ?
Published online:
19 February 2007Table 相似文献
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Modern synchrotron-based X-ray fluorescence microscopy (XFM) has become a critical tool for many a research program, addressing extremely broad and highly relevant scientific questions. Their ability to map trace elemental content and probe local chemical state has been applied to numerous scientific areas in the life sciences [1–3], the environmental and earth sciences [4, 5], the materials sciences, as well as in cultural heritage studies. The newest generation of instruments utilizes high-brightness X-ray sources and incorporate state-of-the-art focusing optics and detector systems. Advances in X-ray sources and nanofocusing optics, for example, have allowed these instruments to achieve spatial resolutions of 20–30 nm using diffractive optics such as Fresnel zone plates and 200 nm using reflective optics such as Kirkpatrick-Baez mirrors. New beamlines, now in the design stage, aim to achieve similar (and better) resolutions within the next five years. 相似文献
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Henry N. Chapman 《Synchrotron Radiation News》2015,28(6):20-24
X-ray free-electron lasers produce brief flashes of X-rays that are of about a billion times higher peak brightness than achievable from storage ring sources. Such a tremendous jump in X-ray source capabilities, which came in 2009 when the Linac Coherent Light Source began operations, was unprecedented in the history of X-ray science. Protein structure determination through the method of macromolecular crystallography has consistently benefited from the many increases in source performance from rotating anodes to all generations of synchrotron facilities. But when confronted with the prospects of such bright beams for structural biology, enthusiastic proposals were tempered by trepidation of the effects of such beams on samples and challenges to record data [1]. A decade after these discussions (and others in the USA) on the applications of X-ray FELs for biology, the first experiments took place at LCLS, giving results that fulfilled many of the dreams of the early visionaries. In particular, the concept that diffraction representing the pristine object could be recorded before the X-ray pulse completely vaporizes the object was validated [2], confirming predictions [3] that established dose limits could be vastly exceeded using femtosecond-duration pulses. The first experiments illuminated a path to achieve room-temperature structures free of radiation damage, from samples too small to provide useful data at synchrotron facilities, as well as providing the means to carry out time-resolved crystallography at femtoseconds to milliseconds. In the five years since, progress has been substantial and rapid, invigorating the field of macromolecular crystallography [4, 5]. This phase of development is far from over, but with both the LCLS and the SPring-8 Ångström Compact Free-electron Laser (SACLA) providing facilities for measurements, the benefits of X-ray FELs are already being translated into new biological insights. 相似文献
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Kai-Bin Fu 《哲学杂志》2013,93(15):1873-1882
Abstract Si et al. [1] pointed out that it was inappropriate to use continuity of displacement at interfaces during phase transitions or in the case of reactions at interfaces as in the case of oxidation, since appropriate reference configurations cannot be identified. They instead derived a new compatibility constraint, when atleast one of the adjoining phases is crystalline. The test of these ideas offered by Slattery et al. [2] was successful, but it likely was too simple, since the deformations were so small. A more stringent and successful test has recently been offered by [3]. Here, we analyze oxidation on the surface of a cylinder both using an extension of the compatibility constraint and using continuity of displacement, comparing the results with the experimental observations of Imbrie and Lagoudas [4]. 相似文献
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Eugenio Ferrari 《Synchrotron Radiation News》2016,29(3):4-9
The advent of FEL sources delivering two synchronized pulses of different wavelengths has made available a whole range of novel pump-probe experiments [1], allowing the exploration of the dynamics of matter driven to extreme non-equilibrium states by an intense ultrashort X-ray pulse and then probing the sample response at variable time delay with a second pulse [2]. 相似文献
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Uwe Mueller Marjolein Thunnissen Jie Nan Mikel Eguiraun Fredrick Bolmsten Antonio Milàn-Otero 《Synchrotron Radiation News》2017,30(1):22-27
The outstanding success of structural biology within the last two decades is closely related to the development and evolution of macromolecular crystallography (MX) beamlines. Indeed, many of today's synchrotron-based MX experimental sessions aim for fast but rigorous evaluations and data collections from very large numbers of samples [1–7]. To facilitate this, sample changing on most MX beamlines is now carried out by robots and the centering of a crystal in the X-ray beam to micrometer precision is now automatically performed using either optical or diffraction-based techniques [8]. Once a crystal is centered, users have a wide array of options at their disposal to prepare any given experiment. This includes: X-ray fluorescence (XRF) [9] analysis to confirm the presence of anomalous scatterers in crystals; X-ray absorption near-edge scans (XANES) to determine the best X-ray wavelengths for MAD/SAD data collection [10]; and the probing of the diffraction properties of crystals to determine the best crystal, or area of a crystal [11], for data collection. All of these operations are now also automated, as is the collection of the final diffraction data set either from single or multiple crystals and the subsequent data analysis and reduction. 相似文献
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Kensuke Tono 《Synchrotron Radiation News》2017,30(1):12-15
X-ray free electron lasers (XFELs) produce femtosecond X-ray pulses with nearly full spatial coherence and a peak power in the order of tens of GW [1, 2]. At present, two XFEL facilities, the Linac Coherent Light Source (LCLS) and SPring-8 Angstrom Compact free electron LAser (SACLA), offer research opportunities to scientists from various fields. The novel characteristics of XFELs necessitate new experimental styles, which are very different from those for conventional X-ray sources. Since many users were not familiar with these new styles in the early stage of scientific applications, user-friendly experimental systems were necessary to boost the XFEL applications into a state of practical use. XFELs are now producing unique results in, for example, structural biology [3–6], nonlinear X-ray optics [7, 8], ultrafast physics and chemistry [9–11], and high-energy-density science [12]. 相似文献
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Ambient-pressure X-ray photoelectron spectroscopy (APXPS) has contributed greatly to a wide range of research fields, including environmental science [1], catalysis [2], and electrochemistry [3], to name a few. The use of this technique at synchrotron facilities primarily focused on probing the solid/gas interface; however, it quickly advanced to the probing of liquid/vapor interfaces [4, 5] and solid/liquid interfaces through an X-ray-transparent window [6–8]. Most recently, combining APXPS with “Tender” X-rays (~2.5 keV to 8 keV) on beamline 9.3.1 at the Advanced Light Source in Lawrence Berkeley National Laboratory (which can generate photoelectrons with much longer inelastic mean free paths) has enabled us to probe the solid/liquid interface without needing a window [9]. This innovation allows us to probe interfacial chemistries of electrochemically controlled solid/liquid interfaces undergoing charge transfer reactions [9]. These advancements have transitioned APXPS from a traditional surface science tool to an essential interface science technique. 相似文献
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A new Lagrangian conditional moment closure (CMC) model is developed for multiple Lagrangian groups of sequentially evaporating fuel in turbulent spray combustion. Flame group interaction is taken into account as premixed combustion by the eddy breakup (EBU) model in terms of the probability of finding flame groups in the burned and the unburned state. Evaporation source terms are included in the two phase conditional transport equations, although they turn out to have negligible influence on the mean temperature field during combustion. The Lagrangian CMC model is implemented in OpenFOAM [1] and validated for test cases in the Engine Combustion Network (ECN) [2,3]. Similar ignition delays and lift-off lengths are predicted by the incompletely stirred reactor (ISR) and the Eulerian CMC models due to relatively uniform conditional flame structure in the domain. The improved Lagrangian CMC model shows no abrupt reaction or oscillatory behaviour with an appropriate model constant K and gives results in better agreement with measurements lying between the predictions by ISR and Lagrangian CMC without flame group interaction. 相似文献
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Yasumasa Joti Kyo Nakajima Takashi Kameshima Mitsuhiro Yamaga Toshinori Abe Kensuke Okada 《Synchrotron Radiation News》2017,30(1):16-21
X-ray free-electron lasers (XFELs) with full spatial coherence, extreme brilliance, and ultra-fast pulse duration [1, 2] allow the investigation of complex phenomena in physics, chemistry, and biology with angstrom and femtosecond resolutions. In particular, a concept of “diffraction before destruction” [3] has been demonstrated for serial femtosecond crystallography (SFX) [4, 5] and coherent diffractive imaging (CDI) [6]. Using femto-second XFEL pulses, diffraction data are collected before radiation damage to samples has time to occur. Since samples are exchanged for each XFEL pulse, shot-to-shot data acquisition (DAQ) is mandatory to correlate the recorded data with the sample characteristics. The shot-to-shot DAQ must be synchronized with the repetition rate of the XFEL source, typically several tens to one hundred Hz for the machine based on normal-conducting accelerators. Shot-to-shot recording of XFEL pulse characteristics is also essential because they fluctuate due to the stochastic nature of a self-amplified spontaneous emission (SASE). We must carefully analyze a huge data set taking the fluctuation into account. 相似文献
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P. Bhatia V. R. Katta S. S. Krishnan Y. Zheng P. B. Sunderland J. P. Gore 《Combustion Theory and Modelling》2013,17(5):774-798
Steady-state global chemistry calculations for 20 different flames were carried out using an axisymmetric Computational Fluid Dynamics (CFD) code. Computational results for 16 flames were compared with flame images obtained at the NASA Glenn Research Center. The experimental flame data for these 16 flames were taken from Sunderland et al. [4] which included normal and inverse diffusion flames of ethane with varying oxidiser compositions (21, 30, 50, 100% O2 mole fraction in N2) stabilised on a 5.5 mm diameter burner. The test conditions of this reference resulted in highly convective inverse diffusion flames (Froude numbers of the order of 10) and buoyant normal diffusion flames (Froude numbers ~0.1). Additionally, six flames were simulated to study the effect of oxygen enhancement on normal diffusion flames. The enhancement in oxygen resulted in increased flame temperatures and the presence of gravity led to increased gas velocities. The effect of gravity-variation and oxygen enhancement on flame shape and size of normal diffusion flames was far more pronounced than for inverse diffusion flames. For normal-diffusion flames, their flame-lengths decreased (1 to 2 times) and flames-widths increased (2 to 3 times) when going from earth-gravity to microgravity, and flame height decreased by five times when going from air to a pure oxygen environment. 相似文献
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Kiyoshi Ueda 《Synchrotron Radiation News》2016,29(5):3-7
In recent years, short wavelength free electron lasers (FELs) have opened up access to ultrafast electronic and structural dynamics in matter. Currently, four FEL facilities are in operation in the world. FLASH [1] in Germany and FERMI [2] in Italy cover the range from extreme ultraviolet (EUV) to soft X-rays, while LCLS [3] in the U.S. and SACLA [4] in Japan provide pulses in the hard X-ray regime. In addition, an upgrade version of SCSS [5], nicknamed SCSS+, has also just started user operation as a beamline of SACLA [6]. These FELs deliver coherent pulses combining unprecedented power densities up to ~1020 W/cm2 and extremely short pulse durations down to a few femtoseconds. The intense coherent FEL pulse focused down to ~1 μm2 makes single-shot diffractive imaging of nano-crystals or even non-crystallized bio-samples as well as other small objects a reality. Time-resolved spectroscopic and structural studies on the timescale of femtoseconds, having FEL pulses as a probe, allow us to probe electrons and atoms in action. Additionally, since FEL pulses are in a new regime of intensity, they are opening up new research fields that exploit the interaction between intense short wavelength pulses and matter, leading to matter at extremely high energy. Relevant theories dealing with such extreme conditions are also rapidly growing. 相似文献
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Ray Conley Nathalie Bouet Yong S. Chu Xiaojing Huang Hyon Chol Kang Albert T. Macrander 《Synchrotron Radiation News》2016,29(4):16-20
X-rays are intrinsically capable of being used for the study of non-periodic objects with atomic resolution, with high penetration, in applied electromagnetic fields, and in fluids and gases. For direct imaging via nanofocused X-ray beams, reflective [1], refractive [2], and diffractive [3, 4] optics are used in various approaches for high-resolution imaging. Diffractive X-ray optics are endowed with the highest numerical aperture, in principle allowing focusing of X-rays to sub-nanometer dimensions. Lithographically produced Fresnel zone plates (FZP) find broad deployment around the globe, in both nanofocusing and full-field imaging approaches, and have, for many years, been workhorse optics in both synchrotron-based and laboratory-based X-ray imaging systems [4]. A FZP consists of a series of radially symmetric rings, which are known as Fresnel zones, which alternate between transparent and opaque. Radiation traversing into the FZP diffracts around the opaque zones, which are placed in an arrangement where light constructively interferes at the focal plane. 相似文献
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Hirokatsu Yumoto Takahisa Koyama Satoshi Matsuyama Yoshiki Kohmura Kazuto Yamauchi Tetsuya Ishikawa 《Synchrotron Radiation News》2016,29(4):27-31
Ellipsoidal mirror optics can produce a smaller, two-dimensional focus with diffraction-limited properties than is possible when using mirror optics in Kirkpatrick–Baez (K–B) geometry [1]. This is because ellipsoidal focusing mirrors can be designed such that they have a larger numerical aperture in the sagittal focusing direction as compared to that in the meridional focusing direction. Although ellipsoidal focusing mirrors have this crucial advantage over K–B optics, K–B optics are widely utilized as micro-/nano-focusing devices [2–8] in synchrotron radiation facilities and X-ray free electron laser facilities [9, 10]. Figure 1 shows a schematic of focusing mirror optics; Figure 1(a) shows the ellipsoidal mirror and Figure 1(b) the K–B mirror arrangement. In K–B geometry, two mirrors with a one-dimensionally curved surface, such as an elliptical cylinder, are orthogonally arranged in tandem to reflect and focus light independently in a direction perpendicular to each other under grazing-incidence conditions. Ellipsoidal focusing mirrors, which can generate a two-dimensional focusing beam by a single reflection, have a highly sloped surface with a two-dimensional aspherical shape, when compared to elliptical-cylinder mirrors that are used for line-focusing in K–B geometry. In addition, surface shapes of nano-focusing mirrors must be fabricated with nanometer-level accuracy. Therefore, fabrication of ellipsoidal nano-focusing mirrors is extremely difficult. There are no reports on ellipsoidal nano-focusing mirrors in the hard X-ray region with superior performances to provide diffraction-limited beams. 相似文献