182 research outputs found
Spatiotemporal Response of Crystals in X-ray Bragg Diffraction
The spatiotemporal response of crystals in x-ray Bragg diffraction resulting
from excitation by an ultra-short, laterally confined x-ray pulse is studied
theoretically. The theory presents an extension of the analysis in symmetric
reflection geometry [1] to the generic case, which includes Bragg diffraction
both in reflection (Bragg) and transmission (Laue) asymmetric scattering
geometries. The spatiotemporal response is presented as a product of a
crystal-intrinsic plane wave spatiotemporal response function and an envelope
function defined by the crystal-independent transverse profile of the incident
beam and the scattering geometry. The diffracted wavefields exhibit amplitude
modulation perpendicular to the propagation direction due to both angular
dispersion and the dispersion due to Bragg's law. The characteristic measure of
the spatiotemporal response is expressed in terms of a few parameters: the
extinction length, crystal thickness, Bragg angle, asymmetry angle, and the
speed of light. Applications to self-seeding of hard x-ray free electron lasers
are discussed, with particular emphasis on the relative advantages of using
either the Bragg or Laue scattering geometries. Intensity front inclination in
asymmetric diffraction can be used to make snapshots of ultra-fast processes
with femtosecond resolution
Takagi-Taupin Description of X-ray Dynamical Diffraction from Diffractive Optics with Large Numerical Aperture
We present a formalism of x-ray dynamical diffraction from volume diffractive
optics with large numerical aperture and high aspect ratio, in an analogy to
the Takagi-Taupin equations for strained single crystals. We derive a set of
basic equations for dynamical diffraction from volume diffractive optics, which
enable us to study the focusing property of these optics with various grating
profiles. We study volume diffractive optics that satisfy the Bragg condition
to various degrees, namely flat, tilted and wedged geometries, and derive the
curved geometries required for ultimate focusing. We show that the curved
geometries satisfy the Bragg condition everywhere and phase requirement for
point focusing, and effectively focus hard x-rays to a scale close to the
wavelength.Comment: 18 pages, 12 figure
Spontaneous spin bifurcations and ferromagnetic phase transitions in a spinor exciton-polariton condensate
We observe a spontaneous parity breaking bifurcation to a ferromagnetic state in a spatially trapped exciton-polariton condensate. At a critical bifurcation density under nonresonant excitation, the whole condensate spontaneously magnetizes and randomly adopts one of two elliptically polarized (up to 95% circularly polarized) states with opposite handedness of polarization. The magnetized condensate remains stable for many seconds at 5 K, but at higher temperatures, it can flip from one magnetic orientation to another. We optically address these states and demonstrate the inversion of the magnetic state by resonantly injecting 100- fold weaker pulses of opposite spin. Theoretically, these phenomena can be well described as spontaneous symmetry breaking of the spin degree of freedom induced by different loss rates of the linear polarizations.Publisher PDFPeer reviewe
Theory and Applications of X-ray Standing Waves in Real Crystals
Theoretical aspects of x-ray standing wave method for investigation of the
real structure of crystals are considered in this review paper. Starting from
the general approach of the secondary radiation yield from deformed crystals
this theory is applied to different concreat cases. Various models of deformed
crystals like: bicrystal model, multilayer model, crystals with extended
deformation field are considered in detailes. Peculiarities of x-ray standing
wave behavior in different scattering geometries (Bragg, Laue) are analysed in
detailes. New possibilities to solve the phase problem with x-ray standing wave
method are discussed in the review. General theoretical approaches are
illustrated with a big number of experimental results.Comment: 101 pages, 43 figures, 3 table
Olfactory Jump Reflex Habituation in Drosophila and Effects of Classical Conditioning Mutations
Habituation is a nonassociative learning mechanism, in which an initial response toward repeated stimuli gradually wanes. This is amongst the simplest and most widespread forms of behavioral plasticity. So far, neither the underlying molecular mechanisms nor the precise neural networks of habituation are well understood. We have developed a novel paradigm to quantify habituation of the olfactory jump reflex in Drosophila. We present data demonstrating several behavioral properties of this phenomenon, generally observed in other species. We also show that the dunce and rutabaga memory mutants behave abnormally in this assay, suggesting that this assay might be used in behavioral screens for new mutants with defects in this simpler form of behavioral plasticity
Manifestation of the effects of dynamic diffraction in the coherent X-ray radiation of relativistic electrons in a periodic layered medium
Orthogonalities and functional equations
In this survey we show how various notions of orthogonality appear in the theory of functional equations. After introducing some orthogonality relations, we give examples of functional equations postulated for orthogonal vectors only. We show their solutions as well as some applications. Then we discuss the problem of stability of some of them considering various aspects of the problem. In the sequel, we mention the orthogonality equation and the problem of preserving orthogonality. Last, but not least, in addition to presenting results, we state some open problems concerning these topics. Taking into account the big amount of results concerning functional equations postulated for orthogonal vectors which have appeared in the literature during the last decades, we restrict ourselves to the most classical equations
Ubiquitous molecular substrates for associative learning and activity-dependent neuronal facilitation.
Recent evidence suggests that many of the molecular cascades and substrates that contribute to learning-related forms of neuronal plasticity may be conserved across ostensibly disparate model systems. Notably, the facilitation of neuronal excitability and synaptic transmission that contribute to associative learning in Aplysia and Hermissenda, as well as associative LTP in hippocampal CA1 cells, all require (or are enhanced by) the convergence of a transient elevation in intracellular Ca2+ with transmitter binding to metabotropic cell-surface receptors. This temporal convergence of Ca2+ and G-protein-stimulated second-messenger cascades synergistically stimulates several classes of serine/threonine protein kinases, which in turn modulate receptor function or cell excitability through the phosphorylation of ion channels. We present a summary of the biophysical and molecular constituents of neuronal and synaptic facilitation in each of these three model systems. Although specific components of the underlying molecular cascades differ across these three systems, fundamental aspects of these cascades are widely conserved, leading to the conclusion that the conceptual semblance of these superficially disparate systems is far greater than is generally acknowledged. We suggest that the elucidation of mechanistic similarities between different systems will ultimately fulfill the goal of the model systems approach, that is, the description of critical and ubiquitous features of neuronal and synaptic events that contribute to memory induction
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