148 research outputs found
Detecting Orientational Order in Model Systems by X-ray Cross Correlation Methods
We present the results of a computational X-ray cross correlation analysis
(XCCA) study on two dimensional polygonal model structures. We show how to
detect and identify the orientational order of such systems, demonstrate how to
eliminate the influence of the "computational box" on the XCCA results and
develop new correlation functions that reflect the sample's orientational order
only. For this purpose, we study the dependence of the correlation functions on
the number of polygonal clusters and wave vector transfer for various types
of polygons including mixtures of polygons and randomly placed particles. We
define an order parameter that describes the orientational order within the
sample. Finally, we determine the influence of detector noise and non-planar
wavefronts on the XCCA data which both appear to affect the results
significantly and have thus to be considered in real experiments
Low dose X-ray speckle visibility spectroscopy reveals nanoscale dynamics in radiation sensitive ionic liquids
X-ray radiation damage provides a serious bottle neck for investigating
{\mu}s to s dynamics on nanometer length scales employing X-ray photon
correlation spectroscopy. This limitation hinders the investigation of real
time dynamics in most soft matter and biological materials which can tolerate
only X-ray doses of kGy and below. Here, we show that this bottleneck can be
overcome by low dose X-ray speckle visibility spectroscopy. Employing X-ray
doses of 22 kGy to 438 kGy and analyzing the sparse speckle pattern of count
rates as low as 6.7x10-3 per pixel we follow the slow nanoscale dynamics of an
ionic liquid (IL) at the glass transition. At the pre-peak of nanoscale order
in the IL we observe complex dynamics upon approaching the glass transition
temperature TG with a freezing in of the alpha relaxation and a multitude of
milli-second local relaxations existing well below TG. We identify this fast
relaxation as being responsible for the increasing development of nanoscale
order observed in ILs at temperatures below TG.Comment: 7 pages, 5 figure
Amortized Bayesian Inference of GISAXS Data with Normalizing Flows
Grazing-Incidence Small-Angle X-ray Scattering (GISAXS) is a modern imaging
technique used in material research to study nanoscale materials.
Reconstruction of the parameters of an imaged object imposes an ill-posed
inverse problem that is further complicated when only an in-plane GISAXS signal
is available. Traditionally used inference algorithms such as Approximate
Bayesian Computation (ABC) rely on computationally expensive scattering
simulation software, rendering analysis highly time-consuming. We propose a
simulation-based framework that combines variational auto-encoders and
normalizing flows to estimate the posterior distribution of object parameters
given its GISAXS data. We apply the inference pipeline to experimental data and
demonstrate that our method reduces the inference cost by orders of magnitude
while producing consistent results with ABC
Faster chiral versus collinear magnetic order recovery after optical excitation revealed by femtosecond XUV scattering
While chiral spin structures stabilized by Dzyaloshinskii-Moriya interaction
(DMI) are candidates as novel information carriers, their dynamics on the fs-ps
timescale is little known. Since with the bulk Heisenberg exchange and the
interfacial DMI two distinct exchange mechanisms are at play, the ultra-fast
dynamics of the chiral order needs to be ascertained and compared to the
dynamics of the conventional collinear order. Using an XUV free-electron laser
we determine the fs-ps temporal evolution of the chiral order in domain walls
in a magnetic thin film sample by an IR pump - X-ray magnetic scattering probe
experiment. Upon demagnetisation we observe that the dichroic (CL-CR) signal
connected with the chiral order correlator in the domain walls
recovers significantly faster than the (CL+CR) sum signal representing the
average collinear domain magnetisation . We explore possible
explanations based on spin structure dynamics and reduced transversal
magnetisation fluctuations inside the domain walls and find that the latter can
explain the experimental data leading to different dynamics for collinear
magnetic order and chiral magnetic order.Comment: 28 pages, 14 figure
Coherent X-ray Scattering Reveals Nanoscale Fluctuations in Hydrated Proteins
Hydrated proteins undergo a transition in the deeply supercooled regime,
which is attributed to rapid changes in hydration water and protein structural
dynamics. Here, we investigate the nanoscale stress relaxation in hydrated
lysozyme proteins stimulated and probed by X-ray Photon Correlation
Spectroscopy (XPCS). This approach allows us to access the nanoscale dynamic
response in the deeply supercooled regime (T = 180 K) which is typically not
accessible through equilibrium methods. The relaxation time constants exhibit
Arrhenius temperature dependence upon cooling with a minimum in the
Kohlrausch-Williams-Watts exponent at T = 227 K. The observed minimum is
attributed to an increase in dynamical heterogeneity, which coincides with
enhanced fluctuations observed in the two-time correlation functions and a
maximum in the dynamic susceptibility quantified by the normalised variance
. Our study provides new insights into X-ray stimulated stress
relaxation and the underlying mechanisms behind spatio-temporal fluctuations in
biological granular materials
Indirect excitation of ultrafast demagnetization
Does the excitation of ultrafast magnetization require direct interaction between the photons of the optical pump pulse and the magnetic layer? Here, we demonstrate unambiguously that this is not the case. For this we have studied the magnetization dynamics of a ferromagnetic cobalt/palladium multilayer capped by an IR-opaque aluminum layer. Upon excitation with an intense femtosecond-short IR laser pulse, the film exhibits the classical ultrafast demagnetization phenomenon although only a negligible number of IR photons penetrate the aluminum layer. In comparison with an uncapped cobalt/palladium reference film, the initial demagnetization of the capped film occurs with a delayed onset and at a slower rate. Both observations are qualitatively in line with energy transport from the aluminum layer into the underlying magnetic film by the excited, hot electrons of the aluminum film. Our data thus confirm recent theoretical predictions
Observation of ultrafast solid-density plasma dynamics using femtosecond X-ray pulses from a free-electron laser
The complex physics of the interaction between short pulse high intensity
lasers and solids is so far hardly accessible by experiments. As a result of
missing experimental capabilities to probe the complex electron dynamics and
competing instabilities, this impedes the development of compact laser-based
next generation secondary radiation sources, e.g. for tumor therapy
[Bulanov2002,ledingham2007], laboratory-astrophysics
[Remington1999,Bulanov2015], and fusion [Tabak2014]. At present, the
fundamental plasma dynamics that occur at the nanometer and femtosecond scales
during the laser-solid interaction can only be elucidated by simulations. Here
we show experimentally that small angle X-ray scattering of femtosecond X-ray
free-electron laser pulses facilitates new capabilities for direct in-situ
characterization of intense short-pulse laser plasma interaction at solid
density that allows simultaneous nanometer spatial and femtosecond temporal
resolution, directly verifying numerical simulations of the electron density
dynamics during the short pulse high intensity laser irradiation of a solid
density target. For laser-driven grating targets, we measure the solid density
plasma expansion and observe the generation of a transient grating structure in
front of the pre-inscribed grating, due to plasma expansion, which is an
hitherto unknown effect. We expect that our results will pave the way for novel
time-resolved studies, guiding the development of future laser-driven particle
and photon sources from solid targets
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