269 research outputs found
Common Arc Method for Diffraction Pattern Orientation
Very short pulses of x-ray free-electron lasers opened the way to obtain
diffraction signal from single particles beyond the radiation dose limit. For
3D structure reconstruction many patterns are recorded in the object's unknown
orientation. We describe a method for orientation of continuous diffraction
patterns of non-periodic objects, utilizing intensity correlations in the
curved intersections of the corresponding Ewald spheres, hence named Common Arc
orientation. Present implementation of the algorithm optionally takes into
account the Friedel law, handles missing data and is capable to determine the
point group of symmetric objects. Its performance is demonstrated on simulated
diffraction datasets and verification of the results indicates high orientation
accuracy even at low signal levels. The Common Arc method fills a gap in the
wide palette of the orientation methods.Comment: 16 pages, 10 figure
Dynamics in a cluster under the influence of intense femtosecond hard x-ray pulses
In this paper we examine the behavior of small cluster of atoms in a short
(10-50 fs) very intense hard x-ray (10 keV) pulse. We use numerical modeling
based on the non-relativistic classical equation of motion. Quantum processes
are taken into account by the respective cross sections. We show that there is
a Coulomb explosion, which has a different dynamics than one finds in classical
laser driven cluster explosions. We discuss the consequences of our results to
single molecule imaging by the free electron laser pulses.Comment: 14 pages, 13 figure
Nonlinear hydrodynamic theory of crystallization
We present an isothermal fluctuating nonlinear hydrodynamic theory of
crystallization in molecular liquids. A dynamic coarse-graining technique is
used to derive the velocity field, a phenomenology, which allows a direct
coupling between the free energy functional of the classical Density Functional
Theory and the Navier-Stokes equation. Contrary to the Ginzburg-Landau type
amplitude theories, the dynamic response to elastic deformations is described
by parameter-free kinetic equations. Employing our approach to the free energy
functional of the Phase-Field Crystal model, we recover the classical spectrum
for the phonons and the steady-state growth fronts. The capillary wave spectrum
of the equilibrium crystal-liquid interface is in a good qualitative agreement
with the molecular dynamics simulations
Holographic Methods as Local Probes of the Atomic Order in Solids
In the last fifteen years several techniques based on the holographic
principle have been developed for the study of the 3D local order in solids.
These methods use various particles: electrons, hard x-ray photons, gamma
photons, or neutrons to image the atoms. Although the practical realisation of
the various imaging experiments is very different, there is a common thread;
the use of inside reference points for holographic imaging. In this paper we
outline the basics of atomic resolution holography using inside reference
points, especially concentrating to the hard x-ray case. Further, we outline
the experimental requirements and what has been practically realized in the
last decade. At last we give examples of applications and future perspectives.Comment: 14 pages, 6 figure
Faceting and branching in 2D crystal growth
The official published version of the Article can be accessed from the link below - Copyright @ 2011 APSUsing atomic scale time-dependent density functional calculations we confirm that both diffusion-controlled and diffusionless crystallization modes exist in simple 2D systems. We provide theoretical evidence that a faceted to nonfaceted transition is coupled to these crystallization modes, and faceting is governed by the local supersaturation at the fluid-crystalline interface. We also show that competing modes of crystallization have a major influence on mesopattern formation. Irregularly branched and porous structures are emerging at the crossover of the crystallization modes. The proposed branching mechanism differs essentially from dendritic fingering driven by diffusive instability.This work has been supported by the EU FP7
Collaborative Project ENSEMBLE under Grant
Agreement NMP4-SL-2008-213669 and by the
Hungarian Academy of Sciences under Contract
No. OTKA-K-62588
Hydrodynamic theory of freezing: Nucleation and polycrystalline growth
Structural aspects of crystal nucleation in undercooled liquids are explored
using a nonlinear hydrodynamic theory of crystallization proposed recently [G.
I. Toth et al., J. Phys.: Condens. Matter 26, 055001 (2014)], which is based on
combining fluctuating hydrodynamics with the phase-field crystal theory. We
show that in this hydrodynamic approach not only homogeneous and heterogeneous
nucleation processes are accessible, but also growth front nucleation, which
leads to the formation of new (differently oriented) grains at the solid-liquid
front in highly undercooled systems. Formation of dislocations at the
solid-liquid interface and interference of density waves ahead of the
crystallization front are responsible for the appearance of the new
orientations at the growth front that lead to spherulite-like nanostructures
A fázisprobléma megoldásának új módszerei = New methods for solving the phase problem
A kristályos anyagok szerkezetĂ©nek ab initio meghatározásához megkerĂĽlhetetlen a krisztallográfiai fázisproblĂ©ma megoldása. Kutatásunk során erre a feladatra dolgoztunk ki Ăşj eljárást, amely a klasszikus direkt mĂłdszertĹ‘l teljesen kĂĽlönbözĹ‘ elven működik. A töltĂ©salternálĂł (charge flipping) mĂłdszer meglepĹ‘en merĂ©sz alapfeltevĂ©se az, hogy kellĹ‘en jĂł felbontás esetĂ©n a megoldáshoz nincs szĂĽksĂ©g sem elĹ‘zetes kĂ©miai vagy szimmetria ismeretekre, sem az atomicitás elvĂ©re vagy statisztikus fázisösszefĂĽggĂ©sekre. SzĂĽksĂ©gesek viszont a cella ĂĽressĂ©gĂ©t kihasználĂł gyenge perturbáciĂłk, amelyekkel a hiányzĂł fázisok terĂ©nek nagy dimenziĂłszámát le lehet csökkenteni. A megvalĂłsĂtott algoritmus a valĂłs Ă©s reciprok terek között gyors Fourier-transzformáciĂłval iterál, a valĂłs tĂ©rben az elektronsűrűsĂ©g elĹ‘jelĂ©t egy kis kĂĽszöbszint alatt megváltoztatja, mĂg a reciprok tĂ©rben csak a gyenge reflexiĂłk fázisait tolja el. Az iteráciĂłs folyamat nem igĂ©nyel kĂĽlsĹ‘ beavatkozást: a megoldást valamely jĂłsági tĂ©nyezĹ‘ hirtelen zuhanása jelzi. Könnyű adaptálhatĂłságának köszönhetĹ‘en a töltĂ©salternálás mĂłdszere az elsĹ‘ közlemĂ©ny megjelenĂ©se Ăłta rohamosan terjed. AlkalmazhatĂłsága mára a diffrakciĂłs szerkezetmeghatározás több terĂĽletĂ©n nyert bizonyosságot; a listán egykristály Ă©s pordiffrakciĂłs adatok, periodikus, pszeudoszimmetrikus Ă©s modulált szerkezetek, ill. kvázikristályok is vannak. Az algoritmust már három felhasználĂłi programcsomag kĂnálja alternatĂvakĂ©nt nehĂ©z esetek elemzĂ©sĂ©re. | The solution of the crystallographic phase problem is essential for the ab inito determination of crystalline structures. For this purpose we developed a new method called charge flipping. It is based on principles very different from that of classical direct methods. The primary assumption of charge flipping is the following: given sufficiently high-resolution diffraction data, we need neither preliminary information on chemical composition or symmetry, nor the principle of atomicity or statistical phase relations. Instead, we must rely on the emptiness of the unit cell and decrease the dimensionality of the missing phases through small perturbations. The new algorithm iterates between the real and reciprocal spaces by the fast Fourier transform, in real space it changes the sign of electron density below a small positive threshold, while in reciprocal space it shifts the phases of weak reflections. The whole process is deterministic and proceeds without user intervention, the solution is signified by a sharp drop of some figure of merit. Due to its deceptive simplicity and easy implementation the charge flipping method quickly propagates. Its applicability is already proven in several fields of structural science, the list contains using single crystal and powder data of periodic, pseudo-symmetric and modulated structures and also quasicrystals. The algorithm is already offered as an alternative by three user programs for the treatment of difficult cases
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