Yuri P. Stetsko

Electron diffractive imaging of nano-objects using a guided method with a dynamic support

We present a phase recovery technique that utilizes a guiding method with a dynamic support to reconstruct the shape and exit wave of a single MgO nanoparticle in a coherent electron diffraction experiment. The proposed method provides an optimal solution deduced from the electron diffraction pattern alone. The recovered shape has spatial resolution 3.1 nm. The complex exit wave encodes the projected atomic structure of the nanocrystal with resolution about 0.15 nm, and agrees with a multislice simulation. The possibility of imaging nanosized objects at diffraction-limited resolution using a field emission electron microscope is thus demonstrated.

Time-delayed beam splitting with energy separation of x-ray channels

We introduce a time-delayed beam splitting method based on the energy separation of x-ray photon beams. It is implemented and theoretically substantiated on an example of an x-ray optical scheme similar to that of the classical Michelson interferometer. The splitter/mixer uses Bragg-case diffraction from a thin diamond crystal. Another two diamond crystals are used as back-reflectors. Because of energy separation and a minimal number (three) of optical elements, the split-delay line has high efficiency and is simple to operate. Due to the high transparency of diamond crystal, the split-delay line can be used in a beam sharing mode at x-ray free-electron laser facilities.

Size dependence of tetrahedral bond lengths in CdSe nanocrystals

The structural characteristics of organically passivated CdSe nanocrystals (NCs) were investigated with x-ray diffraction and extended x-ray absorption fine structure. As the NC size decreases, the axial bond length R(1) for an atomic tetrahedron extends but the equatorial bond length R(2) contracts, with a similar tendency of distortion for the lattice parameters of the wurtzite structure. The authors suggest that the observed hexagonal distortion is attributed to the surface stress of the NCs related to the organic passivation effect and the relaxation of atomic positions at the stacking fault interface.

The phenomenon of polarization suppression of X-ray Umweg multiple waves in crystals

The phenomenon of the polarization suppression of X-ray Umweg multiple waves in Renninger scans [Renninger (1937). Z. Kristallogr. 97, 107–121] of crystals, showing intensity decrease due to properly chosen wavelength and polarization of incident radiation, is observed. That is, one of the participating wave components in the multiple-wave interference is reduced considerably so that the intensity of multiple diffraction is decreased. The condition for total suppression of the multiple-wave interaction in crystals is derived theoretically from the Born approximation and verified with exact dynamical calculation and experiments. Partial suppression of the strong Umweg interfered component is demonstrated using elliptically or linearly polarized synchrotron radiation. The suppressed multiple-wave intensity distribution reveals high sensitivity to X-ray reflection phase. This multiple-diffraction technique under partial polarization suppression provides an alternative way of enhancing the visibility of multiple-wave interference in crystals for direct phase determination.

Electron coherent diffraction tomography of a nanocrystal

Coherent diffractive imaging (CDI) with electron or x-ray sources is a promising technique for investigating the structure of nanoparticles down to the atomic scale. In electron CDI, a two-dimensional reconstruction is demonstrated using highly coherent illumination from a field-emission gun as a source of electrons. In a three-dimensional (3D) electron CDI, we experimentally determine the morphology of a single MgO nanocrystal using the Bragg diffraction geometry. An iterative algorithm is applied to invert the 3D diffraction pattern about a (200) reflection of the nanoparticle measured at an angular range of 1.8°. The results reveal a 3D image of the sample at ∼8 nm resolution, and agree with a simulation. Our work demonstrates an alternative approach to obtain the 3D structure of nanocrystals with an electron microscope.

Polarization-resolved output analysis of X-ray multiple-wave interaction

The polarization suppression of the interfering components in X-ray multiple-wave interaction is observed for the first time by using a polarization analyzer with an arbitrary inclination of the diffraction plane with respect to that of the investigated crystal. The condition for total suppression of the multiple-wave interaction outside the investigated crystals by a polarization analyzer is derived theoretically from the modified Born approximation. By means of the partial suppression of the strong interfering component, the increase in the visibility of multiple-wave interference is experimentally and theoretically demonstrated. The proposed experimental polarization-resolved technique provides an operational way to enhance the visibility of X-ray multiple-wave interaction outside the investigated crystals for direct phase determination.

Quasi-universality and scaling behaviour of three-wave X-ray interference in crystals: A novel way for the quantitative determination of X-ray reflection phases

Abstract A quasi-universal function describing the intensity distribution of three-wave X-ray interaction in single crystals is derived for the first time from X-ray dynamical theory under the Bethe approximation for inversion-symmetry related reflections. The intensity distributions of three-wave interference can be scaled down to this dimensionless universal function with proper scaling factors. A newly derived intensity-ratio formula provides a new way of analyzing three-wave diffraction data for direct quantitative determination of X-ray reflection phases of complex systems involving macromolecules.

Direct measurement of spatial distortions of charge density waves in K0.3MoO3

Using x-ray scattering and multiple diffraction on a charge density wave (CDW) material, K0.3MoO3, under applied voltages, we demonstrate that the occurrence of nonlinear conductivity caused by the periodic media is through the internal deformation of the CDW lattice, i.e., a phase jump of 2π, as the applied voltage exceeds the threshold. From the evolution of the measured peak width of satellite reflections as a function of the field strength, we also report that the CDW lattice can be driven to move and undergo a dynamic phase transition from the disordered pinning state to ordered moving solid state and then to disordered moving liquid.

X-ray multiple-wave coherent interaction in a quasi-two-dimensional material NbSe2-2H.

The first observation of the X-ray multiple-wave interaction in an incommensurate charge-density-wave (CDW) modulated structure at low temperatures is reported for an example of a quasi-two-dimensional material, NbSe(2)-2H. Via the coherent interaction between the X-ray waves propagating in the CDW-modulated structure and the host structure, the phase-dependent intensity variations of a CDW reflection were detected. In accord with a centrosymmetric structure, the phases of the structure-factor triplets of two CDW reflections and a Bragg reflection of the host structure were determined to be either 0 or 180 degrees, and not to vary with temperature. Relative phase differences of the two CDW reflections are also deduced.

Anomalous Dispersion Behavior of Multiple-Wave X-Ray Diffraction at Absorption Edges

The effects of anomalous dispersion (resonance) on multiple reflection of x rays and their interference in crystals at atomic absorption edges are studied. Intensity ratios of two inversion-symmetry-related multiple diffractions at or near absorption edges exhibit highly phase-sensitive profiles with strong asymmetric characteristics, unlike those far from the edges. A new resonance perturbation Bethe approach is developed to explain this behavior. This leads to direct determination of the phase change for x-ray reflections at resonance.