Ultrafast Structural Dynamics of Photo-Reactions Revealed by Model-Independent X-ray Cross-Correlation Analysis

We applied angular X-ray Cross-Correlation analysis (XCCA) to scattering images from a femtosecond resolution LCLS X-ray free-electron laser (XFEL) pump-probe experiment with solvated PtPOP ([Pt$_2$(P$_2$O$_5$H$_2$)$_4$]$^{4-}$) metal complex molecules. The molecules were pumped with linear polarized laser pulses creating an excited state population with a preferred orientational (alignment) direction. Two time scales of $1.9\pm1.5$ ps and $46\pm10$ ps were revealed by model-independent XCCA, associated with an internal structural changes and rotational dephasing, respectively. Our studies illustrate the potential of XCCA to reveal hidden structural information in a model independent analysis of time evolution of solvated metal complex molecules.Comment: 8 pages, 5 figures, 50 reference

Local structure of semicrystalline P3HT films probed by nanofocused coherent x-rays

We present results of an x-ray study of structural properties of semicrystalline polymer films using nanofocused x-ray beam. We applied the x-ray cross-correlation analysis (XCCA) to scattering data from blends of poly(3-hexylthiophene) (P3HT) embedded with gold nanoparticles (AuNPs). Spatially resolved maps of orientational distribution of crystalline domains allow us to distinguish sample regions of predominant face-on morphology,with a continuous transition to edge-on morphology. The average size of crystalline domains was determined to be of the order of 10 nm. As compared to pristine P3HT film, the P3HT/AuNPs blend is characterized by substantial ordering of crystalline domains, which can be induced by Au nanoparticles. The inhomogeneous structure of the polymer film is clearly visualized on the spatially resolved nanoscale 2D maps obtained using XCCA. Our results suggest that the observed changes of the polymer matrix within crystalline regions can be attributed to nanoconfinement in the presence of gold nanoparticles.Comment: 10 pages, 6 figures, 53 reference

Quantum Imaging with Incoherently Scattered Light from a Free-Electron Laser

The advent of accelerator-driven free-electron lasers (FEL) has opened new avenues for high-resolution structure determination via diffraction methods that go far beyond conventional x-ray crystallography methods. These techniques rely on coherent scattering processes that require the maintenance of first-order coherence of the radiation field throughout the imaging procedure. Here we show that higher-order degrees of coherence, displayed in the intensity correlations of incoherently scattered x-rays from an FEL, can be used to image two-dimensional objects with a spatial resolution close to or even below the Abbe limit. This constitutes a new approach towards structure determination based on incoherent processes, including Compton scattering, fluorescence emission or wavefront distortions, generally considered detrimental for imaging applications. Our method is an extension of the landmark intensity correlation measurements of Hanbury Brown and Twiss to higher than second-order paving the way towards determination of structure and dynamics of matter in regimes where coherent imaging methods have intrinsic limitations

A hybrid optoelectronic Mott insulator

The coupling of electronic degrees of freedom in materials to create "hybridized functionalities" is a holy grail of modern condensed matter physics that may produce versatile mechanisms of control. Correlated electron systems often exhibit coupled degrees of freedom with a high degree of tunability which sometimes lead to hybridized functionalities based on external stimuli. However, the mechanisms of tunability and the sensitivity to external stimuli are determined by intrinsic material properties which are not always controllable. A Mott metal-insulator transition (MIT) is technologically attractive due to the large changes in resistance, tunable by doping, strain, electric fields, and orbital occupancy but not, in and of itself, controllable with light. Here, an alternate approach is presented to produce optical functionalities using a properly engineered photoconductor/strongly correlated hybrid heterostructure. This approach combines a photoconductor, which does not exhibit an MIT, with a strongly correlated oxide, which is not photoconducting. Due to the intimate proximity between the two materials, the heterostructure exhibits giant volatile and nonvolatile, photoinduced resistivity changes with substantial shifts in the MIT transition temperatures. This approach can be extended to other judicious combinations of strongly correlated materials

Angular X-ray Cross-Correlation Analysis (AXCCA): Basic Concepts and Recent Applications to Soft Matter and Nanomaterials

Angular X-ray cross-correlation analysis (AXCCA) is a technique which allows quantitative measurement of the angular anisotropy of X-ray diffraction patterns and provides insights into the orientational order in the system under investigation. This method is based on the evaluation of the angular cross-correlation function of the scattered intensity distribution on a two-dimensional (2D) detector and further averaging over many diffraction patterns for enhancement of the anisotropic signal. Over the last decade, AXCCA was successfully used to study the anisotropy in various soft matter systems, such as solutions of anisotropic particles, liquid crystals, colloidal crystals, superlattices composed by nanoparticles, etc. This review provides an introduction to the technique and gives a survey of the recent experimental work in which AXCCA in combination with micro- or nanofocused X-ray microscopy was used to study the orientational order in various soft matter systems

Analysis of the shape of x-ray diffraction peaks originating from the hexatic phase of liquid crystal films

X-ray diffraction studies of the bond-orientational order in the hexatic-B phase of 75OBC and 3(10)OBC compounds are presented. The temperature evolution of an angular profile of a single diffraction peak is analyzed. Close to the hexatic-B–smectic-A transition these profiles can be approximated by the Gaussian function. At lower temperatures in the hexatic-B phase the profiles are better fitted by the Voigt function. Theoretical analysis of the width of diffraction peaks in three-dimentional (3D) hexatics is performed on the basis of the effective Hamiltonian introduced by Aharony and Kardar. Theoretical estimations are in good agreement with the results of x-ray experiments

Angular structure factor of the hexatic-B liquid crystals: bridging theory and experiment

We report results from X-ray scattering studies of the angular structure factor of liquid crystal hexatic-B films. According to the sixfold rotational symmetry of the hexatic-B phase, its characteristic scattering splits into six reflections. The shape of the radial and angular cross-sections of these reflections and their temperature evolution are analyzed. We find that over a wide temperature range of the hexatic-B phase existence the angular profiles of the in-plane X-ray scattering are well fitted by the Voigt function, which is a convolution of the Gaussian and Lorentzian functions. This result is supported by the known theoretical considerations of the hexatic structure factor below the smectic-hexatic phase transition temperture. Similar predictions for the angular shape of the hexatic peak in the vicinity of the smectic-hexatic phase transition temperature follow from the multicritical scaling theory of the hexatic-B phase in three dimensions. We find that the specific shape of the hexatic structure factor can be explained by the interplay of two distinct contributions to the free energy of the system, a liquid-like density term and a coupling term between the bond-orientational order and short-range density fluctuations

Angular X-ray Cross-Correlation Analysis (AXCCA): Basic Concepts and Recent Applications to Soft Matter and Nanomaterials

Angular X-ray cross-correlation analysis (AXCCA) is a technique which allows quantitativemeasurement of the angular anisotropy of X-ray diraction patterns and provides insights into theorientational order in the system under investigation. This method is based on the evaluation of theangular cross-correlation function of the scattered intensity distribution on a two-dimensional (2D)detector and further averaging over many diraction patterns for enhancement of the anisotropicsignal. Over the last decade, AXCCA was successfully used to study the anisotropy in varioussoft matter systems, such as solutions of anisotropic particles, liquid crystals, colloidal crystals,superlattices composed by nanoparticles, etc. This review provides an introduction to the techniqueand gives a survey of the recent experimental work in which AXCCA in combination with micro- ornanofocused X-ray microscopy was used to study the orientational order in various soft matter systems