Femtosecond diffractive imaging of structures, charge and spin textures

Abstract

The study of the dynamical evolution of a system, e.g. a chemical reaction or a phase transition, has become nowadays a subject of growing scientific interest. The direct observation of the steps and the structural changes through which chemical reactions and phase transitions occur requires the study of out-of-equilibrium intermediate states, with a typical time scale of femtoseconds (10─15s) and with high spatial resolution (10─10m), for molecules as well as for complex biological samples and for condensed matter systems. In this frame, Ultrafast Electron Diffraction (UED) allows for the observation of changes in the response function of different systems through phase transitions and chemical reactions, with atomic resolution, at the femtosecond (fs) time scale. In this thesis the implementation of a flexible UED set-up capable of working in both transmission and RHEED geometries with 30 keV electrons is described; the experiment is characterized by 300 fs time-resolution for bunches containing up to 105 electrons at 20 kHz, and constitutes the first demonstration of far-field and small-angle diffraction (from few A to a few tens of nm spatial resolution) with electrons in a single diffractometer. The transverse coherence of the probing electrons can be tuned at the cost of brightness, enabling the observation of the speckle pattern originating from the constructive and destructive interference of the scattered waves from a sample. Diffractive Imaging is carried out to study complex ordering/disordering phenomena in structure, charge and spin textures, with time resolution. The investigation of the dynamics of a supracrystal of alkanethiol-coated gold nanoparticles is reported, and its photo-induced thermal disorder is reconstructed in a real-space movie alongside with an ordering transition of the organic ligands in the picosecond (ps) time scale, by means of UED. The same principles developed for the analysis of the gold nanoparticles diffraction patterns, based on Angular Cross-Correlation and Angular Normalized Intensity Analysis, are applied to the speckle pattern from spin textures detected in Lorentz microscopy. With this aim, magnetic field-dependent diffractive imaging of spin textures in the Dzyaloshinkii-Moriya chiral insulator Cu2OSeO3 is carried out with cryo- Lorentz TEM. Following a similar approach, ultrafast diffractive imaging on the charge density modulation in the Charge Density Waves system 1T-TaS2 is demonstrated. Finally, the main conclusions are summarized and the potential for extending the applicability of the UED system to the regime of ultrafast coherent diffractive imaging and inline holography using 30 keV electrons on a table-top is described.