Spatio-temporal observation of dynamical structures in order-disorder phenomena and phase transitions via Ultrafast Electron Diffraction

The development of ultrafast time-resolved techniques in the last decades allowed the direct observation of out-of-equilibrium transient states and dynamical processes up to then hidden in many different fields, ranging from chemistry to solid state physics and biology. Amongst them, Ultrafast Electron Diffraction (UED) allows the retrieval of direct information on the structural dynamics of solids, surfaces, nanostructures and molecular crystals with femtosecond (10^-15 s) and atomic (Ã…, 10^-10 m) temporal and spatial resolution. The UED setup implemented at EPFL is characterised by 30 keV, 20 kHz pulses containing up to 105 electrons each and can work in both transmission and reflection geometry. A sub-500 fs time resolution in transmission geometry is achieved by means of a radio frequency compression cavity for electrons. This time resolution, combined with the high transverse coherence of our electrons, allowed the observation of order-disorder phenomena in 2D-supracrystals of functionalised gold nanoparticles. The insurgence of ligand-dependent photo- mechanical stiffness phenomena was observed and characterised in the aforementioned systems. In reflection geometry, the time resolution suffered severe limitations - especially when probing mm2-sized samples - due to the velocity mismatch between the pump and the probe beam. To overcome this limitation, an optimal optical front tilt scheme for the pump pulse was designed, implemented and characterised, thus disclosing a sub-500 fs temporal resolution also in reflection geometry. The newly achieved time resolution allowed the direct visualisa- tion of ultrafast structural dynamics in two systems of current interest: graphite and magnetite. In graphite, the selective excitation via electron-phonon coupling of a subset of phonon modes called Strongly Coupled Optical Phonons - with a characteristic timescale of ~ 500 fs - was experimentally resolved. In magnetite, the photoinduction of the Verwey phase transition was achieved, unveiling phase segregation intermediate states during the transformation and disclosing structural differences between the photoinduced and the thermal phase transition mechanism.

Femtosecond diffractive imaging of structures, charge and spin textures

Giulia Fulvia Mancini

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 Å 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.

EPFL2015

Glass transition associated with the jamming of vibrated granular matter

Gérard Gremaud, Daniele Mari, Alessandro Luigi Sellerio

We investigate a vibrated granular system composed of millimeter-size glass beads. When the system is submitted to a perturbation with decreasing intensity, below the fluidization limit, it evolves in a way similar to glass-forming liquids until it reaches an amorphous jammed state. This jamming transition is observed by the means of an immersed oscillator, either in the free or forced mode, while the granular medium is submitted to an external perturbation at a fixed frequency or within a frequency band. The complex susceptibility of the oscillator is measured as a function of the probe forcing frequency or as a function of the perturbation intensity. Data show that the jamming dynamics is "activated," similarly to thermal systems. The empirical control parameter is found proportional to the square root of the vibration intensity and inversely proportional to the vibration frequency. In the case of broadband external vibration, the average frequency of the power spectrum has to be considered.

2011

Unified analysis of topological defects in 2D systems of active and passive disks

Pasquale Digregorio, Ignacio Pagonabarraga Mora

We provide a comprehensive quantitative analysis of localized and extended topological defects in the steady state of 2D passive and active repulsive Brownian disk systems. We show that, both in and out-of-equilibrium, the passage from the solid to the hexatic is driven by the unbinding of dislocations, in quantitative agreement with the KTHNY singularity. Instead, extended clusters of defects largely dominate below the solid-hexatic critical line. The latter percolate in the liquid phase very close to the hexatic-liquid transition, both for continuous and discontinuous transitions, in the homogeneous liquid regime. At critical percolation the clusters of defects are fractal with statistical and geometric properties that are independent of the activity and compatible with the universality class of uncorrelated critical percolation. We also characterize the spatial organization of point-like defects and we show that the disclinations are not free, but rather always very near more complex defect structures. At high activity, the bulk of the dense phase generated by Motility-Induced Phase Separation is characterized by a density of point-like defects, and statistics and morphology of defect clusters, set by the amount of activity and not the packing fraction. Hexatic domains within the dense phase are separated by grain-boundaries along which a finite network of topological defects resides, interrupted by gas bubbles in cavitation. This structure is dynamic in the sense that the defect network allows for an unzipping mechanism that leaves free space for gas bubbles to appear, close, and even be released into the dilute phase.

ROYAL SOC CHEMISTRY2021

Femtosecond diffractive imaging of structures, charge and spin textures

Giulia Fulvia Mancini

EPFL2015