Giulia F. Mancini

Filming the formation and fluctuation of skyrmion domains by cryo-Lorentz transmission electron microscopy

Significance The need for denser storage devices calls for new materials and nanostructures capable of confining single bits of information in a few nanometers. A new topological distribution of spins termed skyrmions is emerging, which promises to robustly confine a small magnetization in a few-nanometers-wide circular domain. A great deal of attention is being devoted to the understanding of these magnetic patterns and their manipulation. We manufactured a large nanoslice supporting over 70,000 skyrmions, and film their evolution in direct-space via cryo-Lorentz transmission electron microscopy. We reveal the octagonal distortion of the skyrmion lattice and show how these distortions and other defects impact its long-range order. These results pave the way to the control of a large two-dimensional array of skyrmions. Magnetic skyrmions are promising candidates as information carriers in logic or storage devices thanks to their robustness, guaranteed by the topological protection, and their nanometric size. Currently, little is known about the influence of parameters such as disorder, defects, or external stimuli on the long-range spatial distribution and temporal evolution of the skyrmion lattice. Here, using a large (7.3×7.3 μm2) single-crystal nanoslice (150 nm thick) of Cu2OSeO3, we image up to 70,000 skyrmions by means of cryo-Lorentz transmission electron microscopy as a function of the applied magnetic field. The emergence of the skyrmion lattice from the helimagnetic phase is monitored, revealing the existence of a glassy skyrmion phase at the phase transition field, where patches of an octagonally distorted skyrmion lattice are also discovered. In the skyrmion phase, dislocations are shown to cause the emergence and switching between domains with different lattice orientations, and the temporal fluctuation of these domains is filmed. These results demonstrate the importance of direct-space and real-time imaging of skyrmion domains for addressing both their long-range topology and stability.

Order/Disorder Dynamics in a Dodecanethiol-Capped Gold Nanoparticles Supracrystal by Small-Angle Ultrafast Electron Diffraction

The design and the characterization of functionalized gold nanoparticles supracrystals require atomically resolved information on both the metallic core and the external organic ligand shell. At present, there is no known approach to characterize simultaneously the static local order of the ligands and of the nanoparticles, nor their dynamical evolution. In this work, we apply femtosecond small-angle electron diffraction combined with angular cross-correlation analysis, to retrieve the local arrangement from nanometer to interatomic scales in glassy aggregates. With this technique we study a two-dimensional distribution of functionalized gold nanoparticles deposited on amorphous carbon. We show that the dodecanethiol ligand chains, coating the gold cores, order in a preferential orientation on the nanoparticle surface and throughout the supracrystal. Furthermore, we retrieve the dynamics of the supracrystal upon excitation with light and show that the positional disorder is induced by light pulses, while its overall homogeneity is surprisingly found to transiently increase. This new technique will enable the systematic investigation of the static and dynamical structural properties of nanoassembled materials containing light elements, relevant for several applications including signal processing and biology.

Temperature-dependent electron-phonon coupling in La2−xSrxCuO4 probed by femtosecond x-ray diffraction

The strength of the electron-phonon coupling parameter and its evolution throughout a solids phase diagram often determines phenomena such as superconductivity, charge- and spin-density waves. Its experimental determination relies on the ability to distinguish thermally activated phonons from those emitted by conduction band electrons, which can be achieved in an elegant way by ultrafast techniques. Separating the electronic from the out-of-equilibrium lattice subsystems, we probed their reequilibration by monitoring the transient lattice temperature through femtosecond x-ray diffraction in La2-xSrxCuO4 single crystals with x = 0.1 and 0.21. The temperature dependence of the electron-phonon coupling is obtained experimentally and shows similar trends to what is expected from the ab initio calculated shape of the electronic density of states near the Fermi energy. This study evidences the important role of band effects in the electron-lattice interaction in solids, in particular, in superconductors.

The role of the coherence in the cross-correlation analysis of diffraction patterns from two-dimensional dense mono-disperse systems.

The investigation of the static and dynamic structural properties of colloidal systems relies on techniques capable of atomic resolution in real space and femtosecond resolution in time. Recently, the cross-correlation function (CCF) analysis of both X-rays and electron diffraction patterns from dilute and dense aggregates has demonstrated the ability to retrieve information on the sample’s local order and symmetry. Open questions remain regarding the role of the beam coherence in the formation of the diffraction pattern and the properties of the CCF, especially in dense systems. Here, we simulate the diffraction patterns of dense two-dimensional monodisperse systems of different symmetries, varying the transverse coherence of the probing wave, and analyze their CCF. We study samples with different symmetries at different size scale, as for example, pentamers arranged into a four-fold lattice where each pentamer is surrounded by triangular lattices, both ordered and disordered. In such systems, different symmetry modulations are arising in the CCF at specific scattering vectors. We demonstrate that the amplitude of the CCF is a fingerprint of the degree of the ordering in the sample and that at partial transverse coherence, the CCF of a dense sample corresponds to that of an individual scattering object.

Quantitative Chemically Specific Coherent Diffractive Imaging of Reactions at Buried Interfaces with Few Nanometer Precision

We demonstrate quantitative, chemically specific imaging of buried nanostructures, including oxidation and diffusion reactions at buried interfaces, using nondestructive tabletop extreme ultraviolet (EUV) coherent diffractive imaging (CDI). Copper nanostructures inlaid in SiO2 are coated with 100 nm of aluminum, which is opaque to visible light and thick enough that neither visible microscopy nor atomic force microscopy can image the buried interface. Short wavelength high harmonic beams can penetrate the aluminum layer, yielding high-contrast images of the buried structures. Quantitative analysis shows that the reflected EUV light is extremely sensitive to the formation of multiple oxide layers, as well as interdiffusion of materials occurring at the metal-metal and metal-insulator boundaries deep within the nanostructure with few nanometers precision.

Spatial, spectral, and polarization multiplexed ptychography.

We introduce a novel coherent diffraction imaging technique based on ptychography that enables simultaneous full-field imaging of multiple, spatially separate, sample locations. This technique only requires that diffracted light from spatially separated sample sites be mutually incoherent at the detector, which can be achieved using multiple probes that are separated either by wavelength or by orthogonal polarization states. This approach enables spatially resolved polarization spectroscopy from a single ptychography scan, as well as allowing a larger field of view to be imaged without loss in spatial resolution. Further, we compare the numerical efficiency of the multi-mode ptychography algorithm with a single mode algorithm.

Spatial, Spectral, and Polarization Multiplexed Ptychography

We demonstrate ptychographic imaging of multiple areas of a sample simultaneously with no loss of resolution, by using different wavelengths or polarizations to collect independent diffraction patterns in parallel. This significantly reduces imaging times.

Reflection Mode Tabletop Coherent Diffraction Imaging of Buried Nanostructures

We image a nanostructured sample through a visibly-opaque 100nm layer of aluminum using lensless Fresnel Ptychography and a tabletop high harmonic source. The reconstructed amplitude-contrast image uncovered the presence of interfacial diffusion non-destructively.

Multiple beam ptychography for large field-of-view, high throughput, quantitative phase contrast imaging

The ability to record large field-of-view images without a loss in spatial resolution is of crucial importance for imaging science. For most imaging techniques however, an increase in field-of-view comes at the cost of decreased resolution. Here we present a novel extension to ptychographic coherent diffractive imaging that permits simultaneous full-field imaging of multiple locations by illuminating the sample with spatially separated, interfering probes. This technique allows for large field-of-view imaging in amplitude and phase while maintaining diffraction-limited resolution, without an increase in collected data i.e. diffraction patterns acquired.

Design and implementation of an optimal laser pulse front tilting scheme for ultrafast electron diffraction in reflection geometry with high temporal resolution

Ultrafast electron diffraction is a powerful technique to investigate out-of-equilibrium atomic dynamics in solids with high temporal resolution. When diffraction is performed in reflection geometry, the main limitation is the mismatch in group velocity between the overlapping pump light and the electron probe pulses, which affects the overall temporal resolution of the experiment. A solution already available in the literature involved pulse front tilt of the pump beam at the sample, providing a sub-picosecond time resolution. However, in the reported optical scheme, the tilted pulse is characterized by a temporal chirp of about 1 ps at 1 mm away from the centre of the beam, which limits the investigation of surface dynamics in large crystals. In this paper, we propose an optimal tilting scheme designed for a radio-frequency-compressed ultrafast electron diffraction setup working in reflection geometry with 30 keV electron pulses containing up to 105 electrons/pulse. To characterize our scheme, we performed optical cross-correlation measurements, obtaining an average temporal width of the tilted pulse lower than 250 fs. The calibration of the electron-laser temporal overlap was obtained by monitoring the spatial profile of the electron beam when interacting with the plasma optically induced at the apex of a copper needle (plasma lensing effect). Finally, we report the first time-resolved results obtained on graphite, where the electron-phonon coupling dynamics is observed, showing an overall temporal resolution in the sub-500 fs regime. The successful implementation of this configuration opens the way to directly probe structural dynamics of low-dimensional systems in the sub-picosecond regime, with pulsed electrons.