Microscopie électronique en transmission

vignette|upright=1.5|Principe de fonctionnement du microscope électronique en transmission. vignette|Un microscope électronique en transmission (1976). La microscopie électronique en transmission (MET, ou TEM pour l'anglais transmission electron microscopy) est une technique de microscopie où un faisceau d'électrons est « transmis » à travers un échantillon très mince. Les effets d'interaction entre les électrons et l'échantillon donnent naissance à une image, dont la résolution peut atteindre 0,08 nanomètre (voire ). Les images obtenues ne sont généralement pas explicites, et doivent être interprétées à l'aide d'un support théorique. L'intérêt principal de ce microscope est de pouvoir combiner cette grande résolution avec les informations de l'espace de Fourier, c'est-à-dire la diffraction. Il est aussi possible d'identifier la composition chimique de l'échantillon en étudiant le rayonnement X provoqué par le faisceau électronique. Contrairement aux microscopes optiques, la résolutio

Real space and reciprocal space investigations of the spin dynamics in skyrmion-hosting materials

Le Yu

Magnetic skyrmions are whirl-like spin configurations with particle-like properties protected by non-trivial topology. Due to their unique spin structures and dynamical properties, they have attracted tremendous interests over the past decade, from fundamental science to technical applications. However, experimental methods to study skyrmion dynamics in real space are limited until present. The goal of this thesis is to both apply and develop novel methodologies to firstly discover new skyrmion lattice hosting systems through neutron scattering experiments, and then directly observe the dynamics in these materials in real space by time-resolved imaging. The main content of this thesis is divided into two parts: firstly, I will demonstrate by means of inelastic neutron scattering how the magnon spectrum was measured in VOSe2O5, one of the few known systems hosting a Néel-type skyrmion lattice. By comparing the measured spectra with SpinW simulations, the exchange interactions and magnetic anisotropy were extracted so that the spin Hamiltonian in this system could be constructed. The results provide positive feedback to the recently developed quantum chemistry calculations. Further, the reasonable consistency among experiments, simulations and calculations may enable the community to use quantum chemistry as a novel tool to predict skyrmion hosting materials and understand their formation. The second part of this thesis will introduce my work in integrating a microwave system into a time-resolved transmission electron microscope (TEM). In this project, a pump-probe technique using microwave to coherently excite spin wave dynamics in skyrmion lattice hosting materials in the GHz frequency range was developed and implemented. The time-resolved TEM will enable the direct observation of excited spin waves and skyrmion dynamics in real space. This instrumental development will also allow the study of the spatial homogeneity of resonant modes, and their behavior around individual impurities or grain boundaries. The insights gained from such experiments would help the community to understand microscopic aspects of skyrmion dynamics and tailor spintronic devices using skyrmions in the future. The chapter layout is presented as follows: in Chapter One, I introduce fundamental concepts about skyrmions, and then discuss their general magnon spectrum and how they can be measured. Chapter Two describes methodologies adopted during this thesis work, including experimental techniques and modelling methods. The scattering experiment details and results obtained at various beamlines are presented in Chapter Three. The progress concerning the development of the new technique in time-resolved real space imaging starts from Chapter Four. Here, the implementation of a microwave system in ultrafast TEM is demonstrated systematically. The last chapter contains the conclusion and outlook.

EPFL2022

Demonstration of Near-field THz Spectroscopy Using Ultrafast Electron Microscopy

Fabrizio Carbone, Ido Kaminer, Giovanni Maria Vanacore, Kangpeng Wang

We probe near-field THz pulses with nm-fs spatio-temporal resolution, through their interaction with free-electron pulses in an ultrafast transmission electron microscope. Our experiment can help understand THz generation and nanoscale THz phenomena. (C) 2021 The Author(s)

IEEE2021

Oocyte CD9 is enriched on the microvillar membrane and required for normal microvillar shape and distribution

Surabhi Gupta, Henning Paul-Julius Stahlberg

Microvilli are found on the surface of many cell types, including the mammalian oocyte, where they are thought to act in initial contact of sperm and oocyte plasma membranes. CD9 is currently the only oocyte protein known to be required for sperm-oocyte fusion. We found CD9 is localized to the oocyte microvillar membrane using transmission electron microscopy (TEM). Scanning electron microscopy (SEM) showed that CD9 null oocytes, which are unable to fuse with sperm, have an altered length, thickness and density of their microvilli. One aspect of this change in morphology was quantified using TEM by measuring the radius of curvature at the microvillar tips. A small radius of curvature is thought to promote fusibility and the radius of curvature of microvillar tips on CD9 wild-type oocytes was found to be half that of the CD9 null oocytes. We found that oocyte CD9 co-immunoprecipitates with two Ig superfamily cis partners, EWI-2 and EWI-F, which could have a role in linking CD9 to the oocyte microvillar actin core. We also examined latrunculin B-treated oocytes, which are known to have reduced fusion ability, and found altered microvillar morphology by SEM and TEM. Our data suggest that microvilli may participate in sperm-oocyte fusion. Microvilli could act as a platform to concentrate adhesion/fusion proteins and/or provide a membrane protrusion with a low radius of curvature. They may also have a dynamic interaction with the sperm that serves to capture the sperm cell and bring it into close contact with the oocyte plasma membrane. (c) 2006 Elsevier Inc. All rights reserved.

Elsevier BV2007

Real space and reciprocal space investigations of the spin dynamics in skyrmion-hosting materials

Le Yu

EPFL2022