The project is divided into two main areas: Holographic tomography of physical fields in materials and Quantum state reconstruction of inelastically scattered electrons for comprehensive analysis of dielectric response.
In order achieve atomic-resolution in the 3D reconstruction of electrostatic fields we combined two ideas. First, we holographically reconstruct the maximal information extractable from a tomographic elastic scattering experiment, i.e. the electron wave function. Second, we identified a suitable (nonlinear) mapping of that holographic tilt series into a projection series linearly depending on the 3D electrostatic potential. This scheme has been successfully applied to simulated tilt series of wave functions, which had been scattered on nanoobjects. Several major breakthroughs have been accomplished in the development of magnetic vector field tomography for nanoscale magnetic textures (Fig. 1). We developed dedicated multiple tilt axis holographic series acquisition and reconstruction routines for the magnetization texture in 3D nanomagnets (such as skyrmions, Bloch points, etc.). Moreover, we extended the method to cryogenic conditions and developed a insitu magnetic field holder, which allowed us to reconstruct for the first time the 3D magnetic texture pertaining to Bloch skyrmion tubes in a helimagnet. The problem of 3D strain field reconstruction as been tackled by our ERC project partner from the TU Berlin, who developed and tested several tomographic schemes for the 3D reconstruction of strain fields including the beam tilt method proposed in AToM. We developed a novel method, which allows to distinguish between mesoscale potentials due to space charge regions (purely electronic potentials) and atomic potentials reflecting the chemical composition (i.e. mean inner potentials), by combining electron holographic tomography and high-angle annular dark field tomography. This method has been applied to multi-quantum-well semiconductor nanowires, where the 3D morphology of the wells determine its optical properties. We developed a novel automatic residual aberration correction scheme dedicated to 2D materials, which allows study the charge distribution in a dose optimized way. We completed the development of the JEOL 2010F JEM Dresden-special insitu TEM. We installed the continuous-flow liquid He cryostat and performed the first cryogenic experiments on topical materials (e.g. charge density waves at low temperatures, magnetic cycloic and helical modulation formation in lacunar spinels and LaTiO3 thin films as well as the formation of mesoscale polar domain patterns). The experiences gathered from this experimental setup are now exploited for a the development and set-up of a novel high-resolution transmission electron microscope dedicated to electron energy loss spectroscopy (HR3-EELS), which will be equipped with a largely improved liquid Helium cooling stage.
One original goal in this area is the measurement of all Cartesian components of the probability current of the inelastically scattered electron beam as a means to measure transient electric and magnetic fields pertaining charge excitations. To that end we developed the inelastic momentum transfer method, where we scan a focused electron probe over the sample and measure its deflection in an energy resolved way. We have employed the technique to study various aluminum based plasmonic nanostructures, plasmonic wave guides based on nanoparticle chains as well as thin gold networks. Recently, the technique has been significantly improved by recording the deflection data in the omega-q resolved way, facilitating better control / correction of the energy drift as well as mitigation of the elastic vignetting problem hampering the interpretation of the deflection data in terms of transient electric and magnetic fields. Moreover, we showed, how the fully non-local response can be completely reconstructed by employing inelastic electron ptychography, which is currently not possible to obtain from other spectroscopic techniques. In order to develop a fully relativistic inelastic electron scattering code, we elaborated on the theoretical foundations of inelastic electron scattering. We particularly studied the ramifications of 3D effects (typically neglected in current inelastic scattering implementations) and provided a unified framework for core-loss and low-loss scattering incorporating current-current coupling as the leading order relativistic effects in both core-loss and low-loss scattering. Recently, we incorporated the implications of axion electrodynamics occurring at the surface of topological insulators in this scheme.
