Development of a high-energy-, high-spatial-, and high-momentum-resolution electron energy loss spectrometer with liquid-Helium cooling stage (HR3-EEL spectrometer)

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The last two decades have witnessed substantial development of spectroscopy techniques that probe low-energy excitations in solids, which, in many cases, lead to important breakthroughs in the understanding of fundamental solid-state effects. Prominent examples are Angular Resolved Photoemission Spectroscopy (ARPES) or Scanning Tunneling Spectroscopy (STS), which, amongst others, demonstrated the existence of gapless surface states in topological insulators and anisotropic gaps in high-temperature superconductors. That was possible because these techniques probe excitations at the surface with an excellent energy resolution (meV regime and below depending on cryogenic conditions) and high spatial or angular resolution. Nevertheless, a spectroscopic technique for analyzing electronic and vibrational excitations in the bulk (and surface) with simultaneous high-energy, high-momentum, and high-spatial resolution down to deep cryogenic conditions (liquid Helium cooling) is still missing. Such a tool would, for instance, allow studying the diverse class of low-dimensional structures (e.g., interfaces, van der Waals materials) with high momentum resolution in the extended dimensions, while maintaining high-spatial resolution in the inhomogeneous direction. It would allow deliberately tailoring spatial and momentum resolution for investigating the details of the dielectric response in complex nanostructures (e.g. nanophotonic and electronic devices, organic semiconductors, but also defects) including non-local dielectric response effects such as spatial variations in the dielectric dispersion. It is the aim of this project to realize this ambition by building a comprehensively improved Transmission Electron Energy Loss spectrometer. Crucial ingredients are a newly developed magnetic ground potential monochromator, a new continuous flow L-He sample stage, an improved sector magnet energy filter, and fast 2D CMOS electron detectors, which are integrated into an adapted state-of-the-art TEM platform including a cold field emission gun and a comprehensive, freely configurable condenser and projection optics, amongst others. The instrument will reach sub 10 meV energy resolution, few nm spatial resolution, sub 1/µm momentum resolution, and sub 10 K temperature in parallel in EEL spectroscopy mode as well as atomic resolution, electric and magnetic field sensing in other operation modes. This breakthrough in various key figures allows drastically widen our basic understanding of the charge density response function in various topical materials such as high-Tc superconductors, topological insulators, Weyl and Dirac semimetals, plasmonic nanostructures, organic semiconductors, soft biologic matter, or correlated matter, providing insight into their specific and universal ground state and dynamic properties (e.g., charge transport, dielectric response, electronic and phononic band structure, charge ordering).