Contact person: László Óvári PhD, email: laszlo.ovari_at_eli-alps.hu
The NanoESCA endstation (Fig. 1) is a state-of-the-art powerful tool to study the electronic structure of surfaces and the condensed phase. Our aim for the near future is to conduct NIR pump - XUV probe measurements in order to reveal surface dynamics, like ultrafast processes in topological insulators, ultrafast magnetism, electron transfer processes, band structure evolution, etc. Although the connection of laser sources to the NanoESCA is not yet ready, we can accept proposals relying on the internal cw light sources of NanoESCA allowing for cutting edge static measurements. The system consists of a Preparation Chamber and an Analysis Chamber.
Fig. 1 The NanoESCA Endstation at the ELI-ALPS
The information depth is around 5nm, therefore the sample surface must be clean, properly prepared and characterized. The preparation chamber is equipped with:
Fig. 2 LEED pattern of Au(111) with the herringbone reconstruction
For each NanoESCA measurement modes the internal light sources available at the moment are: HIS 14 He VUV lamp (21.2 and 40.8 eV lines) and Hg lamp. The analysis chamber has a 4-axis LHe cooled sample stage for low temperature studies (≤30 K).
NanoESCA is a sort of electron microscope, which uses the sample as the electron source. The (photo) electrons excited by a photon source are collected by the immersion lens. At the backfocal plane contrast apertures can be positioned to limit the acceptance angle, thus reducing spherical aberrations and yielding better lateral resolution. In the first image plane an iris is located by which the region of interest can be limited (Fig. 3). In PEEM mode this image is projected on a 2D detector by projective lenses without any energy filtering.
For energy filtered imaging a hemispherical analyzer can be used and the PEEM column can be considered as the entrance lens of the analyzer. Hemispherical electron analyzers select the kinetic energy of photoemitted electrons by a spherical potential bending the electron pathways such that the kinetic energy distribution becomes dispersed in one direction.
Fig. 3 Schematic design of the IDEA (Imaging Dispersive Energy Analyser). (Ref: Physics’ Best, April 2019)
Evidently the energy dispersion introduces severe aberrations in the image, which is compensated by a second hemisphere (IDEA design) analogous to Kepler’s orbitals. In real space imaging the first image plane is projected onto the entrance slit of the first hemisphere, while in k-space imaging the backfocal plane is projected onto the analyser entrance (Fig. 4). The aberration compensated electron beam passed through the analyzers is then imaged on the second 2D detector.
Fig. 4 Demonstration of the real space and k-space imaging; left: Polycrystalline iron sample real space image (FOV 114 µm); right: 2D Fermi surface of the Au(111) at 300 K, the first Brillouin zone is clearly visible
By sweeping the energy E of the imaged electrons it becomes possible to get the whole kx-ky-E image stack. This image stack represents a complete 3D-image of the band structure of a crystalline sample Fig. 5).
Fig. 5 Demonstration of the whole k-space parabola of the Ag(111).
the spin filter is positioned after the exit slit of the IDEA double hemispherical. The method is based on spin polarization dependent reflection. An Ir(001) scattering crystal covered by a monolayer of Au is used. The inert nature of Au allows longer measurement times without cleaning. The filter is selective along one spin direction parallel to the sample surface. This powerful method can be used for both modes of imaging: direct space for the analysis of magnetic domains and their orientations as well as determining the spin orientation of the electron states in momentum space (Fig. 6).
Fig. 6 Demonstration of the spin contrast on a polycrystalline iron sample in real space mode (FOV 130 µm).
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