Available Equipments (ELI-ERIC)

Contact person: László Óvári PhD, email: laszlo.ovari_at_eli-alps.hu

Brief description of the available set up

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

I. Preparation chamber

The information depth is around 5nm, therefore the sample surface must be clean, properly prepared and characterized. The preparation chamber is equipped with:

• X-ray photoelectron spectroscopy (XPS) for chemical surface analysis:
  • with a Scienta Omicron XM 1000 monochromated Al Kα X-ray source and an Argus CU hemispherical electron analyzer
  • FWHM for Ag 3d_5/2 is better than 0.6 eV
  • with imaging capability (lateral resolution ~100 micron)
• Low energy electron diffraction (LEED) for structural surface analysis (Fig. 2) using the BDL 600IR 4-grid rear view LEED of OCI Vacuum Microengineering, capable also for Auger electron spectroscopy (AES)
• Ar⁺ sputtering gun for surface cleaning (IS 40C1 from Prevac)
• Preparation manipulator with low and high temperature stages 100 to 1500 K 
• Mass spectrometer up to 300 amu (RGA300, Stanford Research Systems)
• Metal evaporator for the preparation of ultrathin films, monolayers etc. (EFM3, Focus)
• Gas dosing with a microcapillary array even for liquids with low vapor pressure 

Fig. 2 LEED pattern of Au(111) with the herringbone reconstruction

II. Analysis chamber

Main NanoESCA operation modes:

• Photoemission Electron Microscopy (PEEM) mode: laterally resolved microscopy of the sample surface
• Energy filtered real space imaging mode (Imaging Photoelectron Spectroscopy mode): laterally and energy resolved microscopy of the sample surface (Fig. 3-4)
• Momentum microscopy (k-space) mode: measuring the momentum distribution of photoelectrons originated from a preselected small area (~40 µm) of the sample surface with energy resolution (Fig. 3-5)
• Spin filtered imaging mode: with a state-of-the-art Au/Ir(100) imaging spin filter. The detected electrons can be filtered according to thnanoaeir spin state at both real space and momentum space (k-space) imaging (Fig. 6).

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).

 Specifications:

NanoESCA real and k-space modes:

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 k_x-k_y-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). 

Spin filtered imaging mode:

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).

 References
[1] M. Escher et al., J. Phys.: Condens. Matter 17 (2005) S1329 https://doi.org/10.1088/0953-8984/17/16/004
[2] D. Vasilyev et al., J. Electron Spectr. Relat. Phenom. 199 (2015) 10