Synchrotron ARPES System with PHOIBOS Analyzer

Three-dimensional (3D) electronic band structure is fundamental for understanding a vast diversity of physical phenomena in solid-state systems, including topological phases, interlayer interactions in van der Waals materials, dimensionality-driven phase transitions, etc.  Interpretation of ARPES data in terms of 3D electron dispersions is commonly based on the free-electron approximation for the  photoemission final states. Our soft-X-ray ARPES data on Ag metal reveals, however, that even at high excitation energies the final states can be a way more complex, incorporating several Bloch waves with different out-of-plane momenta. Such multiband final states manifest themselves as a complex structure and added broadening of the spectral peaks from 3D electron states. We analyse the origins of this phenomenon, and trace
it to other materials such as Si and GaN. Our findings are essential for accurate determination of the 3D band structure over a wide range of materials and excitation energies in the ARPES experiment.

V. N. Strocov, L. L. Lev, F. Alarab, P. Constantinou, X. Wang, T. Schmitt, T. J. Z. Stock, L. Nicolaï, J. Očenášek, J. Minár
Nature Communications, Volume 14, Article number: 4827 (2023)

The photoelectron momentum microscope (PMM) in operation at BL6U, an undulator-based soft x-ray beamline at the UVSOR Synchrotron
Facility, offers a new approach for μm-scale momentum-resolved photoelectron spectroscopy (MRPES). A key feature of the PMM is that it
can very effectively reduce radiation-induced damage by directly projecting a single photoelectron constant energy contour in reciprocal
space with a radius of a few Å−1 or real space with a radius of a few 100 μm onto a two-dimensional detector. This approach was applied to
three-dimensional valence band structure E(k) and E(r) measurements (“stereography”) as functions of photon energy (hν), its polarization
(e), detection position (r), and temperature (T). In this study, we described some examples of possible measurement techniques using a soft
x-ray PMM. We successfully applied this stereography technique to μm-scale MRPES to selectively visualize the single-domain band structure
of twinned face-centered-cubic Ir thin films grown on Al2O3(0001) substrates. The photon energy dependence of the photoelectron intensity
on the Au(111) surface state was measured in detail within the bulk Fermi surface. By changing the temperature of 1T-TaS2, we clarified
the variations in the valence band dispersion associated with chiral charge-density-wave phase transitions. Finally, PMMs for valence band
stereography with various electron analyzers were compared, and the advantages of each were discussed.

Fumihiko Matsui, Kenta Hagiwara, Eiken Nakamura, Takayuki Yano, Hiroyuki Matsuda,
Yasuaki Okano, Satoshi Kera, Eri Hashimoto, Shinji Koh, Keiji Ueno, Takahiro Kobayashi,
Emi Iwamoto, Kazuyuki Sakamoto, Shin-ichiro Tanaka, and Shigemasa Suga
Rev. Sci. Instrum. 94, 083701 (2023)