NSRRC Activity Report 2023

Facility Development and Status 107 In 2017, the approval of a novel nanoARPES beamline marked a significant step forward. Two branches, each employing different beam-focusing methods, were conceptualized to cater to research needs and broaden the scope of emergent quantum materials. The micro- focusing μARPES branch ( TPS 39A1 ) was constructed first, supported by the Taiwan Consortium of Emergent Crystalline Materials and the NSRRC. Simultaneously, plans for a nano-focusing ARPES beamline ( TPS 39A2 ) were initiated in 2019. The design of the TPS 39A nanoARPES beamline’s focusing optical systems relied on Kirkpatrick–Baez (KB) mirrors and zone-plate techniques. A new endstation with a scanner stage aimed to enable high-resolution ARPES at the micrometer and nanometer scale. The monochromator type, an active mirror–plane grating monochromator (AM–PGM), was developed by the NSRRC to ensure high energy resolution, photon flux, and a broader photon energy range. Distinct focusing methods were employed for the two branches, achieving spot sizes of 10 μm for TPS 39A1 and 100 nm for TPS 39A2 . Led by Den-Sung Lin from National Tsing Hua University and NSRRC teams, the construction of the TPS 39A1 μARPES branch was completed in early 2018. Due to delays in beamline optics shipment, restructured work on the μARPES endstation was conducted in 2021 to optimize experimental efficiency. The first synchrotron beam was achieved for beamline commissioning by the end of 2022. An elliptically polarized undulator of a 168 mm period served as the photon source, covering the vacuum ultraviolet and soft X-ray photon energy range. Despite COVID-19-related delays in beamline optics shipment, construction work remained on track. The AM–PGM underwent testing and was installed on-site in March 2022. A safety interlock system was established in December 2021, and all beamline optics were installed in April 2022 for the commissioning stage. Leveraging the advantages of bendable optics developed by the NSRRC, the incident synchrotron beam size can be precisely focused down to 20 μm with fully opened slits. This capability enables us to achieve ultrahigh energy resolution for individual photon energy. A comprehensive table detailing photon energy versus undulator gap was established during the commissioning stage. We extend an invitation to potential users to actively participate in ARPES experiments and encourage them to provide valuable feedback for optimizing the performance of the endstation. Figure 1 displays the excellent resolution band structure of bilayer graphene obtained at the TPS 39A1 µ ARPES branch. With these advancements, this novel endstation is poised to facilitate users in conducting both spin-resolved ARPES and conventional ARPES in the near future. For the TPS 39A2 nanoARPES branch, the design of scanning stages, including zone plate, order-sorting aperture, and sample stage, was completed in July 2020. Figure 2 illustrates the operation interface of the entire system. Assembly commenced in June 2021, with motor and analyzer integration scheduled for April 2022. The µ -metal chamber was shipped to the NSRRC in December 2021, and functional tests of the zone plate stages are planned for June 2022. Full system assembly, including chamber, analyzer, and all stages, is anticipated to be conducted in April 2024, with commissioning expected to begin before the middle of 2024. The advancements in nanoARPES beamline facilities presented in this article underscore the ongoing commitment to unraveling the mysteries of emergent quantum materials. These developments hold significant promise for the future of condensed-matter physics and materials science, offering researchers and potential users the tools to conduct cutting-edge experiments, including spin-resolved ARPES and conventional ARPES, in the near future. (Reported by Cheng-Maw Cheng)