Status of spin- and angle-resolved photoelectron spectroscopy withlaser light at Laser and Synchrotron Research Laboratory
Spin- and angle-resolved photoelectron spectroscopy (SARPES) is a powerful technique toinvestigate the spin-dependent electronic states in solids. For example, one looks at the exchange
splitting of ferromagnetic materials. Also, recently, strongly spin-orbit coupled materials, such as
Rashba spin-split systems and topological insulators have been intensively studied. We developed aSARPES apparatus with a vacuum-ultraviolet (VUV) laser at Laser and Synchrotron ResearchLaboratory in the Institute for Solid State Physics, named LOBSTER (Laser-Optics-BasedSpin-vecTor Experimental Research) machine. The LOBSTER machine is utilized to obtain precise
information on spin-dependent electronic structures near the Fermi level in solids. We started a project
to construct the LOBSTER machine from FY 2014 and joint researches at this station have started
from FY 2015.
Figure 1 represents an overview of the LOBSTER machine [1]. The apparatus consists of ananalysis chamber, a sample-bank chamber connected to a load-lock chamber, and a molecular beam
epitaxy (MBE) chamber, which are kept in an ultra-high vacuum (UHV) environment and areconnected with each other via UHV gate valves. The hemispherical electron analyzer is aFig. 1. Overview of the LOBSTER machine developed at the Laser and Synchrotron ResearchLaboratory at the Institute for Solid State Physics [1].

custom-made ScientaOmicron DA30-L, modified to attach the very-low-energy-electron-diffraction
type spin detectors. The electrons are excited by 6.994-eV photons, yielded by 6th harmonic of a
Nd:YVO4 quasi-continuous wave laser with repetition rate of 120 MHz. A helium discharge lamp (VGScienta, VUV5000) is also available as a photon source. At the MBE chamber, samples can be heatedby a direct current heating or electron bombardment. The surface evaluating and preparing instruments,
such as evaporators, low energy electron diffraction, sputter-gun and quartz microbalance, can beinstalled. At the carousel chamber, 16 samples can be stocked in the UHV environment.
In FY 2018, eight research papers were published. Among these, one of the most significant papersis the discovery of a weak topological insulator (WTI) state in a bismuth iodide, reported by Prof.
Kondo group [2]. In this study, the authors were performed ARPES and SARPES with high-energy
and spatial resolutions using the VUV laser for the bismuth iodide. They demonstrated aquasi-one-dimensional Dirac surface state. They also found that a crystal transition from the β-phaseto the α-phase drives a topological phase transition from a nontrivial WTI to a normal insulator atroughly room temperature. The authors concluded that the weak topological phase can open a newpathway for developing novel spintronic devices that benefits from highly directional, dense spincurrents that are protected against backscattering.

Reference[1] K. Yaji, A. Harasawa, K. Kuroda, S. Toyohisa, M. Nakayama, Y. Ishida, A. Fukushima, S. Watanabe, C.-T.
Chen, F. Komori and S. Shin, Rev. Sci. Instrum. 87, 053111 (2016).
[2] R. Noguchi, T. Takahashi, K. Kuroda, M. Ochi, T. Shirasawa, M. Sakano, C. Bareille, M. Nakayama, M. D.
Watson, K. Yaji, A. Harasawa, H. Iwasawa, P. Dudin, T. K. Kim, M. Hoesch, S. Shin, R. Arita, T. Sasagawa andT. Kondo, Nature 588, 518-522 (2019).