Phys. Rev. B. 107, L041102 (2023).
The electronic structure changes of SmS surfaces under potassium (K) doping are elucidated using synchrotron-based core-level photoelectron spectroscopy and angle-resolved photoelectron spectroscopy (ARPES). The Sm core-level and ARPES spectra indicate that the Sm mean valence of the surface increased from the nearly divalent to trivalent states, with increasing K deposition. Carrier-induced valence transition (CIVT) from Sm²⁺ to Sm³⁺ exhibits a behavior opposite to that under conventional electron doping. Excess electrons from K atoms are transferred to S sites and the liberated electrons from Sm³⁺ ions due to CIVT at the surface are trapped like local excitons around the Sm³⁺ ions, which is inconsistent with the phase transition from the black insulator with Sm²⁺ to the gold metal with Sm³⁺ under pressure. This CIVT helps to clarify the pressure-induced black-to-golden phase transition in this material, which originates from the Mott transition of excitons.

Nat. Commun. 13, 5600 (2022).
The topology and spin-orbital polarization of two-dimensional (2D) surface electronic states have been extensively studied in this decade. One major interest in them is their close relationship with the parities of the bulk (3D) electronic states. In this context, the surface is often regarded as a simple truncation of the bulk crystal. Here we show breakdown of the bulk-related inplane rotation symmetry in the topological surface states (TSSs) of the Kondo insulator SmB₆ . Angle-resolved photoelectron spectroscopy (ARPES) performed on the vicinal SmB₆ (001)-p(2×2) surface showed that TSSs are anisotropic and that the Fermi contour lacks the fourfold rotation symmetry maintained in the bulk. This result emphasizes the important role of the surface atomic structure even in TSSs. Moreover, it suggests that the engineering of surface atomic structure could provide a new pathway to tailor various properties among TSSs, such as anisotropic surface conductivity, nesting of surface Fermi contours, or the number and position of van Hove singularities in 2D reciprocal space.

Phys. Rev. B 105, 235141 (2022). [Editors' suggestion]
Spin-orbit interaction in low-dimensional systems, namely, Rashba systems and the edge states of topological materials, has been extensively studied in this decade as a promising source to realize various fascinating spintronic phenomena, such as the source of the spin current and spin-mediated energy conversion. Here, we show the odd fluctuation in the spin-orbital texture in a surface Rashba system on Bi/InAs(110)-(2×1) by spin- and angle-resolved photoelectron spectroscopy and a numerical simulation based on a density-functional theory (DFT) calculation. The surface state shows a paired parabolic dispersion with the spin degeneracy lifted by the Rashba effect. Although its spin polarization should be fixed in a particular direction based on the Rashba model, the observed spin polarization varies greatly and even reverses its sign depending on the wave number. DFT calculations also reveal that the spin directions of two inequivalent Bi chains on the surface change from nearly parallel (canted parallel) to antiparallel in real space in the corresponding wave vector region. These results point out an oversimplification of the nature of spin in Rashba and Dirac systems and provide more freedom than expected for spin manipulation of photoelectrons.

Phys. Rev. B 104, 245116 (2021).
The electronic structures of the heavy-fermion superconductor CeRh2As2 with local inversion symmetry breaking and the reference material LaRh2As2 have been investigated by using experimental optical conductivity [σ1(ω)] spectra and first-principles density functional theory calculations. The low-temperature σ1(ω) spectra of LaRh2As2 revealed a broad peak at ∼0.1 eV and a sharp peak at ∼0.5 eV after a subtraction of the Drude contribution of free carriers. The peak features and the background intensity were nicely reproduced in calculated σ1(ω) spectra from DFT calculations, implying a conventional metallic nature. In CeRh2As2, two mid-IR peaks developed with decreasing temperature, which suggests the emergence of hybridization states between the at ¯hω ∼ 0.12 and 0.4 eV corresponding to the unoccupied Ce 4 f5/2 and 4 f7/2 states, respectively, were strongly conduction and 4 f electrons. We compared the temperature dependence of the mid-IR peaks of CeRh2As2 with corresponding data from CeCu2Si2 and CeNi2Ge2 in a ThCr2Si2-type structure to examine the possible impact of local inversion symmetry breaking on electronic structures.

Rev. Sci. Instrum. 92, 093103 (2021).
We have developed spin-resolved resonant electron energy-loss spectroscopy with the primary energy of 0.3–1.5 keV, which corresponds to the core excitations of 2p–3d absorption of transition metals and 3d–4f absorption of rare-earths, with the energy resolution of about 100 meV using a spin-polarized electron source as a GaAs/GaAsP strained superlattice photocathode. Element- and spin-selective carrier and valence plasmons can be observed using the resonance enhancement of core absorptions and electron spin polarization. Furthermore, bulk- sensitive electron energy-loss spectroscopy spectra can be obtained because the primary energy corresponds to the mean free path of1–10 nm. The methodology is expected to provide us with novel information about elementary excitations by resonant inelastic x-ray scattering and resonant photoelectron spectroscopy.

Main experimental setup

	ARPES / SAR-EELS/ED / LEED [#D206] Photoelectron analyzer (A-1 w/ deflector, MBScientific) He discharge lamp+monochromator (VUV5000 + VUV5040, GAMMA-DATA Scienta) LEED/AES (ErLEED 150, SPECS) Spin-polarized electron source 6-axes cold manipulator A-1's vacuum (Lab-site use only)
	IPES / LE-EELS / XPS [#D208-1] Photoelectron analyzer (SES100, VG Scienta) Inverse Photoelectron spectrometer (IPES, PSP Vacuum Technology) X-ray source (XR-50, SPECS) LHe-flow cryostat (ST-400, JANIS) SES100's vacuum (Lab-site use only) EGUNs vacuum (Lab-site use only)
	IR / THz spectrometer [#D206] FTIR (JASCO FTIR6100, hv: 5meV - 1.5eV) FT-FIR (JASCO FARIS-1, hv = 2--50meV) LHe-flow cryostat (Oxford Optstat, T > 4 K) 3He cryostat (Oxford HelioxAC-V, T: 0.4 - 10 K, for FARIS-1) Closed-cycle He cryostat (IWATANI M-310, T: 8 - 350K) Si bolometer (3 units, Infrared Laboratory) MCT detector Recycle He gas purity (Lab-site use only) Status of FTIR Status of FARIS
	IR microscope [#D208-2] FTIR (JASCO FTIR6600, hv: 50meV - 1.5eV) IR microscope (JASCO IRT5200) LHe-flow cryostat (Oxford MicrostatHe, T > 4 K)
	Femto-second pulse laser #1 [#D208-2] Ti:Sa laser + Regenerator (Coherent Libra) Optical parametric amplifier: OPA (Coherent)
	Femto-second pulse laser #2 [#D206] Yb:KGW laser (PHAROS, LIGHT CONVERSION) Optical parametric amplifier: OPA (ORPHEUS, LIGHT CONVERSION)

Beamlines we have used.

UVSOR-III BL7U	Low-energy ARPES
UVSOR-III BL5U	micro-SX-ARPES
UVSOR-III BL6B	IR and THz microscope
UVSOR-III BL3B/7B	VUV spectroscopy
SPring-8 BL07LSU	time-resolved XAS
KEK-PF BL-2A	Wide photon energy range SX-ARPES
HiSOR BL9B	Spin-resolved ARPES
SOLEIL CASSIOPEE, France	ARPES
SAGA-LS BL13	Time-resolved ARPES
AichiSR BL7U	micro-SX-ARPES

• Molecular beam epitaxy (EW-3, EIKO)
  
  • K-cell (4 units)
  • RHEED
• Mini MBE (Originally developed)
  
  • K-cell (4 units)
  • RHEED
• Muffle furnace
  
  • Maximum temperature: 1300 deg.
• Evapolator (Originally developed)
  
  • ohmic heating of tungsten basket
  • vacuum: ~10^-6 Pa
• Diamond cutter
  
• Metal polisher
  
• Glove box
  

• Ultrasonic cleaner
  
• Spot welder
  
• Laser processing machine
  
• Hand crane
  
• Hand pallet truck (BISHAMON)
  
• Engine crane
  
• Bogie 3 units