Shunsuke Tsuda

We present practical data-analysis procedures for multichannel spin detection using a SPLEED (spin-polarized low-energy electron diffraction)-type spin detector implemented in our imaging-type spin-resolved photoemission microscopy. In contrast to conventional single-channel Mott or very-low-energy electron diffraction detectors, SPLEED-based spin analysis requires combining photoelectron intensities measured at two distinct scattering energies with opposite effective Sherman-function signs. We describe a quantitative workflow to extract two-dimensional spin polarization maps, including normalization to correct detector inhomogeneity and energy-dependent reflectivity variations of the spin filter. As a benchmark, we apply the method to the Rashba-split Shockley surface state on Au(111). The spin-integrated momentum-space image clearly resolves the characteristic double-ring Fermi surface, and the derived spin-polarization map reveals the expected helical spin texture with opposite tangential spin components for the inner and outer branches. The full dataset is acquired within two hours, demonstrating the high efficiency of the multichannel SPLEED detection system. The analysis protocol outlined here is directly applicable to other multichannel SPLEED instruments and can be extended to real-space operando spin microscopy for device-relevant surface and interface studies.

Emergent ferromagnetism on the surface of two-dimensional (2D) MXene is investigated by X-ray magnetic circular dichroism (XMCD) and angle-dependent hard X-ray photoemission spectroscopy (HAXPES). Focusing on Cr₂N as one of the 2D-MXenes, high quality bilayers of Cr₂N/Co and Cr₂N/Pt are prepared by a magnetron sputtering technique. XMCD reveals the induced magnetic moment of Cr in the Cr₂N/Co interface, while it is not observed in the Cr₂N/Pt interface at room temperature. In order to distinguish the possible origins of either the interlayer magnetic exchange coupling or the charge transfer model as the source of ferromagnetism at the interface, the additional controlled Cr₂N/Cu bilayer, whose work function of Cu is consistent with Co, is prepared. HAXPES spectra for the Cr 2p core level near the interface of Cr₂N/Cu are consistent with that of Cr₂N/Co, indicating that the induced magnetic moment of Cr observed by XMCD for Cr₂N/Co can be attributed to the model of interlayer magnetic exchange coupling, rather than the charge transfer model, leading to emergent ferromagnetism at the interface with 2D-MXene.

We demonstrate high-throughput evaluation of the half-metallicity of Co₂MnSi Heusler alloys by spin-integrated hard X-ray photoelectron spectroscopy (HAXPES) of composition-spread films performed with high-brilliance synchrotron radiation at NanoTerasu, which identifies the optimum composition showing the best half-metallicity. Co_75–xMn_xSi₂₅ composition-spread thin films for x = 10–40% with a thickness of 30 nm are fabricated on MgO(100) substrates using combinatorial sputtering technique. The L2₁-ordering and (001)-oriented epitaxial growth of Co₂MnSi are confirmed by X-ray diffraction for x = 18–40%. The valence band HAXPES spectra exhibit a systematic compositional dependence and the smallest photoemission intensity at the Fermi level (E_F) for a slightly Mn-rich composition of x = 27%. The density of states (DOS) for L2₁-ordered Co₂MnSi with different Mn compositions obtained from first-principles calculation also show the smallest total DOS at E_F for x = 27% because of the formation of a clear half-metallic gap in the minority spin channel and the less localized d-states in the majority spin channel, indicating the best half-metallic nature of this composition. Our experimental results demonstrate that high-throughput evaluation of half-metallicity is possible even with spin-integrated HAXPES by capturing systematic changes in the electronic structures through the measurements on the composition-spread film. Moreover, the anisotropic magnetoresistance (AMR) of the composition-spread film is measured for electric current directions along the [110] and [100] of Co₂MnSi. Previous studies indicated that a larger negative AMR ratio is a signature of a higher spin polarization. The largest negative AMR ratio is observed for x = 27% for both current directions, which also supports the best half-metallicity for this off-stoichiometric composition.

Moiré superlattices consisting of two-dimensional materials have attracted immense attention because of emergent phenomena such as flat band-induced Mott insulating states and unconventional superconductivity. However, the effects of spin-orbit coupling on these materials have not yet been fully explored. Here, we show that single- and double-bilayer antimony honeycomb lattices, referred to as antimonene, form moiré superlattices on a Bi(111) substrate due to lattice mismatch. Scanning tunnelling microscopy (STM) measurements reveal the presence of spectral peaks near the Fermi level, which are spatially modulated with the moiré period. Angle-resolved photoemission spectroscopy (ARPES) combined with density functional theory calculations clarify the surface band structure with saddle points near the Fermi level, which allows us to attribute the observed STM spectral peaks to the van Hove singularity. Moreover, spin-resolved ARPES measurements reveal that the observed surface states are Rashba-type spin-polarized. The present work has significant implications in that Fermi surface instability and symmetry breaking may emerge at low temperatures, where the spin degree of freedom and electron correlation also play important roles.

We have developed a novel photoemission microscopy apparatus employing a vacuum ultraviolet laser. This setup combines photoemission electron microscopy (PEEM) with a time-of-flight detector, facilitating rapid visualization of electronic states in both real and momentum space. Achieving a spatial resolution of 70 nm, attributed to the PEEM lens system, we showcase the full band mapping of a Bi(111) single crystal film using angle-resolved photoemission spectroscopy within a short acquisition time.

Fe–Al–Si-based thermoelectric (FAST) materials are non-toxic and low-cost materials that can be used for autonomous power supplies to drive internet-of-things wireless sensor devices. The conduction type can be controlled by changing the Al/Si ratio, which is suitable for fabricating reliable thermoelectric power-generation modules consisting of materials with similar thermal expansion coefficients. In this work, we evaluated the electronic structures of p- and n-type FAST materials with relatively large absolute values of the Seebeck coefficient by photoemission spectroscopy to obtain deeper insight into controlling the p-n characteristics of FAST materials. The core-level spectra suggested that the FAST materials have a covalent bonding nature. The chemical-potential shift should be the dominant factor of the core-level shift, which is consistent with the expected behavior of carrier doping of thermoelectric semiconductors, that is, rigid-band-like behavior. The size of the core-level shift of ∼0.15 eV is close to the band gap of ∼0.18 eV obtained from transport measurements. The observed electronic structure can qualitatively explain the experimental results.