Katsuaki Sugawara

Bilayer graphene (BLG) exhibits unique physical properties that strongly depend on the twist angle, such as superconductivity at magic angles, moiré superlattices, and quasicrystal structures. However, the influence of the twist angle on the chemical properties remains unclear. Here, we investigated the chemical reactivity of twist-angle-controlled BLG toward ultraviolet (UV)/ozone oxidation. For the systematic study of the angle dependence, two sheets of monolayer graphene were stacked with controlled stacking angles from 0° to 30° with 5° intervals. Raman spectroscopy revealed that the degree of oxidation is strongly dependent on the twist angle and it increases with the twist angle, indicating a strong influence of the interlayer coupling on the reactivity. Scanning transmission electron microscopy and X-ray photoelectron spectroscopy revealed that the oxidation proceeds through the formation of localized oxygen functional groups in the graphene lattice. Density functional theory calculations suggested that functionalization becomes energetically more favorable at higher twist angles. Layer-specific 12C/13C isotope labeling demonstrated that the oxidation occurs mostly at the top graphene layer, while the bottom layer remains almost intact. Furthermore, the generality of the twist-angle-dependent reactivity was confirmed via molecular functionalization with 4-nitrobenzenediazonium molecules. Our work highlights the importance of twist angle in the chemistry of graphene, paving a way to tune its electronic structure and carrier transport properties.

Two-dimensional transition-metal dichalcogenides provide an ideal platform for exploring emergent electronic and magnetic phenomena inaccessible in bulk materials. Here, we realize a metastable monolayer CrS2 film on bilayer graphene/SiC(0001) by combining molecular-beam epitaxy with a topotactic chemical reaction. Angle-resolved photoemission spectroscopy (ARPES), together with first-principles calculations, supports the assignment of monolayer CrS2 to the octahedral 1T structure. ARPES reveals a gap-like semiconducting state that is not reproduced by the GGA+U calculations. X-ray magnetic circular dichroism detects no ferromagnetic response in monolayer 1T-CrS2 down to 160 K, consistent with paramagnetic or antiferromagnetic behavior, in stark contrast to its isostructural counterpart, monolayer CrTe2, which exhibits metallic ferromagnetic behavior. These results establish metastably stabilized monolayer CrS2 as a distinct member of Cr-based two-dimensional materials and provide a foundation for exploring ultrathin semiconductor devices.

Spin-valley magnetic ordering is restricted to layered van der Waals type transition-metal dichalcogenides with ordering temperatures below 55 K. Recent theoretical studies on non-van der Waals structures have predicted spin-valley polarization induced semiconducting ferromagnetic ground states, but experimental validation is missing. We report high-Curie temperature (T_C ~ 225 K) metallic ferromagnetism with spontaneous spin-valley polarization in monolayer Cr₂Se₃ on graphene. Angle-resolved photoemission spectroscopy (ARPES) reveals systematic temperature-dependent energy shifts and splitting of localized Cr 3 d^↑-t_2g bands, accompanied by occupancy of the itinerant Cr 3d-e_g valleys. The t_2g-e_g spin-valley coupling at the K/K’ points of hexagonal Brillouin zone leads to ferromagnetic ordering. Circular dichroism in ARPES shows clear evidence of spin-valley polarized states. Comparison with bilayer and trilayer Cr₂Se₃ reveals the crucial role of valley carrier density in enhancing T_C and provides a guiding principle to realize 2D ferromagnetism at higher temperatures in non-van der Waals materials.

Although the surface plasmon resonance excited by a Dirac electron system has gained considerable attention in the terahertz region, studies have been limited to grating-coupling surface plasmon resonance (GC-SPR) in Bi₂Se₃. In this study, we experimentally demonstrated terahertz prism-coupled surface plasmon resonance (PC-SPR) and GC-SPR in topological insulator systems, viz. Bi₂Se₃ (GC-SPR), Bi₂Se₂Te (GC-SPR), and Bi₂Se₃ (PC-SPR). The physical properties measured using angle-resolved photoemission spectroscopy indicate that the presence of surface state near the Fermi level is insufficient for SPR excitation; linearity of the Dirac cone is essential for SPR excitation in the THz range.

Achieving a large spin splitting is highly desirable for spintronic devices, which often requires breaking of the inversion symmetry. However, many atomically thin films are centrosymmetric, making them unsuitable for spintronic applications. Here, we report a strategy to achieve inversion symmetry breaking from a centrosymmetric transition metal dichalcogenide (TMDC) bilayer PtTe₂, leading to a giant Rashba spin splitting. Specifically, the thermal annealing turns one layer of PtTe₂ sample into a transition metal monochalcogenide (TMMC) PtTe through Te extraction, thus forming PtTe/PtTe₂ heterostructure with inversion symmetry breaking. In this naturally-formed PtTe/PtTe₂ heterostructure, we observe a giant Rashba spin splitting with Rashba coefficient of α _R = 1.8 eV ⋅ Å, as revealed by spin- and angle-resolved photoemission spectroscopy measurements. Our work demonstrates a convenient and effective pathway for achieving pronounced Rashba splitting in centrosymmetric TMDC thin films by creating TMMC/TMDC heterostructure, thereby extending their potential applications to spintronics.

Monolayer atomic thin films of group-V elements have a high potential for application in spintronics and valleytronics because of their unique crystal structure and strong spin–orbit coupling. We fabricated Sb and Bi monolayers on a SiC(0001) substrate by the molecular-beam-epitaxy method and studied the electronic structure by angle-resolved photoemission spectroscopy (ARPES) and first-principles calculations. The fabricated Sb film shows the (√3 × √3)R30° superstructure associated with the formation of α-Sb, and exhibits a semiconducting nature with a band gap of more than 1.8 eV. Spin-resolved ARPES measurements of isostructural α-Bi revealed the in-plane spin polarization for the topmost valence band, demonstrating its Rashba-splitting nature due to the space-inversion-symmetry breaking. We discuss the origin of observed characteristic band structure and its similarity and difference between Sb and Bi.

We selectively fabricated monolayers of octahedral (1H) and distorted trigonal (1T') WTe2 on graphene/SiC(0001) by controlling the substrate temperature during epitaxy. Angle-resolved photoemission spectroscopy, combined with first-principles band-structure calculations, has revealed several drastic differences between these two polymorphs. The 1T' phase exhibits a semiconducting character with a nearly-zero energy gap, while the 1H phase shows a large band gap and the band splitting at the K/K' point. The present results pave a pathway toward developing nanoelectronic devices based with WTe2.