Kouji Segawa

We report a combined microfocused angle-resolved photoemission spectroscopy and first-principles study of the rare-earth tetraborides R B 4 ( R = Tb , Gd ) , which host a Shastry-Sutherland lattice. We identify extended bulk nodal lines at the tetragonal Brillouin-zone boundaries, whose fourfold degeneracies are enforced by the nonsymmorphic glide-mirror symmetries even in the presence of spin-orbit coupling (SOC), unlike conventional nodal lines that SOC typically gaps. Systematically evolving Dirac-like dispersions resolved by momentum-distribution-curve analysis support this symmetry-enforced nodal line. We further identify a related nodal-line feature that disperses toward the Fermi level ( E F ) ; although the calculated SOC-induced gap for this feature (0.2–4.6 meV) lies below the experimental resolution, its proximity to E F offers a band-structure basis consistent with the emergence of large magnetoresistance. First-principles Wilson-loop calculations establish a nontrivial π Berry phase for the bulk nodal line, and we observe a photon-energy-independent surface-derived feature whose dispersion is consistent with the calculated surface state. Our results identify R B 4 as a platform for nonsymmorphic-symmetry-enforced nodal-line physics with potential relevance to large magnetoresistance and topological surface states.

The Dzyaloshinskii-Moriya (DM) interaction has been considered essential for skyrmion formation, however, the discovery of skyrmion lattices (SkLs) in nominally centrosymmetric materials where the DM interaction is forbidden, such as Eu(Ga_1-xAl_x)₄, has challenged this established view. Recent structural investigations of Eu(Ga_1-xAl_x)₄ have further complicated this issue by revealing that the charge-density wave breaks local symmetry, theoretically allowing DM interaction. This raises a fundamental question: are the complex magnetic phases driven by the DM interaction or by alternative mechanisms? Here, using soft-x-ray angle-resolved photoemission spectroscopy, we determine the three-dimensional bulk electronic structure of Eu(Ga_1-xAl_x)₄, and elucidate the electronic origins of its rich magnetic orders. We directly observe an x-dependent Lifshitz transition leading to the emergence of a Fermi-surface pocket. Importantly, multiple nesting vectors derived from this pocket match the symmetries and periodicities of the multiple SkLs. Moreover, these nesting vectors can also account for other magnetic orders, such as the zero-field helical magnetism, suggesting a common electronic origin of the complex magnetic phases. These findings suggest that competing nesting-induced Ruderman-Kittel-Kasuya-Yosida interactions and their engineering can generate and control various SkLs and related topological spin textures.

Introducing the concept of topology has revolutionized materials classification, leading to the discovery of topological insulators and Dirac–Weyl semimetals^1,2,3. One of the most fundamental theories underpinning topological materials is the Su–Schrieffer–Heeger (SSH) model^4,5, which was developed in 1979—decades before the recognition of topological insulators—to describe conducting polymers. Distinct from the vast majority of known topological insulators with two and three dimensions^1,2,3, the SSH model predicts a one-dimensional analogue of topological insulators, which hosts topological bound states at the endpoints of a chain^4,5,6,7,8. To establish this unique and pivotal state, it is crucial to identify the low-energy excitations stemming from bound states, but this has remained unknown in solids because of the absence of suitable platforms. Here we report unusual electronic states that support the emergent bound states in elemental tellurium, the single helix of which was recently proposed to realize an extended version of the SSH chain^9,10. Using spin- and angle-resolved photoemission spectroscopy with a micro-focused beam, we have shown spin-polarized in-gap states confined to the edges of the (0001) surface. Our density functional theory calculations indicate that these states are attributed to the interacting bound states originating from the one-dimensional array of SSH tellurium chains. Helices in solids offer a promising experimental platform for investigating exotic properties associated with the SSH chain and exploring topological phases through dimensionality control.

Realization of topological superconductors (TSCs) hosting Majorana fermions is an exciting challenge in materials science. Majorana fermions are predicted to emerge at vortex cores of two-dimensional (2D) TSCs and at both ends of one-dimensional (1D) TSCs; these two types of TSCs have been explored independently in different materials. Here, a system which has a potential to access both 1D and 2D TSCs in a single platform, Pb(111)/Bi(111) heterostructure, is proposed. One to twenty bilayers (BLs) of Bi(111) ultra-thin films are epitaxially fabricated on TlBiSe₂, and an intriguing evolution of electronic states upon variation of Bi-layer thickness is revealed by angle-resolved photoemission spectroscopy. The metallic quantum-well states at 1–2BLs are found to turn into the Rashba states at 5–20BLs, via the semiconducting states at 3BL. Fabrication of a Pb(111) film on 20BL Bi(111) enables observation of the proximity-induced superconductivity in Bi(111) as evident from a 1 meV energy gap at 5 K; these energy and temperature scales are considerably larger than those of Rashba superconductors. The Pb/Bi heterostructure serves as a versatile platform to study the interplay among proximity-induced superconductivity, band structure, and topology.

Quasiparticle - a key concept to describe interacting particles - characterizes electron-electron interaction in metals (Fermi liquid) and electron pairing in superconductors. While this concept essentially relies on the simplification of hard-to-solve many-body problem into one-particle picture and residual effects, a difficulty in disentangling many-body effects from experimental quasiparticle signature sometimes hinders unveiling intrinsic low-energy dynamics, as highlighted by the fierce controversy on the origin of Dirac-band anomaly in graphene and dispersion kink in high-temperature superconductors. Here, we propose an approach to solve this fundamental problem - the Bayesian modelling of quasiparticles. We have chosen a topological insulator TlBi(S,Se)₂ as a model system to formulate an inverse problem of quasiparticle spectra with semiparametric Bayesian analysis, and successfully extracted one-particle and many-body characteristics, i.e. the intrinsic energy gap and unusual lifetime in Dirac-quasiparticle bands. Our approach is widely applicable to clarify the quasiparticle dynamics of quantum materials.

In this article, practical methods for synthesizing Tl-based ternary III-V-VI2 chalcogenide TlBi(S1-xSex)(2) are described in detail, along with characterization by x-ray diffraction and charge transport properties. The TlBi(S1-xSex)(2) system is interesting because it shows a topological phase transition, where a topologically nontrivial phase changes to a trivial phase without changing the crystal structure qualitatively. In addition, Dirac semimetals whose bulk band structure shows a Dirac-like dispersion are considered to exist near the topological phase transition. The technique shown here is also generally applicable for other chalcogenide topological insulators, and will be useful for studying topological insulators and related materials.