Electronic Structure of the Dark Surface of the Weak Topological Insulator Bi14Rh3I9

Experimental Realization of Two-Dimensional Weak Topological Insulators

We report the experimental realization of a two-dimensional (2D) weak topological insulator (WTI) in spinless Su-Schrieffer-Heeger circuits with parity-time and chiral symmetries. Strong and weak Z2 topological indexes are adopted to explain the experimental findings that a Dirac semimetal (DSM) phase and four WTI phases emerge in turn when we modulate the centrosymmetric circuit deformations. In the DSM phase, it is found that the Dirac cone is highly anisotropic and that it is not pinned to any high-symmetry points but can widely move within the Brillouin zone, which eventually leads to the phase transition between WTIs. In addition, we observe a pair of flat-band domain wall states by designing spatially inhomogeneous node connections. Our work provides the first experimental evidence for 2D WTIs, which significantly advances our understanding of the strong and weak nature of topological insulators, the robustness of flat bands, and the itinerant and anisotropic features of Dirac cones.

The Weak 3D Topological Insulator Bi12 Rh3 Sn3 I9

Topological insulators (TIs) gained high interest due to their protected electronic surface states that allow dissipation-free electron and information transport. In consequence, TIs are recommended as materials for spintronics and quantum computing. Yet, the number of well-characterized TIs is rather limited. To contribute to this field of research, we focused on new bismuth-based subiodides and recently succeeded in synthesizing a new compound Bi₁₂ Rh₃ Sn₃ I₉ , which is structurally closely related to Bi₁₄ Rh₃ I₉ - a stable, layered material. In fact, Bi₁₄ Rh₃ I₉ is the first experimentally supported weak 3D TI. Both structures are composed of well-defined intermetallic layers of _∞ ² [(Bi₄ Rh)₃ I]²⁺ with topologically protected electronic edge-states. The fundamental difference between Bi₁₄ Rh₃ I₉ and Bi₁₂ Rh₃ Sn₃ I₉ lies in the composition and the arrangement of the anionic spacer. While the intermetallic 2D TI layers in Bi₁₄ Rh₃ I₉ are isolated by _∞ ¹ [Bi₂ I₈ ]^2- chains, the isoelectronic substitution of bismuth(III) with tin(II) leads to _∞ ² [Sn₃ I₈ ]^2- layers as anionic spacers. First transport experiments support the 2D character of this material class and revealed metallic conductivity.

Signature of Large-Gap Quantum Spin Hall State in the Layered Mineral Jacutingaite

Quantum spin Hall (QSH) insulators host edge states, where the helical locking of spin and momentum suppresses backscattering of charge carriers, promising applications from low-power electronics to quantum computing. A major challenge for applications is the identification of large gap QSH materials, which would enable room temperature dissipationless transport in their edge states. Here we show that the layered mineral jacutingaite (Pt₂HgSe₃) is a candidate QSH material, realizing the long sought-after Kane-Mele insulator. Using scanning tunneling microscopy, we measure a band gap in excess of 100 meV and identify the hallmark edge states. By calculating the [Formula: see text] invariant, we confirm the topological nature of the gap. Jacutingaite is stable in air, and we demonstrate exfoliation down to at least two layers and show that it can be integrated into heterostructures with other two-dimensional materials. This adds a topological insulator to the 2D quantum material library.

Crystal Chemistry and Bonding Patterns of Bismuth-Based Topological Insulators

Bismuth is gaining importance as a key element of functional quantum materials. The effects of spin-orbit coupling (SOC) are at the heart of many exciting proposals for next-generation quantum technologies, including topological materials for efficient information transmission and energy-saving applications. The "heavy" element bismuth and its compounds are predestined for SOC-induced topological properties, but materials design is challenged by a complex link between them and the chemical composition and crystal structure. Nevertheless, a lot can be learned about a certain property by testing its limits with compositional and/or structure modifications. We survey a handful of topological bismuth-based materials that bear structural and chemical semblance to the early topological insulators, antimony-doped elemental bismuth, Bi₂Se₃ and Bi₂Te₃. Chemical bonding via p orbitals and modular structure underlie all considered bismuth chalcogenides, subhalides, and chalcogenide halides and allow us to correlate the evolution of chemical bonding and structure with variability of the topological properties, although materials design should not be regarded as a building blocks set. Over the past decade, material discoveries have unearthed a plethora of topological properties, and bismuth is very fertile as a progenitor of a rich palette of exotic quantum materials, ranging from strong and weak 3D and crystalline topological insulators over topological metals and semimetals to magnetic topological insulators, while preserving the general layered structure motif.

Chemical Gating of a Weak Topological Insulator: Bi14Rh3I9

The compound Bi₁₄Rh₃I₉ has recently been suggested as a weak three-dimensional topological insulator on the basis of angle-resolved photoemission and scanning-tunneling experiments in combination with density functional (DF) electronic structure calculations. These methods unanimously support the topological character of the headline compound, but a compelling confirmation could only be obtained by dedicated transport experiments. The latter, however, are biased by an intrinsic n-doping of the material's surface due to its polarity. Electronic reconstruction of the polar surface shifts the topological gap below the Fermi energy, which would also prevent any future device application. Here, we report the results of DF slab calculations for chemically gated and counter-doped surfaces of Bi₁₄Rh₃I₉. We demonstrate that both methods can be used to compensate the surface polarity without closing the electronic gap.

Bi1Te1 is a dual topological insulator

New three-dimensional (3D) topological phases can emerge in superlattices containing constituents of known two-dimensional topologies. Here we demonstrate that stoichiometric Bi₁Te₁, which is a natural superlattice of alternating two Bi₂Te₃ quintuple layers and one Bi bilayer, is a dual 3D topological insulator where a weak topological insulator phase and topological crystalline insulator phase appear simultaneously. By density functional theory, we find indices (0;001) and a non-zero mirror Chern number. We have synthesized Bi₁Te₁ by molecular beam epitaxy and found evidence for its topological crystalline and weak topological character by spin- and angle-resolved photoemission spectroscopy. The dual topology opens the possibility to gap the differently protected metallic surface states on different surfaces independently by breaking the respective symmetries, for example, by magnetic field on one surface and by strain on another surface.

Coexistence of a topological insulator phase and a topological crystalline insulator phase helps to maintain topological properties under a controlled symmetry breaking perturbation. Here, Eschback et al. report a superlattice of Bi and Bi₂Te₃ to be such a dual topological insulator.