Interaction Effects in a 1D Flat Band at a Topological Crystalline Step Edge.

Step edges of topological crystalline insulators can be viewed as predecessors of higher-order topology, as they embody one-dimensional edge channels embedded in an effective three-dimensional electronic vacuum emanating from the topological crystalline insulator. Using scanning tunneling microscopy and spectroscopy, we investigate the behavior of such edge channels in Pb1-xSnxSe under doping. Once the energy position of the step edge is brought close to the Fermi level, we observe the opening of a correlation gap. The experimental results are rationalized in terms of interaction effects which are enhanced since the electronic density is collapsed to a one-dimensional channel. This constitutes a unique system to study how topology and many-body electronic effects intertwine, which we model theoretically through a Hartree-Fock analysis.

Topological Surface State Annihilation and Creation in SnTe/Crx(BiSb)2-xTe3 Heterostructures.

Topological surface states are a new class of electronic states with novel properties, including the potential for annihilation between surface states from two topological insulators at a common interface. Here, we report the annihilation and creation of topological surface states in the SnTe/Crx(BiSb)2-xTe3 (CBST) heterostructures as evidenced by magneto-transport, polarized neutron reflectometry, and first-principles calculations. Our results show that topological surface states are induced in the otherwise topologically trivial two-quintuple-layers thick CBST when interfaced with SnTe, as a result of the surface state annihilation at the SnTe/CBST interface. Moreover, we unveiled systematic changes in the transport behaviors of the heterostructures with respect to changing Fermi level and thickness. Our observation of surface state creation and annihilation demonstrates a promising way of designing and engineering topological surface states for dissipationless electronics.

Ultra-Broadband Flexible Photodetector Based on Topological Crystalline Insulator SnTe with High Responsivity.

Topological crystalline insulators (TCIs) are predicted to be a promising candidate material for ultra-broadband photodetectors ranging from ultraviolet (UV) to terahertz (THz) due to its gapless surface state and narrow bulk bandgap. However, the low responsivity of TCIs-based photodetectors limits their further applications. In this regard, a high-performance photodetector based on SnTe, a recently developed TCI, working in a broadband wavelength range from deep UV to mid-IR with high responsivity is reported. By taking advantage of the strong light absorption and small bandgap of SnTe, photodetectors based on the as-grown SnTe crystalline nanoflakes as well as specific short channel length achieve a high responsivity (71.11 A W-1 at 254 nm, 49.03 A W-1 at 635 nm, 10.91 A W-1 at 1550 nm, and 4.17 A W-1 at 4650 nm) and an ultra-broad spectral response (254-4650 nm) simultaneously. Moreover, for the first time, a durable flexible SnTe photodetector fabricated directly on a polyethylene terephthalate film is demonstrated. These results prove the great potential of TCIs as a promising material for integrated and flexible optoelectronic devices.

Crystal field effect induced topological crystalline insulators in monolayer IV-VI semiconductors.

Two-dimensional (2D) topological crystalline insulators (TCIs) were recently predicted in thin films of the SnTe class of IV-VI semiconductors, which can host metallic edge states protected by mirror symmetry. As thickness decreases, quantum confinement effect will increase and surpass the inverted gap below a critical thickness, turning TCIs into normal insulators. Surprisingly, based on first-principles calculations, here we demonstrate that (001) monolayers of rocksalt IV-VI semiconductors XY (X = Ge, Sn, Pb and Y = S, Se, Te) are 2D TCIs with the fundamental band gap as large as 260 meV in monolayer PbTe. This unexpected nontrivial topological phase stems from the strong crystal field effect in the monolayer, which lifts the degeneracy between p(x,y) and p(z) orbitals and leads to band inversion between cation pz and anion px,y orbitals. This crystal field effect induced topological phase offers a new strategy to find and design other atomically thin 2D topological materials.

Low-Dimensional Topological Crystalline Insulators.

Topological crystalline insulators (TCIs) are recently discovered topological phase with robust surface states residing on high-symmetry crystal surfaces. Different from conventional topological insulators (TIs), protection of surface states on TCIs comes from point-group symmetry instead of time-reversal symmetry in TIs. The distinct properties of TCIs make them promising candidates for the use in novel spintronics, low-dissipation quantum computation, tunable pressure sensor, mid-infrared detector, and thermoelectric conversion. However, similar to the situation in TIs, the surface states are always suppressed by bulk carriers, impeding the exploitation of topology-induced quantum phenomenon. One effective way to solve this problem is to grow low-dimensional TCIs which possess large surface-to-volume ratio, and thus profoundly increase the carrier contribution from topological surface states. Indeed, through persistent effort, researchers have obtained unique quantum transport phenomenon, originating from topological surface states, based on controllable growth of low-dimensional TCIs. This article gives a comprehensive review on the recent progress of controllable synthesis and topological surface transport of low-dimensional TCIs. The possible future direction about low-dimensional TCIs is also briefly discussed at the end of this paper.

Two-atom-thin topological crystalline insulators lacking out of plane inversion symmetry.

A two-dimensional topological crystalline insulator (TCI) with a single unit cell (u.c.) thickness is demonstrated here. To that end, one first shows that tetragonal (C4in-plane) symmetry is not a necessary condition for the creation of zero-energy metallic surface states on TCI slabs of finite-thicknesses, because zero-energy states persist even as all the in-plane rotational symmetries-furnishing topological protection-are completely removed. In other words, zero-energy levels on the model are not due to (nor are they protected by) topology. Furthermore, effective two-fold energy degeneracies taking place at few discretek-points away from zero energy in the bulk Hamiltonian-that are topologically protected-persist at the u.c. thickness limit. The chiral nature of the bulk TCI Hamiltonian permits creating a2×2square Hamiltonian, whose topological properties remarkably hold invariant at both the bulk and at the single u.c. thickness limits. The identical topological characterization for bulk and u.c.-thick phases is further guaranteed by a calculation involving Pfaffians. This way, a two-atom-thick TCI is deployed hereby, in a demonstration of a topological phase that holds both in the bulk, and in two dimensions.

Pressure induced topological and topological crystalline insulators.

Research on topological and topological crystalline insulators (TCIs) is one of the most intense and exciting topics due to its fascinating fundamental science and potential technological applications. Pressure (strain) is one potential pathway to induce the non-trivial topological phases in some topologically trivial (normal) insulating or semiconducting materials. In the last ten years, there have been substantial theoretical and experimental efforts from condensed-matter scientists to characterize and understand pressure-induced topological quantum phase transitions (TQPTs). In particular, a promising enhancement of the thermoelectric performance through pressure-induced TQPT has been recently realized; thus evidencing the importance of this subject in society. Since the pressure effect can be mimicked by chemical doping or substitution in many cases, these results have opened a new route to develop more efficient materials for harvesting green energy at ambient conditions. Therefore, a detailed understanding of the mechanism of pressure-induced TQPTs in various classes of materials with spin-orbit interaction is crucial to improve their properties for technological implementations. Hence, this review focuses on the emerging area of pressure-induced TQPTs to provide a comprehensive understanding of this subject from both theoretical and experimental points of view. In particular, it covers the Raman signatures of detecting the topological transitions (under pressure), some of the important pressure-induced topological and TCIs of the various classes of spin-orbit coupling materials, and provide future research directions in this interesting field.

Facet Engineering to Regulate Surface States of Topological Crystalline Insulator Bismuth Rhombic Dodecahedrons for Highly Energy Efficient Electrochemical CO2 Reduction.

Bismuth (Bi) is a topological crystalline insulator (TCI), which has gapless topological surface states (TSSs) protected by a specific crystalline symmetry that strongly depends on the facet. Bi is also a promising electrochemical CO2 reduction reaction (ECO2 RR) electrocatalyst for formate production. In this study, single-crystalline Bi rhombic dodecahedrons (RDs) exposed with (104) and (110) facets are developed. The Bi RDs demonstrate a very low overpotential and high selectivity for formate production (Faradic efficiency >92.2%) in a wide partial current density range from 9.8 to 290.1 mA cm-2 , leading to a remarkably high full-cell energy efficiency (69.5%) for ECO2 RR. The significantly reduced overpotential is caused by the enhanced *OCHO adsorption on the Bi RDs. The high selectivity of formate can be ascribed to the TSSs and the trivial surface states opening small gaps in the bulk gap on Bi RDs, which strengthens and stabilizes the preferentially adsorbed *OCHO and mitigates the competing adsorption of *H during ECO2 RR. This study describes a promising application of Bi RDs for high-rate formate production and high-efficiency energy storage of intermittent renewable electricity. Optimizing the geometry of TCIs is also proposed as an effective strategy to tune the TSSs of topological catalysts.

Realization of Epitaxial Thin Films of the Topological Crystalline Insulator Sr3 SnO.

Topological materials are derived from the interplay between symmetry and topology. Advances in topological band theories have led to the prediction that the antiperovskite oxide Sr3 SnO is a topological crystalline insulator, a new electronic phase of matter where the conductivity in its (001) crystallographic planes is protected by crystallographic point group symmetries. Realization of this material, however, is challenging. Guided by thermodynamic calculations, a deposition approach is designed and implemented to achieve the adsorption-controlled growth of epitaxial Sr3 SnO single-crystal films by molecular-beam epitaxy (MBE). In situ transport and angle-resolved photoemission spectroscopy measurements reveal the metallic and electronic structure of the as-grown samples. Compared with conventional MBE, the used synthesis route results in superior sample quality and is readily adapted to other topological systems with antiperovskite structures. The successful realization of thin films of Sr3 SnO opens opportunities to manipulate topological states by tuning symmetries via strain engineering and heterostructuring.

Superconductivity of Topological Surface States and Strong Proximity Effect in Sn1- x Pbx Te-Pb Heterostructures.

Superconducting topological crystalline insulators are expected to form a new type of topological superconductors to host Majorana zero modes under the protection of lattice symmetries. The bulk superconductivity of topological crystalline insulators can be induced through chemical doping and the proximity effect. However, only conventional full gaps are observed, so the existence of topological superconductivity in topological crystalline insulators is still controversial. Here, the successful fabrication of atomically flat lateral and vertical Sn1- x Pbx Te-Pb heterostructures by molecular beam epitaxy is reported. The superconductivity of the Sn1- x Pbx Te-Pb heterostructures can be directly investigated by scanning tunneling spectroscopy. Unconventional peak-dip-hump gap features and fourfold symmetric quasiparticle interference patterns taken at the zero energy in the superconducting gap support the presence of the topological superconductivity in superconducting Sn1- x Pbx Te. Strong superconducting proximity effect and easy preparation of various constructions between Sn1- x Pbx Te and Pb make the heterostructures to be a promising candidate for topological superconducting devices to detect and manipulate Majorana zero modes in the future.

Rational Design of Ultralarge Pb1-x Snx Te Nanoplates for Exploring Crystalline Symmetry-Protected Topological Transport.

Ultralarge topological crystalline insulator Pb1-x Snx Te nanoplates are developed by controlling substrate surface chemical properties in a cost-efficient chemical vapor deposition (CVD) process. Dominant topological surface transport is demonstrated by a gate-voltage-controlled weak (anti)localization effect, indicating the potential application of these nanoplates to low-dissipation topological transistors.