Higher‐order topological insulators (HOTIs) belong to a new class of materials with unusual topological phases. They have garnered considerable attention due to their capabilities in confining energy ...at the hinges and corners, which is entirely protected by the topology, and have thus become attractive structures for acoustic wave studies and control. However, for most practical applications at audible and low frequencies, compact and subwavelength implementations are desirable in addition to providing robust guiding of sound beyond a single‐frequency operation. Here, a holey HOTI capable of sustaining deeply confined corner states 50 times smaller than the wavelength is proposed. A remarkable resilience of these surface‐confined acoustic states against defects is experimentally observed, and topologically protected sound is demonstrated in three different frequency regimes. Concerning this matter, the findings will thus have the capability to push forward exciting applications for robust acoustic imaging way beyond the diffraction limit.
Higher‐order topological insulators belong to a new class of materials with unusual topological phases supporting lower‐dimensional boundary states. Along this frontier, a deep‐subwavelength acoustic second‐order topological insulator is proposed to explore corner states in three different frequency ranges. Besides the topological robustness against fabrication imperfections, a programmable device is designed to achieve acoustic imaging beyond the diffraction limit.
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Breaking time‐reversal symmetry by introducing magnetic order, thereby opening a gap in the topological surface state bands, is essential for realizing useful topological properties such as the ...quantum anomalous Hall and axion insulator states. In this work, a novel topological antiferromagnetic (AFM) phase is created at the interface of a sputtered, c‐axis‐oriented, topological insulator/ferromagnet heterostructure—Bi2Te3/Ni80Fe20 because of diffusion of Ni in Bi2Te3 (Ni‐Bi2Te3). The AFM property of the Ni‐Bi2Te3 interfacial layer is established by observation of spontaneous exchange bias in the magnetic hysteresis loop and compensated moments in the depth profile of the magnetization using polarized neutron reflectometry. Analysis of the structural and chemical properties of the Ni‐Bi2Te3 layer is carried out using selected‐area electron diffraction, electron energy loss spectroscopy, and X‐ray photoelectron spectroscopy. These studies, in parallel with first‐principles calculations, indicate a solid‐state chemical reaction that leads to the formation of Ni−Te bonds and the presence of topological antiferromagnetic (AFM) compound NiBi2Te4 in the Ni‐Bi2Te3 interface layer. The Neél temperature of the Ni‐Bi2Te3 layer is ≈63 K, which is higher than that of typical magnetic topological insulators (MTIs). The presented results provide a pathway toward industrial complementary metal−oxide−semiconductor (CMOS)‐process‐compatible sputtered‐MTI heterostructures, leading to novel materials for topological quantum devices.
Study of topological magnetic phases formed in topological insulators coupled with a ferromagnet is performed on a high‐quality c‐axis‐oriented Bi2Te3 thin film grown using a magnetron sputtering process. A topological antiferromagnetic (AFM) phase of Ni‐Bi2Te3 is revealed in the interface of Bi2Te3 coupled with the ferromagnet, Ni80Fe20. The Ni‐Bi2Te3 layer is found to contain Ni−Te bonds and shows evidence of the formation of the topological AFM compound NiBi2Te4.
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Robust multiband photonic topological edge states are of great importance for photonic applications, including nonlinear wavelength conversion. In particular, higher‐order photonic topological states ...provide the realizability of photonic nanoresonators with high robustness against structural disorder of photonic crystals. This work reveals that multiband photonic topological valley‐Hall edge states and second‐order corner states can be observed in square lattice photonic crystals consisting of triangular dielectric rods. For small sizes of the triangles, multiband gapless edge modes propagate through the photonic topological waveguide. Their transmission characteristics and robustness against the structural defects have been evaluated for linear and Z‐shaped interfaces. When the size of the triangles increases, most of edge bands become gapped and one can obtain disorder‐immune multiband second‐order topological corner states, which is the core result of this report. The results obtained in this work can find important applications for nonlinear topological frequency conversion.
Multiband photonic topological valley‐Hall edge states and second‐order corner states can be observed with square lattice photonic crystals consisting of triangular dielectric rods. For small triangles, dual band gapless edge states exist. With increasing their sizes, one can obtain multiband gapped edge states and corner states which are immune to the structural defects.
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An all-Si photonic structure emulating the quantum-valley-Hall effect is proposed. We show that it acts as a photonic topological insulator (PTI), and that an interface between two such PTIs can ...support edge states that are free from scattering. The conservation of the valley degree of freedom enables efficient in- and out-coupling of light between the free space and the photonic structure. The topological protection of the edge waves can be utilized for designing arrays of resonant time-delay photonic cavities that do not suffer from reflections and cross-talk.
2D second‐order topological insulators (SOTIs) have sparked significant interest, but currently, the proposed realistic 2D materials for SOTIs are limited to nonmagnetic systems. In this study, for ...the first time, a single layer of chalcogenide CrSiTe3—an experimentally realized transition metal trichalcogenide is proposed with a layer structure—as a 2D ferromagnetic (FM) SOTI. Based on first‐principles calculations, this study confirms that the CrSiTe3 monolayer exhibits a nontrivial gapped bulk state in the spin‐up channel and a trivial gapped bulk state in the spin‐down channel. Based on the higher‐order bulk–boundary correspondence, it demonstrates that the CrSiTe3 monolayer exhibits topologically protected corner states with a quantized fractional charge (e3$\frac{e}{3}$) in the spin‐up channel. Notably, unlike previous nonmagnetic examples, the topological corner states of the CrSiTe3 monolayer are spin‐polarized and pinned at the corners of the sample in real space. Furthermore, the CrSiTe3 monolayer retains SOTI features when the spin–orbit coupling (SOC) is considered, as evidenced by the corner charge and corner states distribution. Finally, by applying biaxial strain and hole doping, this study transforms the magnetic insulating bulk states into spin‐gapless semiconducting and half‐metallic bulk states, respectively. Importantly, the topological corner states persist in the spin‐up channel under these conditions.
A single layer of chalcogenide CrSiTe3 is found to be a 2D ferromagnetic second‐order topological insulator. The CrSiTe3 monolayer exhibits a nontrivial gapped bulk state in the spin‐up channel and a trivial gapped bulk state in the spin‐down channel. The topological corner states of the CrSiTe3 monolayer are spin‐polarized and pinned at the corners of the sample in real space.
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A d-dimensional second-order topological insulator (SOTI) can host topologically protected (d-2)-dimensional gapless boundary modes. Here, we show that a 2D non-Hermitian SOTI can host zero-energy ...modes at its corners. In contrast to the Hermitian case, these zero-energy modes can be localized only at one corner. A 3D non-Hermitian SOTI is shown to support second-order boundary modes, which are localized not along hinges but anomalously at a corner. The usual bulk-corner (hinge) correspondence in the second-order 2D (3D) non-Hermitian system breaks down. The winding number (Chern number) based on complex wave vectors is used to characterize the second-order topological phases in 2D (3D). A possible experimental situation with ultracold atoms is also discussed. Our work lays the cornerstone for exploring higher-order topological phenomena in non-Hermitian systems.
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Unlike conventional topological materials that carry topological states at their boundaries, higher‐order topological materials are able to support topological states at boundaries of boundaries, ...such as corners and hinges. While band topology has been recently extended into thermal diffusion for thermal metamaterials, its realization is limited to a 1D thermal lattice, lacking access to the higher‐order topology. In this work, the experimental realization is reported of a higher‐order thermal topological insulator in a generalized 2D diffusion lattice. The topological corner states for thermal diffusion are observed in the bandgap of diffusion rate of the bulk, as a consequence of the anti‐Hermitian nature of the diffusion Hamiltonian. The topological protection of these thermal corner states is demonstrated with the stability of their diffusion profile in the presence of amorphous deformation. This work constitutes the first realization of higher‐order topology in purely diffusive systems and opens the door for future thermal management with topological protection beyond 1D geometries.
Experimental realization of higher‐order thermal topological states in a generalized 2D diffusion lattice is reported. The topological edge and corner states for thermal diffusion are observed in the bandgap of diffusion rate spectrum that isolated from the bulk. Topological protection of these thermal domain‐wall states is demonstrated with the stability of their diffusion profiles in the presence of amorphous deformation.
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Floquet engineering is the concept of tailoring a system by a periodic drive, and it is increasingly employed in many areas of physics. Ultracold atoms in optical lattices offer a particularly large ...toolbox to design a variety of driving schemes. A strong motivation for developing these methods is the prospect to study the interplay between topology and interactions in a system where both ingredients are fully tunable. We review the recent successes of Floquet engineering in realizing new classes of Hamiltonians in quantum gases, such as Hamiltonians including artificial gauge fields, topological band structures and density-dependent tunnelling. The creation of periodically driven systems also gives rise to phenomena without static counterparts such as anomalous Floquet topological insulators. We discuss the challenges facing the field, particularly the control of heating mechanisms, which currently limit the preparation of many-body phases, as well as the potential future developments as these obstacles are overcome.The freedom to manipulate quantum gases with external fields makes them an ideal platform for studying many-body physics. Floquet engineering using time-periodic modulations has greatly expanded the range of accessible models and phenomena.
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GEOZS, IJS, IMTLJ, KISLJ, NLZOH, NUK, OILJ, PNG, SAZU, SBCE, SBMB, UL, UM, UPUK, ZAGLJ
We introduce two-dimensional topological insulators in proximity to high-temperature cuprate or iron-based superconductors as high-temperature platforms of Majorana Kramers pairs of zero modes. The ...proximity-induced pairing at the helical edge state of the topological insulator serves as a Dirac mass, whose sign changes at the sample corner because of the pairing symmetry of high-T_{c} superconductors. This sign changing naturally creates at each corner a pair of Majorana zero modes protected by time-reversal symmetry. Conceptually, this is a topologically trivial superconductor-based approach for Majorana zero modes. We provide quantitative criteria and suggest candidate materials for this proposal.
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Valley‐Hall photonic topological insulators (VPTIs) provide an intriguing approach to suppress backscattering, enhance the robustness of transport, and shrink the footprints of the topological ...devices. However, previous works focused on the transmission characteristics of the symmetric topological waveguides with few reports on functional devices involving edge mode coupling, which hinders the implementation of various functional devices such as optical couplers/splitters, switches, filters, and etc. In this paper, a new method to manipulate the mode field distributions in topological waveguides is demonstrated by introducing asymmetric edge states, thus the mode coupling with an arbitrary coupling ratio can be achieved between topological waveguides. As an example, a topological power splitter with a 33:67 splitting ratio is demonstrated experimentally. In addition, to verify the robust transmission property of the topological photonic devices, a high‐speed data transmission experiment for a topological power splitter is performed and a high data rate of 128 Gb s−1 is achieved. These results promise a new method to manipulate the mode field distribution in topological waveguides and devices that may find wide applications in diverse fields such as optical communications, nanophotonics, and quantum information processing.
Proper design of asymmetric topological valley edge states can effectively manipulate the mode profile distributions, thus achieving mode coupling with arbitrary coupling ratios between topological waveguides. As an example, an on‐chip topological power splitter (TPS) is designed and fabricated with a 33:67 splitting ratio, and a first‐time high‐speed transmission experiment is carried out through the topological device.
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