Search

• Novel topological heterostructrues based on chiral lattices are proposed. • Robust high-capacity in-plane elastic wave transport is realized. • Multi-functional topological heterostructures are demonstrated. • Temperature can act as a switch for topological waveguide states. Novel 2D tri-chiral metastructures with mass inclusion are proposed in this work. Compared to conventional 2D honeycomb metastructures, these superior metastructures have a wide in-plane low-frequency bandgap (BG) and single Dirac cone (DC) simultaneously. Ligament width and inclusion density are both key factors for tuning the DC and low-frequency BGs. Due to the superior dispersion properties, metamolecules analog of quantum spin Hall effects (QSHEs) are built by the band folding method, and topological phase transition is obtained by shrinking/expanding distance between the mass inclusion and metamolecule center. Topological interface states (TISs) are observed between the two domains with distinct topological properties. To further enhance energy capacity of in-plane elastic wave transport, a 2D heterostructure is constructed by doping waveguiding layer at the topological interface. As expected, robust high capacity in-plane elastic wave transport is realized, named as topological waveguide states (TWSs). While TWS velocity remains unaffected, an increasing number of waveguiding layers additionally leads to a reduced bandgap width and transition from TWSs to conventional edge states (CESs). Average transmitted energy is also observed to increase almost linearly with the thickness of waveguide layer. By virtue of the robust high-capacity wave transport, two potential applications for energy focusing and beam splitting are clearly demonstrated. Besides, the temperature field is introduced into the 2D topological heterostructure to widen the operating frequency of TWSs. Fortunately, TWSs can be tuned to the lower frequency range by increasing temperature, and retain gapless and high-capacity characteristics. Last but not least, we demonstrate that temperature can be used as a switch for in-plane topological wave transport. The proposed 2D chiral metastructures have great potentials to serve as building blocks for multifunctional topological devices. [ABSTRACT FROM AUTHOR]

• Germanene shares many properties with graphene. • Germanene is buckled and has a sizeable spin-orbit gap. • Germanene is a two dimensional topological insulator. • Germanene exhibits the quantum spin Hall effect. • A topological phase transition from two dimensional topological insulator to trivial band insulator can be induced by an electric field. The great moment of fame for germanium was in December 1947. In that year the first transistor was made by a research team of Bell Laboratories. Owing to some problems with germanium, it was soon supplanted by silicon. Currently, germanium is still used in the microelectronic industry for opto-electronic and solar electric applications, but its role is very minor compared to its big brother silicon. After the rise of graphene, germanium received renewed interest because of the predicted stability of the graphene-like allotrope of germanium. Germanene, the germanium analogue of graphene, shares many properties with graphene, but there are also a few interesting differences that makes this material very appealing for device applications. In this contribution, I will give a brief historical overview of germanene, discuss the pros and cons of germanene and elaborate on its potential for future device applications. [Display omitted] [ABSTRACT FROM AUTHOR]

The photonic quantum spin Hall effect in time reversal invariant photonic system shows two pseudospin-up and-down photonic states preserved pseudo-time-reversal symmetry in C 6 v symmetry, and forms a typical of unidirectional helical edge modes within a topological bulk photonic bands gap. Here we demonstrate topological unidirectional helical edge states with robustness and pseudospin-locking in all-dielectric topological photonic crystals with honeycomb lattice. Based on intrinsic and local chirality of the topological helical edge modes, we calculate the directionality D to characterize directionality of topological unidirectional helical edge modes. The unidirectional excitation of topological helical edge states is position-dependent of chiral source, which is checked out by the full-wave simulations. A specific multifunctional waveguide with tunable cross-channel, V-splitting and four-channel in a waveguide structure with cross-like route is achieved by changing the location of chiral source based on chirality map. A reprogrammable system to recode information (pseudospin-polarized and location) of chiral source is presented by us, and realize multi-switching for integrated photonic via circuits external circuits control in computer. Our work has great potential for applications in on-chip optical circuits with recoding and modularized information processing using pseudospin-dependent one-way transport. • This research paper mainly works and highlights are as following: • The chirality map of topological helical edge modes is calculated and checked out. • A specific multifunctional waveguide with tunable cross-channel, V-splitting and four-channel in a waveguide structure with cross-like route is achieved. • A reprogrammable system to recode information (pseudospin-polarized and location) of chiral source is presented by us. [ABSTRACT FROM AUTHOR]

Valley Hall topological insulators (VHTIs) hold great promise for enhancing the manipulation of elastic wave propagation by their intrinsic topologically protected mechanism. Different from most of VHTIs designed by the deterministic Dirac degeneracy, a more flexible design of VHTIs is proposed by the accidental Dirac degeneracy to steer elastic wave propagation. Based on the accidental Dirac degeneracy, a kind of hexagonal phononic crystal is designed to independently control the topological phase transitions at different frequency ranges. Consequently, a two-channel topological demultiplexer is designed with the function of frequency separation for flexural waves, and its effectivity is verified by numerical simulations and experimental testing. Comparing with traditional designs of demultiplexers, the VHTIs-based demultiplexer possesses a series of advantages in robust performance, easy fabrication and low energy leakage, and sheds light on developing new generation of elastic wave devices. [Display omitted] • Topological phase transitions at different frequencies are independently controlled. • Frequency dependent edge states are obtained to transmit flexural waves robustly. • A topological demultiplexer is designed to separating mixing frequency components. [ABSTRACT FROM AUTHOR]

The honeycomb lattice iridate Na2IrO3 shows frustrated magnetism and can potentially display Kitaev-like exchange interactions. Recently, it was shown that the electronic properties of the surface of crystalline Na2IrO3 can be tuned by Ar plasma treatment in a controlled manner, leading to various phases of matter ranging from a fully gapped to a metallic surface, where the possibility of a charge-density wave like transition has been suggested. Here, through direct imaging with an atomic force microscope (AFM) in air, we show that the surface of crystalline Na2IrO3 evolves rapidly as elemental Na effuses out of the interleave planes to the surface and undergoes sublimation, thereby disappearing from the surface gradually over time. Using conductive AFM, we recorded a series of topographs and surface current maps simultaneously and found that the modification of the surface leads to change in the electronic properties in a dynamic fashion until the whole system reaches a dynamic equilibrium. These observations are important in the context of the exotic electronic and magnetic properties that the surface of Na2IrO3 displays. Published by AIP Publishing. [ABSTRACT FROM AUTHOR]

The honeycomb lattice iridate Na2IrO3 shows frustrated magnetism and can potentially display Kitaev-like exchange interactions. Recently, it was shown that the electronic properties of the surface of crystalline Na2IrO3 can be tuned by Ar plasma treatment in a controlled manner, leading to various phases of matter ranging from a fully gapped to a metallic surface, where the possibility of a charge-density wave like transition has been suggested. Here, through direct imaging with an atomic force microscope (AFM) in air, we show that the surface of crystalline Na2IrO3 evolves rapidly as elemental Na effuses out of the interleave planes to the surface and undergoes sublimation, thereby disappearing from the surface gradually over time. Using conductive AFM, we recorded a series of topographs and surface current maps simultaneously and found that the modification of the surface leads to change in the electronic properties in a dynamic fashion until the whole system reaches a dynamic equilibrium. These observations are important in the context of the exotic electronic and magnetic properties that the surface of Na2IrO3 displays. Published by AIP Publishing. [ABSTRACT FROM AUTHOR]

A large bandgap is critical for the applications of quantum spin Hall (QSH) insulators at room temperature. Based on the first-principles calculations, we predict that the methyl and ethynyl functionalized TlSb monolayers, namely, TlSb(CH3)2 and TlSb(C2H)2 films, own QSH states with large bandgaps of 0.13 and 0.272 eV, which possess potential applications at room temperature. For TlSb(CH3)2, the QSH phase arises from the spin-orbit coupling (SOC) induced s-p band inversion, while for TlSb(C2H)2, the QSH phase results from the SOC induced p-p bandgap opening. The QSH effect is further characterized by the Z2 topological invariant and topologically protected edge states. Significantly, the QSH states in TlSb(CH3)2 and TlSb(C2H)2 films are robust against external strain and various methyl/ethynyl coverages, making them especially flexible in the substrate selection. Besides, we find that h-BN is an ideal substrate for TlSb(CH3)2 and TlSb(C2H)2 films to keep QSH states with large bandgaps. Thus, the methyl and ethynyl functionalized TlSb films may be good QSH effect platforms for the design and fabrication of topological electronic devices. [ABSTRACT FROM AUTHOR]

The spin reorientation in rare-earth intermetallics involves distinctive magnetic morphologies commencing with spontaneous skyrmion textures without an external magnetic field. Here, we present the sputtering growth of CaCu5-type NdCo5−xCux thin films on MgO (110) substrates. Our films exhibit two successive spin reorientation transitions between the ab-plane and the c-axis, close to the room temperature, apart from the non-uniaxial behavior below 150 K. The corresponding modulations of magnetocrystalline anisotropy at reorientation temperatures lead to the large topological Hall effect, which can be maintained up to 250 K with a maximum Hall resistivity of 210 nΩ cm. These results of robust topological signals will provide platforms for realizing room-temperature topological magnetic textures. [ABSTRACT FROM AUTHOR]

Magnetic skyrmions are nanoscale topologically nontrivial spin texture that offer great promise as information carriers for the next-generation spintronic computing schemes. However, the current skyrmion-based logic gates require precise control of skyrmion collisions through clocked synchronizers, which leads to the complexity of logic implementation. To address this challenge, we propose a clockless skyrmion logic gate that can be implemented into large-scale computing networks without skyrmions synchronization. The clockless operations are achieved in a cross-shaped skyrmion track based on the skyrmion gating method with the aid of the voltage-controlled magnetic anisotropy gate. A complete set of Boolean operations and a cascaded full adder can be realized through the different combinations of inputs and interconnections of these logic gates. This computing paradigm paves the way for the design of a highly efficient and robust computing architecture using fully skyrmion-based logic devices. [ABSTRACT FROM AUTHOR]

The properties of topological edge states of resonant photonic lattices are reported. The canonical 1D lattice embodies all chief qualities necessary for a complete comprehension of the variety of fundamental physical processes at play. Radiating topological edge states are understood in terms of leaky‐mode resonance stimulated by counter‐propagating Bloch modes. As a free‐space electromagnetic wave illuminates the junction between band‐flipped photonic lattices, quasi‐leaky resonant light is localized at the interface, which embodies the characteristics of the closed‐band state of the composite lattice. Rigorous numerical examples with Ge/ZnSe‐based lattices with a finite number of periods and having homogeneous sublayers illustrate the resonance properties of the constituent lattices, the band‐closed lattice, and the composite lattice hosting the radiant edge state. The topological interface is seen to radiate effectively near the wavelength where the band closes. With the insight gained from the 1D analysis, a representative 2D lattice is similarly treated. As orthogonal leaky modes are supported by 2D lattices, the interface‐bound energy flow picture is more intriguing. Thus, lateral TE and TM modes can be excited to run along the topological interface or normal to it. Versatile power flow, energy density, and radiation characteristics are then available at the 2D topological interface. [ABSTRACT FROM AUTHOR]

The article presents a study which discovered that the topology of structures fundamentally dictates the topological properties of optical fields and offers a new dimension to exploit for optical functionalities that are irrelevant to specific material constituents or structural geometries. Topics include polarization singularities (PSs) protected by surface topology of structures, integration of topology and mirrors symmetry, and integration of topology and generalized rotational symmetry.

Quantum anomalous Hall effect (QAHE) and quantum spin Hall effect (QSHE) are two interesting physical manifestations of 2D materials that have an intrinsic nontrivial band topology. In principle, they are ground-state equilibrium properties characterized by Fermi level lying in a topological gap, below which all the occupied bands are summed to a non-zero topological invariant. Here, we propose theoretical concepts and models of ‘excited’ QAHE (EQAHE) and EQSHE generated by dissociation of an excitonic insulator (EI) state with complete population inversion (CPI), a unique many-body ground state enabled by two yin-yang flat bands (FBs) of opposite chirality hosted in a diatomic Kagome lattice. The two FBs have a trivial gap in between, i.e. the system is a trivial insulator in the single-particle ground-state, but nontrivial gaps above and below, so that upon photoexcitation the quasi-Fermi levels of both electrons and holes will lie in a nontrivial gap achieved by the CPI-EI state, as demonstrated by exact diagonalization calculations. Then dissociation of singlet and triplet EI state will lead to EQAHE and EQSHE, respectively. Realizations of yin-yang FBs in real materials are also discussed. [ABSTRACT FROM AUTHOR]

Based on the concept of band topology in phonon dispersion, we designed a topological phononic crystal in a thin plate for developing an efficient elastic waveguide. Despite that various topological phononic structures have been actively proposed, a quantitative design strategy of the phononic band and its robustness assessment in an elastic regime are still missing, hampering the realization of topological acoustic devices. We adopted a snowflake-like structure for the crystal unit cell and determined the optimal structure that exhibited the topological phase transition of the planar phononic crystal by changing the unit cell structure. The bandgap width could be adjusted by varying the length of the snow-side branch, and a topological phase transition occurred in the unit cell structure with threefold rotational symmetry. Elastic waveguides based on edge modes appearing at interfaces between crystals with different band topologies were designed, and their transmission efficiencies were evaluated numerically and experimentally. The results demonstrate the robustness of the elastic wave propagation in thin plates. Moreover, we experimentally estimated the backscattering length, which measures the robustness of the topologically protected propagating states against structural inhomogeneities. The results quantitatively indicated that degradation of the immunization against the backscattering occurs predominantly at the corners in the waveguides, indicating that the edge mode observed is a relatively weak topological state. [ABSTRACT FROM AUTHOR]

In insulating crystals, it was previously shown that defects with two fewer dimensions than the bulk can bind topological electronic states. We here further extend the classification of topological defect states by demonstrating that the corners of crystalline defects with integer Burgers vectors can bind 0D higher-order end (HEND) states with anomalous charge and spin. We demonstrate that HEND states are intrinsic topological consequences of the bulk electronic structure and introduce new bulk topological invariants that are predictive of HEND dislocation states in solid-state materials. We demonstrate the presence of first-order 0D defect states in PbTe monolayers and HEND states in 3D SnTe crystals. We relate our analysis to magnetic flux insertion in insulating crystals. We find that π-flux tubes in inversion- and time-reversal-symmetric (helical) higher-order topological insulators bind Kramers pairs of spin-charge-separated HEND states, which represent observable signatures of anomalous surface half quantum spin Hall states. Many topological crystalline phases have unknown physical responses. Here, the authors systematically extend the theory of defect and flux responses to predict zero-dimensional (0D) states in topological crystalline materials, including 2D PbTe monolayers and 3D SnTe. [ABSTRACT FROM AUTHOR]

Photonic and phononic topological insulators (TIs) offer numerous opportunities for manipulating light and sound with high efficiency and resiliency. On the other hand, inverse design methodologies, such as gradient-based approaches, evolutionary approaches, and deep-learning methods, provide a cost-effective strategy for developing photonic and phononic structures with unique features in steering light and sound. Here, we discuss recent advances and achievements in the development of photonic and phononic TIs employing inverse design methodologies, including one-dimensional TIs, TIs based on the quantum spin Hall effect (QSHE) and quantum valley Hall effect (QVHE), and high-order TIs in lattices with diverse symmetries. Several inversely designed photonic and phononic TIs with superior performance are exhibited. In addition, we offer our perspectives on the future of this emerging study field. [ABSTRACT FROM AUTHOR]

The study on quantum spin Hall effect and topological insulators formed the prologue to the surge of research activities in topological materials in the past decade. Compared to intricately engineered quantum wells, three-dimensional weak topological insulators provide a natural route to the quantum spin Hall effect, due to the adiabatic connection between them and a stack of quantum spin Hall insulators, and the convenience in exfoliation of samples associated with their van der Waals-type structure. Despite these advantages, both theoretical prediction and experimental identification of weak topological insulators remain scarce. Here, based on first-principles calculations, we show that AuTe2Br locates at the boundary between a strong and a weak topological semimetal state. We identify the key structural parameter that dictates the traversal of the topological transition, which can be easily realized in experiments. More interestingly, the critical topology of AuTe2Br persists up to an applied pressure of ~15.4 GPa before a structural phase transition accompanied by a change of electronic topology and the onset of superconductivity. Our results establish AuTe2Br as a new candidate for an effective tuning between weak and strong topological phases in a single material, with the potential to realize various other topological phases of matter. [ABSTRACT FROM AUTHOR]

We consider a disordered topological insulator thin film placed on the top of a ferromagnetic insulator with a perpendicular exchange field M and subjected to a perpendicular electric field. The presence of ferromagnetic insulator causes that bottom surface states of the topological insulator thin film become spin polarized and the electric field provides a potential difference V between the two surface states, resulting in breaking of time-reversal and inversion symmetry in the system. Using Kubo formalism and employing the first Born approximation as well as the self-consistent Born approximation, we calculate the spin Hall conductivity. We find that for small values of V, a large spin conductivity can be generated through large values of M away from the charge neutrality point. But for large values of V, the spin conductivity can be promoted even with small values of M around the charge neutrality point. The effect of vertex corrections and the stability of the obtained large spin conductivity against disorders are also examined. [ABSTRACT FROM AUTHOR]

Although recent studies have established a powerful framework to search for and classify topological phases based on symmetry indicators, there exists a large class of fragile topology beyond the description. The Euler class characterizing the topology of two-dimensional real wave functions is an archetypal fragile topology underlying some important properties. However, as a minimum model of fragile topology, the two-dimensional topological Euler insulator consisting of three bands remains a significant challenge to be implemented in experiments. Here, we experimentally realize a three-band Hamiltonian to simulate a topological Euler insulator with a trapped-ion quantum simulator. Through quantum state tomography, we successfully evaluate the Euler class, Wilson loop flow, entanglement spectra and Berry phases to show the topological properties of the Hamiltonian. The flexibility of the trapped-ion quantum simulator further allows us to probe dynamical topological features including skyrmion-antiskyrmion pairs and Hopf links in momentum-time space from quench dynamics. A new class of topological systems has been recently discovered, it has been dubbed the Euler insulator and can manifest in two-dimensional systems. The authors present an experimental implementation for realizing the Euler insulator with 171Yb+ ion trapped by an electrode-surface trap where the topological indicators such as the Euler class, Wilson loop flow and entanglement spectrum for the theoretical and experimental models clearly show the presence of a fragile phase [ABSTRACT FROM AUTHOR]

The introduction of topological concepts to the design of photonic crystal cavities holds great promise for applications in integrated photonics due to the prospect of topological protection. This study examines the signatures of topological light confinement in the leakage radiation of 2D topological photonic crystal cavities. The cavities are implemented in an all-dielectric platform that features the photonic quantum spin Hall effect at telecom wavelengths and supports helical edge states that are weakly coupled to the radiation continuum. The modes of resonators scaling down to single point defects in the surrounding bulk lattice are characterized via spectral position and multipolar nature of the eigenstates. The mode profiles in real and momentum space are mapped using far-field imaging and Fourier-spectropolarimetry, revealing how certain properties of the cavity modes reflect on their origin in the topological bandstructure. This includes band-inversion-induced confinement and inverted scaling of mode spectra for trivial and topological defect cavities. Furthermore, hallmarks of topological protection in the loss rates are demonstrated, which are largely unaffected by cavity shape and size. The results constitute an important step toward the use of radiative topological cavities for on-chip confinement of light, control of emitted wave fronts, and enhancement of light–matter interactions. [ABSTRACT FROM AUTHOR]

Quantum spin Hall (QSH) insulators have unique electronic properties, comprising a band gap in their 2D interior and 1D spin-polarized edge states in which current flows ballistically. In scanning tunneling microscopy (STM), the edge states manifest themselves as an enhanced local density of states (LDOS). However, there is a significant research gap between the observation of edge states in nanoscale spectroscopy, and the detection of ballistic transport in edge channels which typically relies on transport experiments with microscale lithographic contacts. Here, few-layer films of the 3D topological insulator (BixSb1−x)2Te3 are studied, for which a topological transition to a 2D topological QSH insulator phase has been proposed. Indeed, an edge state in the LDOS is observed within the band gap. Yet, in nanoscale transport experiments with a four-tip STM, two-quintuple-layer films do not exhibit a ballistic conductance in the edge channels and thus no QSH edge states. This demonstrates that the detection of edge states in spectroscopy can be misleading with regard to the identification of a QSH phase. In contrast, nanoscale multi-tip transport experiments are a robust method for effectively pinpointing ballistic edge channels, as opposed to trivial edge states, in quantum materials. [ABSTRACT FROM AUTHOR]

To determine the topological quantum numbers of fractional quantum Hall (FQH) states hosting counter-propagating (CP) downstream (Nd) and upstream (Nu) edge modes, it is pivotal to study quantized transport both in the presence and absence of edge mode equilibration. While reaching the non-equilibrated regime is challenging for charge transport, we target here the thermal Hall conductance GQ, which is purely governed by edge quantum numbers Nd and Nu. Our experimental setup is realized with a hexagonal boron nitride (hBN) encapsulated graphite gated single layer graphene device. For temperatures up to 35 mK, our measured GQ at ν = 2/3 and 3/5 (with CP modes) match the quantized values of non-equilibrated regime (Nd + Nu)κ0T, where κ0T is a quanta of GQ. With increasing temperature, GQ decreases and eventually takes the value of the equilibrated regime ∣Nd − Nu∣κ0T. By contrast, at ν = 1/3 and 2/5 (without CP modes), GQ remains robustly quantized at Ndκ0T independent of the temperature. Thus, measuring the quantized values of GQ in two regimes, we determine the edge quantum numbers, which opens a new route for finding the topological order of exotic non-Abelian FQH states. The knowledge of quantum numbers of the edge modes is essential for understanding fractional Hall states containing counter-propagating downstream and upstream modes. Here the authors identify the edge quantum numbers by probing a crossover from non-equilibrated to equilibrated edge mode regime in thermal conductance. [ABSTRACT FROM AUTHOR]

It is necessary but challenging to verify topological surface states of α-Sn by electrical transport. In this work, we demonstrate conclusive transport evidence on topological properties of an α-Sn film grown on a CdTe substrate by molecular beam epitaxy. A Berry phase determined from Shubnikov–de Haas oscillations is 0.98π. A two-dimensional (2D) Fermi surface is clearly demonstrated by angle-dependent oscillations. We believe the nontrivial topology originates from the 2D Dirac fermions of the topological surface states. In addition, both anisotropic magneto-resistance and planar Hall effect have negative amplitudes at higher fields, which we attribute to the spin-flip backscattering in the topological surface states. We also show that these topological surface states have a long relaxation time of ∼95 fs, making α-Sn a potential candidate for high-efficiency spintronics. [ABSTRACT FROM AUTHOR]

The substituted Mn2Sb-based ferrimagnetic (FIM) alloys by other elements have attracted much attention recently due to excellent magneto-functional properties. However, almost all investigations have been concentrated on the first-order phase transition between antiferromagnetic and FIM state. In this work, a large topological Hall effect (THE) associated with the spin reorientation transition is observed in the Mn2Sb0.9Bi0.1 alloy. Strikingly, the sign reversal of topological Hall resistivity ( ρ x y T ) is found before and after the SRT temperature TSR ∼260 K. Above the TSR, a positive ρ x y T is observed, e.g., ∼0.52 μΩ cm at 320 K. By reducing temperature to 275 K, the ρ x y T value decreases to ∼0.42 μΩ cm. It is interesting that, however, the negative ρ x y T value is found below the TSR, e.g., ∼−0.66 μΩ cm at 175 K. The physical mechanism about this peculiar THE is discussed in Bi-doped Mn2Sb-based alloys. [ABSTRACT FROM AUTHOR]

Spin–orbit torque (SOT) has been extensively applied to magnetization manipulation in low power consumption logic and memory devices. However, it is believed that materials with strong spin–orbit coupling (SOC) are indispensable for magnetic torque generation. Recently, theoretical studies have indicated that the oxides of light materials with weak SOC can provide a sizable orbital torque (OT), inducing magnetization switching. Here, we experimentally report the extreme enhancement of torque efficiency and spin Hall angle through the natural oxidation of Cu with weak SOC in the perpendicularly magnetized Pt/Co/Cu–CuOx multilayers. The values of torque efficiency and spin Hall angle increase by approximately five times by tuning the surface oxidation at room temperature. The comparative analysis of the effective field reveals that the significant enhancement mainly originates from the collaborative drive of the OT at the Cu/CuOx interface and the SOT from the Pt layer. This finding provides a powerful way to engineer the high-efficient spintronic devices through combining OT and SOT to improve the torque efficiency. [ABSTRACT FROM AUTHOR]

Chiral magnetic phases with an unconventional topological twist in the magnetization are of huge interest due to their potential in spintronics applications. Here, we present a general method to induce such exotic magnetic phases using interfacial phase frustration within artificially grown superlattices. To demonstrate our method, we consider a multilayer with two different chiral magnetic phases as the competing orders at the top and bottom and show, using Monte Carlo calculations, that the interfacial phase frustration is realized at the central layer. In particular, we obtain three unconventional phases: a checkerboard skyrmion crystal, an incommensurate skyrmion stripe, and a ferrimagnetic skyrmion crystal. In these frustration-induced phases, the spin chirality-driven topological Hall conductivity can be largely enhanced. This method provides a playground to realize unconventional magnetic phases in any family of materials that can be grown in superlattices. [ABSTRACT FROM AUTHOR]

We studied the emergence of the quantum spin Hall (QSH) states for the pseudo-gap (PG) phase of Bi2212 bilayer system, assumed to be D-density wave (DDW) ordered, starting with a strong Rashba spin-orbit coupling (SOC) armed, and the time reversal symmetry (TRS) complaint Bloch Hamiltonian. The presence of strong SOC gives rise to non-trivial, spin-momentum locked spin texture tunable by electric field. The emergence of quantum anomalous Hall effect with TRS broken Chiral DDW Hamiltonian of Das Sarma et al. is found to be possible. [ABSTRACT FROM AUTHOR]

• An equivalent mathematical model developed for underwater acoustic metamaterials. • Observation of mode shape transition for both acoustic pressure and displacement fields. • Occurrence of two frequency domains for topologically protected interface modes. • Investigation of active control effects on the working frequency of the system. • Various waveguide paths designs and acoustic switch designs. This study presents a new type of active underwater local resonance metamaterial mathematical model for topologically protected acoustic wave propagation. The underwater acoustic metamaterial consists of arrays of core cylinders coated with soft shells periodically located in a water environment. An efficient but simple computational model is established to analytically predict the system band behavior. Compared to a system with only Bragg scatters, two double Dirac cones (DDCs) are observed in the dispersion curve due to the local resonance mechanism. The result obtained from numerical analysis shows great consistency with analytical prediction. By tuning structural parameters, the four-fold degeneracy is realized at the center of the irreducible Brillouin zone, i.e., the Γ point. It leads to the generation of two dipolar states and two quadrupolar states. Motivated by the theoretical prediction, locations of the cylinders are tuned and the closing-reopening phenomenon of bandgaps in both higher frequency region and lower region is observed with topologically protected phase transition. Besides, it is found that two frequency domains for topologically protected interface modes (TPIMs) occur. It provides potentials for two passive working frequency ranges without structural reconstruction or active tuning of the system. The effects of temperature on TPIM behavior are also demonstrated. Both working frequencies of TPIMs can be easily tuned to different ranges without influencing the quality of TPIMs. Besides, robustness and defect-immune properties of the topologically protected wave propagation are exhibited. With temperature control, this smart system can be performed as an acoustic metamaterial switch device. [Display omitted]. [ABSTRACT FROM AUTHOR]

The emergence of an intrinsic quantum anomalous Hall (QAH) insulator with long-range magnetic order triggers unprecedented prosperity for combining topology and magnetism in low dimensions. Here, based on stacked two-dimensional LiFeTe, we confirm that magnetic coupling and topological electronic states can be simultaneously manipulated by just changing the layer numbers. Monolayer LiFeTe shows intralayer ferrimagnetic coupling, behaving as a QAH insulator with Chern number C = 2. Beyond the monolayer, the odd and even layers of LiFeTe correspond to uncompensated and compensated interlayer antiferromagnets, resulting in unexpected QAH and quantum spin Hall (QSH) states, respectively. Moreover, the spin Chern number is proportional to the stacking layer numbers in even-layer LiFeTe, proving that the spin Hall conductivity can be continuously enhanced by increasing layer numbers. Therefore, the odd-even-layer-dependent QAH and QSH effects found in LiFeTe topological insulators offer new insight into regulating quantum states in two-dimensional topological materials.

In this study, we present a theoretical study of the quantum spin Hall effect in InN/InGaN coupled multiple quantum wells with the individual well widths equal to two atomic monolayers. We consider triple and quadruple quantum wells in which the In content in the interwell barriers is greater than or equal to the In content in the external barriers. To calculate the electronic subbands in these nanostructures, we use the eight-band k∙p Hamiltonian, assuming that the effective spin–orbit interaction in InN is negative, which represents the worst-case scenario for achieving a two-dimensional topological insulator. For triple quantum wells, we find that when the In contents of the external and interwell barriers are the same and the widths of the internal barriers are equal to two monolayers, a topological insulator with a bulk energy gap of 0.25 meV can appear. Increasing the In content in the interwell barriers leads to a significant increase in the bulk energy gap of the topological insulator, reaching about 0.8 meV. In these structures, the topological insulator can be achieved when the In content in the external barriers is about 0.64, causing relatively low strain in quantum wells and making the epitaxial growth of these structures within the range of current technology. Using the effective 2D Hamiltonian, we study the edge states in strip structures containing topological triple quantum wells. We demonstrate that the opening of the gap in the spectrum of the edge states caused by decreasing the width of the strip has an oscillatory character regardless of whether the pseudospin-mixing elements of the effective Hamiltonian are omitted or taken into account. The strength of the finite size effect in these structures is several times smaller than that in HgTe/HgCdTe and InAs/GaSb/AlSb topological insulators. Therefore, its influence on the quantum spin Hall effect is negligible in strips with a width larger than 150 nm, unless the temperature at which electron transport is measured is less than 1 mK. In the case of quadruple quantum wells, we find the topological insulator phase only when the In content in the interwell barriers is larger than in the external barriers. We show that in these structures, a topological insulator with a bulk energy gap of 0.038 meV can be achieved when the In content in the external barriers is about 0.75. Since this value of the bulk energy gap is very small, quadruple quantum wells are less useful for realizing a measurable quantum spin Hall system, but they are still attractive for achieving a topological phase transition and a nonlocal topological semimetal phase.

In recent years, the spin Hall effect has received great attention because of its potential application in spintronics and quantum information processing and storage. However, this effect is usually studied under the external homogeneous electric field. Understanding how the inhomogeneous electric field affects the spin Hall effect is still lacking. Here, we investigate a two-dimensional two-band time-reversal symmetric system and give an expression for the intrinsic spin Hall conductivity in the presence of the inhomogeneous electric field, which is shown to be expressed through the geometric quantities: quantum metric and interband Berry connection. We show that for Rashba and Dresselhaus systems, the inhomogeneous intrinsic spin Hall conductivity can be tuned with the Fermi energy. On the other hand, when people get physical intuition on transport phenomena from the wave packet, one issue appears. It is shown that the conductivity obtained from the conventional wave packet approach cannot be fully consistent with the one predicted by the Kubo-Greenwood formula. Here, we attempt to solve this problem. The spin Hall effect is a transport phenomenon with significant importance for spintronics since it can be used to design ways of generating and detecting spin currents. Here, the authors investigate the intrinsic spin Hall effect induced by an inhomogeneous electric field and show that it can be engineered by tuning the Fermi energy in both Rashba and Dresselhaus systems. [ABSTRACT FROM AUTHOR]

Topological materials have significant potential for spintronic applications owing to their superior spin–charge interconversion. Here, the spin‐to‐charge conversion (SCC) characteristics of epitaxial Bi1−xSbx films is investigated across the topological phase transition by spintronic terahertz (THz) spectroscopy. An unexpected, intense spintronic THz emission is observed in the topologically nontrivial semimetal Bi1−xSbx films, significantly greater than that of Pt and Bi2Se3, which indicates the potential of Bi1−xSbx for spintronic applications. More importantly, the topological surface state (TSS) is observed to significantly contribute to SCC, despite the coexistence of the bulk state, which is possible via a unique ultrafast SCC process, considering the decay process of the spin‐polarized hot electrons. This means that topological material‐based spintronic devices should be fabricated in a manner that fully utilizes the TSS, not the bulk state, to maximize their performance. The results not only provide a clue for identifying the source of the giant spin Hall angle of Bi1−xSbx, but also expand the application potential of topological materials by indicating that the optically induced spin current provides a unique method for focused‐spin injection into the TSS. [ABSTRACT FROM AUTHOR]

Spin-orbit coupling plays an important role in the spin Hall effect and topological insulators. Bose-Einstein condensates with spin-orbit coupling show remarkable quantum phase transition. In this work we control an exciton polariton condensate – a macroscopically coherent state of hybrid light and matter excitations – by virtue of the Rashba-Dresselhaus (RD) spin-orbit coupling. This is achieved in a liquid-crystal filled microcavity where CsPbBr3 perovskite microplates act as the gain material at room temperature. Specifically, we realize an artificial gauge field acting on the CsPbBr3 exciton polariton condensate, splitting the condensate fractions with opposite spins in both momentum and real space. Besides the ground states, higher-order discrete polariton modes can also be split by the RD effect. Our work paves the way to manipulate exciton polariton condensates with a synthetic gauge field based on the RD spin-orbit coupling at room temperature. Engineered spin-orbit coupling can induce novel quantum phases in a Bose-Einstein condensate, however such demonstrations have been limited to cold atom systems. Here the authors realize a exciton-polarion condensate with tunable spin-orbit coupling in a liquid crystal microcavity at room temperature. [ABSTRACT FROM AUTHOR]

Electronic band-gap is a key factor in applying two-dimensional (2D) topological insulators into room-temperature quantum spin Hall effect (QSH) spintronic devices. Employing pseudopotential plane-wave first-principles calculations, we investigate spin-orbit coupling (SOC) electronic structures of the novel 2D topological insulator series of antimony (Sb) and bismuth (Bi) monolayers (isolated double atomic layers) functionalized by organic-groups (methyl, amino and hydroxyl). Cohesive energies and phonon frequency dispersion spectra indicate that these organic-group decorated Sb and Bi monolayers possess structural stability in both energetic statics and lattice dynamics. The giant electronic band-gaps adequate for room-temperature applications are attributed to the effective SOC enhancement of group functionalization. The nontrivial topology of these novel 2D monolayer materials is verified by the Z2 invariant derived from wave-function parity and edge-states of their nanoribbons, which is prospective for QSH spintronic devices. The chemical functional group changes the p-orbital component of Fermi level electrons, leading to strong intra-layer spin-orbit coupling, opening a large band-gap of approaching 1.4 eV at Dirac-cone point and resulting in a global indirect band-gap of 0.75 eV, which, even underestimated, is adequate for room-temperature operations. Sb and Bi monolayers functionalized by organic groups are also predicted to maintain stable nontrivial topology under in-layer biaxial strain, which is suitable for epitaxy technology to realize QSH spintronic devices. [ABSTRACT FROM AUTHOR]

The scientific interest in two-dimensional topological insulators (2D TIs) is currently shifting from a more fundamental perspective to the exploration and design of novel functionalities. Key concepts for the use of 2D TIs in spintronics are based on the topological protection and spin-momentum locking of their helical edge states. In this study we present experimental evidence that topological protection can be (partially) lifted by pairwise coupling of 2D TI edges in close proximity. Using direct wave function mapping via scanning tunneling microscopy/spectroscopy (STM/STS) we compare isolated and coupled topological edges in the 2D TI bismuthene. The latter situation is realized by natural lattice line defects and reveals distinct quasi-particle interference (QPI) patterns, identified as electronic Fabry-Pérot resonator modes. In contrast, free edges show no sign of any single-particle backscattering. These results pave the way for novel device concepts based on active control of topological protection through inter-edge hybridization for, e.g., electronic Fabry-Pérot interferometry. New functionalities of two-dimensional topological insulators (2DTI) are of current scientific interest. Here, the authors show that topological protection can be lifted by pairwise coupling of 2DTI bismuthene edges in close proximity. [ABSTRACT FROM AUTHOR]

Topological photonics provides exceptional opportunities to control electromagnetic waves with a great potential for applications. Most of the proposed photonic systems support topological edge states with fixed parameters, thus hindering their practical applications. The study of nonlinear and tunable effects in topological systems enlarges applications of topological phenomena. Here, we propose an approach for the manipulation of photonic topological edge states based on temperature tuning. We design and demonstrate experimentally topological zigzag arrays composed of high-index resonators. The resonators are fabricated from ferroelectrics that brings an opportunity to dynamically change their permittivity by heating. We study the emergence of topological edge states in zigzag arrays of ferroelectric particles supporting the Mie resonances and demonstrate the topological transition induced by heating individual resonators in the array. [ABSTRACT FROM AUTHOR]

The combination of nontrivial topology and charge density wave (CDW) has been proposed as a powerful resource for realizing novel quantum phenomena such as axion electrodynamics and the anomalous Hall effect. Hence, topological materials with CDW states attract great interest, yet they are still very rare, particularly in the 2D limit. Here, it is predicted that monolayer NbTe2 in its high‐symmetry 1T phase stabilized by anharmonicity at room temperature exhibits nontrivial topology sensitive to electronic interactions: it changes from a quantum spin Hall (QSH) state to a quantum anomalous Hall (QAH) state when the Hubbard potential U exceeds a critical value. At low temperature, a 4 × 4 CDW order emerges and coexists with the nontrivial topology. Meanwhile, the critical U increases because CDW reduces density of states at Fermi level. More interestingly, in contrast to the high‐symmetry structure that actually is a topologically nontrivial metal, the CDW structure shows an insulating nontrivial gap either in the QSH or the QAH phase, indicating CDW is an effective means to modulate the topological state for developing new functions and devices. These discoveries establish NbTe2 as a promising candidate to explore exotic quantum states at the confluence of nontrivial topology, electronic correlation, and CDW. [ABSTRACT FROM AUTHOR]

The combination of nontrivial topology and charge density wave (CDW) has been proposed as a powerful resource for realizing novel quantum phenomena such as axion electrodynamics and the anomalous Hall effect. Hence, topological materials with CDW states attract great interest, yet they are still very rare, particularly in the 2D limit. Here, it is predicted that monolayer NbTe2 in its high‐symmetry 1T phase stabilized by anharmonicity at room temperature exhibits nontrivial topology sensitive to electronic interactions: it changes from a quantum spin Hall (QSH) state to a quantum anomalous Hall (QAH) state when the Hubbard potential U exceeds a critical value. At low temperature, a 4 × 4 CDW order emerges and coexists with the nontrivial topology. Meanwhile, the critical U increases because CDW reduces density of states at Fermi level. More interestingly, in contrast to the high‐symmetry structure that actually is a topologically nontrivial metal, the CDW structure shows an insulating nontrivial gap either in the QSH or the QAH phase, indicating CDW is an effective means to modulate the topological state for developing new functions and devices. These discoveries establish NbTe2 as a promising candidate to explore exotic quantum states at the confluence of nontrivial topology, electronic correlation, and CDW. [ABSTRACT FROM AUTHOR]

Quantum matter (novel phases of matter at zero temperature with exotic properties) is a growing field with applications in its own domain, and in providing foundational support to quantum sciences fields more generally. The ability to characterize and manipulate matter at the smallest scales continues to advance in fundamental ways. This review provides a plain-language, non-technical description of contemporary activity in quantum matter for a general science audience, and an example of these methods applied to quantum neuroscience. Quantum matter is the study of topologically governed phases of matter at absolute zero temperature that exhibit new kinds of emergent order and exotic properties related to topology and symmetry, entanglement, and electronic charge and magnetism, which may be orchestrated to create new classes of materials and computational devices (including in the areas of spintronics, valleytronics, and quantum computing). The paper is organized to discuss recent developments in quantum matter on the topics of short-range topologically protected materials (namely, topological semimetals), long-range entangled materials (quantum spin liquids and fractional quantum Hall states), and codes for characterizing and controlling quantum systems. A key finding is that a shift in the conceptualization of the field of quantum matter may be underway to expand the core focus on short-range topologically protected materials to also include geometry-based approaches and long-range entanglement as additionally important tools for the understanding, characterization, and manipulation of topological materials. [ABSTRACT FROM AUTHOR]

The survival of the quantum spin Hall edge channels in presence of an external magnetic field has been a subject of experimental and theoretical research. The inversion of Landau levels that accommodates the quantum spin Hall effect is destroyed at a critical magnetic field, and a trivial insulating gap appears in the spectrum for stronger fields. In this work, we report the absence of this transport gap in disordered two dimensional topological insulators in perpendicular magnetic fields of up to 16 T. Instead, we observe that a topological edge channel (from band inversion) coexists with a counterpropagating quantum Hall edge channel for magnetic fields at which the transition to the insulating regime is expected. For larger fields, we observe only the quantum Hall edge channel with transverse resistance close to h/e2. By tuning the disorder using different fabrication processes, we find evidence that this unexpected ν = 1 plateau originates from extended quantum Hall edge channels along a continuous network of charge puddles at the edges of the device. The quantum spin Hall effect disappears at high magnetic fields when the band inversion is lifted. The authors demonstrate that in contrast, in disordered samples, counter-propagating topological and quantum Hall edge channels prevent the detection of the trivial gap, explaining a previous observation. [ABSTRACT FROM AUTHOR]

The paper presents the experimental results of the electrical resistance, magnetic susceptibility, and Hall effect of the intermetallic compounds of the Gd−In system in a wide temperature range. The normal and anomalous Hall effect is determined as well as the electronic band parameters and the effective parameter of the spin-orbit interaction of the described intermetallic compounds. [ABSTRACT FROM AUTHOR]

Higher-order topological insulator hosts both gapped edge states and in-gap corner states, which has garnered considerable attentions in the field of condensed matter physics, and most recently is further extended to the classical wave systems. Conventional acoustic metamaterials have intrinsic material and design limitations that prevent them from being used to create such states in subwavelength scale with function reconfigurability. Recently, the acoustic second-order topological insulators (SOTIs) composed of locally resonant metamaterials were reported to solve the problem, where the topological phase transition is induced by shrinking/expanding metamolecules. Here, we propose an acoustic SOTI in subwavelength scale by another protocol, i.e. gliding metamolecules, and the SOTI is pinned in the nontrivial region without the regular topological phase transition. Soda cans metamaterials in free space are utilized to support the spoof surface acoustic waves. With varying the introduced glided angle, the switching from the bulk to edge and corner states can be achieved accordingly. Furthermore, we not only experimentally observe this state switching process, but also illustrate the robustness of the topological corner states against various defects. Our results provide versatile ways to launch acoustic lower-dimensional topological states that might lead to interesting sound concentration applications. [ABSTRACT FROM AUTHOR]

Among efforts made to improve thermoelectric efficiency, the use of structurally modified graphene nanomaterials as thermoelectric matter are one of the promising strategies owing to their fascinating physical and electrical properties, and these materials are anticipated to be less thermally conductive than regular graphene structures, as a result of an additional phonon scattering introduced at the modified surfaces. In this study, we explore the thermal conductivity behaviors of strain-induced rippled graphene sheets by varying the ripple amplitude, periodicity, and dimensions of the structure. We introduce a technique which enables creation of a graphene sheet with evenly distributed ripples in molecular dynamics simulation, and the Green-Kubo linear response theory is used to calculate the thermal conductivity of the structures of interest. The results reveal the reduction of thermal conductivity with the greater degree of strain, the smaller system dimension, and the shorter ripple wavelength, which, in turn, could lead to the thermoelectric efficiency enhancement. This work has significance in that it presents the capability of generating repeated and controllable patterns in molecular dynamics, and so, it enables the atomic-level transport study in the regularly patterned two-dimensional surface or in any structures with a specified degree of strain. [ABSTRACT FROM AUTHOR]

By using the Green's function method, we have investigated spin Hall conductance (SHC) in a four terminal quantum spin Hall insulator. The results show that the intrinsic spin orbit coupling in a HgTe/CdTe hetero-structure interface naturally leads to separate probability distributions for the two spins in coordinate space, which leads to the spin Hall effect in our proposed device. We also find that the SHC of bulk states exhibits an oscillatory behavior as a function of the device width and persists at a broad device width. In addition, we calculate the effects of disorder on the SHC of the bulk states of non-trivial and trivial topology. The results indicate that the spin up and spin down conductances show different degrees of suppression by disorder; thus the SHC could be significantly enhanced by the disorder. This kind of property has a great value to practical applications. [ABSTRACT FROM AUTHOR]

• The electrical and optical properties of MnBi 2 Te 4 were studied. • The anisotropy of the electrical conductivity of MnBi 2 Te 4 was found. • The optical spectrum of MnBi 2 Te 4 is formed by interband transitions of charge carriers. The electrical conductivity σ 0 (T) of single-crystal and polycrystalline samples of the intrinsic magnetic topological insulator MnBi 2 Te 4 was measured in the temperature range from 5 to 300 K. The optical characteristics σ(ω), ɛ 1 (ω) , ɛ 2 (ω) and R(λ) of MnBi 2 Te 4 were studied in the spectral range from 1250 to 36000 cm−1 at room temperature. The anisotropy of the electrical conductivity of MnBi 2 Te 4 was found, which arises due to the additional contribution from scattering on layer boundaries. The optical spectrum of MnBi 2 Te 4 is formed predominantly due to interband absorption of a light wave. Despite the qualitatively similar behavior of the optical characteristics, there is some difference between poly- and single crystals in infrared region. [ABSTRACT FROM AUTHOR]

• Topological edge states of various waves modes are realized in plate-like systems. • Ideal mode separation of in-plane and out-of-plane waves is achieved. • Topological whispering-gallery mode further enhances the wave energy localization. • Potential applications of mode separation for energy harvesting are explored. Topological metamaterials in condensed matter systems have shown significant potential for information and energy applications due to their novel wave propagation behaviors. Recently, there has been an increasing focus on studying topological metamaterials in classical wave systems. Elastic waves, as full-vector classical waves, possess more intricate mechanical characteristics compared to sound waves and electromagnetic waves. This complexity not only offers diverse practical application possibilities but also presents notable research challenges in the field of topological metamaterials. In this study, our main objective is to achieve the ideal mode separation of elastic waves, specifically separated into the in-plane and out-of-plane components, and explore its potential applications. Firstly, we design phononic crystals with dual Dirac cones. Through geometric perturbation, we successfully separate the valley Hall phases of in-plane and out-of-plane modes. By superimposing different topological phases, we achieve three types of topological edge states: in-plane waves, out-of-plane waves, and hybrid waves. Combining these different topological edge states enables us to effectively separate the elastic waves into in-plane and out-of-plane components. Furthermore, we investigate the potential application of mode separation in energy harvesting. We achieve the energy localization of different wave modes based on the topological whispering-gallery mode. Additionally, by utilizing piezoelectric materials, we realize the subregional energy harvesting of in-plane and out-of-plane modes. The mechanical properties resulting from mode separation and topological whispering-gallery mode hold great potential for signal filtering and energy storage, thereby opening up new possibilities for controlling and utilizing elastic waves in various engineering scenarios. [Display omitted] [ABSTRACT FROM AUTHOR]

This study showcases the intriguing electronic, optical, and thermodynamic properties of Nb 3 Cl 8 , an exceptional class of two-dimensional (2D) crystalline materials renowned for their Kagome structure and an exceptional band arrangement featuring remarkably flat energy bands. Nb 3 Cl 8 was found to be a semiconductor with a narrow bandgap energy of 1.23 eV. Our study reveals pronounced anisotropic behavior in various optical aspects, extending within and perpendicular to the ab-plane. We show a distinctive anisotropy in parameters like the refractive index (4.5 in-plane compared to 2.75 out-of-plane), the optical conductivity (8000 (Ω cm)−1 in-plane compared to 100 (Ω cm)−1 in out-of-plane), and the absorption coefficient (105 cm−1 within the plane as opposed to 104 cm−1 in the out-of-plane direction). In addition, the thermodynamic calculations unveil a transition, distinguished by a Schottky anomaly occurring in the specific heat capacity at approximately 100K which is closely associated with the Néel transition of the Nb 3 Cl 8 compound. The presence of robust interactions among quantum spins within Nb 3 Cl 8 was supported by its low thermal conductivity, ranging from 6 to 0.7 W m−1. K−1. This study not only deepens our comprehension of the optical, electronic, and thermodynamic properties of Nb 3 Cl 8 but also establishes a basis for future innovative technological applications. [ABSTRACT FROM AUTHOR]

• Nonlinear coupled cavity-waveguide system. • Sharp resonance due to coherent coupling of topological edge and corner states. • Flexible control of nonlinear spectral response by two-color pumping. • Topologically protected optical bistability and multistability. • Multi-level all-optical switching based on multi-bistability and multistability. Robust optical bistability and multistability in topological coupled cavity-waveguide systems with one- and two-color pumping are analytically and numerically demonstrated, taking chalcogenide valley-Hall photonic crystal structure supporting valley-dependent topological edge and corner states, as an example. Due to inherently narrow linewidth of corner states, the photonic system exhibits a sharp resonance with Q-factor and intensity enhancement, which are in the order of 104. Thus, it displays topologically protected hysteresis and optical bistability for moderate pump power, enabling robust all-optical switching with near-unity contrast. The analytical model based on nonlinear temporal coupled-mode theory well reproduces the numerical results. In particular, peculiar nonlinear spectral responses are observed for two-color pumping. For proper pump parameters, the nonlinear resonant nanostructure supports multiple bistability and multistability, exhibiting multi-level optical switching. The results presented in this work offer a new route for realizing flexibly controllable optical multistability, enabling robust all-optical switches and logic gates against structural disorder and defects. [ABSTRACT FROM AUTHOR]

A light beam reflected and refracted at a sharp interface can acquire a momentum-dependent Pancharatnam–Berry (PB) phase, which produces a topological phase transition from one kind of spin–orbit interaction (e.g., spin-controlled vortex generation) to another (e.g., photonic spin-Hall effect). However, this process is extremely inefficient and difficult to observe directly in experiments, which also hinders its applications. Here, we propose to enhance significantly the topological phase transitions by c-cut uniaxial crystals. We first give a full-wave theory to describe the spin–orbit interactions of a beam passing through a c-cut uniaxial crystal and experimentally observe the topological phase transition process of the transmitted beam when the angle between the beam propagation direction and the optical axis direction changes. It is found that the efficiency of the spin–orbit interactions caused by the momentum-dependent PB phase can be increased as high as 50%, which is much larger than that at isotropic sharp interfaces. Our findings provide an alternative approach for manipulating the spin and orbital angular momenta of light and exhibit potential applications in the future spin–orbit photonic components. [ABSTRACT FROM AUTHOR]

WTe2 nanostructures have intrigued much attention due to their unique properties, such as large non-saturating magnetoresistance, quantum spin Hall effect and topological surface state. However, the controllable growth of large-area atomically thin WTe2 nanostructures remains a significant challenge. In the present work, we demonstrate the controllable synthesis of 1T′ atomically thin WTe2 nanoflakes (NFs) by water-assisted ambient pressure chemical vapor deposition method based on precursor design and substrate engineering strategies. The introduction of water during the growth process can generate a new synthesized route by reacting with WO3 to form intermediate volatile metal oxyhydroxide. Using WO3 foil as the growth precursor can drastically enhance the uniformity of as-prepared large-area 1T′ WTe2 NFs compared to WO3 powders. Moreover, highly oriented WTe2 NFs with distinct orientations can be obtained by using a-plane and c-plane sapphire substrates, respectively. Corresponding precursor design and substrate engineering strategies are expected to be applicable to other low dimensional transition metal dichalcogenides, which are crucial for the design of novel electronic and optoelectronic devices. [ABSTRACT FROM AUTHOR]