Topological Phase Research Articles

Temperature tunability of topological phase transitions and edge states in two-dimensional acoustic topological insulators

Topological insulators are characterized by exhibiting an internal insulating state but a surface conductor state, which makes them advantageous for applications in novel devices. However, for most given acoustic topological metamaterials, the operating frequency is relatively fixed and the effect of temperature on their topological properties is rarely considered. Therefore, a temperature-tunable acoustic topological insulator is constructed in this paper. The quadruple degenerate Dirac cone formed at Γ\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\Gamma$$\\end{document} point can be opened by adjusting the temperature, causing topological band inversion between the doubly degenerate dipolar and quadrupole states, and achieving topological phase transition. The evolution of its topological state with temperature is numerically investigated and a novel topological acoustic waveguide is constructed. The switching effect of temperature on the waveguide device is verified by numerical simulation and experiment. Non-contact active modulation of edge states in the structure is achieved by temperature-controlled topological phases, exhibiting acoustic switching effects. This study can provide corresponding references for the intelligent control of acoustic topology in noise, vibration, and other aspects.

The Nested Topological Band-Gap Structure for the Periodic Domain Walls in a Photonic Super-Lattice

We study the nested topological band-gap structure of one-dimensional (1D) photonic super-lattices. One cell of the super-lattice is composed of two kinds of photonic crystals (PhCs) with different topologies so that there is a domain wall (DW) state at the interface between the two PhCs. We find that the coupling of periodic DWs could form a new band-gap structure inside the original gap. The new band-gap structure could be topologically nontrivial, and a topological phase transition can occur if the structural or material parameters of the PhCs are tuned. Theoretically, we prove that the Hamiltonian of such coupled DWs can be reduced to the simple Su–Schrieffer–Heeger (SSH) model. Then, if two super-lattices carrying different topological phases are attached, a new topological interface state can occur at the interface between the two super-lattices. Finally, we find the nested topological band-gap structure in two-dimensional (2D) photonic super-lattices. Consequently, such nested topological structures can widely exist in complex super-lattices. Our work improves the topological study of photonic super-lattices and provides a new way to realize topological interface states and topological phase transitions in 1D and 2D photonic super-lattices. Topological interface states in super-lattices are sensitive to frequency and have high accuracy, which is desired for high-performance filters and high-finesse cavities.

Polarization textures in crystal supercells with topological bands

Two-dimensional materials are a highly tunable platform for studying the momentum space topology of the electronic wavefunctions and real space topology in terms of skyrmions, merons, and vortices of an order parameter. Such textures for electronic polarization can exist in moiré heterostructures. A quantum-mechanical definition of local polarization textures in insulating supercells was recently proposed. Here, we propose a definition for local polarization that is also valid for systems with topologically nontrivial bands. We introduce semilocal hybrid polarizations, which are valid even when the Wannier functions in a system cannot be made exponentially localized in all dimensions. We use this definition to explicitly show that nontrivial real-space polarization textures can exist in topologically nontrivial systems with nonzero Chern number under (1) an external superlattice potential, and (2) under a stacking-induced moiré potential. In the latter, we find that while the magnitude of the local polarization decreases discontinuously across a topological phase transition from trivial to topologically nontrivial, the polarization does not completely vanish. Our findings suggest that band topology and real-space polar topology may coexist in real materials. Published by the American Physical Society 2024

Pseudogap phases, chiral anomaly, and topological order with quantum loop entanglement

Pseudogap phases, chiral anomaly, and topological order with quantum loop entanglement

Improved performance of temperature sensors based on the one-dimensional topological photonic crystals comprising hyperbolic metamaterials

This paper seeks to progress the field of topological photonic crystals (TPC) as a promising tool in face of construction flaws. In particular, the structure can be used as a novel temperature sensor. In this regard, the considered TPC structure comprising two different PC designs named PC1 and PC2. PC1 is designed from a stack of multilayers containing Silicon (Si) and Silicon dioxide (SiO2), while layers of SiO2 and composite layer named hyperbolic metamaterial (HMM) are considered in designing PC2. The HMM layer is engineered using subwavelength layers of Si and Bismuth Germinate, or BGO (Bi4Ge3O12\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$${\ ext{Bi}}_{4}{\ ext{Ge}}_{3}{\ ext{O}}_{12}$$\\end{document}). The mainstay of our suggested temperature sensor is mainly based on the emergence of some resonant modes inside the transmittance spectrum that provide the stability in the presence of the geometrical changes. Meanwhile, our theoretical framework has been introduced in the vicinity of transfer matrix method (TMM), effective medium theory (EMT) and the thermo-optic characteristics of the considered materials. The numerical findings have extensively introduced the role of some topological parameters such as layers’ thicknesses, filling ratio through HMM layers and the periodicity of HMM on the stability or the topological features of the introduced sensor. Meanwhile, the numerical results reveal that the considered design provides some topological edge states (TESs) of a promising robustness and stability against certain disturbances or geometrical changes in the constituent materials. In addition, our sensing tool offers a relatively high sensitivity of 0.27 nm/°C.

Controllable Goos-Hänchen Shift in Photonic Crystal Heterostructure Containing Anisotropic Graphene

In this study, we investigate the electrically and magnetically tunable Goos–Hänchen (GH) shift of a reflected light beam at terahertz frequencies. Our study focuses on a photonic crystal heterostructure incorporating a monolayer anisotropic graphene. We observe a tunable and enhanced GH shift facilitated by a drastic change in the reflected phase at the resonance angle owing to the excitation of the topological edge state. Considering the quantum response of graphene, we demonstrate the ability to switch positive and negative GH shifts through the manipulation of graphene’s conductivity properties. Moreover, we show that the GH shift can be actively tuned by the external electric field and magnetic field, as well as by controlling the structural parameters of the system. We believe that this tunable and enhanced GH shift scheme offers excellent potential for preparing terahertz shift devices.

ExSitu Production and Storage of Exciton-Polariton Vortices in Higher-Order Topological Corner Modes.

We propose a scheme for producing and manipulating quantized exciton-polariton vortices in the higher-order topological corner modes of a two-dimensional array of micropillars. By nonresonantly exciting p-orbital condensates with different orientations at two input corners, polariton vortices carrying the required topological charges can be controllably created at output corners away from the pumping spots. Besides, polariton vortices formed at input corners can be copied to the output corners through the topological edge states. Our scheme provides topological double insurance for intrinsic binary information memory and holds potential applications in remote information processing.

Floquet topological phases and skin effects in periodically driven non-Hermitian systems

Floquet topological phases and skin effects in periodically driven non-Hermitian systems

Kinetics-controlled epitaxial growth and bipolar transport properties of semimetal Bi4Se3

(Bi2)m(Bi2Se3)n (m, n: integers) compounds with an infinitely adaptive superlattice structure exhibit several fascinating topological phases. Here, we study kinetics-controlled epitaxial growth of Bi4Se3 on mica by co-evaporating Bi and Se using molecular beam epitaxy technique, as well as the transport properties of Bi4Se3. By precisely controlling the beam fluxes of Bi and Se and growth temperature, we can tune the growth modes from van der Waals' condensation to spiral growth, thus achieving single-crystalline Bi4Se3 of dislocation-free microplate or mounded thin-film morphologies. This reflects a transition from near-thermodynamic-equilibrium to non-thermodynamic-equilibrium growth processes of single-crystalline Bi4Se3. Thin-film Bi2+xSe3 (1.7 < x < 2) solid-solution phases consisting of randomly stacked Bi2 and Bi2Se3 units are also prepared as comparative samples. Hall and thermopower properties suggest that the as-grown Bi4Se3 films exhibit semimetallic bipolar conduction behaviors while the Bi2+xSe3 films present typical semiconducting transport characteristics with a n-type polarity. Due to semiconductor band structures, the Bi2+xSe3 films show superior thermopower to that of semimetal Bi4Se3.

Reconfigurable acoustic topological valley-locked waveguide states transport based on nestable sonic crystals

Topological waveguide states in condensed matter physics systems show significant potential for applications, such as high-capacity information communication and energy regulation of complex physical fields owing to the phenomenon of large-area wave transport. The lack of tunability design for sonic crystals makes it difficult to change the structure and function of existing topological waveguide states transport structures after sample fabrication, especially in terms of the freedom of width and tunability on the transport path, and thus hinders further engineering applications of topological waveguide states. To address these challenges, the design method based on nested difference theory to achieve topological phase transitions in sonic crystals is proposed, which in turn leads to the construction of fully reconfigurable acoustic topological waveguide states and explores their potential applications. Firstly, a reconfigurable nested scatterer structure is designed, which is split into a snowflake-like core and an outer shell. This scatterer is demonstrated to be able to break the spatial inversion symmetry and realize the transition of sonic crystals between three valley topological phases by only changing the nesting mode of the nested outer shell. Based on reconfigurable sonic crystals, the fully reconfigurable acoustic topological valley-locked waveguide states are constructed. Simulations and experiments further confirm that such waveguides are characterized by high-capacity transport, acoustic focusing, and robust transport over large inflection corners. In addition, we explore the potential applications of reconfigurable waveguides for acoustic channels and acoustic logic gates. The precise distribution of the waveguide states energy is achieved in the acoustic channel by adjusting the rotation angle of the channel. Moreover, the logical computational relationships of the transport structure of the topological waveguide states are determined using different acoustic excitation modes. The excellent transport properties generated by fully reconfigurable acoustic topological waveguide states will have potential applications in both field enhancement and energy harvesting. This provides the possibility for the development of novel acoustic devices capable of modulating waves in complex physical scenarios.

Crystal Chemistry of Synthetic Mg(Si1−xGex)O3 Pyroxenes: A Single-Crystal X-ray Diffraction Study

Germanate-pyroxenes often are used as model systems to study the stability and phase relationships of analog silicate systems. Based on such analyses, it is assumed that silicates and germanates behave ideally in terms of mixing. A systematic study was performed to monitor in detail the changes introduced by a Si4+ through Ge4+ replacement in the important rock-forming pyroxene enstatite MgSiO3. Well-shaped, idiomorphic singe crystals of a MgSi1−xGexO3 pyroxene solid solution were grown at ambient pressure from a high-temperature flux-assisted synthesis. Structural analysis using single-crystal X-ray diffraction methods revealed orthorhombic symmetry, Pbca, Z = 8, for the complete solid-solution series. Long-term storage over a period of 8 years at ambient conditions or annealing at 525 °C over a period of 10 weeks did not change the symmetry of the proposed thermodynamically stable monoclinic polymorph. Within the solid-solution series, lattice parameters increased almost linearly with increasing Si4+ by Ge4+ substitution. The main changes occurred on the tetrahedral sites, which showed an almost linear increase in individual and average bond lengths but also in distortion parameters. The refined site occupancy of Si4+ and Ge4+ showed a distinct preference of Ge4+ for the TB site. The altered topology and kinking state in the tetrahedral chains also imposed significant changes to the bonding topology and geometry of the neighboring M1 and M2 sites.

Flat bands and topological phase transition in entangled Su–Schrieffer–Heeger chains

Flat, nondispersive bands and topological phase transition in multiple Su–Schrieffer–Heeger (SSH) chains, cross-linked via periodically arranged nodal points are explored within a tight binding framework. We give an analytic prescription, based on a real space decimation scheme, which extracts the energy eigenvalues corresponding to the flat bands along with their degeneracy. The topological phase transition is confirmed through the existence of quantized Zak phase for all the Bloch bands, and the edge states that are protected by chiral symmetry, consistent with the bulk-boundary correspondence. In addition to the edge states, the entangled systems are shown to give rise to clusters of localized eigenstates in the bulk of the system, in contrast to a purely one-dimensional SSH system.

Emergence of critical thickness and multifaceted role of stacking faults in high misfit hexagonal ferrites films

Polarization in improper ferroelectrics is believed to be robust against the depolarizing field. However, this characteristic appears to be disrupted in hexagonal ferrite and manganite, leading to a controversial debate regarding the origin of the critical thickness. In this study, we design hexagonal TbFeO3 (h-TFO) films which exhibits a critical thickness exceeding 5 nm at room temperature and an exceptionally long suppression layer in thicker films. It is explained that the critical thickness effect originates from the synergistic interplay of the interface, strain, and vacancies, which is confirmed by combining scanning transmission electron microscopy (STEM), electron energy-loss spectroscopy (EELS) and density functional theory (DFT). This explanation is equally applicable to the polarization suppression near the stacking fault (SF). Finally, we find that the SF reduces the suppression length to an extent, and contributes to changes in the topological order of the domain in h-TFO film. Our study provides a more comprehensive understanding of the origin of critical thickness and the multifaceted role of the SF in ferroelectrics.

Topological Anderson phases in heat transport

Topological Anderson phases (TAPs) offer intriguing transitions from ordered to disordered systems in photonics and acoustics. However, achieving these transitions often involves cumbersome structural modifications to introduce disorders in parameters, leading to limitations in flexible tuning of topological properties and real-space control of TAPs. Here, we exploit disordered convective perturbations in a fixed heat transport system. Continuously tunable disorder-topology interactions are enabled in thermal dissipation through irregular convective lattices. In the presence of a weak convective disorder, the trivial diffusive system undergos TAP transition, characterized by the emergence of topologically protected corner modes. Further increasing the strength of convective perturbations, a second phase transition occurs converting from TAP to Anderson phase. Our work elucidates the pivotal role of disorders in topological heat transport and provides a novel recipe for manipulating thermal behaviors in diverse topological platforms.

Photonic topological phase transition induced by material phase transition.

Photonic topological insulators (PTIs) have been proposed as an analogy to topological insulators in electronic systems. In particular, two-dimensional PTIs have gained attention for the integrated circuit applications. However, controlling the topological phase after fabrication is difficult because the photonic topology requires the built-in specific structures. This study experimentally demonstrates the band inversion in two-dimensional PTI induced by the phase transition of deliberately designed nanopatterns of a phase change material, Ge2Sb2Te5 (GST), which indicates the first observation of the photonic topological phase transition in two-dimensional PTI with changes in the Chern number. This approach allows us to directly alter the topological invariants, which is achieved by symmetry-breaking perturbation through GST nanopatterns with different symmetry from original PTI. The success of our scheme is attributed to the ultrafine lithographic alignment technologies of GST nanopatterns. These results demonstrate how to control photonic topological properties in a reconfigurable manner, providing insight into the possibilities for reconfigurable photonic processing circuits.

Density-matrix mean-field theory

Mean-field theories have proven to be efficient tools for exploring diverse phases of matter, complementing alternative methods that are more precise but also more computationally demanding. Conventional mean-field theories often fall short in capturing quantum fluctuations, which restricts their applicability to systems with significant quantum effects. In this article, we propose an improved mean-field theory, density-matrix mean-field theory (DMMFT). DMMFT constructs effective Hamiltonians, incorporating quantum environments shaped by entanglements, quantified by the reduced density matrices. Therefore, it offers a systematic and unbiased approach to account for the effects of fluctuations and entanglements in quantum ordered phases. As demonstrative examples, we show that DMMFT can not only quantitatively evaluate the renormalization of order parameters induced by quantum fluctuations, but can also detect the topological quantum phases. Additionally, we discuss the extensions of DMMFT for systems at finite temperatures and those with disorders. Our work provides an efficient approach to explore phases exhibiting unconventional quantum orders, which can be particularly beneficial for investigating frustrated spin systems in high spatial dimensions.

Vortex superlattice induced second-order topological mid-gap states in first-order topological insulators and superconductors

Topological defects such as vortex and dislocations, support zero-energy localized states as a reflection of the bulk topology, in first-order topological insulators and superconductors. Furthermore, emergent first-order topological mid-gap states have been discovered driven by the magnetic vortex superlattice. However, whether the higher-order topological mid-gap states would emerge from the first-order topological insulators and superconductors with the vortex superlattice remains elusive. In this work, we propose vortex superlattice could induce second-order topological mid-gap states with staggered lattice spacings for vortices in first-order topological insulators and superconductors. These higher-order topological mid-gap states originate from the staggered tunneling between vortex-induced bound states and the emergent π flux on vortex superlattices, as an intrinsic exhibition of the interplay between vortices and bulk topology for the first-order topological states. Our work uncovers higher-topological characteristics of topological-defect superlattice in first-order topological states, and develops a controllable environment for the creation and exploration of higher-order topological states.

Unveiling the thermodynamic landscape of liquid Ti–Al–Ni alloys through first-principles simulations

We conducted ab initio molecular dynamics simulations to systematically examine the composition-dependent thermodynamic properties and atomic-scale interactions in liquid Ti–Al–Ni alloys throughout the entire ternary phase space at 2033 K. The calculated enthalpies of mixing demonstrated exothermic tendencies, with a distinct minimum in the composition of Ti0.0Al0.50Ni0.50, indicating significant Al–Ni attractive interactions. Incorporating ternary interaction parameters into the Redlich-Kister-Muggianu equation enabled accurate modeling of the complex variations in mixing enthalpy. Analysis of partial pair correlation functions and structure factors revealed chemical and topological short-range ordering (SRO), as well as medium-range ordering, within the liquid alloy. Quantifying deviations from ideal configurational entropy clarifies the coupling between chemical SRO and topological SRO, significantly impacting the overall Gibbs energy of mixing. This comprehensive atomistic study provides insights into the thermodynamics of Ti–Al–Ni alloys, paving the way for tailoring their properties for high-performance applications.

Hybrid-order Weyl semimetal and its acoustic realizations

Abstract Hybrid-order topological insulators combine first- and higher-order topological properties and host topological boundary states with codimension one and more than one in different bandgaps. A Weyl semimetal (WSM) can possess two types of Weyl points: one class of Weyl points terminates the Fermi arc surface states, while another class of Weyl points not only launch Fermi arc surface states but also hinge arc states, exhibiting the hybrid-order topology. Here, we propose a hybrid-order WSM by stacking two-dimensional rhomboid lattices based on chiral nearest-neighbor and double-helix next-nearest interlayer couplings. The first type of Weyl point that only truncates the Fermi arc surface states exists at the crossing of any two-fold degeneracy of two adjacent bands, and the second type of Weyl point that connects the hinge arc states only appears at the crossing of the two middle bands. Our findings enrich the classification of topological semimetals in condensed matter physics.

Anisotropic weak antilocalization in thin films of the Weyl semimetal TaAs

Device applications of topological semimetals await the development of epitaxial films in the ultrathin limit. Weak antilocalization (WAL) has been extensively utilized in the understanding of surface states in topological insulators and shows promise for use in elucidating the properties of thin film topological semimetals. Here, we report insights from WAL in the surface state and interface transport properties of our recently synthesized single-crystal-like thin films of the Weyl semimetal TaAs(001) grown on GaAs(001). We observe robust, anisotropic WAL in the magnetoconductance from 2 to 20 K in films from 10 to 200 nm thick. We link the anisotropic WAL magnetoconductance to anisotropic mobility stemming from film topography. We conclude that WAL in the films likely originates from the antilocalization of topological surface states. The WAL magnetoconductance is impacted by the film thickness and topography, solidifying the useful role of WAL in the study of topological semimetal/semiconductor heterointerfaces. Published by the American Physical Society 2024