Topological Phase Research Articles

Probing topological phase transitions in the Aubry-Andre-Harper model via high-harmonic generation

Probing topological phase transitions in the Aubry-Andre-Harper model via high-harmonic generation

Anomalous Hall effect and electronic correlation in a spin-reoriented kagome antiferromagnet LuFe6Sn6

Abstract The kagome lattice system has been identified as a fertile ground for the emergence of a number of new quantum states, including superconductivity, quantum spin liquids, and topological electronic states. This has attracted significant interest within the field of condensed matter physics. Here, we present the observation of an anomalous Hall effect in an iron-based kagome antiferromagnet LuFe6Sn6, which implies a non-zero Berry curvature in this compound. By means of extensive magnetic measurements, a high Neel temperature, T N = 552 K, and a spin reorientation behavior were identified and a simple temperature-field phase diagram was constructed. Furthermore, this compound was found to exhibit a large Sommerfeld coefficient of γ = 87 mJ⋅mol−1⋅K−2, suggesting the presence of a strong electronic correlation effect. Our research indicates that LuFe6Sn6 is an intriguing compound that may exhibit magnetism, strong correlation, and topological states.

Floquet-Engineering Topological Phase Transition in Graphene Nanoribbons by Light

Abstract Quasi-one-dimensional (1D) graphene nanoribbons (GNRs) play a crucial role in advancement of next-generation devices. Recent studies have suggested their potential to exhibit unique symmetry-protected topological phases defined by a Z 2 invariant. By employing both the tight-binding model and the Floquet theory, our investigation demonstrates the effective control of the topological phase within quasi-1D armchair GNRs (AGNRs) using elliptically polarized light, unveiling rich topological phase diagrams. Specifically, we observe that varying the amplitude of the light can induce transitions in the band gap (E g) of AGNRs, leading to multiple changes in the system’s Z 2 invariant. Furthermore, for heterojunctions composed of different AGNR segments, the junction state can be either created or eliminated by the application of elliptically polarized light.

Topological phases in population dynamics with rock-paper-scissors interactions.

Topological phases have arisen great interests of physicists. Though most works focus on quantum systems, topological phases can also be found in nonquantum systems. In this work, we study an antisymmetric Lotka-Volterra dynamics defined on a chain of two-site cells with open boundary conditions. We find two edge-localization states, left edge-localization state, and right edge-localization state. In an edge-localization state, there exists a boundary region in which mass distribution displays an exponential decay with the distance away from the boundary. The two edge-localization states are connected by a sharp transition. To comprehend the edge-localization states, we transform the population dynamics into a non-Hermitian quantum system. Based on the generalized topological band theory of the non-Hermitian system with periodic boundary conditions, we use winding number to distinguish the left and the right edge-localization states, and the transition between these two states is identified to be a topological one.

Linear response theory for transport in non-Hermitian [formula omitted]-symmetric models

We analyzed the influence of dissipative non-Hermitian terms on electrical transport in one-dimensional non-Hermitian PT-symmetric models, which presents chiral symmetry. The combination of periodic modulation and non-Hermitian effects opens up exciting avenues for exploring new phases of matter and understanding of the role of non-Hermitian physics in topological systems. The effective Hermitian Hamiltonian has always a higher dimension than the corresponding non-Hermitian one. We analyzed the effect of periodic hopping modulation on Su-Schrieffer-Heeger chain, that exhibits the non-Hermiticity in the presence of an complex staggered potential, where this dissipative non-Hermitian extension modifies the features of the topological trivial phase and the topological nontrivial phase.

Valley selections and control of topological phases by bicircular laser field in transition-metal dichalcogenides

The energy spectra of transition metal dichalcogenides are influenced by three orbitals, forming a three-band model with strong spin-orbit interaction. Analyzing these bands through a two-band projection suffices for exploring electronic and optical properties. We examine topological phases and the spin-valley Hall effect, derived from spin-valley Chern numbers. Spin-orbit interaction breaks inversion symmetry but preserves time-reversal symmetry, resulting in non-zero spin-valley-dependent Hall conductivities, crucial for both spin and valley Hall effects. Interestingly, direct optical manipulation of the valley degree of freedom can be achieved using a trefoil field generated by a bicircular field. Selectable valley conductivity occurs when the field orientation matches the lattice symmetry, leading to a quantum anomalous Hall effect without magnetization. This approach suggests the potential for valley selection via field orientation, with applications in valleytronics and spintronics devices, enabling qubit encoding.

Higher-order topological phase with subsystem symmetries

Abstract A wide variety of higher-order symmetry-protected topological phases (HOSPT) with gapless corners or hinges have been proposed as descendants of topological crystalline insulators protected by spatial symmetry. In this work, we address a new class of higher-order topological states that do not require crystalline symmetries but instead rely on subsystem symmetry for protection. We propose several strongly interacting models with gapless hinges or corners based on a decorated hinge-wall condensate picture. The hinge-wall, which appears as the defect configuration of a Z 2 paramagnet, is decorated with a lower-dimensional SPT state. Such a unique hinge-wall decoration structure leads to gapped surfaces separated by gapless hinges. The non-trivial nature of the hinge modes can be captured by a 1 + 1 D conformal field theory with a Wess–Zumino–Witten term. Moreover, we establish a no-go theorem to demonstrate the ungappable nature of the hinges by making a connection between the generalized Lieb–Schultz–Mattis theorem and the boundary anomaly of the HOSPT state. This universal correspondence engenders a comprehensive criterion to determine the existence of HOSPT under certain symmetries, regardless of the microscopic Hamiltonian.

The de Haas–van Alphen quantum oscillations in the kagome metal RbTi3Bi5

Abstract The kagome system has attracted great interest in condensed matter physics due to its unique structure that can host various exotic states such as superconductivity (SC), charge density waves (CDWs) and nontrivial topological states. The topological semimetal RbTi3Bi5 consisting of a Ti kagome layer shares a similar crystal structure to the topological correlated materials AV3Sb5 (A = K, Rb, Cs) but without the absence of CDW and SC. Systematic de Haas–van Alphen oscillation measurements are performed on single crystals of RbTi3Bi5 to pursue nontrivial topological physics and exotic states. Combining this with theoretical calculations, the detailed Fermi surface topology and band structure are investigated. A two-dimensional Fermi pocket β is revealed with a light effective mass, consistent with the semimetal predictions. The Landau fan diagram of RbTi3Bi5 reveals a zero Berry phase for the β oscillation in contrast to that of CsTi3Bi5. These results suggest that kagome RbTi3Bi5 is a good candidate for exploring nontrivial topological exotic states and topological correlated physics.

Topology and curvature effects in the photonics of ring – split ring – cuboid transitions

Topological transitions in various materials are actively being studied, including topological quantum phase transitions, going beyond the Landau theory and the concept of the order parameter. Here we propose the concept of a transition between two structures with different topology using the example of the transition between a flat dielectric ring and a split ring and its further unbending into a rectangular Fabry-Pérot resonator. Experimentally and theoretically, we discovered the lifting of the degeneracy of the CW-CCW photonic modes of the ring and the formation of two families: topological, which acquire an additional phase π, equal to the Berry phase in a thin Möbius strip, and ordinary ones, which do not acquire an additional phase. Topological modes arise due to the gradual “cutting” of one antinode of the field by a gap into two antinodes as the angular size of the gap increases from zero to one degree. Thus, using a topological Fabry-Pérot resonator with variable curvature and fixed length, resonant modes with an arbitrary non-integer number of waves are realized and a new generation of resonators is created with the prospect of unique classical and quantum applications.

Single pair of charge-2 Dirac and charge-2Weyl phonons in GeO2

Abstract The presence of a pair of Weyl and Dirac points (WP–DP) in topological semimetal states is intriguing and sought after due to the effects associated with chiral topological charges. However, identifying these states in real materials poses a significant challenge. In this study, by means of first-principles calculations we predict the coexistence of charge-2 Dirac and charge-2 Weyl phonons at high-symmetry points within a noncentrosymmetric P41212 space group. Furthermore, we propose GeO2 as an ideal candidate for realizing these states. Notably, we observe two distinct surface arcs that connect the Dirac and Weyl points across the entire Brillouin zone, which could facilitate their detection in future experimental investigations. This study not only presents a tangible material for experimentalists to explore the topological properties of WP–DP states but also opens up new avenues in the quest for ideal platforms to study chiral particles.

Valley-polarized edge plasmons in graphene p–n junctions with pseudomagnetic fields

Abstract Owing to the inherent characteristics of collective excitations in graphene, electrical control of edge plasmons is highly desirable for nanoplasmonic applications. This study investigates valley-polarized edge pseudomagneplasmons in a graphene p–n junction subjected to a strain-induced pseudomagnetic field. A four-component hydrodynamic model is employed and solved via the Wiener–Hopf method, revealing the coexistence of three plasmon modes, including counterpropagating acoustic edge modes, gapless topological edge states, and zero modes. The valley polarization, as determined from the numerically exact solution, is stronger than that predicted by the approximate models. Notably, the confinement of edge plasmons at the graphene p–n junction significantly exceeds that at the graphene/vacuum interface, possibly because of the electron–hole attraction. Furthermore, gate-controlled subwavelength confinement is successfully achieved by applying an appropriate gate voltage, thereby highlighting a unique and promising attribute of edge pseudomagnetoplasmons in graphene p–n junctions.

Three-dimensional topological crystalline insulator without spin–orbit coupling in nonsymmorphic photonic metacrystal

Abstract By including certain point group symmetry in the classification of band topology, Fu proposed a class of three-dimensional topological crystalline insulators (TCIs) without spin–orbit coupling in 2011. In Fu’s model, surface states (if present) doubly degenerate at Γ ¯ and M ¯ when time-reversal and C 4 symmetries are preserved. The analogs of Fu’s model with surface states quadratically degenerate at M ¯ are widely studied, while surface states with quadratic degeneracy at Γ ¯ are rarely reported. In this study, we propose a three-dimensional TCI without spin–orbit coupling in a judiciously designed nonsymmorphic photonic metacrystal. The surface states of photonic TCIs exhibit quadratic band degeneracy in the (001) surface Brillouin zone (BZ) center ( Γ ¯ point). The gapless surface states and their quadratic dispersion are protected by C 4 and time-reversal symmetries, which correspond to the nontrivial band topology characterized by Z 2 topological invariant. Moreover, the surface states along lines from Γ ¯ to the (001) surface BZ boundary exhibit zigzag feature, which is interpreted from symmetry perspective by building composite operators constructed by the product of glide symmetries with time-reversal symmetry. The metacrystal array surrounded with air possesses high order hinge states with electric fields highly localized at the hinge that may apply to optical sensors. The gapless surface states and hinge states reside in a clean frequency bandgap. The topological surface states emerge at the boundary of the metacrystal and perfect electric conductor (PEC), which provide a pathway for topologically manipulating light propagation in photonic devices.

Realizations of Su-Schrieffer-Heeger (SSH) edge states in two-dimensional hydrocarbon systems

The Su-Schrieffer-Heeger (SSH) model of one-dimensional (1D) diatomic and four-atom chains, exhibit a topological phase transition characterized by the Zak phase. However, a challenge arises from the inherent difficulty of maintaining strong structural stability in real 1D nanostructures. Here, we show how to realize periodic 1D chains, reminiscent of the SSH model, in a two-dimensional (2D) system. These chains form a quasi-1D chain topological insulator (CTI) where the interchain coupling can be neglected. Based on first-principles calculations, we proposed that such CTIs can be realized in dumbbell (DB) C40H14 and DB C40H12 monolayers. The monolayers are CTIs, with a type of weak topological state, and the topological phase transition can be achieved by unit cell transformation or the application of 2D strain. Furthermore, increasing the number of DB C10H4 rings can enlarge the distance between the chains, corresponding to line defects within the monolayer, providing a possible strategy for experimental synthesis.

Topological equivalence and phase transition rate in holographic thermodynamics of regularized Maxwell theory

Utilizing the holographic dictionary from the proposal that treats Newton’s constant as a thermodynamic variable, we establish a thermodynamic topological equivalence between the AdS black holes in the bulk and the thermal states in the dual CFT. The findings further reveal that the thermodynamic topological characteristics of the RegMax AdS black holes are strongly influenced by the characteristic parameter of the regularized Maxwell theory. Additionally, we investigate the phase transition between low and high entropy thermal states within a canonical ensemble in the dual CFT. Our observations indicate that the phase transition behavior of the thermal states mirrors that of the black holes. By modeling the phase transition process as a stochastic process, we are able to calculate the rates of phase transition between the thermal states. This result enhances our understanding of the dominant processes involved in the phase transition of the thermal states in the dual CFT.

Dynamics of Polar Vortex Crystallization.

Vortex crystals are commonly observed in ultrathin ferroelectrics. However, a clear physical picture of origin of this topological state is currently lacking. Here, we show that vortex crystallization in ultrathin Pb(Zr_{0.4},Ti_{0.6})O_{3} films stems from the softening of a phonon mode and can be described as a Z_{2}×SU(1) symmetry-breaking transition. This result sheds light on the topology of the polar vortex patterns and bridges polar vortices with smectic phases, spin spirals, and other modulated states. Finally, we predict a resonant switching of the vortex tube orientation driven by midinfrared laser light which could enable new technologies.

Optical properties of cylindrical topological photonic crystal heterostructures

This paper uses a modified transfer matrix method to investigate the optical properties of a cylindrical topological photonic crystal heterostructure composed of two cylindrical photonic crystals. Topological photonic crystals are novel structures with topological edge states capable of field confinement and robust propagation. Numerical results showed that when the sum of the phases of the reflection coefficients of the two cylindrical photonic crystals is zero, a topological edge state occurs inside their overlapping band gaps. In the linear regime, the peak frequency of the topological edge states undergoes a redshift as the incidence angle increases. An increase in the incidence angle leads to a decrease (increase) in the Full width at half maximum of the E-polarized (H-polarized) topological edge states. As the incidence angle increases, the frequency separation between the E-polarized and H-polarized topological edge states increases, causing the cylindrical heterostructure to work as a polarizer. The performance of the cylindrical topological photonic crystal heterostructure as a polarizer is evaluated in the linear and nonlinear regimes. We showed that the peak frequency of the topological edge states undergoes a redshift irrespective of their polarization state as the intensity of the input light increases. We found that the structure has a good performance in the nonlinear regime due to the higher displacement in E-polarized topological edge states compared to H-polarized topological edge states. The findings of this paper might be beneficial in the construction of polarization-maintaining optical fiber, which has specific applications in telecommunications, fiber optic sensing, interferometry, and quantum key distribution.

Construction of topological quantum magnets from atomic spins on surfaces.

Artificial quantum systems have emerged as platforms to realize topological matter in a well-controlled manner. So far, experiments have mostly explored non-interacting topological states, and the realization of many-body topological phases in solid-state platforms with atomic resolution has remained challenging. Here we construct topological quantum Heisenberg spin lattices by assembling spin chains and two-dimensional spin arrays from spin-1/2 Ti atoms on an insulating MgO film in a scanning tunnelling microscope. We engineer both topological and trivial phases of the quantum spin model and thereby realize first- and second-order topological quantum magnets. We probe the many-body excitations of the quantum magnets by single-atom electron spin resonance with an energy resolution better than 100 neV. Making use of the atomically localized magnetic field of the scanning tunnelling microscope tip, we visualize various many-body topological bound modes including topological edge states, topological defects and higher-order corner modes. Our results provide a bottom-up approach for the simulation of exotic quantum many-body phases of interacting spins.

Van Hove Singularity-Enhanced Raman Scattering and Photocurrent Generation in Twisted Monolayer-Bilayer Graphene.

Twisted monolayer-bilayer graphene (TMBG) has recently emerged as an exciting platform for exploring correlated physics and topological states with rich tunability. Strong light-matter interaction was realized in twisted bilayer graphene, boosting the development of broadband graphene photodetectors from the visible to infrared spectrum with high responsivity. Extending this approach to the case of TMBG will help design advanced quantum nano-optoelectronic devices because of the reduced symmetry of the system. Here, we observe the formation of van Hove singularities (VHSs) in TMBG by monitoring the significant enhancement of the Raman intensity of the G peak and the intensity ratio of G and 2D peaks. The strong interlayer coupling also leads to the appearance of twist-angle-dependent Raman R and R' peaks in TMBG. Furthermore, the constructed graphene photodetectors from 13.5°-TMBG show significantly enhanced photoresponsivity (∼31 folds of monolayer graphene and ∼15 folds of trilayer graphene) when the energy of incident photons matches the interval energy between the two VHSs in the conduction and valence bands. Our findings establish TMBG as a tunable platform for investigating the light-matter interaction and designing high-performance graphene photodetectors with combined high responsivity and high selectivity.

Magnetic Exchange Mechanism and Quantized Anomalous Hall Effect in Bi2Se3 Film with a CrWI6 Monolayer.

Magnetizing the surface states of topological insulators without damaging their topological features is a crucial step for realizing the quantum anomalous Hall (QAH) effect and remains a challenging task. The TI-ferromagnetic material interface system was constructed and studied by the density functional theory (DFT). A two-dimensional magnetic semiconductor CrWI6 has been proven to effectively magnetize topological surface states (TSSs) via the magnetic proximity effect. The non-trivial phase was identified in the Bi2Se3 (BS) films with six quantum layers (QL) within the CrWI6/BS/CrWI6 heterostructure. BS thin films exhibit the generation of spin splitting near the TSSs, and a band gap of approximately 2.9 meV is observed at the Γ in the Brillouin zone; by adjusting the interface distance of the heterostructure, we increased the non-trivial band gap to 7.9 meV, indicating that applying external pressure is conducive to realizing the QAH effect. Furthermore, the topological non-triviality of CrWI6/6QL-BS/CrWI6 is confirmed by the nonzero Chern number. This study furnishes a valuable guideline for the implementation of the QAH effect at elevated temperatures within heterostructures comprising two-dimensional (2D) magnetic monolayers (MLs) and topological insulators.

Synthesis, structure analysis, biological activity evaluation and pharmacological aspects prediction of two newly synthesized compounds: N-methylbenzylammonium bromide and 2-amino-5-ammoniopyridinium dichloride

Medical and pharmaceutical demands have driven the development of new synthetic materials. This study explores bioinspired materials from various intriguing biological compositions, focusing on functional and structural categories. In this context, two novel salts, (C8H12N),Br (I) and (C5H9N3),2Cl (II) were synthesized via slow evaporation at ambient temperature and characterized using multiple techniques. Their crystal packing is stabilized by non-covalent interactions, including N–H…Br, C–H…Br, C–H…Cl and N–H…Cl hydrogen bonds. Three-dimensional Hirshfeld surface analysis followed by two-dimensional fingerprint plots provides insights into the intermolecular interactions within the crystalline structure. The thermal decomposition of the compounds was analyzed using TGA-DTA techniques. The synthesized products were also evaluated for in vitro antifungal, antioxidant and antibacterial activities, revealing notable antioxidant properties. Additionally, molecular docking studies were conducted against the bacterial DNA gyrase of Staphylococcus aureus, Listeria monocytogenes, Salmonella enterica Typhimurium and Pseudomonas aeruginosa as well as the fungal chitin synthase enzyme of Aspergillus niger and Fusarium oxysporum. Based on the obtained free energies of binding (ΔG), compounds I and II demonstrated significant effects against the tested microorganisms by disrupting the topological state of bacterial genetic material and inhibiting fungal chitin formation. ADME (Absorption, Distribution, Metabolism and Excretion) prediction using SwissADME webserver suggest that these compounds have suitable pharmacokinetic profiles and could serve as promising drug candidates against various microbial infections.