Nontrivial Topology Research Articles

Electronic correlation-driven quantum anomalous valley Hall effect in intrinsic ferrovalley FeClBr

Both ferrovalley and quantum anomalous valley Hall effect (QAVHE) are practically desirable and inherently fascinating for new-style device applications. However, works realizing the intrinsic ferrovalley and QAVHE in a single ferromagnetic system with a high Curie temperature are still lacking. We predict that monolayer FeClBr exhibits the ferrovalley phase with a substantial valley polarization of 116 meV and high Curie temperature of approximately 565 K. When considering electronic correlation effects, within the range of Ueff = 0.87 eV and Ueff = 1.13 eV, the QAVHE of nontrivial topology is present. A Chern number of C = −1 is confirmed by chiral edge states and an anomalous Hall conductivity. Intriguingly, the emergence of out-of-plane easy magnetization when Ueff < 1.13 eV is conducive to realizing the intrinsic ferrovalley and QAVHE. The QAVHE is also present in monolayers of FeClI and FeBrI. Our study offers potential candidate materials for the advancement of multifunctional quantum devices in topology and valleytronics.

Two-fold symmetry of the in-plane resistance in kagome superconductor Cs(V1–xTax)3Sb5 with enhanced superconductivity

Abstract The kagome superconductor CsV3Sb5 has attracted widespread attention due to its rich correlated electron states including superconductivity, charge density wave (CDW), nematicity, and pair density wave. Notably, the modulation of the intertwined electronic orders by the chemical doping is significant to illuminate the cooperation/competition between multiple phases in kagome superconductors. In this study, we have synthesized a series of tantalum-substituted Cs(V1−x Ta x )3Sb5 by a modified self-flux method. Electrical transport measurements reveal that CDW is suppressed gradually and becomes undetectable as the doping content of x is over 0.07. Concurrently, the superconductivity is enhanced monotonically from T c ∼ 2.8 K at x = 0 to 5.2 K at x = 0.12. Intriguingly, in the absence of CDW, Cs(V1−x Ta x )3Sb5 (x = 0.12) crystals exhibit a pronounced two-fold symmetry of the in-plane angular-dependent magnetoresistance (AMR) in the superconducting state, indicating the anisotropic superconducting properties in the Cs(V1−x Ta x )3Sb5. Our findings demonstrate that Cs(V1−x Ta x )3Sb5 with the non-trivial band topology is an excellent platform to explore the superconductivity mechanism and intertwined electronic orders in quantum materials.

Holographic torus correlators in AdS3 gravity coupled to scalar field

This paper investigates holographic torus correlators of generic operators at conformal infinity and a finite cutoff within AdS3 gravity coupled with a free scalar field. Using a near-boundary analysis and solving the gravitational boundary value problem, we solve Einstein’s equation and calculate mixed correlators for massless and massive coupled scalar fields. The conformal Ward identity on the torus has been reproduced holographically, which can be regarded as a consistency check. Further, recurrence relations for a specific class of higher-point correlators are derived, validating AdS3/CFT2 with non-trivial boundary topology. While the two-point scalar correlator is accurately computed on the thermal AdS3 saddle, the higher-point correlators associated with scalar and stress tensor operators are explored.

Magnetic topological phases in the double exchange model with spin–orbit coupling

The double exchange model with the spin–orbit coupling is studied by the dynamical mean field theory. It reveals a competition between the magnetic, charge orderings and the non-trivial topology of the ground state. The spin exchange tends to maintain the magnetic ordering, and at the same time it tries to suppress the charge ordering. The spin–orbit coupling maintains nontrivial topology of the ground state, whereas the magnetic ordering tries to destroy it. As a result, a rich phase diagram is obtained. The competition leads to a half topological ground state, where spin-up electrons form a nontrivial topological state, while spin-down electrons are in the topological trivial insulating state.

Strong-coupling approach to temperature dependence of competing orders of superconductivity: Possible time-reversal symmetry breaking and nontrivial topology

Strong-coupling approach to temperature dependence of competing orders of superconductivity: Possible time-reversal symmetry breaking and nontrivial topology

Intrinsic valley-polarized quantum anomalous Hall effect in a two-dimensional germanene/MnI2 van der Waals heterostructure

Abstract Valley-polarized quantum anomalous Hall effect (VQAHE), combined nontrivial band topology with valleytronics, is of importance for both fundamental sciences and emerging applications. However, the experimental realization of this property is challenging. Here, by using first-principles calculations and modal analysis, we predict a mechanism of producing VQAHE in two-dimensional ferromagnetic van der Waals germanene/MnI2 heterostructure. This heterostructure exhibits both valley anomalous Hall effect and VQAHE due to the joint effects of magnetic exchange effect and spin–orbital coupling with the aid of anomalous Hall conductance and chiral edge state. Moreover interestingly, through the electrical modulation of ferroelectric polarization state in In2Se3, the germanene/MnI2/In2Se3 heterostructure can undergo reversible switching from a semiconductor to a metallic behavior. This work offers a guiding advancement for searching for VQAHE in ferromagnetic van der Waals heterostructures and exploiting energy-efficient devices based on the VQAHE.

Superconducting diode effect and interference patterns in kagome CsV3Sb5.

The interplay among frustrated lattice geometry, non-trivial band topology and correlation yields rich quantum states of matter in kagome systems1,2. A series of recent members in this family, AV3Sb5 (A = K, Rb or Cs), exhibit a cascade of symmetry-breaking transitions3, involving the 3Q chiral charge ordering4-8, electronic nematicity9,10, roton pair density wave11 and superconductivity12. The nature of the superconducting order is yet to be resolved. Here we report an indication of dynamic superconducting domains with boundary supercurrents in intrinsic CsV3Sb5 flakes. The magnetic field-free superconducting diode effect is observed with polarity modulated by thermal histories, suggesting that there are dynamic superconducting order domains in a spontaneous time-reversal symmetry-breaking background. Strikingly, the critical current exhibits double-slit superconductivity interference patterns when subjected to an external magnetic field. The characteristics of the patterns are modulated by thermal cycling. These phenomena are proposed as a consequence of periodically modulated supercurrents flowing along certain domain boundaries constrained by fluxoid quantization. Our results imply a time-reversal symmetry-breaking superconducting order, opening a potential for exploring exotic physics, for example, Majorana zero modes, in this intriguing topological kagome system.

Emerging topological states in EuMn2Bi2: A first principles prediction

New materials with magnetic order driven topological phases are hugely sought after for their immense application potential. In this work, we propose a new compound EuMn2Bi2 from our first principles density functional theory calculations to host novel topological phases such as Dirac/Weyl semimetal and topological insulator in its different magnetic states which are energetically close to one another. The new compound EuMn2Bi2 is found to be dynamically stable from our phonon calculations supporting its experimental preparation in future. By comparing the total energies of various possible magnetic structures of EuMn2Bi2, we identified the ground state. The magnetic ground state is found to be a normal insulator with tiny band gap which is an order of magnitude less than the same in another well studied sister compound EuMn2As2. However, there exist other magnetic states energetically very close to the ground state in EuMn2Bi2 which display remarkable non-trivial band topology such as Dirac/Weyl points close to the Fermi level and topological insulator state driven by magnetic order which is not observed in EuMn2As2. The energetic proximity of these magnetic order driven topological phases makes them tunable via external handle such as magnetic field indicating that the proposed new material EuMn2Bi2 would be a very versatile magnetic topological material if prepared experimentally in future.

Tunable magnetism in titanium-based kagome metals by rare-earth engineering and high pressure

Rare-earth engineering is an effective way to introduce and tune magnetism in topological kagome materials, which have been acting as a fertile platform to investigate the quantum interactions between geometry, topology, spin, and correlation. Here, we report the synthesis, structure, and physical properties of titanium-based kagome metals RETi3Bi4 (RE = Yb, Pr, and Nd) with various magnetic states. They all crystallize in the orthogonal space group Fmmm (No. 69), featuring distorted titanium kagome lattices and rare-earth zig-zag chains. By changing the rare earth atoms in the zig-zag chains, the magnetism can be tuned from nonmagnetic YbTi3Bi4 to short-range ordered PrTi3Bi4 (Tanomaly ~ 8.2 K), and finally to ferromagnetic NdTi3Bi4 (Tc ~ 8.5 K). In-situ resistance measurements of NdTi3Bi4 under high pressure further reveal a tunable ferromagnetic ordering temperature. These results highlight RETi3Bi4 as a promising family of kagome metals to explore nontrivial band topology and exotic phases.

Coupled Electronic and Magnonic Topological States in Two-Dimensional Ferromagnets.

Magnetic materials offer a fertile playground for fundamental physics discovery, with not only electronic but also magnonic topological states intensively explored. However, one natural material with both electronic and magnonic nontrivial topologies is still unknown. Here, we demonstrate the coexistence of first-order topological magnon insulators (TMIs) and electronic second-order topological insulators (SOTIs) in 2D honeycomb ferromagnets, giving rise to the nontrivial corner states being connected by the charge-free magnonic edge states. We show that, with symmetry, the phase factor ± ϕ caused by the next nearest-neighbor Dzyaloshinskii-Moriya interaction breaks the pseudo-spin time-reversal symmetry , which leads to the split of magnon bands, i.e., the emergence of TMIs with a nonzero Chern number of , in experimentally feasible candidates of MoI3, CrSiTe3, and CrGeTe3 monolayers. Moreover, protected by the symmetry, the electronic SOTIs characterized by nontrivial corner states are obtained, bridging the topological aspect of fermions and bosons with a high possibility of innovative applications in spintronics devices.

Coexistence of superconductivity and topological phase in kagome metals ANb3Bi5 (A = K, Rb, Cs)

The AV3Sb5 prototype kagome materials have been demonstrated as a versatile platform for exploring exotic properties in condensed matter physics, including charge density waves, superconductivity, non-trivial electron topology, as well as topological superconductivity. Here we identify that ANb3Bi5 (A = K, Rb, Cs) exhibit non-trivial coexisting superconductivity and topological properties via first-principles calculations. The negative formation energy and the absence of imaginary phonon dispersion demonstrate both thermodynamics and dynamics stabilities of ANb3Bi5 (A = K, Rb, Cs) under ambient conditions. By analytically solving the Allen-Dynes-modified McMillan formula, the superconducting transition temperatures are predicted to be 2.11, 2.15 and 2.21 K for KNb3Bi5, RbNb3Bi5, and CsNb3Bi5, respectively. More importantly, the kagome materials proposed here can be classified into Z2\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$${{\\mathbb{Z}}}_{2}$$\\end{document} topological metals due to the non-trivial topological index and the obvious surface states around the Fermi level. Such coexistence of superconductivity and non-trivial band characters in ANb3Bi5 (A = K, Rb, Cs) offer us more insights to study the relationship between superconductivity and topological properties, and to design innate topological superconductors.

Spin-orbit coupled Hubbard skyrmions

When the material phases exhibit topological quantum numbers, they host defects protected by the nontrivial topology. Magnetic skyrmions are such “quantized” objects, and although many of them are metals, they had been most likely treated in theories as purely classical spin states. Here, we show that the electrons described by the Hubbard model with strong spin-orbit couplings can exhibit various nano- or flake-size skyrmions in their ground state. Importantly, the conducting electrons forming Fermi pockets themselves carry textured magnetic moments in both real and momentum space and on top of that possess Chern numbers in their energy bands. This quantum and conducting skyrmion is related to small skyrmions observed in atomic-layered compounds. We clarify how the effective magnetic interactions and magnetic anisotropies are tied to the spin-orbit coupling, and how they influence the stability of skyrmions beyond the phenomenology. Published by the American Physical Society 2024

Topological phase transition in compressed van der Waals superlattice heterostructure BiTeCl/HfTe2

Abstract Based on first-principles calculations, we investigate the electronic band structures and topological properties of heterostructure BiTeCl/HfTe2 under c-direction strain. In the primitive structure, this material undergoes a phase transition from an insulator with a narrow indirect gap to a metal by strong spin–orbital coupling. When strain effect is considered, band inversion at time-reversal invariant point Z is responsible for the topological phase transition. These nontrivial topologies are caused by two different types of band crossings. The observable topological surface states in (110) surface also support that this material experiences topological phase transition twice. The layered heterostructure with van der Waals force provides us with a new desirable platform upon which to control topological phase transition and construct topological superconductors.

Surface-dominated conductance scaling in Weyl semimetal NbAs

Protected surface states arising from non-trivial bandstructure topology in semimetals can potentially enable advanced device functionalities in compute, memory, interconnect, sensing, and communication. This necessitates a fundamental understanding of surface-state transport in nanoscale topological semimetals. Here, we investigate quantum transport in a prototypical topological semimetal NbAs to evaluate the potential of this class of materials for beyond-Cu interconnects in highly-scaled integrated circuits. Using density functional theory (DFT) coupled with non-equilibrium Green’s function (NEGF) calculations, we show that the resistance-area RA product in NbAs films decreases with decreasing thickness at the nanometer scale, in contrast to a nearly constant RA product in ideal Cu films. This anomalous scaling originates from the disproportionately large number of surface conduction states which dominate the ballistic conductance by up to 70% in NbAs thin films. We also show that this favorable RA scaling persists even in the presence of surface defects, in contrast to RA sharply increasing with reducing thickness for films of conventional metals, such as Cu, in the presence of surface defects. These results underscore the potential of topological semimetals as future back-end-of-line (BEOL) interconnect metals.

Topological Signature of Stratospheric Poincaré-Gravity Waves

Abstract The rotation of Earth breaks time-reversal and reflection symmetries in an opposite sense north and south of the equator, leading to a topological origin for certain atmospheric and oceanic equatorial waves. Away from the equator, the rotating shallow-water and stably stratified primitive equations exhibit Poincaré inertia–gravity waves that have nontrivial topology as evidenced by their strict superinertial time scale and a phase singularity in frequency–wavevector space. This nontrivial topology then predicts, via the principle of bulk-interface correspondence, the existence of two equatorial waves along the equatorial interface, the Kelvin and Yanai waves. To directly test the nontrivial topology of Poincaré-gravity waves in observations, we examine ERA5 data and study cross correlations between the wind velocity and geopotential height of the midlatitude stratosphere at the 50 hPa height. We find the predicted vortex and antivortex in the relative phase of the geopotential height and velocity at the high frequencies of the waves. By contrast, lower-frequency planetary waves are found to have trivial topology also as expected from theory. These results demonstrate a new way to understand stratospheric waves and provide a new qualitative tool to investigate waves in other components of the climate system.

Memristive switching in the surface of a charge–density–wave topological semimetal

Owing to the outstanding properties provided by nontrivial band topology, topological phases of matter are considered as a promising platform towards low-dissipation electronics, efficient spin-charge conversion, and topological quantum computation. Achieving ferroelectricity in topological materials enables the non-volatile control of the quantum states, which could greatly facilitate topological electronic research. However, ferroelectricity is generally incompatible with systems featuring metallicity due to the screening effect of free carriers. In this study, we report the observation of memristive switching based on the ferroelectric surface state of a topological semimetal (TaSe4)2I. We find that the surface state of (TaSe4)2I presents out-of-plane ferroelectric polarization due to surface reconstruction. With the combination of ferroelectric surface and charge-density-wave-gapped bulk states, an electric-switchable barrier height can be achieved in (TaSe4)2I-metal contact. By employing a multi-terminal-grounding design, we manage to construct a prototype ferroelectric memristor based on (TaSe4)2I with on/off ratio up to 103, endurance over 103 cycles, and good retention characteristics. The origin of the ferroelectric surface state is further investigated by first-principles calculations, which reveal an interplay between ferroelectricity and band topology. The emergence of ferroelectricity in (TaSe4)2I not only demonstrates it as a rare but essential case of ferroelectric topological materials, but also opens new routes towards the implementation of topological materials in functional electronic devices.

Control of non-Hermitian skin effect by staggered synthetic gauge fields

Synthetic gauge fields introduce an unconventional degree of freedom for studying many fundamental phenomena in different branches of physics. Here, we propose a scheme to use staggered synthetic gauge fields for control of the non-Hermitian skin effect (NHSE). A modified Su–Schrieffer–Heeger model is employed, where two dimer chains with non-reciprocal coupling phases are coupled, exhibiting non-trivial point-gap topology and the NHSE. In contrast to previous studies, the skin modes in our model are solely determined by the coupling phase terms associated with the staggered synthetic gauge fields. By manipulating such gauge fields, we can achieve maneuvering of skin modes as well as the bipolar NHSE. As a typical example, we set up a domain wall by imposing different synthetic gauge fields on two sides of the wall, thereby demonstrating flexible control of the non-Hermitian skin modes at the domain wall. Our scheme opens a new avenue for the creation and manipulation of NHSE by synthetic gauge fields, which may find applications in beam shaping and non-Hermitian topological devices.

Half-quantized Hall states with C=3/2 exited by chiral photons

The topology of Hilbert space in semiconductor quantum wells (QW) can become non-trivial when excited by an electromagnetic wave. It means the quasi-energy bands acquire a nonzero integer Floquet-Chern number even if QW is trivial in the equilibrium. We analyze electron-photon interaction in the Floquet state and identify dynamical quantum phases characterized by half-quantized Floquet-Chern numbers. The non-integer topology depends on which equilibrium state is exposed to illumination. Trivial III-V semiconductor QW's such as AlGaAs and AlGaN, being resonantly illuminated, generate the dynamical index |CF|=1/2, similar to a gapless quantum anomalous semimetal (QAS) in equilibrium. Excitation of 2D topological insulator CdHgTe reveals |CF|=3/2 - the index stemmed exclusively from interaction with chiral photons. Comparing results obtained in continuous and full-Brillouin zone (BZ) models, we conclude that high-momentum regions in BZ contribute to the dynamical half-quantized topology. Results indicate that by manipulating parameters of external force, one can bring inverted-gap QW into a non-equilibrium state with a high topological non-integer charge |CF|=3/2. We present the phase diagram, which includes all dynamical states of non-trivial topology.

Incoherent non-Hermitian skin effect in photonic quantum walks

The non-Hermitian skin effect describes the concentration of an extensive number of eigenstates near the boundaries of certain dissipative systems. This phenomenon has raised a huge interest in different areas of physics, including photonics, deeply expanding our understanding of non-Hermitian systems and opening up new avenues in both fundamental and applied aspects of topological phenomena. The skin effect has been associated to a nontrivial point-gap spectral topology and has been experimentally demonstrated in a variety of synthetic matter systems, including photonic lattices. In most of physical models exhibiting the non-Hermitian skin effect full or partial wave coherence is generally assumed. Here we push the concept of skin effect into the fully incoherent regime and show that rather generally (but not universally) the non-Hermitian skin effect persists under dephasing dynamics. The results are illustrated by considering incoherent light dynamics in non-Hermitian photonic quantum walks.

Betti Functionals as Probes for Cosmic Topology

The question of the global topology of the Universe (cosmic topology) is still open. In the ΛCDM concordance model, it is assumed that the space of the Universe possesses the trivial topology of R3, and thus that the Universe has an infinite volume. As an alternative, in this paper, we study one of the simplest non-trivial topologies given by a cubic 3-torus describing a universe with a finite volume. To probe cosmic topology, we analyze certain structure properties in the cosmic microwave background (CMB) using Betti functionals and the Euler characteristic evaluated on excursions sets, which possess a simple geometrical interpretation. Since the CMB temperature fluctuations δT are observed on the sphere S2 surrounding the observer, there are only three Betti functionals βk(ν), k=0,1,2. Here, ν=δT/σ0 denotes the temperature threshold normalized by the standard deviation σ0 of δT. The analytic approximations of the Gaussian expectations for the Betti functionals and an exact formula for the Euler characteristic are given. It is shown that the amplitudes of β0(ν) and β1(ν) decrease with an increasing volume V=L3 of the cubic 3-torus universe. Since the computation of the βk’s from observational sky maps is hindered due to the presence of masks, we suggest a method that yields lower and upper bounds for them and apply it to four Planck 2018 sky maps. It is found that the βk’s of the Planck maps lie between those of the torus universes with side-lengths L=2.0 and L=3.0 in units of the Hubble length and above the infinite ΛCDM case. These results give a further hint that the Universe has a non-trivial topology.