Quantized octupole acoustic **topological** **insulator**

**topological**

**insulator**(TI) phase. Expand abstract.

**topological**

**insulator**(TI) phase. Recent theories have generalized such quantization from dipole to higher multipole moments, giving rise to the discovery of multipole TIs, which exhibit a cascade hierarchy of multipole topology at boundaries of boundaries: A quantized octupole moment in the three-dimensional (3D) bulk can induce quantized quadrupole moments on its two-dimensional (2D) surfaces, which then produce quantized dipole moments along 1D hinges. The model of 2D quadrupole TI has been realized in various classical structures, exhibiting zero-dimensional (0D) in-gap corner states. Here we report on the realization of a quantized octupole TI on the platform of a 3D acoustic metamaterial. By direct acoustic measurement, we observe 0D corner states, 1D hinge states, 2D surface states, and 3D bulk states, as a consequence of the

**topological**hierarchy from octupole moment to quadrupole and dipole moment. The critical conditions of forming a nontrivial octupole moment are further demonstrated by comparing with another two samples possessing a trivial octupole moment. Our work thus establishes the multipole topology and its full cascade hierarchy in 3D geometries.

7/10 relevant

arXiv

Demonstration of a quantized acoustic octupole **topological** **insulator**

**topological**

**insulators**(QMTIs) describe higher-order multipole moments, lying in nested Wilson loops, which are inherently quantized by lattice symmetries. Expand abstract.

**topological**

**insulators**(QMTIs) describe higher-order multipole moments, lying in nested Wilson loops, which are inherently quantized by lattice symmetries. Overlooked in the past, QMTIs reveal new types of gapped boundaries, which themselves represent lower-dimensional

**topological**phases and host topologically protected zero-dimensional (0D) corner states. Inspired by these pioneering theoretical predictions, tremendous efforts have been devoted to the experimental observation of

**topological**quantized quadrupole phase in a variety of two dimensional (2D) metamaterials. However, due to stringent requirements of anti-commuting reflection symmetries in crystals, it has been challenging to achieve higher-order quantized multipole moments, such as octupole moments, in a realistic three-dimensional (3D) structure. Here, we overcome these challenges, and experimentally realize the acoustic analogue of a quantized octupole

**topological**

**insulator**(QOTIs) using negatively coupled resonators. We confirm by first-principle studies that our design possesses a quantized octupole

**topological**phase, and experimentally demonstrate spectroscopic evidence of a

**topological**hierarchy of states in our metamaterial, observing 3rd order corner states, 2nd order hinge states and 1st order surface states. Furthermore, we reveal

**topological**phase transitions from higher- to lower-order multipole moments in altered designs of acoustic TIs. Our work offers a new pathway to explore higher-order

**topological**states (HOTSs) in 3D classical platforms.

10/10 relevant

arXiv

Quasi-periodic dynamical phase transitions in multi-band **topological**
**insulators** and connections with entanglement entropy and fidelity
susceptibility

**topological**

**insulator**models typically considered previously. Expand abstract.

**topological**

**insulator**. For this purpose we introduce a new solvable multi-band model based on the Su-Schrieffer-Heeger model, generalised to unit cells containing many atoms but with the same symmetry properties. Such models have a richer structure of dynamical phase transitions than the simple two-band

**topological**

**insulator**models typically considered previously. We also investigate the boundary contributions from the presence of the topologically protected edge states of this model, and consider the fidelity susceptibility as an indicator of the

**topological**phase transitions. Finally we investigate the dynamics of the entanglement entropy generated after a quench, and its potential relation to dynamical phase transitions.

9/10 relevant

arXiv

Observation of optical absorption correlated with surface state of
**topological** **insulator**

**topological**surface band of Bi2Se3. Expand abstract.

**topological**surface band of Bi2Se3. The optical population of surface band is of significant importance not only for fundamental study but also for TI-based optoelectronic device application.

7/10 relevant

arXiv

Finite-dimensional bistable **topological** **insulators**: From small to large

**topological**

**insulators**built from small-size honeycomb arrays of microcavity pillars, we illustrate how

**topologic**al properties of the system build up upon gradual increase of its dimensionality. Expand abstract.

**topological**

**insulators**supporting unidirectional topologically protected edge states represent attractive platform for realization of disorder- and backscattering-immune transport of edge excitations in both linear and nonlinear regimes. In many realizations of

**topological**

**insulators**structured periodic materials are used, since they may admit specific Dirac degeneracy in the spectrum, around which unidirectional edge states appear under the action of physical effects breaking time-reversal symmetry. While properties of the edge states at unclosed interfaces of two bulk media with different topology are known, the existence of the edge states in practical finite-dimensional

**topological**

**insulators**fully immersed in nontopological environment remains largely unexplored. In this work using as an example realistic polariton

**topological**

**insulators**built from small-size honeycomb arrays of microcavity pillars, we illustrate how

**topological**properties of the system build up upon gradual increase of its dimensionality. To account for dissipative nature of polariton condensate forming in the array of microcavity pillars, we consider the impact of losses and resonant pump leading to rich bistability effects in this system. We describe the mechanism in accordance with which trivial-phase pump "selects" and excites specific nonlinear

**topological**edge states circulating along the periphery of the structure in the azimuthal direction dictated by the direction of the external applied magnetic field. We also show the possibility of utilization of vortex pump with different

**topological**charges for selective excitation of different edge currents.

10/10 relevant

arXiv

Electronic transport in one-dimensional Floquet **topological** **insulators**
via **topologic**al- and non-topological edge states

**topological**phase diagram of one-dimensional Floquet

**topologic**al

**insulators**with Rashba spin-orbit interaction [Kennes \emph{et al.}, Phys. Expand abstract.

**topological**phase diagram of one-dimensional Floquet

**topological**

**insulators**with Rashba spin-orbit interaction [Kennes \emph{et al.}, Phys. Rev. B {\bf 100}, 041104(R) (2019)]. Using the Keldysh-Floquet formalism, we compute electronic transport properties of these nanowires, where we propose to couple the leads in such a way, as to primarily address electronic states with a large relative weight at one edge of the system. By tuning the Fermi energy of the leads to the center of the

**topological**gap, we are able to directly address the

**topological**edge states, granting experimental access to the

**topological**phase diagram. Surprisingly, we find conductance values similar or even larger in magnitude to those corresponding to

**topological**edge states, when tuning the lead Fermi energy to special values in the bulk, which coincide with bifurcation points of the dispersion relation in complex quasimomentum space. These peaks reveal the presence of narrow bands of states whose wave functions are linear combinations of delocalized bulk states and exponentially localized edge states, where the amplitude of the edge-state component is sharply peaked at the aforementioned bifurcation point, resulting in an unusually large relative edge-weight. We discuss the transport properties of these \emph{non-

**topological**edge states} and explain their emergence in terms of an intuitive yet quantitative physical picture. The mechanism giving rise to these states is not specific to the model we consider here, suggesting that they may be present in a wide class of systems.

10/10 relevant

arXiv

Influence of point defects on the electronic and **topological** properties
of monolayer WTe$_2$

**topological**insulators, such as graphene and WTe$_2$, band inversion originates from chemical bonding and space group symmetry, in contrast to materials such as Bi$_2$Se$_3$, where the band inversion derives from relativistic effects in the atoms. In the former, band inversion is susceptible to changes of the chemical environment, e.g. by defects, while the latter are less affected by defects due to the larger energy scale associated with atomic relativistic effects. Motivated by recent experiments, we study the effect of Te-vacancies and Te-adatoms on the electronic properties of WTe$_2$. We find that the Te-vacancies have a formation energy of $2.21$ eV, while the formation energy of the Te-adatoms is much lower with $0.72$ eV. The vacancies strongly influence the band structure and we present evidence that band inversion is already reversed at the nominal composition of WTe$_{1.97}$. In contrast, we show that the adatoms do not change the electronic structure in the vicinity of the Fermi level and thus the

**topological**properties. Our findings indicate that Te-adatoms should be present in thin films that are grown in a Te-rich environment, and we suggest that they have been observed in scanning tunneling microscopy experiments.

4/10 relevant

arXiv

Efficient spin-orbit torque switching with non-epitaxial chalcogenide heterostructures

**topological**

**insulators**(TIs) have gained increasing attention in recent years. These TIs, which are typically formed by epitaxially grown chalcogenides, possess extremely high SOT efficiencies and have great potential to be employed in the next-generation spintronics devices. However, epitaxy of these chalcogenides is required to ensure the existence of topologically-protected surface state (TSS), which limits the feasibility of using these materials in industry. In this work, we show that non-epitaxial Bi$_{x}$Te$_{1-x}$/ferromagnet heterostructures prepared by conventional magnetron sputtering possess giant SOT efficiencies even without TSS. Through harmonic voltage measurement and hysteresis loop shift measurement, we find that the damping-like SOT efficiencies originated from the bulk spin-orbit interactions of such non-epitaxial heterostructures can reach values greater than 100% at room temperature. We further demonstrate current-induced SOT switching in these Bi$_{x}$Te$_{1-x}$-based heterostructures with thermally stable ferromagnetic layers, which indicates that such non-epitaxial chalcogenide materials can be potential efficient SOT sources in future SOT magnetic memory devices.

4/10 relevant

arXiv

Giant unidirectional magnetoresistance in **topological** **insulator** --
ferromagnetic semiconductor heterostructures

**topological**

**insulator**-- GaMnAs ferromagnetic semiconductor. We obtained a large UMR ratio of 1.1%, and found that this giant UMR is governed not by the giant magnetoresistance (GMR)-like spin-dependent scattering, but by magnon emission/absorption and strong spin-disorder scattering in the GaMnAs layer. Our results provide new insight into the complex physics of UMR, as well as a strategy for enhancing its magnitude for device applications.

7/10 relevant

arXiv

Lateral lattice coherence lengths in thin films of bismuth telluride
**topological** **insulators**, with overview on polarization factors for X-ray
dynamical diffraction in monochromator crystals

**Topological**Insulators(J. Phys. Chem. C 2019, 123, 24818-24825, doi: 10.1021/acs.jpcc.9b05377), several topics on X-ray diffraction analysis of thin films were developed or revisited. A simple equation to determine lateral lattice coherence lengths in thin films stands as the main development (section S4 - Lateral lattice coherence length in thin films), while X-ray dynamical diffraction simulation in monochromator crystals stands as an interesting overview on how the ratio between $\pi$ and $\sigma$ polarization components is affected by whether diffraction takes place under kinematical or dynamical regime (section S3 - Polarization factor).

8/10 relevant

arXiv