Topological Phase Research Articles

Guided structure learning of DAGs for count data

In this paper, we tackle structure learning of directed acyclic graphs (DAGs), with the idea of exploiting available prior knowledge of the domain at hand to guide the search of the best structure. In particular, we assume to know the topological ordering of variables in addition to the given data. We study a new algorithm for learning the structure of DAGs, proving its theoretical consistency in the limit of infinite observations. Furthermore, we experimentally compare the proposed algorithm to several popular competitors, to study its behaviour in finite samples. Biological validation of the algorithm is presented through the analysis of non-small cell lung cancer data.

Topological data analysis assisted machine learning for polar topological structures in oxide superlattices

Ferroelectric topological phases and phase transitions have been extensively investigated recently due to the rich physical insights and potential applications in next-generation electronic devices. However, precisely predicting the topological phase transitions under different internal and external conditions in polar oxide superlattice systems is challenging due to the complex energy competitions and highly nonlinear kinetics involved. Herein, we adopted a state-of-the-art mathematical concept called “persistent homology” from topological data analysis to extract the essential topological features for the polarization data in various topological structures. By implementing the persistent image as the descriptor, support vector regression (SVR) based convolutional neural network (CNN) models are developed for the automated and high precision classification and regression of topological states based on high-dimensional phase-field simulation data of the PTO/STO superlattice. Using this method, we can automatically construct the strain and electric field phase diagrams in seconds with high throughput phase-field data. We hope to spur further interest in the integration of state-of-the-art mathematical tools, machine learning algorithms, and condensed matter physics for predictions of topological phase transitions.

Image Steganography Approaches and Their Detection Strategies: A Survey

Steganography is the art and science of hidden (or covered) communication. In digital steganography, the bits of image, video, audio and text files are tweaked to represent the information to hide. This article covers the current methods for hiding information in images, alongside steganalysis methods that aim to detect the presence of steganography. By reviewing 456 references, this article discusses the different approaches that can be taken toward steganography and its much less widely studied counterpart. Currently in research older steganography approaches are more widely used than newer methods even though these show greater potential. New methods do have flaws; therefore, more research is needed to make these practically applicable. For steganalysis one of the greatest challenges is the generalisability. Often one scheme can detect the presence of one specific hiding method. More research is needed to combine current schemes and/or create new generalisable schemes. To allow readers to compare results between different papers in our work, performance indications of all steganalysis methods are outlined and a comparison of performance is included. This comparison is given using ‘topological sorting’ graphs, which compares detection results from all papers (as stated in the papers themselves) on different steganographic schemes.

Experimental Detection of Topological Electronic State and Large Linear Magnetoresistance in SrSn4 Superconductor

AbstractWhile recent experiments confirm the existence of hundreds of topological electronic materials, only a few exhibit the coexistence of superconductivity (SC) and a topological electronic state. These compounds attract significant attention in forefront research because of the potential for the existence of topological SC, paving the way for future technological advancements. SrSn4 is known for exhibiting unusual SC below the transition temperature (TC) of 4.8 K. Recent theory predicts a topological electronic state in this compound, which is yet to be confirmed by experiments. Systematic and detailed studies of the magnetotransport properties of SrSn4 and its Fermi surface characterizations are also absent. For the first time, a quantum oscillation study reveals a nontrivial 𝝅‐Berry phase, very light effective mass, and high quantum mobility of charge carriers in SrSn4. Magnetotransport experiment unveils large linear transverse magnetoresistance (TMR) of more than 1200% at 5 K and 14 T. Angle‐dependent transport experiments detect anisotropic and fourfold symmetric TMR, with the maximum value (≈2000%) occurring when the angle between the magnetic field and the crystallographic b‐axis is 45°. The results suggest that SrSn4 is the first topological material with SC above the boiling point of helium that displays such high magnetoresistance.

Topological superconductivity in Fibonacci quasicrystals

We investigate the properties of a Fibonacci quasicrystal (QC) arrangement of a one-dimensional topological superconductor, such as a magnetic atom chain deposited on a superconducting surface. We uncover a general mutually exclusive competition between the QC properties and the topological superconducting phase with Majorana bound states (MBS): there are no MBS inside the QC gaps and the MBS never behave as QC subgap states and, likewise, no critical or winding QC subgap states exist inside the topological superconducting gaps. Surprisingly, despite this competition, we find that the QC is still highly beneficial for realizing topological superconductivity with MBS. It both leads to additional large nontrivial regions with MBS in parameter space, that are topologically trivial in crystalline systems, and increases the topological gap protecting the MBS. We also find that shorter approximants of the Fibonacci QC display the largest benefits. As a consequence, our results promote QCs, and especially their short approximants, as an appealing platform for improved experimental possibilities to realize MBS as well as generally highlight the fundamental interplay between different topologies. Published by the American Physical Society 2024

Strain-modulated antiferromagnetic Chern insulator in NiOsCl$_6$ monolayer

Abstract Recently, Chern insulators in an antiferromagnetic (AFM) phase have been suggested theoretically and predicted in a few materials. However, the experimental observation of two-dimensional AFM quantum anomalous Hall effect is still a challenge to date. In this work, we propose that an AFM Chern insulator can be realized in a two-dimensional monolayer of NiOsCl$_6$ modulated by a compressive strain. Strain modulation is accessible experimentally and used widely in predicting and tuning topological nontrivial phases. With first-principles calculations, we have investigated the structural, magnetic and electronic properties of NiOsCl$_6$. Its stability has been confirmed through molecular dynamical simulations, elasticity constant and phonon spectrum. It has a collinear AFM order, with opposite magnetic moments of 1.3 $\mu_B$ on each Ni/Os atom, respectively, and the Néel temperature is estimated to be 93 $K$. In the absence of strain, it functions as an AFM insulator with a direct gap with spin-orbital coupling included. Compressive strain will induce a transition from a normal insulator to a Chern insulator characterized by a Chern number $C = 1$, with a band gap of about 30 meV. This transition is accompanied by a structural distortion. Remarkably, the Chern insulator phase persists within the 3$\%$-10$\%$ compressive strain range, offering an alternative platform for the utilization of AFM materials in spintronic devices.

Manipulating Charge-to-Spin conversion via insertion layer control at the interface of topological insulator and ferromagnet

Strong spin–orbit coupling and highly spin-polarized surface states in topological insulators (TIs) are key parameters that explain their extremely high charge-to-spin conversion (CSC) efficiency at interfaces with ferromagnetic materials (FMs). This study focused on the influence of the insertion layer on the proximity effect occurring in a Co4Fe4B2/Bi2Se3 interface. Various insertion layers, including Au, MgO, and Se, were introduced to modulate the proximity effect from TI to FM and vice versa. X-ray photoelectron spectroscopy and transmission electron microscopy revealed that the Se insertion layer effectively suppresses the formation of an additional Bi layer, reducing intermixing against Co4Fe4B2. Electrical transport properties such as RXX and RXY under a vertical magnetic field show that the Se-inserted structure features the lowest anomalous Hall angle and exhibits a pristine topological surface state, indicating its potential for improving CSC efficiency. The Se-inserted structure exhibits the highest spin Hall angle among various heterostructures, according to results obtained from spin-torque ferromagnetic resonance. These findings highlight the importance of selecting an insertion layer and controlling the interface to optimize the spin-transport properties of TI-based spintronic devices and provide insights into the design of future spin devices.

Bose Metals, from Prediction to Realization.

Bose metals are metals made of Cooper pairs, which form at very low temperatures in superconducting films and Josephson junction arrays as an intermediate phase between superconductivity and superinsulation. We predicted the existence of this 2D metallic phase of bosons in the mid 1990s, showing that they arise due to topological quantum effects. The observation of Bose metals in perfectly regular Josephson junction arrays fully confirms our prediction and rules out alternative models based on disorder. Here, we review the basic mechanism leading to Bose metals. The key points are that the relevant vortices in granular superconductors are core-less, mobile XY vortices which can tunnel through the system due to quantum phase slips, that there is no charge-phase commutation relation preventing such vortices from being simultaneously out of condensate with charges, and that out-of-condensate charges and vortices are subject to topological mutual statistics interactions, a quantum effect that dominates at low temperatures. These repulsive mutual statistics interactions are sufficient to increase the energy of the Cooper pairs and lift them out of condensate. The result is a topological ground state in which charge conduction along edges and vortex movement across them organize themselves so as to generate the observed metallic saturation at low temperatures. This state is known today as a bosonic topological insulator.

Tunable Topological Wetting State of Water Droplets on Planar Surfaces: Closed-Loop Chemical Heterogeneity by Design.

Understanding droplet wetting on surfaces has broad implications for surface science and engineering. Here, we report a joint theoretical/experimental study of the topological wetting states of water droplets on chemically heterogeneous closed-loop and planar surfaces. Interestingly, we provide both simulation and experimental evidence of biloop or even multiloop transition wetting states of water droplets. Specifically, in our molecular dynamics simulations, we designed surfaces patterned with alternating closed-loop superhydrophilic and hydrophobic nanobands. On these surfaces, we find that the contact shape and contact angle of water nanodroplets can be tailored by changing the shape of the nanobands. Overall, the contact angle of the droplets is dependent on the initial location of the water droplet, the interaction between the water molecules and hydrophobic particles, and the width of the superhydrophilic and hydrophobic nanobands. The wetting-state transition dynamics is also dependent on the nanoband shape. In the biloop or multiloop transition wetting states, the three-phase contact line is not limited to only one loop nanoband but can be located at two or more distinct loops. Guided by the simulation results, the corresponding experiments confirmed the presence of topological wetting states and multiloop wetting states on planar chemically heterogeneous surfaces with closed-loop microbands. Importantly, we provide an explanation of the mechanism of the topological wetting state formation and transition. Our study facilitates a deeper understanding of the droplet-surface interactions and offers an alternative way to tune the droplet shape and contact angle on planar surfaces by engineering chemically heterogeneous surface textures.

Valley-Polarized Topological Phases with In-Plane Magnetization.

The coexistence of valley polarization and topology has considerably facilitated the applications of 2D materials toward valleytronics device technology. However, isolated and distinct valleys are required to observe the valley-related quantum phenomenon. Herein, we report a new mechanism to generate in-plane magnetization direction-dependent isolated valley carriers by preserving or breaking the mirror symmetry in a 2D system. First-principle calculations are carried out on a prototype material, W2MnC2O2 MXene, to demonstrate the mechanism. A valley-coupled topological phase transition among Weyl semimetal, valley-polarized quantum anomalous Hall insulator, and topological semimetal is observed by manipulating the in-plane magnetization directions in W2MnC2O2. Monte Carlo simulations of W2MnC2O2 show that the estimated Curie temperature is around 170 K, indicating the possibility of observing valley-polarized topological states at higher temperatures. Our finding provides a generalized platform for investigating the valley and topological physics, which is extremely important for future quantum information processing applications.

Observation of spatiotemporal dynamics for topological surface state with on-demand dispersion

Observation of spatiotemporal dynamics for topological surface state with on-demand dispersion

Braids and higher-order exceptional points from the interplay between lossy defects and topological boundary states

We show that the perturbation of the Su-Schrieffer-Heeger chain by a localized lossy defect leads to higher-order exceptional points (HOEP). Depending on the location of the defect, third- and fourth-order exceptional points (EP3 and EP4) appear in the space of Hamiltonian parameters. On the one hand, they arise due to the non-Abelian braiding properties of exceptional lines (EL) in parameter space. Namely, the HOEPs lie at intersections of mutually noncommuting ELs. On the other hand, we show that such special intersections happen due to the fact that the delocalization of edge states, induced by the non-Hermitian defect, hybridizes them with defect states. These can then coalesce together into an EP3. When the defect lies at the midpoint of the chain, a special symmetry of the full spectrum can lead to an EP4. In this way, our model illustrates the emergence of interesting non-Abelian topological properties in the multiband structure of non-Hermitian perturbations of topological phases. Published by the American Physical Society 2024

Topological phases, van Hove singularities, and spin texture in magic-angle twisted bilayer graphene in the presence of proximity-induced spin-orbit couplings

Topological phases, van Hove singularities, and spin texture in magic-angle twisted bilayer graphene in the presence of proximity-induced spin-orbit couplings

Multiband topological acoustic waveguide with multistage toroidal resonant cavities

In recent years, there has been significant development in acoustic waveguides through the introduction of topological phase correlation concepts in acoustics. Many studies have shown the properties of acoustic unidirectional transmission. This paper presents a new ring-shaped acoustic topological insulator that is multi-band, unlike previous structures. The structure improves internal space utilization with the same dimensions, reducing acoustic wave conduction frequency in air environments. Additionally, the introduction of a multistage ring-shaped resonant cavity enables the conduction of acoustic waves in multiple frequency bands. This paper employs the rotational scattering mechanism to invert the topological phase of the lattice. This allows for the construction of two crystals with opposite phases, which can be used to create a topological channel for the transmission of acoustic waves. At the topological interface, the acoustic transmission losses within the four bands are small. Furthermore, this paper verifies by simulation that the defects have little effect on acoustic transmission. The paper's research offers potential for multiband acoustic control.

Dual-band large-area topological edge states and higher-order corner states in a valley Hall photonic crystal

Dual-band large-area topological edge states and higher-order corner states in a valley Hall photonic crystal

Signature of pressure-induced topological phase transition in ZrTe5

The layered van der Waals material ZrTe5 is known as a candidate topological insulator (TI), however its topological phase and the relation with other properties such as an apparent Dirac semimetallic state is still a subject of debate. We employ a semiclassical multicarrier transport (MCT) model to analyze the magnetotransport of ZrTe5 nanodevices at hydrostatic pressures up to 2 GPa. The temperature dependence of the MCT results between 10 and 300 K is assessed in the context of thermal activation, and we obtain the positions of conduction and valence band edges in the vicinity of the chemical potential. We find evidence of the closing and re-opening of the band gap with increasing pressure, which is consistent with a phase transition from weak to strong TI. This matches expectations from ab initio band structure calculations, as well as previous observations that CVT-grown ZrTe5 is a weak TI in ambient conditions.

Topological valley waveguide state transport based on the nesting difference

Acoustic topological waveguide states are characterized by topologically protected transport. Existing acoustic topological valley-locked waveguide state transport structures can only realize single-scene acoustic field modulation due to the limitation of structural tunability, and it is difficult to realize width degree of freedom and reconfigurable transport paths of acoustic waves by adjusting the transport structure for complex acoustic field. Therefore, the transport structures become a conceptualized idea in terms of tunability. Our study introduces a nesting method in the design of the sonic crystal scatterer, which splits the scatterer structure into a removable outer shell and a core, and three topological phase transitions can be realized by only changing the nesting method of the sonic crystal shell, this approach constructs a fully reconfigurable topological waveguide. The results demonstrate that changing the nesting of the sonic crystals induces effective spatial inversion symmetry breaking, which leads to the valley topological phase transitionSimulations and experiments demonstrate that this transport structure has excellent robust transport properties with large inflection corners, and it is capable of realizing acoustic focusing, acoustic channeling, and acoustic logic gates. This fully reconfigurable sonic crystal design method can provide implications for the modulation of other classical waves.

Topological interface states in deep-subwavelength phononic beams with acoustic black hole configurations

We present deep-subwavelength phononic beams engineered with unit cells incorporating acoustic black hole (ABH) configurations. The ABH design enables the realization of topological interface states (TISs) within the first band gap at low frequencies, where the lattice constant (a) is far smaller than the wavelength (λ) of the controlled wave. Manipulating the ABH configuration and breaking the unit-cell symmetry allow us to control Zak phases for the topological beams. Two topologically distinct phononic beams are interconnected to produce the TIS within their low-frequency band gaps. Through numerical and experimental demonstrations, we confirm the existence of TISs at frequencies ranging from 4.3 to 8.5 Hz using the designed topological beams of which a/λ is less than 1/20. We also offer the potential to further reduce the value of a/λ by rearranging the ABH unit cells.

Manipulation of edge states in non-Hermitian acoustic Su Schrieffer Heeger chains

A distinctive feature of topology is its unique ability to obtain and control robust interface states. Among other elements, it has been shown that non-Hermiticity alone can trigger topological phase transition in physical systems. In this work, we construct acoustic Su Schrieffer Heeger(SSH) chains using different unit cells where only the non Hermitian components differ due to different distributions of onsite losses. Our study focus on exploring the emergence of edge states at the interfaces between two chains composed of different unit cells. Our findings delineate three distinct types of edge modes. The topological, cavity and Parity-Time edge modes originate from the topological phase of the unit cells, the local parity symmetry at the interface and the existence of a Parity Time symmetric domain wall, respectively. We then compare the robustness of these different edge modes against disorder, confirming the robustness of the topological edge modes. Parity Time edge states demonstrate superior robustness compared to classical cavity modes for low level of disorder. However, beyond a certain threshold, their eigenfrequencies importantly diverge.

Topological robust transmission of elastic waves in heterogeneous phononic crystal plates

Topological insulators possessing robust transmissions in the topologically protected edge have attract increasing attention. Conventionally, the topological edge states are highly concentrated at the edge with limited width, putting restrictions on the potential phononic applications. In this work, a broadened topological interface is reported in the phononic crystal plate with Y-shaped perforated holes, by adding a phononic crystal featuring Dirac points between two phononic crystals with different phases. The band structure of topological edge state together with typical displacement field is presented. Using a line source to excite on the left side of the topological interface, the topological transports of elastic waves are visualized within a broadband frequency range. Moreover, based on the broadened interface including multiple cavities at different spatial locations, the robust wave propagation is investigated by displacement fields. These results give a straightforward platform to explore the elastic wave manipulation in heterostructures for versatile advanced phononic devices.