Hai Yang

A significant paradigm shift in condensed matter physics has been precipitated by the emergence of higherorder topological insulators. We design a higher-order Kagome acoustic crystal incorporating balanced gain and loss factors, thereby emulating non-Hermitian characteristics. This system exhibits the coexistence of two distinct corner states localized at the same angular positions within a supercell composed of two triangular domains with different topological phases, and the radius of one cylinder can control the frequency of the corner state. By constructing a polygonal structure formed by the expanded and shrunk lattices with distinct topological phases, we theoretically demonstrate that acoustic waves of various frequencies can be selectively trapped at a series of specific angles of this polygon. Moreover, these corner states are locked to the corner order, corresponding to the clockwise or anticlockwise orientation of the polygon, which leads to the higher-order topological rainbow. This work opens a new avenue for guiding and trapping multi-frequency acoustic energy, with potential applications in advanced acoustic devices.

Biphenylene network, a two-dimensional material, has recently been extensively studied, owing to its potential applications. Here, we introduce an experimentally synthesized configuration of hexagonal biphenylene network (h-BPN) as a second-order topological insulator (SOTI). We begin by discussing the higher-order topological origin of h-BPN, which is an extended model of the Kekulé lattice characterized by a multi-node π-conjugation. Through first-principles calculations and tight-binding model, we reveal SOTI in h-BPN, characterized by non-zero fractional corner charges. Furthermore, we show that by varying the number of biphenylene units, h-BPN exhibits adjustable second-order topological phases with alternating parity. Specifically, odd biphenylene indices in the h-BPN system correspond to a SOTI, while even indices result in a trivial insulator. We propose that the h-BPN evolves from the Kekulé lattice. These materials will have important applications in two-dimension devices as higher-order topological materials.

The synthetic dimension provides us with a powerful platform to explore the topological properties of matter. In this paper, we choose two physical dimensions (kx and ky) and one geometric structure parameter (the height of the air cavity δ) to construct a synthetic space. The simulation results display that the three couples of Weyl points and Fermi arcs appear in the synthetic space. We find that changing the position of the boundary truncation of a sonic crystal can flexibly regulate the frequency range of valley chiral edge states. Both theoretically and experimentally, we demonstrate the sonic topological valley-locked whispering gallery, in which the valley states propagate along the interfaces between a sonic crystal and a hard wall. In the valley-locked whispering gallery, the sonic waves propagate unidirectionally along the closed channel, with high scattering efficiency and small diffraction.

Recently, researchers have devoted their intense efforts to investigating Weyl physics in synthetic space. In this Letter, we study the intriguing topological rotated Weyl physics in a three-dimensional parameter space, which consists of two extra structural parameters and the wave vector of a simple one-dimensional sonic crystal. In our ultrasonic experiments, we observe that the topological interface states propagate along the interface formed by two sonic crystals of distinct chirality caused by the rotated Weyl points. We detect the rotated synthetic Weyl points and measure the singularities of the reflection phase, which results in the robustness of the interface states. At the same time, it also shows the advantages of synthetic dimensions in exploring high-dimensional physics problems in low-dimensional systems.

As a quantum state of frequency extrema in the momentum space of acoustic systems, sonic valley pseudospin provides a new degree of freedom for controlling acoustic waves. Higher-order topological insulators (HOTIs) have extended the traditional bulk-edge correspondence principle and are a crucial concept for classic wave regulation. However, HOTIs in valley sonic crystals (VSCs) only appear in a single bandgap, which limits the multi-frequency selectivity of the corner state and is not conducive to the design of multi-frequency acoustic communication devices. Here, we demonstrate “Y-shaped” acoustic crystals with C3 symmetry that form a double-band VSC, and the topological phase transitions in both low- and high-frequency band gaps coincide. We realize theoretically and experimentally higher-order states in dual-band valley sonic crystals. Our work enriches the application of HOTIs in acoustic multi-frequency regulatory systems and provides different avenues for designing of multi-band acoustic devices.

In this work, five two-dimensional (2D) noble-transition-metal chalcogenide (NTMC) semiconductors, namely β-NX (N = Au, Ag; X = S, Se, Te), were designed and predicted by first-principles simulations.

The synthetic dimensional provides a powerful platform for us to explore the topological properties of matter. In this paper, we choose two physical dimensions ($k_x$ and $k_y$) and one geometric structure parameter (the height of the air cavity $\delta$) to construct a synthetic space. The simulation results display that the three couples of Weyl points and Fermi arcs appear in the synthetic space. We find that the frequency range of valley chiral edge states can be flexibly regulated by changing the position of boundary truncation of the sonic crystal. Both theoretically and experimentally, we demonstrate that the sonic valley-locked whispering gallery can be realized along the interface between the sonic crystals and hard walls. In the valley-locked whispering gallery, the acoustic waves propagate unidirectionally along the closed channel, with highly scattering efficiency and small diffraction.

In this paper, the first-principles density functional theory and non-equilibrium Green’s function method were used to investigate the effect of strain on the electronic and thermal transport of monolayer black arsenic phosphorus (B-AsP). Through this research, we found that the band gap of the monolayer B-AsP decreased under compressive strain, the electronic transport was revealed by I-V curves, and there was current at a very small voltage. The compressive strain can enhance the electronic transport, but it does not have a favorable effect on the thermal transport. When increasing the temperature at the same time for the tensile strain, the thermal transport has been improved, reflected by the ZT coefficient, which is increased by 7.76% compared with the original system. At the same time, we also used temperature to regulate the ZT coefficient. At a high temperature of 400 K, the ZT coefficient is larger than the one at room temperature by 26.11%.