Guiding robust valley-dependent edge states by surface acoustic waves

Abstract

Recently, the concept of valley pseudospin, labeling quantum states of energy extrema in momentum space, has attracted enormous attention because of its potential as a new type of information carrier. Here, we present surface acoustic wave (SAW) waveguides, which utilize and transport valley pseudospins in two-dimensional SAW phononic crystals (PnCs). In addition to a direct visualization of the valley-dependent states excited from the corresponding chiral sources, the backscattering suppression of SAW valley-dependent edge states transport is observed in sharply curved interfaces. By means of band structure engineering, elastic wave energy in the SAW waveguides can be transported with remarkable robustness, which is very promising for new generations of integrated solid-state phononic circuits with great versatility.Valley, an intriguing and significant concept in condensed matter physics, stems from the extensive research of two dimensional (2D) hexagonal crystals, such as graphene, bilayer graphene, and transition-metal dichalcogenides, in recent years. [1][2][3][4][5][6][7][8][9][10][11][12][13] When the inversion symmetry is broken in two-dimensional (2D) hexagonal lattices, the berry curvatures will acquire opposite signs at K and K' points due to time-reversal symmetry. Subsequently, the electrons in different valleys own opposite anomalous velocities and move towards the opposite directions. Additionally, slow valley relaxation and dephasing processes, compared to electron spin, can be accessed due to the reason that intervalley scattering is suppressed by large momentum separation between different valleys. 2,3,12,14

Topological Metamaterials

“…For instance, topological valley edge modes propagate along the interface between two honeycomb sonic crystals whose triangular scatterers have opposite orientations, taking sharp turns with very little backscattering, − Figure (a). Many platforms for sound, − water waves, and mechanical waves, − Figure (b), even at the scale of a phononic chip, − have been investigated. The robustness of the edge modes is tightly related to the symmetries of the scriptP -broken lattice, hence providing immunity against defects that do not mix the valleys, allowing for directional antennas , (Figure (c)), topological beam splitters (Figure (d)) and acoustic delay lines which benefits from the excellent tunability offered by phononic platforms .…”

Topological sound in two dimensions

“…Topological valley edge modes propagate along the interface between domains with different invariants, taking sharp turns with very little backscattering. Such a straightforward protocol has paved the way to the transposition of phononic valley Hall insulators to a large number of platforms, for sound, [91][92][93][94][95][96][97][98][99][100][101] water waves, and mechanical waves [102][103][104][105][106][107][108][109][110][111][112] (Figure 4B), even at the scale of a phononic chip. [113][114][115] The robustness of edge modes is sustained by the symmetries of the -broken lattice, hence they are protected only against defects that do not mix chiralities, allowing for topological beam splitters 113 (Figure 4E), as well as directional antennas 116,117 (Figure 4D) and acoustic delay lines 118 (Figure 4D) that can benefit from the high degree of tunability offered by phononic platforms.…”

Topological surface wave metamaterials for robust vibration attenuation and energy harvesting

“…The effect has been employed successfully in Lamb wave manipulation with significant robustness [37][38][39][40][41][42][43], revealing a great potential in the ground surface waves manipulation. For the application to surface acoustic waves, Wang et al reported robust guiding valley-dependent edge states for Rayleigh waves excited by chiral sources at around 30 MHz [44]; Topological chiral edge state is also realized on a periodically corrugated surface [45].…”