Figure 3: Topologically protected phononic helical edge modes propagating along the domain wall.
From: Topologically protected elastic waves in phononic metamaterials
(a) Domain wall between two crystals with the counterbore made on opposite faces, shown in two viewing angles. The sign of the effective mass is reversed across the domain wall. (b) Band structures of the phononic helical edge modes at the domain wall and the projected bulk bands (grey). The forward edge mode (red) and the backward edge mode (blue) are of the opposite spin. (c) Displacement fields of the forward edge mode demonstrating reflectionless propagation along a zigzag-shaped domain wall. Colours indicate the amplitude of the displacement.
