Fig. 2.
Calculated phonon band structures for chiral metamaterial beams. The left vertical scales are in absolute units, the right scales, a/λ0, are normalized and scalable to other parameters (cf. Fig. 1). We consider wave propagation with wave number kz in a beam which is infinitely extended along the z-direction and which contains Nx × Ny unit cells (with Nx = Ny) in the two orthogonal directions. The three panel columns correspond to Nx = Ny = 1, 3, and ∞, respectively. The upper row is for chiral (δ = 34.8°) metamaterials beams, the lower row for achiral (δ = 0) ones. Transverse or shear-like bands are shown in red (a lifting of degeneracy reflects acoustical activity), pressure-like modes in blue, and twist-like modes in black. The latter are absent for Nx = Ny = ∞. For clarity, higher bands not relevant here are plotted in gray. The insets illustrate the corresponding geometries. The spatial mode depicted in Fig. 1 is highlighted by a circle. Parameters are as in Fig. 1