Fig. 1: Non-Hermiticity-induced quadrupole topological insulator and its acoustic implementation.
From: Non-Hermitian route to higher-order topology in an acoustic crystal
a Schematic of a bulk quadrupole moment (qxy) with its accompanying edge dipole moments (px and py) and corner charges (Q). b Tight-binding model of a unit cell that consists of 16 sites. Red (blue) sites have an imaginary on-site potential of γ2 (γ1). Solid (dashed) lines denote positive (negative) couplings with strength κ. There is a π flux threading each plaquette due to the negative couplings. c Acoustic design of the lattice model in (b). Blue and red cuboids represent acoustic resonators with only background loss (γ1) and those with deliberately enhanced loss (γ2), respectively. These resonators are coupled by identical thin waveguides. The coupling waveguides that exhibit positive (negative) couplings are colored in yellow (gray). d Photo of the fabricated sample that realizes the acoustic lattice shown in (c). Some resonators have small holes sealed with black absorbing materials to enhance the losses.
