Edge states and corner modes in second-order topological phononic crystal plates.

We realize an elastic second-order topological insulator hosting both one-dimensional gapped edge states and zero-dimensional in-gap corner modes in double-side pillared phononic crystal plates with a square lattice. Changing the width of two neighbor pillars breaks the inversion symmetry and induces band inversion to emulate the quantum spin Hall effect where gapless edge states are obtained. Further breaking the space symmetry at the interface introduces gaps to the gapless edge states, induces edge topological transitions and gives rise to zero-dimensional in-gap corner modes. Our work offers a novel way of elastic wave trapping and robust guiding. [ABSTRACT FROM AUTHOR]