A phononic quadrupole insulator

The modern theory of charge polarization in solids is based on a generalization of Berry's phase. Its quantization lies at the heart of our understanding of all systems with topological band structures that were discovered over the last decades. Recently, the theory of this quantized polarization was extended from the dipole- to higher multipole-moments. In particular, a two-dimensional quantized quadrupole insulator is predicted to have gapped yet topological one-dimensional edge-modes, which in turn stabilize zero-dimensional in-gap corner states. I will report on our measurements of a phononic quadrupole insulator. We experimentally characterize the bulk, edge, and corner physics of a mechanical metamaterial and find the predicted gapped edge and in-gap corner states.