Third-order Orbital Corner State and its Realization in Acoustic Crystals

Three dimensional (3D) third-order topological insulators (TIs) have zero-dimensional (0D) corner states, which are three dimensions lower than bulk. Here we investigate the third-order TIs on breathing pyrochlore lattices with p-orbital freedom. The tight-binding Hamiltonian is derived for the p-orbital model, in which we find that the two orthogonal $\pi$-type (transverse) hoppings are the key to open a band gap and obtain higher-order topological corner states. We introduce the Z4 berry phase to characterize the bulk topology and analysis the phase diagram. The corner states, demonstrated in a finite structure of a regular tetrahedron, exhibit rich 3D orbital configurations. Furthermore, we design an acoustic system to introduce the necessary $\pi$-type hopping and successfully observe the orbital corner states. Our work extends topological orbital corner states to third-order, which enriches the contents of orbital physics and may lead to applications in novel topological acoustic devices.