Fig. 4: Topological classes of the second-order Wannier bands.
From: Demonstration of a quantized acoustic octupole topological insulator
a Diagram of the topological classes for the second-order Wannier bands as a function of \(\left| {\frac{{\lambda _i}}{{\gamma _i}}} \right|,i = x,y,z\). b–d Energy spectra of a finite lattice obtained from first-principle FEM studies with a topological phase in \(( {p_x^{ - z, - y},p_y^{ - x, - z},p_z^{ - y, - x}} ) = ( {\frac{1}{2},0,\frac{1}{2}} )\), c in \(( {p_x^{ - z, - y},p_y^{ - x, - z},p_z^{ - y, - x}} ) = ( {0,0,\frac{1}{2}} )\), and d in \(( {p_x^{ - z, - y},p_y^{ - x, - z},p_z^{ - y, - x}} ) = (0,0,0)\).
