Lattice dynamics and topological surface phonon states in cuprous oxide Cu 2 O

The topological phonon state of quantum matter is an emerging field that has been attracting considerable interest. For instance, Weyl phonons in transition-metal monosilicides have been proposed theoretically and identified experimentally. However, topological phonon nodal net states are not well studied due to the lack of realistic materials. Here, based on first-principles calculations and effective model analysis, we propose an existing material---cuprous oxide---to host the nodal net phonons. The nontrivial phonon surface states and uncovered phononic arcs are clearly visible on $k$-resolved phonon spectra, which are amenable to experimental detection. Our findings offer a possible platform for realizing topologically nontrivial phonon states and their applications.