Sign freedom of non-Abelian topological charges in phononic and photonic topological insulators

Abstract: The topological nature of nodal lines in a three-band system can be described by non-Abelian topological charges called quaternion numbers. Due to the gauge freedom of the eigenstates, the sign of the non-Abelian topological charges can be flipped by performing the gauge transformation, i.e., choosing a different basis of eigenstates. However, the sign flipping has not been explicitly shown in realistic systems such as phononic and photonic topological insulators. Here, we elaborate the sign freedom by visualizing the numerically calculated topological charges in phononic and photonic topological insulators. In doing so, we employ a common reference point method for multiple nodal lines to confirm that the sign flipping does not cause any issue in building the quaternion group.