Higher-order topological corner and bond-localized modes in magnonic insulators

We theoretically investigate a two-dimensional decorated honeycomb lattice framework to realize a second-order topological magnon insulator (SOTMI) phase featuring distinct corner-localized modes. Our study emphasizes the pivotal role of spin-magnon mapping in characterizing bosonic topological properties, which exhibit differences from their fermionic counterparts. We employ a symmetry indicator topological invariant to identify and characterize this SOTMI phase, particularly for systems respecting time-reversal and ${\mathsf{C}}₆$ rotational symmetry. Using a spin model defined on a honeycomb lattice geometry, we demonstrate that introducing ``kekul\'e'' type distortions yields a topological phase. In contrast, ``antikekul\'e'' distortions result in a nontopological magnonic phase. The presence of kekul\'e distortions manifests in two distinct topologically protected bosonic corner modes---an intrinsic and a pseudo, based on the specific edge terminations. On the other hand, antikekul\'e distortions give rise to Tamm/Shockley type bond-localized boundary modes, which are nontopological and reliant on particular edge termination. We further investigate the effects of random out-of-plane exchange anisotropy disorder on the robustness of these bosonic corner modes. The distinction between SOTMIs and their fermionic counterparts arises due to the system-specific magnonic onsite energies, a crucial feature often overlooked in prior literature. Our study unveils exciting prospects for engineering higher-order topological phases in magnon systems and enhances our understanding of their unique behavior within decorated honeycomb lattices.