Geometrically Frustrated Phonons in a Topological Kagome Metal

Geometric frustration on the kagome lattice results in localized real space modes with flat bands in reciprocal space. Because the ratio of potential energy to kinetic energy of these modes is so high, they are promising platforms for studying the effects of strong interactions. Here, we investigate the consequences of localized phonon modes in a topological kagome lattice metal. We use inelastic neutron scattering (INS) to find unambiguous evidence of the flat kagome phonon bands in reciprocal space with low bandwidth across the Brillouin zone, and explore the impact these localized phonons have on bulk thermodynamic properties including heat capacity, thermal conductivity and thermoelectricity. By using an extensive suite of experimental probes along with INS and DFT calculations, we find that the localized kagome phonons interact strongly with the electrons in this system, inducing a novel phase transition. We highlight how engineering flat phonon bands generates phases of fundamental intrest and can be promising for applications in thermal energy management.