Observation of Van Hove singularity interband plasmons in the kagome metal CsV₃Sb₅ for strong light-matter...

Van Hove singularities (VHSs) play a critical role in determining the properties of topological and correlated electronic states. Their associated excitations offer unique opportunities for exploring light-matter interactions reshaped by these correlated states, although experimental observations remain limited. Here, we studied the interaction between plasmons and VHS-related excitations in kagome metal CsV₃Sb₅ films via far-field absorption spectroscopy. Notably, an anticrossing phenomenon was observed in the charge density wave states, with coupling strength approaching the strong coupling regime, indicating the formation of dispersive hybrid VHS interband plasmons. These modes are closely correlated to charge ordering states, with both the fitted coupling strength and the universal screening length of interband transitions exhibiting pronounced anomalies at the transition temperature. Our findings offer critical insights into the role of correlated electronic states in modulating plasmon behavior in kagome metals and unveil promising possibilities for tuning light-matter interactions in correlated materials.