Three-Dimensional Dispersive Excitons in Metallic TbNiC2

Excitons in semiconductors with negligible group velocity have been extensively investigated using optical methodologies. However, one significant characteristic of excitons for information transmission applications is their mobility, which poses a challenge in band-gapped systems. Meanwhile, three-dimensional dispersive excitons formed with electrons in a partially occupied conduction band are rare due to electron screening effect, excitonic instability, and the technical challenge of realizing them. Here, using angle-resolved photoemission spectroscopy and theoretical simulation, we observed dispersive excitonic bound states that are manifested as replica valence bands across a large portion of the three-dimensional Brillouin in the metallic material TbNiC2. Furthermore, our research indicates that the excitonic bound states are promoted by low density of states of free charges induced by electron-phonon coupling, and the resonance between topological surface and bulk states. Our findings provide a new thread to generate and stabilize three dimensional dispersive excitons in metals, which paves the way for the exploration of novel materials potentially suitable as metallic optoelectronic media which could conduct both electronic and excitonic signals simultaneously.