Observation of 3D Flat Bands in the Pyrochlore Metal CaNi2

The realization of flat electronic bands originating from a destructive hopping of wave functions has proven a promising route to designing exotic and correlated electronic states of matter. Recent efforts have demonstrated flat band phase diagrams in prototypical 2D materials including kagome metals and twisted moiré structures. Here we study the pyrochlore network which naturally generalizes the flat band of the kagome lattice to 3D crystal structures. Through angle resolved photoemission spectroscopy, we identify the C15 Laves phase metal CaNi2 as a model realization of the d-electron pyrochlore band structure including finding multiple bands flat across the full 3D Brillouin zone. We additionally observe the characteristic quadratic band touchings and Dirac band features of the pyrochlore band structure. We further demonstrate chemical tuning of a flat band to the Fermi level in CaRh2 that coincides with the appearance of superconductivity. Our theoretical modelling of the multiple flat bands in these structures points to a combination of single-orbital and multi-orbital destructive interference patterns as essential for reaching a full description of the flat bands in these systems. In this talk, I will present our identification and characterization of the flat bands in CaNi2 and CaRh2. We expect that these understandings of the origins of flat bands in 3D materials may allow for a wide array of possibilities in engineering novel correlated and topological states unique to 3D.