Three-Dimensional Multiorbital Flat Band Models and Materials

Flat band (FB) systems are essential for uncovering exotic quantum phenomena associated with strong electron correlations. Here we present a systematic theoretical framework for constructing multiorbital FB models and identifying feasible material candidates. This framework integrates group theory and crystallography into a symmetry-adapted tight-binding model incorporating lattice, site, and orbital degrees of freedom. Using this approach, we unveil a novel three-dimensional (3D) multiorbital FB model in the face-centered-cubic lattice, distinct from well-known single-orbital Lieb and kagome models. Critically, we identify numerous high-quality binary materials with ultraclean 3D FBs near the Fermi level. Furthermore, we explore diverse orbital bases within this model and extend our analysis to other cubic lattices with different space groups, broadening the scope for realizing 3D multiorbital FB systems. Our findings provide a foundational platform for exploring correlated physics in multiorbital FB systems and guiding the discovery of new quantum materials.