Hidden magnetism and the emergence of flat bands in insulator-metal transition in VO₂

Flat bands near the Fermi level offer a platform to tune the interaction between electrons for realizing exotic physical effects such as insulator-metal transition and superconductivity. Most materials with flat bands fall into the category of flat topological band systems such as Kagome crystals that lack a knob to switch the flat bands. Here, we focus on a class of split-off flat bands induced by symmetry breaking, where the latter can be used as knobs to switch on and off the flat bands and demonstrate such flat bands in the quintessential insulator-metal transition system VO₂. Such flat bands in VO₂ are split down into the band gap from the wide principal conduction band by cation-cation dimerization, forming the occupied, rather narrow V-d valence band. We predict rather low energy barrier for the insulator-metal transition and find that strong dimerization will suppress magnetism, leading to the nonmagnetic insulating state, whereas magnetism appears when dimerization is reduced, suggesting hidden magnetism during the insulator-metal transition and a potential magnetic metallic state. This study opens the way to design novel functional quantum materials with symmetry breaking-induced flat bands.