Flat band physics in the charge-density wave state of 1T-TaS₂ and 1T-TaSe₂

1T-TaS₂ is a non-magnetic Mott insulating transition-metal dichalcogenide with an odd number of electrons per unit cell, making it a potential spin-liquid candidate. This behavior arises from miniband reconstructions in the charge density wave state, producing a nearly flat band at half-filling. We revisit its electronic band structure using a nearest-neighbor tight-binding model, emphasizing the importance of often-neglected “spin-flip” terms in the spin-orbit coupling. By comparing with density functional theory calculations, we estimate the strength of these couplings. We also apply our theory to 1T-TaSe₂, which is found to be a promising candidate for a topologically non-trivial flat band. Our findings have significant implications for correlated physics in the flat band, including the emergent spin-spin Hamiltonian at half-filling, identified as a J-K-Γ-Γ′ model on a triangular lattice, and for tuning electronic properties away from half-filling.