Understanding the flat band in 1T-TaS2 using a rotated basis

Electronic flat bands serve as a unique platform to achieve strongly correlated phases. The emergence of a flat band around the Fermi level in 1T-TaS2 accompanied by the development of a 13×13 charge density wave (CDW) superlattice has long been noticed experimentally, but a transparent theoretical understanding remains elusive. We show that without the CDW order, the primary feature of the 1×1 bands can be fitted by a simple trigonometric function, and physically understood by choosing a rotated t2g basis with the principal axes aligning to the tilted TaS6 octahedron. Using this basis, we trace the band evolution in the 13×13 superlattice by progressively including different CDW effects. We point out that the CDW order strongly rehybridizes the three t2g orbitals, which leads to the formation of a well-localized molecular orbital and the flat band.