Fate of the half-filled, strongly correlated topological flat band in 1T-TaS2

Ascertain the fate of electrons in the half-filled flat miniband of 1T-TaS2 under strong correlations when the band carries a non-trivial Z2 topological index, i.e., opposite inversion eigenvalues at the Γ and M points that obstruct a time-reversal-invariant localized Wannier representation; determine the resulting many-body ground state or correlated phase in this regime.

Background

The paper constructs a microscopic tight-binding model for 1T-TaS2 that includes spin-orbit coupling and the commensurate star-of-David charge-density wave, showing that an isolated, half-filled flat band is a robust feature across broad parameter ranges. By viewing the CDW phase as a superlattice of weakly coupled clusters, the authors classify possible flat-band scenarios and identify conditions under which the flat band acquires a non-trivial Z2 topology based on inversion eigenvalues at Γ and M.

In the topologically non-trivial regime, the band lacks a time-reversal-invariant localized Wannier representation, which complicates the understanding of correlation-driven phenomena at half filling. This obstruction raises a fundamental unresolved question regarding the many-body ground state and correlated phases that can emerge from strong electron interactions in such a band, with prior literature indicating nontrivial possibilities but no definitive resolution.

References

Therefore, the fate of the strongly correlated band at half filling is not clear.

— Flat band physics in the charge-density wave state of $1T$-TaS$_2$  (2406.18645 - Dalal et al., 2024) in Section 6 (Discussion)

What the precise identity of this correlated phase is remains not straightforward to answer and requires a much more sophisticated treatment.

— Fractional quantum spin Hall crystals from hidden density-wave order  (2609.19638 - Shammami et al., 17 Sep 2026) in Section IV, subsection \(\nu=1/2\)

We note that the ground state of $1T$-$\text{TaS}_2$ remains debated, the $\sim 0.2$~eV empty-state feature having been attributed either to the UHB of a Mott state or to a stacking- and termination-dependent band edge .

— Imaging atomic scale stochasticity: noise in the nearly commensurate charge density wave of 1T-TaS$_2$ for leaky-integrator-and-fire neuromorphic emulator  (2608.24693 - Verhage, 25 Aug 2026) in Introduction; Supplementary Section S1, “Discussion on the spectral features of the Mott state”