Spin-polarized triplet excitonic insulators in Ta₃ X ₈ (X = I or Br) monolayers

Bose-Einstein condensation of spin-polarized triplet excitons can give rise to an intriguing spin supercurrent, providing a direct experimental signature of exciton condensation and offering a non-dissipative channel for information transfer in spintronic devices. In this work, we predict that Ta₃ X ₈ (X = I or Br) ferromagnetic monolayers are spin-polarized triplet excitonic insulators (EIs), based on the systematic first-principles GW calculations coupled with the Bethe-Salpeter equation (GW + BSE). The single-particle calculations of spin-polarized band structures reveal that these monolayers are bipolar magnetic semiconductors, where the highest valence band and the lowest conduction band possess opposite spin polarization. The two low-energy bands, primarily originating from Ta $d_{z^2}$ orbitals, are almost flat. The same-orbital parity and opposite-spin nature of the band-edge states effectively suppress dielectric screening, promoting the emergence of the EI state. The GW + BSE calculations reveal that the binding energy of the lowest-energy exciton is 1.499 eV for the Ta₃I₈ monolayer and 1.986 eV for the Ta₃Br₈ monolayer, both of which exceed the respective GW band gaps, indicating spin-polarized triplet EIs. A wavefunction analysis confirms that the lowest-energy exciton is a tightly bound Frenkel-like state. Our findings establish an ideal material platform for exploring spin-polarized triplet EIs, with promising implications for spintronic applications, such as spin-current Josephson junctions.