Spin-polarized triplet excitonic insulators in Ta3X8 (X=I, Br) monolayers

TL;DR

This paper predicts that Ta3X8 (X=I, Br) monolayers are spin-polarized triplet excitonic insulators with strong excitonic binding energies, offering a new platform for spintronics based on first-principles calculations.

Contribution

The study introduces the prediction of spin-polarized triplet excitonic insulators in Ta3X8 monolayers using GW+BSE calculations, highlighting their potential for spintronic applications.

Findings

Strong excitonic instability with binding energies exceeding GW band gaps.

Identification of the lowest-energy exciton as a tightly bound, spin-polarized triplet Frenkel-like state.

Monolayers are bipolar magnetic semiconductors with opposite spin polarization in valence and conduction bands.

Abstract

Bose-Einstein condensation of spin-polarized triplet excitons can give rise to an intriguing spin supercurrent, enabling experimental detection of exciton condensation. In this work, we predict that Ta3X8 (X=I, 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 natures 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 Ta3I8 monolayer and 1.986 eV for Ta3Br8 monolayer. Since both values exceed the respective GW band gaps, these results indicate a strong excitonic instability in these monolayers. A wavefunction analysis confirms that the lowest-energy exciton is a tightly bound Frenkel-like state, exhibiting a spin-polarized triplet nature with $S_z=1$. Our findings establish a valuable material platform for investigating spin-polarized triplet EIs, offering promising potential for spintronic applications.