Excitonic Instability in Ta₂Pd₃Te₅ Monolayer

By systematic theoretical calculations, we reveal an excitonic insulator (EI) in the Ta 2 Pd 3 Te 5 monolayer. The bulk Ta 2 Pd 3 Te 5 is a van der Waals (vdW) layered compound, whereas the vdW layer can be obtained through exfoliation or molecular-beam epitaxy. First-principles calculations show that the monolayer is a nearly zero-gap semiconductor with the modified Becke–Johnson functional. Due to the same symmetry of the band-edge states, the two-dimensional polarization α 2D would be finite as the band gap goes to zero, allowing for an EI state in the compound. Using the first-principles many-body perturbation theory, the GW plus Bethe–Salpeter equation calculation reveals that the exciton binding energy is larger than the single-particle band gap, indicating the excitonic instability. The computed phonon spectrum suggests that the monolayer is dynamically stable without lattice distortion. Our findings suggest that the Ta 2 Pd 3 Te 5 monolayer is an excitonic insulator without structural distortion.