Monolayer WTe₂ as an excitonic spin density wave

Recently we have demonstrated that that the two-dimensional bulk of monolayer WTe₂ contains electrons and holes bound by Coulomb attraction—excitons—that spontaneously form in thermal equilibrium [1]. The natural paradigm to interpret the ground state is the long-sought ‘excitonic insulator’ (EI), which breaks the pristine symmetry of the crystal though the Bose-Einstein condensation of excitons. Since lowest-energy excitons have finite momentum, one expects the EI to break the periodicity of the crystal, but no charge order has been observed at low temperature [1]. Here we predict that the EI is a spin density wave, on the basis of a full microscopic theory that builds on the ab-initio treatment of excitons [2]. Key to our claim is the strong spin-orbit interaction of WTe₂, which largely enhances the splitting between spin singlet and triplet excitons, and thus stabilizes the spin order. We further discuss the consistency of the computed band structure, photoemission spectra, and chemical potential of the EI with available experimental data. Finally, we propose paths to unveil the macroscopic quantum coherence possibly hidden in the ground state.