Abstract
We develop a theory of the excitonic phase recently proposed as the zero-field insulating state observed near charge neutrality in monolayer . Using a Hartree-Fock approximation, we numerically identify two distinct gapped excitonic phases: a spin density wave state for weak nonzero interaction strength and spin spiral order at stronger interactions, separated by a narrow window of a nonexcitonic quantum spin Hall insulator. We introduce a simplified model capturing key features of the band structure, in which these phases appear as distinct valley ferromagnetic orders. We link the competition between the excitonic phases to the orbital structure of electronic wave functions at the Fermi surface and hence its proximity to the underlying gapped Dirac point in . We briefly discuss collective modes of the two excitonic states, and comment on implications for experiments.
- Received 18 March 2021
- Revised 1 September 2021
- Accepted 8 September 2021
DOI:https://doi.org/10.1103/PhysRevB.104.125133
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