Theory of competing excitonic orders in insulating WTe${}_2$ monolayers

Abstract

We develop a theory of the excitonic phase recently proposed as the zero-field insulating state observed near charge neutrality in monolayer WTe${}_2$. Using a Hartree-Fock approximation, we numerically identify two distinct gapped excitonic phases: a spin density wave state for weak non-zero interaction strength and spin spiral order at stronger interactions, separated by a narrow window of non-excitonic quantum spin Hall insulator. We introduce a simplified model capturing key features of the WTe${}_2$ 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 wavefunctions at the Fermi surface and hence its proximity to the underlying gapped Dirac point in WTe${}_2$. We briefly discuss collective modes of the two excitonic states, and comment on implications for experiments.

14 pages, 5 figures

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• A gate-tunable quantum phase transition in a topological excitonic insulator
• Breakdown of helical edge state topologically protected conductance in time-reversal-breaking excitonic insulators
• Charge-Neutral Electronic Excitations in Quantum Insulators
• Theory of Glide Symmetry Protected Helical Edge States in WTe${}_2$ Monolayer
• Twisted coupled wire model for a moiré sliding Luttinger liquid
• Low-energy effective theory and anomalous Hall effect in monolayer ${\mathrm{WTe}}_2$
• Artificial moiré engineering for an ideal BHZ model
• Excitonic and magnetic phases in doped WTe${}_2$ monolayers: a Hartree-Fock approach