Is the insulating state of monolayer $\mathrm{WTe_2}$ an excitonic insulator?
In plain words
Monolayer $\mathrm{WTe_2}$ is insulating at charge neutrality and conducts along its edges. Experiments suggest the bulk gap comes from electron-hole pairing, but a gap from ordinary band mixing or disorder has not been ruled out.
Precise statement
Monolayer 1T'-WTe2 has a small bulk gap at charge neutrality that closes with gate-induced doping; 2022 experiments interpreted it as an interaction-driven excitonic insulator, possibly with spontaneous symmetry breaking. Determine whether the gap is an excitonic condensate (order parameter, symmetry breaking, mean-field or BKT-like temperature dependence, collective mode) or a single-particle hybridization gap enhanced by interactions. An answer is the identified gap origin with evidence for or against a broken symmetry.
What would settle it
Direct detection of the predicted broken symmetry (e.g. by optical second-harmonic generation or scanning tunneling spectroscopy of order-parameter domains) or of an excitonic collective mode.
Status in the literature
Unverified note: it names recent work and was partly written without a live literature search; it awaits a check by an editor.
2022 Nature Physics reports gave excitonic signatures; independent evidence of a broken symmetry had not appeared as of 2025.
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