Signatures of the excitonic insulator phase of monolayer WTe₂ from photoemission spectra

Evidence has recently been presented that, in the two-dimensional WTe₂ monolayer, electrons and holes bound by Coulomb attraction—excitons—spontaneously form at thermal equilibrium [1]. The ground state, which has been probed so far mostly by electrical transport measurements, is naturally interpreted within the paradigm of the long-sought-after ‘excitonic insulator’. Here, we report angle-resolved photoemission spectra of WTe₂ monolayers for different temperatures and doping levels, controlled by in-situ electrostatic gating, and compare them with spectral weight maps from a self-consistent many-body theory of the excitonic insulator. This theory, built on density functional theory calculations, takes into account both spin-orbit and inter-valley coupling. The measured and predicted spectra are consistent, signaling that improved experimental intensity should be sufficient to establish the quantitative correctness of the predicted spectral modifications due to the excitonic insulator formation.

[1] Sun et al., Nature Physics 18, 94 (2022).