under pressure

Among correlated insulators, a fascinating paradigm applies to narrow-gap semiconductors, whose low-energy excitations are pairs of electron (e) and hole (h) bound by Coulomb attraction, the excitons. If the binding energy overcomes the gap, then excitons spontaneously condense at thermodynamic equilibrium—similarly to Cooper pairs in a superconductor—giving rise to the ‘excitonic insulator’ (EI). Crucially, breakthrough reports [1] of the EI in transition metal dichalcogenides could not assess at which extent the condensation of eh pairs was due to the formation of bound excitons or to the softening of the phonon responsible of the observed structural change. Here, by means of many-body perturbation theory from first principles, we demonstrate that MoS₂ at high pressure is prone to the condensation of genuine excitons of finite momentum, whereas the phonon dispersion remains regular. The self-consistent electronic charge density of the EI sustains an out-of-plane permanent electric dipole moment with an in-plane anti-ferroelectric texture. At the onset of the EI phase, those optical phonons that share the exciton momentum are folded to the zone center, providing a unique Raman fingerprint that has been observed but not explained yet.

[1] Kogar et al., Science 358, 1314 (2017).