Abstract
Spontaneous condensation of excitons is a long-sought phenomenon analogous to the condensation of Cooper pairs in a superconductor. It is expected to occur in a semiconductor at thermodynamic equilibrium if the binding energy of the excitons-electron (e) and hole (h) pairs interacting by Coulomb force-overcomes the band gap, giving rise to a new phase: the "excitonic insulator" (EI). Transition metal dichalcogenides are excellent candidates for the EI realization because of reduced Coulomb screening, and indeed a structural phase transition was observed in few-layer systems. However, previous work could not disentangle to which extent the origin of the transition was in the formation of bound excitons or in the softening of a phonon. Here we focus on bulk [Formula: see text] and demonstrate theoretically that at high pressure it is prone to the condensation of genuine excitons of finite momentum, whereas the phonon dispersion remains regular. Starting from first-principles many-body perturbation theory, we also predict that the self-consistent electronic charge density of the EI sustains an out-of-plane permanent electric dipole moment with an antiferroelectric texture in the layer plane: At the onset of the EI phase, those optical phonons that share the exciton momentum provide a unique Raman fingerprint for the EI formation. Finally, we identify such fingerprint in a Raman feature that was previously observed experimentally, thus providing direct spectroscopic confirmation of an ideal excitonic insulator phase in bulk [Formula: see text] above 30 GPa.
Highlights
Spontaneous condensation of excitons is a long-sought phenomenon analogous to the condensation of Cooper pairs in a superconductor
Building a selfconsistent effective-mass model on top of ab initio calculations, we show that the true ground state is an ideal, antiferroelectric excitonic insulator” (EI) with a distinctive Raman fingerprint that has already been observed [36]
With respect to the accurate band structure calculated within the GW approximation (Materials and Methods), density functional theory underestimates the gap of about 0.4 eV at P = 0
Summary
Spontaneous condensation of excitons is a long-sought phenomenon analogous to the condensation of Cooper pairs in a superconductor. Starting from first-principles many-body perturbation theory, we predict that the self-consistent electronic charge density of the EI sustains an out-of-plane permanent electric dipole moment with an antiferroelectric texture in the layer plane: At the onset of the EI phase, those optical phonons that share the exciton momentum provide a unique Raman fingerprint for the EI formation. The indirect character of excitons—in reciprocal [20, 21] and real [22, 23] space for TiSe2 and Ta2NiSe5, respectively—led to inherently weaker screening and stronger e-h attraction, potentially stabilizing the EI phase In those systems, the putative transition to the EI was accompanied by a lattice instability [24,25,26,27,28] when lowering the temperature—a singularity in the phonon density of states at vanishing energy—that in turn created e-h pairs through e-phonon interaction. In bulk MoS2, the pressure (P ) closes the indirect gap, G, between the top of the filled valence band, located at the center
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