Giant Kohn Anomaly and Chiral Phonons in the Charge-Density Wave Phase of 1H-NbSe₂: Impact of Phonon Anticrossing

Despite extensive investigations, many aspects of charge-density waves (CDWs) remain elusive, especially the relative roles of electron–phonon coupling and Fermi surface nesting as the underlying driving mechanisms responsible for the emergence of CDW vector Q_CDW. It is puzzling that even though electrons interact strongly with optical phonons in many correlated systems, the actual mode softening is of an acoustic mode. Here, we consider monolayer 1H-NbSe₂ as an exemplar system, and through an accurate computation of the phonon self-energy, including its off-diagonal components, we provide compelling evidence that the relevant mode is a longitudinal optical phonon that softens by anticrossing several intervening phonon bands, i.e., a Kohn ladder that has been only observed previously in high-temperature superconductors. We also show that Q_CDW is fixed by the convolution of the susceptibility and electron–phonon coupling and that the softened phonons are circularly polarized.