Giant Kohn Anomaly and Chiral Phonons in the Charge-Density Wave Phase of 1H-NbSe 2 : 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 QCDW. 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 QCDW is fixed by the convolution of the susceptibility and electron-phonon coupling and that the softened phonons are circularly polarized.