Giant Kohn Anomaly and Chiral Phonons in the Charge-Density Wave Phase of 1H-NbSe<sub>2</sub>: Impact of Phonon Anticrossing.
basic_science · Level V
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- Also identified by DOI 10.1021/acsnano.5c11110.
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Abstract
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 <b><i>Q</i></b><sub>CDW</sub>. 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-NbSe<sub>2</sub> 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 <b><i>Q</i></b><sub>CDW</sub> is fixed by the convolution of the susceptibility and electron-phonon coupling and that the softened phonons are circularly polarized.