Giant Kohn anomaly and chiral phonons in the charge density wave phase of 1H-NbSe$_2$

Susy Exists; Sougata Mardanya; Robert Markiewicz; Tugrul Hakioglu; Jouko Nieminen; Ville J. Härkönen; Cem Sanga; Arun Bansil; Sugata Chowdhury

doi:10.1021/acsnano.5c11110

arXiv · 2501.04821

Giant Kohn anomaly and chiral phonons in the charge density wave phase of 1H-NbSe$_2$

Susy Exists·Sougata Mardanya·Robert Markiewicz·Tugrul Hakioglu·Jouko Nieminen·Ville J. Härkönen·Cem Sanga·Arun Bansil·Sugata Chowdhury

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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 the 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-NbSe$_2$ 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 anti-crossing several intervening phonon bands, i.e. a Kohn ladder which 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.

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Susy Exists, Sougata Mardanya, Robert Markiewicz, Tugrul Hakioglu, Jouko Nieminen, Ville J. Härkönen, Cem Sanga, Arun Bansil, Sugata Chowdhury. 2025-01-08. Giant Kohn anomaly and chiral phonons in the charge density wave phase of 1H-NbSe$_2$. https://doi.org/10.1021/acsnano.5c11110

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