Hidden and mirage collective modes in two dimensional Fermi liquids

This paper challenges the conventional understanding of zero sound (ZS) modes in Fermi liquids, proposing the existence of "hidden" and "mirage" ZS modes in two-dimensional (2D) Fermi liquids. A hidden mode, present even with attractive interactions, doesn't produce a sharp peak in susceptibility but influences long-time transient responses, detectable via pump-probe experiments. A mirage mode, arising from strong repulsion, appears as a peak in susceptibility but isn't a true pole, exhibiting a width exceeding the single-particle scattering rate. These unconventional modes stem from the algebraic non-analyticity of the 2D susceptibility, leading to distinct behavior in time-domain responses.

Avraham Klein, Dmitrii L. Maslov, Andrey V. Chubukov