# Density response and collective modes of semi-holographic non-Fermi liquids

## TL;DR

This paper investigates the density response and collective excitations in semi-holographic non-Fermi liquids, revealing a stable plasmon mode, modified particle-hole continuum, and suppressed Friedel oscillations, advancing understanding of non-Fermi liquid dynamics.

## Contribution

It provides the first detailed analysis of density response and collective modes in semi-holographic non-Fermi liquids, highlighting unique features like a well-defined plasmon and modified screening effects.

## Key findings

- Presence of a stable, linearly dispersing plasmon at large momenta.
- Blurred particle-hole continuum with an inner core retaining Fermi liquid features.
- Deep attractive regions in the dynamic screening potential at high frequencies.

## Abstract

Semi-holographic models of non-Fermi liquids have been shown to have generically stable generalised quasi-particles on the Fermi surface. Although these excitations are broad and exhibit particle-hole asymmetry, they were argued to be stable from interactions at the Fermi surface. In this work, we use this observation to compute the density response and collective behaviour in these systems. Compared to the Fermi liquid case, we find that the boundaries of the particle-hole continuum are blurred by incoherent contributions. However, there is a region inside this continuum, that we call inner core, within which salient features of the Fermi liquid case are preserved. A particularly striking prediction of our work is that these systems support a plasmonic collective excitation which is well-defined at large momenta, has an approximately linear dispersion relation and is located in the low-energy tail of the particle-hole continuum. Furthermore, the dynamic screening potential shows deep attractive regions as a function of the distance at higher frequencies which might lead to long-lived pair formation depending on the behaviour of the pair susceptibility. We also find that Friedel oscillations are present in these systems but are highly suppressed.