Collective density fluctuations of strange metals with critical Fermi surfaces

TL;DR

This paper investigates collective density fluctuations in strange metals with critical Fermi surfaces, revealing potential explanations for experimental anomalies beyond traditional Fermi liquid theory.

Contribution

It provides a theoretical analysis of low-energy collective modes in solvable non-Fermi liquid models with critical Fermi surfaces, connecting to recent experimental findings.

Findings

Certain models exhibit long-lived zero-sound modes consistent with experiments

Decay of collective modes into particle-hole continua matches observed power-laws

Some models explain experimental features beyond Fermi liquid predictions

Abstract

Recent spectroscopic measurements in a number of strongly correlated metals that exhibit non-Fermi liquid like properties have observed evidence of anomalous frequency and momentum-dependent charge-density fluctuations. Specifically, in the strange metallic regime of the cuprate superconductors, there is a featureless particle-hole continuum exhibiting unusual power-laws, and experiments suggest that the plasmon mode decays into this continuum in a manner that is distinct from the expectations of conventional Fermi liquid theory. Inspired by these new experimental developments, we address the nature of low-energy collective modes and the particle-hole continua for different "solvable" lattice models of non-Fermi liquids that host a critical Fermi surface -- a sharp electronic Fermi surface without any low-energy electronic quasiparticles. We scrutinize theoretically the possible existence of a long-lived zero-sound mode, which is renormalized to the plasma frequency in the presence of long-ranged coulomb interactions, and its decay into the continuum over a wide range of frequencies and momenta. Quite remarkably, some of the models analyzed here can account for certain aspects of the universal experimental observations, that clearly lie beyond the purview of standard Fermi liquid theory.