# Crossover of Charge Fluctuations across the Strange Metal Phase Diagram

## TL;DR

This study investigates charge fluctuations in the strange metal phase of Bi-2212, revealing a persistent featureless continuum across the phase diagram and highlighting a crossover in spectral weight behavior at optimal doping.

## Contribution

It demonstrates that plasmons are absent in Bi-2212 and identifies a universal continuum feature in the strange metal phase, with a crossover in spectral weight behavior at optimal doping.

## Key findings

- Continuum persists throughout the strange metal region.
- Spectral weight increases in underdoped samples at low temperature.
- Spectral weight decreases in overdoped samples at low temperature.

## Abstract

A normal metal exhibits a valence plasmon, which is a sound wave in its conduction electron density. The mysterious strange metal is characterized by non-Boltzmann transport and violates most fundamental Fermi liquid scaling laws. A fundamental question is: Do strange metals have plasmons? Using momentum-resolved inelastic electron scattering (M-EELS) we recently showed that, rather than a plasmon, optimally-doped Bi$_{2.1}$Sr$_{1.9}$Ca$_{1.0}$Cu$_{2.0}$O$_{8+x}$ (Bi-2212) exhibits a featureless, temperature-independent continuum with a power-law form over most energy and momentum scales [M. Mitrano, PNAS 115, 5392-5396 (2018)]. Here, we show that this continuum is present throughout the fan-shaped, strange metal region of the phase diagram. Outside this region, dramatic changes in spectral weight are observed: In underdoped samples, spectral weight up to 0.5 eV is enhanced at low temperature, biasing the system towards a charge order instability. The situation is reversed in the overdoped case, where spectral weight is strongly suppressed at low temperature, increasing quasiparticle coherence in this regime. Optimal doping corresponds to the boundary between these two opposite behaviors at which the response is temperature-independent. Our study suggests that plasmons do not exist as well-defined excitations in Bi-2212, and that a featureless continuum is a defining property of the strange metal, which is connected to a peculiar crossover where the spectral weight change undergoes a sign reversal.