Anomalous density fluctuations in a strange metal

TL;DR

This study measures the dynamic charge response of a cuprate strange metal, revealing a featureless continuum with power-law behavior, indicating highly localized excitations and decoupled space-time dynamics.

Contribution

First direct measurement of the charge response function in a cuprate strange metal using momentum-resolved inelastic electron scattering.

Findings

Identified a temperature- and momentum-independent continuum in the strange metal regime.

Observed a power-law behavior in charge fluctuations, suggesting highly localized excitations.

Detected a gap-like feature in overdoped samples, indicating deviation from strange metal behavior.

Abstract

A central mystery in high temperature superconductivity is the origin of the so-called "strange metal," i.e., the anomalous conductor from which superconductivity emerges at low temperature. Measuring the dynamic charge response of the copper-oxides, $\chi''(q,\omega)$, would directly reveal the collective properties of the strange metal, but it has never been possible to measure this quantity with meV resolution. Here, we present the first measurement of $\chi''(q,\omega)$ for a cuprate, optimally doped Bi$_2$Sr$_2$CaCu$_2$O$_{8+x}$ ($T_c=91$ K), using momentum-resolved inelastic electron scattering. In the medium energy range 0.1-2 eV relevant to the strange metal, the spectra are dominated by a featureless, temperature- and momentum-independent continuum persisting to the eV energy scale. This continuum displays a simple power law form, exhibiting $q^2$ behavior at low energy and $q^2/\omega^2$ behavior at high energy. Measurements of an overdoped crystal ($T_c=50$ K) showed the emergence of a gap-like feature at low temperature, indicating deviation from power law form outside the strange metal regime. Our study suggests the strange metal exhibits a new type of charge dynamics in which excitations are local to such a degree that space and time axes are decoupled.