Collective charge excitations studied by electron energy-loss spectroscopy

TL;DR

This paper reviews the use of electron energy-loss spectroscopy (EELS) to study collective charge excitations in materials, highlighting current challenges, conflicting results in complex metals, and future technical improvements for better measurements.

Contribution

It provides a comprehensive overview of EELS techniques for measuring charge susceptibility, compares different geometries, and discusses future technological advancements.

Findings

Conflicting results in strongly interacting and strange metals.

EELS techniques are still developing despite long history.

Future improvements include advanced analyzers and STEM instruments.

Abstract

The dynamic charge susceptibility, $\chi(q,\omega)$, is a fundamental observable of all materials, in one, two, and three dimensions, quantifying the collective charge modes, the ability of a material to screen charge, as well as its electronic compressibility. Here, we review the current state of efforts to measure this quantity using inelastic electron scattering, which historically has been called electron energy-loss spectroscopy (EELS). We focus on comparison between transmission (T-EELS) and reflection (R-EELS) geometries as applied to a selection of 3D conductors. While a great deal is understood about simple metals, measurements of more strongly interacting and strange metals are currently contradictory, with different groups obtaining fundamentally conflicting results, emphasizing the importance of improved EELS measurements. Further, current opportunities for improvement in EELS techniques are vast, with the most promising future developments being in hemispherical and time-of-flight analyzers, as well as STEM instruments configured for high momentum resolution. We conclude that, despite more than half a century of work, EELS techniques are currently still in their infancy