Collective Charge Excitations Studied by Electron Energy-Loss Spectroscopy

The dynamic charge susceptibility, χ(q, ω), is a fundamental observable of all materials, in one, two, and three dimensions, quantifying the collective charge modes and the ability of a material to screen charge, as well as its electronic compressibility. Here, we review the current state of efforts to measure the charge susceptibility of quantum materials 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 three-dimensional and quasi-two-dimensional conductors. Although a great deal is understood about simple metals, measurements of more strongly interacting and strange metals are currently conflicting, with different groups obtaining fundamentally contradictory results, emphasizing the importance of improved EELS measurements. Furthermore, current opportunities for improvement in EELS techniques are vast, with the most promising future development being in hemispherical and time-of-flight analyzers, as well as scanning transmission electron microscope 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.