Spectroscopic measurements of electronic excitations, such as absorption or loss spectroscopies, are routinely used to understand properties of materials. These spectra are
often dominated by interaction effects, and many features cannot be understood in a
picture of independent electrons. Traditionally, one associates plasmons, collective
oscillations of the sea of electrons, to metals, whereas the spectra of insulators often show
dominant excitonic effects, with bound electron-hole pairs whose signature consists in
sharp peaks that appear within the fundamental gap of electron addition and removal. This
talk will present a theoretical investigation of the homogeneous electron gas (HEG) at low
density. The HEG is the standard model for metals with perfect screening, and one would
not expect anything like strong electron-hole interaction effects. This intuition, however, is
based on a picture of macroscopic screening of classical charges, whereas here, one should
consider microscopic scales and quantum effects. This leads to exotic excitations that can
have excitonic character [1]. We will discuss possible experimental fingerprints and
consequences both for the three-dimensional and for the two-dimensional HEG.
[1] J. Koskelo, L. Reining, M. Gatti, Phys. Rev. Lett. 134, 046402 (2025)