Abstract

The characteristic excitation of a metal is its plasmon, which is a quantized sound wave in its valence electron density. In 1965, David Pines predicted that a distinct type of plasmon, which he named a “demon,” could exist in multiband metals that contain more than one species of charge carrier. Consisting of electrons in two different bands beating out-of-phase, demons are acoustic excitations, meaning they are “massless,” meaning their energy tends toward zero as the momentum q → 0. Demons may therefore play a central role in the low-energy physics of multiband metals. However, demons are neutral excitations that do not couple to light, so they have never been observed experimentally, at least in an equilibrium, 3D material.

In this talk I will present the observation of a demon in the multiband metal Sr2RuO4. Formed of electrons in the β and γ bands, the demon is gapless with critical momentum qc = 0.08 reciprocal lattice units and room temperature velocity v = 1.065(120)×105 m/s. This study confirms a 67-year old prediction and suggests that demons may be a widespread feature of multiband metals.

*A. A. Husain, et al., Nature 621, 66 (2023)