The characteristic excitation of a metal is its plasmon, which is a quantized collective oscillation of its electron density. In 1965, David Pines predicted that a distinct type of plasmon, which he called a "demon," could exist in multiband metals containing more than one species of charge carrier. Consisting of out-of-phase movement of electrons in different bands, demons are acoustic, electrically neutral, and do not couple to light, so have never been detected in an equilibrium metal. Nevertheless, demons are believed to be responsible for a wide range of quantum phenomena in condensed matter, including phase transitions in mixed-valence solids, “soundarons” in Weyl semimetals, optical properties of metal nanoparticles, and high temperature superconductivity in metal hydrides.

In this talk, I will present evidence for a demon in the multiband metal, Sr₂RuO₄, from momentum-resolved electron energy-loss spectroscopy (M-EELS). Formed of electrons in the ß and ? bands, the demon is gapless with critical momentum q_c = 0.08 reciprocal lattice units and room temperature velocity v = 1.065(120)×105 m/s, its self-energy being renormalized by coupling to the particle-hole continuum. I will also show that demons intrinsically violate low-energy sum rules, meaning they are always slaved to a conventional plasmon at high energy. Our study confirms a 66-year old prediction and suggests that demons may be a pervasive feature of multiband metals.