Observation of Pines' Demon in Sr₂RuO₄

The characteristic excitation of a metal is its plasmon, which is a quantized collective oscillation of its electron density. In 1956, David Pines predicted that a distinct type of plasmon, dubbed a "demon," could exist in three-dimensional 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, three-dimensional metal. Nevertheless, demons are believed to be critical for diverse phenomena including phase transitions in mixed-valence semimetals, optical properties of metal nanoparticles, soundarons in Weyl semimetals, and high temperature superconductivity in, for example, metal hydrides. Here, we present evidence for a demon in Sr₂RuO₄ from momentum-resolved electron energy-loss spectroscopy (M-EELS). Formed of electrons in the β and γ bands, the demon is gapless with critical momentum q_c = 0.08 reciprocal lattice units and room temperature velocity v = (1.065 ± 0.12) ×10⁵ m/s, which undergoes a 14% renormalization upon cooling to 20 K due to coupling to the particle-hole continuum. The momentum dependence of the intensity of the demon confirms its neutral character. Our study confirms a 66-year old prediction and suggests that demons may be a pervasive feature of multiband metals.