Observation of Pines' Demon in Sr$_2$RuO$_4$The characteristic excitation of a metal is its plasmon, which is a quantizedcollective oscillation of its electron density. In 1956, David Pines predictedthat a distinct type of plasmon, dubbed a "demon," could exist inthree-dimensional metals containing more than one species of charge carrier.Consisting of out-of-phase movement of electrons in different bands, demons areacoustic, electrically neutral, and do not couple to light, so have never beendetected in an equilibrium, three-dimensional metal. Nevertheless, demons arebelieved to be critical for diverse phenomena including phase transitions inmixed-valence semimetals, optical properties of metal nanoparticles,"soundarons" in Weyl semimetals, and high temperature superconductivity in, forexample, metal hydrides. Here, we present evidence for a demon in Sr$_2$RuO$_4$from momentum-resolved electron energy-loss spectroscopy (M-EELS). Formed ofelectrons in the $β$ and $γ$ bands, the demon is gapless with a roomtemperature velocity $v=1.065 \pm 0.12 \times 10^5$ m/s and critical momentum$q_c=0.08$ reciprocal lattice units. Its spectral weight violates low-energypartial sum rules, affirming its neutral character. Our study confirms a66-year old prediction and suggests that demons may be a pervasive feature ofmultiband metals.arxiv.org

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