Pines’ demon observed as a 3D acoustic plasmon in Sr2RuO4

Ali A. Husain (), Edwin W. Huang, Matteo Mitrano, Melinda S. Rak, Samantha I. Rubeck, Xuefei Guo, Hongbin Yang, Chanchal Sow, Yoshiteru Maeno, Bruno Uchoa, Tai C. Chiang, Philip E. Batson, Philip W. Phillips and Peter Abbamonte ()
Additional contact information
Ali A. Husain: University of Illinois
Edwin W. Huang: University of Illinois
Matteo Mitrano: Harvard University
Melinda S. Rak: University of Illinois
Samantha I. Rubeck: University of Illinois
Xuefei Guo: University of Illinois
Hongbin Yang: Rutgers University
Chanchal Sow: Kyoto University
Yoshiteru Maeno: Kyoto University
Bruno Uchoa: University of Oklahoma
Tai C. Chiang: University of Illinois
Philip E. Batson: Rutgers University
Philip W. Phillips: University of Illinois
Peter Abbamonte: University of Illinois

Nature, 2023, vol. 621, issue 7977, 66-70

Abstract: Abstract 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 (3D) metals containing more than one species of charge carrier1. 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, 3D metal. Nevertheless, demons are believed to be critical for diverse phenomena including phase transitions in mixed-valence semimetals2, optical properties of metal nanoparticles3, soundarons in Weyl semimetals4 and high-temperature superconductivity in, for example, metal hydrides3,5–7. Here, we present evidence for a demon in Sr2RuO4 from momentum-resolved electron energy-loss spectroscopy. 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 ± 0.12) × 105 m s−1 that undergoes a 31% renormalization upon cooling to 30 K because of coupling to the particle–hole continuum. The momentum dependence of the intensity of the demon confirms its neutral character. Our study confirms a 67-year old prediction and indicates that demons may be a pervasive feature of multiband metals.

Date: 2023
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DOI: 10.1038/s41586-023-06318-8

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