3:30–4:30 pm Maria Goeppert-Mayer Lecture Hall
Observation of Pines' Demon in Sr2RuO4 with Momentum-Resolved EELS *
Peter Abbamonte, University of Illinois, Urbana-Champaign
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 distinct type of plasmon, dubbed 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 phenomena such as “soundarons” in Weyl semimetals, phase transitions in mixed-valence materials, optical properties of metal nanoparticles, and high temperature superconductivity in, for example, 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 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, which undergoes a 14% renormalization upon cooling due to coupling to the particle-hole continuum. Our study confirms a 66-year old prediction and suggests that demons may be a pervasive feature of multiband metals.