AUST-US Colloquium. Observation of Pines’ Demon in Sr2RuO4 with Momentum-Resolved EELS

Prof. Peter Abbamonte, Fox Family Professor of Engineering

Department of Physics and Materials Research Laboratory, University of Illinois

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The characteristic excitation of a metal is its plasmon, which is a quantized sound wave in its valence electron density. In 1965, David Pines predicted that a distinct type of plasmon, which he named a “demon,” could exist in multiband metals that contain more than one species of charge carrier. Unlike conventional plasmons, demons are acoustic excitations, meaning they are “massless,” i.e., their energy tends toward zero as the momentum q ® 0. So demons may play a central role in the low-energy physics of multiband metals. However, demons are neutral excitations that do not couple to light, so they have never been observed experimentally, at least in a 3D material.

In this talk I will present the discovery of a demon in the multiband metal Sr₂RuO₄. 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(120)×10⁵ m/s. This study confirms a 67-year old prediction and suggests that demons may be a widespread feature of multiband metals.

Prof Abbamonte’s group is dedicated to the study of elementary collective phenomena in condensed matter. These include electron-self organization in strongly correlated electron systems (viz. “stripes”) and other forms of topological order, phase transitions in materials exhibiting quantum criticality, edge and interface states in artificially structured oxides.

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