Demons are charge neutral collective excitations in 3D metals that result from out-of phase electron density fluctuations between two or more bands. Demons where predicted to exist in multi-band metals by David Pines in 1956, but have never been observed to date. Because they are charge neutral, they do not couple to light and their spectrum is described by acoustic plasmons in 3D. We and collaborators at UIUC examined the momentum resolved energy loss electron spectroscopy (MEELS) in the Fermi liquid phase of Sr2RuO4. The analysis of the data based on sum rules indicates the emergence of an acoustic plasmon that is consistent with conventional linear response theory in a 3D Fermi liquid. This result would be the first experimental observation of demons. Read more:  Nature 621, 66 (2023). 

The nature of the strange metal phase in the cuprates is believed to be one of the key puzzles towards understanding high temperature superconductivity. The origin of this phase has been attributed in the past to the proximity of a quantum critical point, where metallic quasiparticles are either marginally defined or entirely absent. In a paper published in Proceedings of the National Academy of Sciences, Proc. Nat. Acad. Sci, 115, 5392 (2018) , we and experimental collaborators at the University of Illinois at Urbana-Champaign addressed the role of charge fluctuations in that enigmatic phase. The experimental results of measurements with momentum resolved inelastic electron scattering indicate the emergence of local scale invariance in the dynamical charge response of the system. This effect is manifested in the appearance of an enigmatic momentum independent energy scale below which an energy and momentum independent continuum of excitations emerges. The results indicate the emergence of possible “local criticality” in the strange metal phase.