Pines' demon

In condensed matter physics, Pines' demon or, simply demon is a collective excitation of electrons which corresponds to electrons in different energy bands moving out of phase with each other. Equivalently, a demon corresponds to counter-propagating currents of electrons from different bands. Named after David Pines, who coined the term in 1956, demons are quantum mechanical excited states of a material belonging to a broader class of exotic collective excitations, such as the magnon, phason, or exciton. Pines' demon was first experimentally observed in 2023 by A. A. Husain et al. within the transition-metal oxide distrontium ruthenate (Sr₂RuO₄).