Equivalently, a demon corresponds to counter-propagating currents of electrons from different bands.
Named after David Pines, who coined the term in 1956,[1] 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.
"[1] Pines explained his terminology by making the term a half backronym because particles commonly have suffix "-on" and the excitation involved distinct electron motion, resulting in D.E.M.on, or simply demon for short.
A. Husain et al.[2] in the unconventional superconducting material Sr2RuO4 using a momentum-resolved variant of high-resolution electron energy-loss spectroscopy.
The plasmon is a quantized vibration of the charge density in a material where all electron bands move in-phase.
Plasmons exist in all conducting materials and play a dominant role in shaping the dielectric function of a metal at optical frequencies.
Early studies of the demon in the context of superconductivity[5] showed, under the two band picture presented by Pines, that superconducting pairing of the light electron band can be enhanced through the existence of demons, while the pairing of the heavy electrons would be more or less unaffected.
However, for the simple case of spherically symmetric metals with two bands, natural realizations of demon-enhanced superconductivity seemed unlikely, as the heavy (d-)electrons play the dominant role in superconductivity of most transition metal considered at the time.
However, more recent studies on high-temperature superconducting metal hydrides, where light electron bands participate in superconductivity, suggest demons may be playing an active role in such systems.