Spin Demons in d-Wave Altermagnets

Demons are a type of plasmons, which consist of out-of-phase oscillations of electrons in different bands. Here, we show that d-wave altermagnets, a recently discovered class of collinear magnetism, naturally realize a spin demon, which consists of out-of-phase movement of the two spin species. The spin demon lives outside of the particle-hole continuum of one of the spin species, and is therefore significantly underdamped, reaching quality factors of >10. We show that the spin demon carries a magnetic moment, which inherits the d-wave symmetry. Finally, we consider both three- and two-dimensional d-wave altermagnets, and show that spin demons exist in both.