Orateur
Description
Nature hosts both massless particles with linear dispersions and massive particles whose energies scale quadratically with momentum and exhibit finite gaps. In condensed matter systems, analogous behaviour appears in the form of collective modes, i.e., measurable excitations with well-defined energy–momentum relations. A long-discussed third possibility is that of a superluminal tachyon, characterised by ill-defined energies at small momentum. In hydrodynamic descriptions of matter, an analogous excitation has been predicted: a mode with a purely imaginary energy at low momentum, reflecting a finite lifetime rather than a propagating excitation. Despite its theoretical anticipation, clear evidence for such a dispersion has remained elusive. By systematically bridging hydrodynamic theory and microscopic models of metallic systems, we show that this unconventional dispersion naturally emerges in correlated quantum matter when momentum relaxation is present, for instance due to impurity-induced breaking of electronic translational symmetry. As a concrete realisation, we analyse the recently reported acoustic plasmon — known as Pines’ demon — in Sr$_2$RuO$_4$. Its experimentally observed dispersion deviates markedly from the massless linear behaviour expected within the random phase approximation. We argue that this deviation is consistent with the first experimental manifestation of a momentum-gapped collective mode.
