Coexisting Strange Metal and Fermi Liquid Collective Excitations in Sr₂RuO₄

The strange metal is an enigmatic phase found in numerous strongly correlated systems distinguished by violations of Fermi liquid and Boltzmann transport theories. Using momentum-resolved EELS (M-EELS), we previously showed that, while charge excitations of a Fermi liquid are known to propagate as plasmons with well-defined energy and momentum, the strange metal Bi_2.1Sr_1.9CaCu₂O_8+x (Bi-2212) exhibits a featureless continuum of non-propagating charge fluctuations exhibiting simple scaling laws. Here, we present M-EELS results on Sr₂RuO₄ showing that it exhibits both a strange metal continuum and a well-defined low-energy collective mode below 100 meV propagating at near the Fermi velocity. Strikingly, the dispersion of this mode is strongly renormalized at low temperature where transport measurements indicate the onset of Fermi liquid behavior. These results suggest that Sr₂RuO₄ exhibits a crossover between Fermi liquid-like behavior at low energy and strange metal behavior at high energy, with the two sectors in coexistence. We also present new transmission M-EELS experiments validating the bulk origin of these observations.