measured with momentum-resolved EELS (M-EELS)*

Measurements of the charge susceptibility χ(q,ω) of the high-T_c cuprate Bi-2212 using Momentum-resolved inelastic electron scattering (M-EELS) have shown that bad-metallic behavior can be characterized by a continuum of non-propagating density fluctuations, in stark contrast to the plasmons of simple metals [1]. Here, we report on M-EELS measurements of Sr₂RuO₄, a bad metal which crosses over to a Fermi Liquid at T=40K. Like Bi-2212, we find that the bad metal phase of Sr₂RuO₄ exhibits a continuum of density fluctuations. However, below 40K, spectral weight is redistributed to form a plasmon-like excitation around 1.4 eV. Surprisingly, we also discover a coherent low-energy (sub-80meV) 2D acoustic plasmon in the bad metal phase, whose velocity is strongly renormalized from 0.7 to 0.4eV Å below the Fermi liquid crossover, indicating a coupled 2D Fermi Liquid and 3D bad metal in a single solid. We will contrast this coupled system to a closely related ruthenate, the Mott-insulator Ca₂RuO₄.

[1] M. Mitrano, A. A. Husain, et al. PNAS 115 (21) 5392-5396 (2018)