The strange metal remains a major unsolved problem for modern condensed matter physics. Since the early development of marginal Fermi liquid phenomenology, it has been clear that progress requires detailed knowledge of the momentum- and frequency-dependent charge susceptibility $\chi (\mathbf{q},\omega )$, particularly at large momenta. Electron energy-loss spectroscopy (EELS), performed in either reflection or transmission geometry, provides the most direct probe of $\chi (\mathbf{q},\omega )$. However, measurements over the past four decades have yielded conflicting results, with authors of some studies reporting a dispersing random phase approximation (RPA)-like plasmon and others observing a strongly overdamped, incoherent response. Here, we report a transmission EELS study of ${\mathrm{Bi}}_2{\mathrm{Sr}}_2{\mathrm{CaCu}}_2{\mathrm{O}}_{8+x}$ (Bi-2212) that simultaneously achieves high-energy resolution ($\mathrm{\Delta }E\approx 30\mathrm{meV}$) and high-momentum resolution ($\mathrm{\Delta }q\approx 0.01{\AA }^{-1}$). To address issues of reproducibility, measurements were repeated 10 times on five different Bi-2212 flakes, benchmarked against aluminum, a well-characterized Fermi liquid, and quantitatively compared with those of prior studies spanning four decades. At momenta $q<0.15{\AA }^{-1}$, we observe a highly damped plasmon whose linewidth is comparable with its energy. At larger momenta $q>0.15{\AA }^{-1}$, this excitation does not disperse but instead evolves into an incoherent continuum, with no evidence for the RPA-like dispersion reported in some earlier works. Comparison with recent resonant inelastic x-ray scattering measurements on Bi-based cuprates supports the view that Bi-2212 is an incoherent metal with strongly damped charge excitations.

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• Charge dynamics
• Plasmons
• Strongly correlated systems
• Electron energy loss spectroscopy
• High-resolution electron energy loss spectroscopy