Momentum garbling, plasmons and the strange metal continuum in Bi_2.1Sr_1.9CaCu₂O_8+x

All the physical properties of strange metals can be unified under the marginal Fermi liquid (MFL) phenomenology, which conjectures that the itinerant electrons are coupled to a continuum of charge fluctuations, of unknown origin. Recently [1,2], we reported momentum-resolved electron energy-loss spectroscopy (M-EELS) measurements from the strange metal Bi_2.1Sr_1.9CaCu₂O_8+x (Bi-2212,T_c=91K) demonstrating that such a continuum exists. However, these measurements *seem* to contradict IR ellipsometry studies showing that this material exhibits a conventional plasmon at ω_p ~ 1 eV. Here, we use Jain and Allen’s model of a layered conductor [3] to make a quantitative comparison between IR data and M-EELS measurements at small momentum, q. We show that the M-EELS data are quantitatively consistent with IR for q<0.02 r.l.u., indicating that both probes see the same plasmon in the small q limit. For q>0.02 r.l.u., we find that this plasmon damps and evolves into an MFL-like continuum. We show that the large q data may be explained by postulating some hidden, elastic multiple scattering mechanism that garbles the momentum of the probe electron.

[1] Mitrano, M., et al. "Anomalous density fluctuations in a strange metal." Proceedings of the National Academy of Sciences 115.21 (2018): 5392-5396.

[2] Husain, Ali A., et al. "Crossover of charge fluctuations across the strange metal phase diagram." Physical Review X 9.4 (2019): 041062.

[3] Jain, Jainendra K., and Philip B. Allen. "Dielectric response of a semi-infinite layered electron gas and Raman scattering from its bulk and surface plasmons." Physical Review B 32.2 (1985): 997.