Plasmon like behavior of optimally doped Bi2212 at small in-plane momentum measured with M-EELS*

Optimally doped Bi_2.1Sr_1.9CaCu₂O_8+x (Bi-2212,T_c=91K) is a so-called “strange metal” at 300K. Using momentum-resolved electron energy-loss spectroscopy (M-EELS) measurements, we recently showed that, while Bi-2212 exhibits a well-defined plasmon spectrum at moderate momentum (q~0.1 r.l.u.), at large q (>0.2 r.l.u.) it exhibits a featureless, frequency-independent continuum^[1]. Here we examine the M-EELS data from Bi-2212 at much smaller momentum, 0.01 < q < 0.1 r.l.u. ^[2]. In this regime the scattering matrix elements are extremely energy-dependent, making interpretation of the spectra difficult. Using Jain and Allen’s model of a layered conductor^[3], accounting for the finite q resolution of the measurement, we show that the M-EELS data are consistent with a 1 eV plasmon over this entire q range. These results are qualitatively consistent with previous ellipsometry and transmission EELS measurements on this material.

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

[2]Husain, Ali Abdullah. Charge fluctuations of the strange metal in space and time. Diss. University of Illinois at Urbana-Champaign, 2020.

[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.