Uncovering the underlying mechanism responsible for the rich phenomena in cuprate superconductors remains an important topic in physics. Over decades of research, extensive experimental efforts have revealed important information about the electronic structures, phonons, and magnetism in cuprates. Further information about collective charge dynamics can shed new light on microscopic behaviors of cuprates. Using resonant inelastic x-ray scattering (RIXS), we study collective charge dynamics in cuprates. In this presentation, I will discuss our observations of acoustic plasmon [1], the charge excitations associated with a coexisting charge order [2], and its paradoxical behaviors in the superconducting state [3], which could be explained by the proliferation of fluctuations near a quantum critical point.

[1] M. Hepting, L. Chaix, E. W. Huang, R. Fumagalli, Y. Y. Peng, B. Moritz, K. Kummer, N. B. Brookes, W. C. Lee, M. Hashimoto, T. Sarkar, J. F. He, C. R. Rotundu, Y. S. Lee, R. L. Greene, L. Braicovich, G. Ghiringhelli, Z. X. Shen, T. P. Devereaux, W. S. Lee, Three-dimensional collective charge excitations in electron-doped copper oxide superconductors, Nature 563, 374 (2018).
[2] L. Chaix, G. Ghiringhelli, Y. Y. Peng, M. Hashimoto, B. Moritz, K. Kummer, N. Brookes, Y. He, S. Chen, S. Ishida, Y. Yoshida, H. Eisaki, L. Braicovich, Z.-X. Shen, T. P. Devereaux, W.-S. Lee, Dispersive charge density wave excitations in underdoped cuprates Bi2Sr2CaCu2O8+d. Nature Physics 13, 952 (2017).
[3] W. S. Lee, K. J. Zhou, M. Hepting, J. Li, A. Nag, A. C. Walters, M. Garcia-Fernandez, H. Robarts, M. Hashimoto, H. Lu, B. Nosarzewski, D. Song, H. Eisaki, Z. X. Shen, B. Moritz, J. Zaanen, T. P. Devereaux, Spectroscopic Fingerprint of Charge Order Melting via Quantum Fluctuations in a Cuprate, Nature Physics (2020) https://doi.org/10.1038/s41567-020-0993-7 (advanced published online)

Keywords: RIXS, cuprates, plasmon, charge order, quantum critical point