Zoom registration: https://harvard.zoom.us/webinar/register/WN_q8rI_YJWS2GiLNyxVADNKA

The relationship between the charge order (CO) and the superconductivity (SC) is an intriguing question in unconventional superconductors, which has motived extensive experimental and theoretical investigations. Recently, resonant inelastic x-ray (RIXS) has emerged to be a powerful tool to study the CO phenomena in quantum materials, which can simultaneously measure CO order parameter and its influence in the inelastic channels. I will highlight our recent RIXS studies on the CO phenomena in double-layered cuprates Bi₂Sr₂CaCu₂O_8+d via high-resolution resonant inelastic x-ray scattering (RIXS). Signatures of CO excitations can be discerned in the RIXS phonon cross-section [1], providing a new perspective for studying the CO phenomena. Interestingly, near optimally doping, we observed anomalous enhancements of the CO excitations deep inside the superconducting state, despite that the CO order parameter suppresses [2].  Tracking this anomalous temperature dependence throughout the phase diagram, we identify a characteristic doping concentration of the CO phenomena [3], whose implications on putative quantum critical points will be discussed. If time permits, I will also talk about our recent discovery of a CO in infinite-layer nickelate superconductors [4] and discuss the similarities and differences to the CO in cuprates.

[1] 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 Bi₂Sr₂CaCu₂O_8+d. Nature Physics 13, 952 (2017).

[2] 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 17, 659 (2021).

[3] H. Lu, M. Hashimoto, S. Chen, S. Ishida, D. Song, H. Eisaki, A. Nag, M Garcia-Fernandez, R. Arpaia, G. Ghiringhelli, L. Braicovich, J. Zaanen, B. Moritz, K. Kummer, N. B. Brookes, K.-J. Zhou, Z.-X. Shen, T. P. Devereaux, and W.-S. Lee, Identification of a Characteristic Doping for Charge Order phenomena in Bi-2212 cuprates via RIXS, Phys. Rev. B 106, 155109 (2022).

[4] M. Rossi, M. Osada, J. Choi, S. Agrestini, D. Jost, Y. Lee, H. Lu, B. Y. Wang, K. Lee, A. Nag, Y. D. Chuang, C. T. Kuo, S. J. Lee, B. Moritz, T. P. Devereaux, Z.X. Shen, J.S. Lee, K.-J. Zhou, H. Y. Hwang and W.S. Lee, A Broken Translational Symmetry State in an Infinite-Layer Nickelate, Nature Physics 18, 869 (2022).