Emergence of charge order and opening of pseudogap are two important phenomena in the normal state of underdoped cuprates, and their relations to superconductivity remain unclear. A key open question is the influence of chemical disorder, which is almost inevitable in most doped cuprates yet exceedingly difficult to capture in theoretical models.

In this poster, I will introduce our recent studies on a homogeneous cuprate superconductor YBa₂Cu₄O₈ (Y124, T_C = 81 K), which is stoichiometric and nearly free of chemical disorder. Using resonant inelastic X-ray scattering (RIXS), we identified two-dimensional charge orders in Y124, whose amplitude exhibits a sharp second-order phase transition at much higher temperature T_CDW = 200 K and coincides with its pseudogap temperature T* [Nat Commun 16, 3579 (2025)].

Next, we utilized high-energy electron irradiation to controllably introduce disorder to the system without altering its hole doping level. After 5.6 C/cm² dose of irradiation, its superconducting transition temperature T_C is suppressed from 81 K to roughly 66 K. Our preliminary resonant X-ray scattering (RXS) data suggest that the CDW peaks in irradiated Y124 appear at momenta q_CDW = (0, ±0.32, L) and seems to onset gradually around 120~160 K, resembling that of Y123 and other cuprates but distinctly different from the sharp transition observed in pristine Y124. In addition, electronic Raman spectroscopy revealed that the pseudogap temperature T* is little affected by irradiation. Combining the X-ray and Raman data, we hope to provide new insight into the relation between pseudogap and charge order in underdoped cuprates, and to clarify the role of disorder in their formation.