The nature of the pseudogap and its relationship to charge order remain central questions in the physics of high- T_c superconducting cuprates, with progress largely hindered by the ubiquitous presence of disorder. In this talk, I will present Cu L₃-edge resonant x-ray scattering (RXS) results on the stoichiometric cuprate YBa₂Cu₄O₈ (Y124), a model system with minimal quenched disorder and exceptional electronic homogeneity.

We directly observe quasi-two-dimensional incommensurate charge order exhibiting a concave, order-parameter-like temperature dependence with a sharp onset at T* ≈ 200 K, coincident with the pseudogap temperature determined by independent probes [1]. This finding supports a scenario in which, in the clean limit, the pseudogap emerges via a genuine thermodynamic phase transition associated with charge ordering. In contrast, the widely observed gradual onset in all other cuprate families, including YBa₂Cu₃O_6+x (Y123), likely arises from disorder-induced spatial inhomogeneity, which smears the transition into a crossover.

Further insights come from preliminary RXS experiments on Y124 samples subjected to controlled electron irradiation, introducing tunable disorder [2]. With increasing irradiation dose, T_c decreases and the sharp charge-order transition evolves into a broadened, gradual temperature dependence below 160K closely resembling that of less homogenous Y123 and suggesting that disorder is acting as the key control parameter. Interestingly, the pseudogap onset and its temperature evolution, as probed by Raman spectroscopy in the irradiated samples, appear largely unaffected.

These results thus provide a unified framework to reinterpret a large body of experimental data across different cuprate families. Overall, our findings establish and clarify the link between charge order and the pseudogap, highlighting the key role of disorder in cuprate physics and positioning Y124 as clean benchmark for future experimental and theoretical investigations.

[1] D. Betto, et al. Nature Communications 16, 3579 (2025).

[2] Y. Liu, M. Minola, et al. in preparation