Coincident onset of charge order and pseudogap in a homogeneous high-temperature superconductor

A key question in research on high-temperature superconductivity is the influence of chemical disorder, which is ubiquitous but exceedingly difficult to model. Using resonant X-ray scattering, we identified two-dimensional charge order in stoichiometric YBa₂Cu₄O₈ (T_c = 80 K), which is nearly free of chemical disorder. The charge order amplitude shows a concave temperature dependence and vanishes sharply at T* = 200 K, the onset of a prominent pseudogap previously determined by spectroscopy, suggesting a causal link between these phenomena. The gradual onset of charge order in other cuprates is thus likely attributable to an inhomogeneous distribution of charge ordering temperatures due to disorder induced by chemical substitution. The relationship between the pseudogap and the disorder-induced gradual freeze-out of charge carriers remains a central issue in research on high-T_c superconductors. We will also present Raman scattering data on phonon anomalies in YBa₂Cu₄O₈, as well as data on samples where disorder is generated in a controlled fashion by electron irradiation or chemical substitution.