Universal Waterfall Feature in Cuprate Superconductors: Evidence of a Momentum-Driven Crossover

We study two related universal anomalies of the spectral function of cuprates, so-called waterfall and high-energy kink features, by a combined cellular dynamical mean-field theory and angle-resolved photoemission study for the oxychloride NaxCa2-xCuO2Cl2 (Na-CCOC). Tracing their origin back to an interplay of spin-polaron and local-correlation effects both in undoped and hole-doped (Na-)CCOC, we establish them as a universal crossover between regions differing in the momentum dependence of the coupling and not necessarily in the related quasiparticles' energies. The proposed scenario extends to doping levels coinciding with the cuprate's superconducting dome and motivates further investigations of the fate of spin polarons in the superconducting phase.