Universality of momentum-dependent charge-transfer excitations in the undoped cuprates measured by resonant inelastic X-ray scattering

From its conception, the study of charge-transfer excitations in the cuprates has driven the majority of resonant inelastic X-ray scattering (RIXS) research. Still, to this day, there exists more variation among published RIXS spectra and their interpretation than common ground. By carefully studying the momentum-dependent spectral weight of three of the most important undoped cuprates -- La$_2$CuO$_4$, Nd$_2$CuO$_4$, and Sr$_2$CuO$_2$Cl$_2$ -- we present evidence of universality of the observed charge-transfer excitations. To clarify and highlight this observation, we construct dispersion diagrams and study the incident energy dependence. Comparison to Hubbard model predictions suggests that the essence of this universality is captured already by one-band physics, while material dependence and cross-sectional effects only obfuscate this behavior.