Collective excitations provide direct information about the interactions that stabilize ordered phases in correlated materials. In cuprate superconductors, spin excitations are well characterized, whereas dispersive charge excitations associated with stripe order have remained difficult to resolve. Their weak spectral weight overlaps with elastic scattering and phonons, obscuring their dispersion and characteristic energy scale. Quantifying these properties is essential for identifying the interactions underlying charge order and assessing how charge dynamics relate to high-temperature superconductivity.

In this work, we used high-resolution oxygen K-edge resonant inelastic X-ray scattering (RIXS) at the I21 beamline of Diamond Light Source to investigate low-energy excitations in stripe-ordered La_1.675Eu_0.2Sr_0.125CuO₄. By calculating the momentum-dependent RIXS intensities of the relevant phonon modes, we separated the phonon and charge contributions and extracted the charge-excitation spectral weight over a broad energy range. A complementary temperature-difference analysis provided model-independent evidence for the non-phonon origin of the dispersive signal.

The charge excitation emerges from the charge-ordering wave vector and disperses to at least 60 meV. Modelling it as a damped bosonic mode yields an energy gap of approximately 18 meV and a velocity of (800 ± 200) meV·Å, comparable to the paramagnon velocity in cuprates. The comparable energy scales support an intimate connection between spin and charge correlations within a fluctuating-stripe picture. The measured dispersion also provides a quantitative estimate of the interaction scale governing charge order and constrains microscopic models of the cuprate phase diagram.

This work appears in Volume 1, Issue 1 of The Innovation Physics (X. Hong, Y. Yan, L. Martinelli, et al., J. Chang^✉, Q. Wang^✉. Unveiling the nature of collective charge excitations in a cuprate superconductor. The Innovation Physics 1, 100011 (2026). DOI). We are grateful to all collaborators for their contributions. A Chinese-language research highlight is available on WeChat.