Morillo-Candas, A. S., Augustin, S., Prat, E., Sarracini, A., Knurr, J., Zerdane, S., Sun, Z., Yang, N., Rebholz, M., Zhang, H., Deng, Y., Xie, X., Zyaee, E., Rohrbach, D., Cannizzo, A., Al-Haddad, A., Schnorr, K., Ott, C., Feurer, T., Bostedt, C., Pfeifer, T., Knopp, G. (2026). Coherent nonlinear X-ray four-photon interaction with core-shell electrons. Nature. https://doi.org/10.1038/s41586-025-09911-1