Intense ultrashort electromagnetic fields are an increasingly important tool to realize and control novel emergent phases in quantum materials. Among a variety of nonthermal excitation pathways, a particularly intriguing route is represented by the direct light-engineering of effective many-body interactions, such as electron hopping amplitudes and electron-electron repulsion. Achieving a light-induced dynamical renormalization of the screened onsite Coulomb repulsion (“Hubbard U”) would have far-reaching implications for high-harmonic generation, attosecond spectroscopy and ultrafast magnetism in the solid state. However, experimental evidence for a dynamically controlled Hubbard U remains scarce. In this talk, I will present a recent demonstration of light-induced renormalization of the Hubbard U in a high-temperature superconductor, La2-xBaxCuO4,  and discuss its implications for the control of superconductivity, magnetism, as well as for the realization of other long-range-ordered phases in light-driven quantum materials. Further, I will discuss its role in the dynamical renormalization of magnetic fluctuations , which can be probed by newly developed ultrafast resonant inelastic x-ray scattering methods.
 D. R. Baykusheva et al. Phys. Rev. X 12, 011013 (2022).
 Y. Wang et al. Commun. Phys. 4, 212 (2021).