Ultrafast renormalization of the onsite Coulomb repulsion in a cuprate superconductor*

Intense ultrafast electromagnetic fields represent an increasingly important tool to stabilize and control emergent phases in quantum materials through the direct light-engineering of effective many-body interactions, such as electron hopping and electron-electron repulsion. Here, we employ time-resolved x-ray absorption spectroscopy (trXAS) to demonstrate the ultrafast renormalization of the Hubbard U parameter in the underdoped cuprate La_1.905Ba_0.095CuO₄ (LBCO, x=9.5%). Our element-specific measurements reveal that intense femtosecond optical pulses (1.55 eV, 50 fs) induce a pronounced shift of the x-ray absorption maxima associated with transitions to the upper Hubbard bands, while the transition energy into Zhang-Rice singlet states near the Fermi level remains unaffected. Based on exact-diagonalization calculations of the time-dependent spectrum within the single- as well as the three-band Hubbard models, we assign these effects to a pump-induced suppression of the Hubbard U on the Cu sites. Our results represent a first demonstration of dynamically-renormalized Hubbard U in strongly correlated oxides and have significant implications for the on-demand engineering of their magnetic interaction spectrum.