Ultrafast modification of the electronic structure of a correlated insulator

Electronic materials properties are determined by the interplay of many competing factors. Electro-magnetic fields strong enough to rival atomic interactions can disturb the balance between kinematic effects due to electrons hopping between lattice sites and the Coulomb repulsion between electrons that limits the band formation. This allows for new insights into quantum phases, as well as the time-scales and energies involved in using quantum effects for possible applications. Here we show that 0.2 V/A ultrashort optical fields in the high harmonic generation regime lead to a pronounced transient inter-site charge transfer in NiO, a prototypical correlated electron insulator. Element-specific transient x-ray absorption spectroscopy detects a negligible change in electron correlations of Ni 3d-states. This behaviour is captured by time-dependent density functional theory and points to a speed limit for the dynamical screening of the Coulomb interaction taking place above our experimental 6.9 femtoseconds optical cycle.