Attosecond screening dynamics mediated by electron localization in transition metals

Transition metals, with their densely confined and strongly coupled valence electrons, are key constituents of many materials with unconventional properties(1), such as high-temperature superconductors, Mott insulators and transition metal dichalcogenides(2). Strong interaction offers a fast and efficient lever to manipulate electron properties with light, creating promising potential for next-generation electronics(3-6.) However, the underlying dynamics is a hard-tounderstand, fast and intricate interplay of polarization and screening effects, which are hidden below the femtosecond timescale of electronic thermalization that follows photoexcitation(7). Here, we investigate the many-body electron dynamics in transition metals before thermalization sets in. We combine the sensitivity of intra-shell transitions to screening effects8 with attosecond time resolution to uncover the interplay of photo-absorption and screening. First-principles time-dependent calculations allow us to assign our experimental observations to ultrafast electronic localization on d orbitals. The latter modifies the electronic structure as well as the collective dynamic response of the system on a timescale much faster than the light-field cycle. Our results demonstrate a possibility for steering the electronic properties of solids before electron thermalization. We anticipate that our study may facilitate further investigations of electronic phase transitions, laser-metal interactions and photo-absorption in correlated-electron systems on their natural timescales.