Ultrafast photoemission spectroscopy of the uranium dioxide UO 2 Mott insulator: Evidence for a robust energy gap structure

Time-resolved photoemission spectroscopy utilizing a probe energy of 32.55 eV and a pump energy of 3.1 and 4.65 eV with 30 fs temporal resolution is used to study the carrier dynamics in the 5f Mott insulator uranium dioxide (UO2). The Mott gap and on-site Coulomb interaction energies are measured directly as Egap=2.5 eV and UC=5 eV, respectively, and the dynamics of the upper Hubbard band is mapped. The f-fMott-Hubbard dynamics involves subpicosecond fluence-dependent relaxation, followed by decay via coupling to the lattice upon formation of excitonic polarons. Instead of an expected metallic transition, we observe a robust Mott gap structure, even at high pump fluences. © 2014 American Physical Society.