We describe a femtosecond pump-probe study of ultrafast hopping dynamics of $5f$ electrons in the Mott insulator ${\mathrm{UO}}_2$ following Mott-gap excitation at temperatures of 5–300 K. Hopping-induced response of the lattice and electrons is probed by transient reflectivity at mid- and above-gap photon energies, respectively. These measurements show an instantaneous hop, subsequent picosecond lattice deformation, followed by acoustic phonon emission and microsecond relaxation. Temperature-dependent studies indicate that the slow relaxation results from Hubbard excitons formed by ${\mathrm{U}}^{3+}-{\mathrm{U}}^{5+}$ pairs.

• Received 29 November 2010

DOI:https://doi.org/10.1103/PhysRevLett.106.207402