Persistent Photo-Enhancement of Molecular Orbital Excitations in the Kondo Insulator FeSb 2

When insulators are excited by photons with above-gap energies, photo-doped carriers are transiently generated. Typically, orbital excitations weaken and broaden due to increased screening by these transient carriers, followed by a few-ps recovery via phonon thermalization. Here we report the observation of anomalous dynamics of many-body molecular orbital excitations in the narrow-gap Kondo insulator FeSb 2. In the pumped X-ray absorption spectra following excitation with 1 μ m photons, we observe a transient transfer of absorption intensity from the delocalized states at the main absorption peak to the localized molecular orbital states at the pre-edge. At the resonance of the low-energy molecular orbital excitations, the pumped resonant inelastic X-ray spectra show an approximately 30% intensity increase at a pump fluence of 11 mJ/cm squared concomitant with the emergence of an energy gain feature. While the energy gain feature disappears after about 1-2 ps, the increased molecular orbital excitation intensities partially persist for at least 50 ps with no discernible energy shift. These observations require the invocation of a many-body excitation picture beyond single particle population changes.