Persistent Photo-Enhancement of Molecular Orbital Excitations in the Kondo Insulator FeSb$_2$

Year: 2026

Type: preprint

Abstract: 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$^2$ 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.

Source: arXiv (Cornell University)

DOI: 10.48550/arxiv.2610.27630

Authors: Xu, Ke-Jun, Giles-Donovan, Nathan, Majchrzak, Paulina, Suthar, Abhi, Hepting, Matthias, Trento, Marco, Ghiringhelli, Giacomo, Skoropata, Elizabeth, Ueda, Hiroki, Paris, Eugenio, Liu, Biaolong, Razzoli, Elia, Lee, Wei-Sheng, Devereaux, Thomas P., Shen, Zhi-Xun, Lee, Dung-Hai, Birgeneau, Robert J.

Institutions: Lawrence Berkeley National Laboratory, University of California, Berkeley, SLAC National Accelerator Laboratory, Stanford University, Max Planck Institute for Solid State Research, Politecnico di Milano, Paul Scherrer Institute

Language: English

Topic: X-ray Spectroscopy and Fluorescence Analysis

Subfield: Radiation

Field: Physics and Astronomy

Domain: Physical Sciences

Open Access status: green

Funders: U.S. Department of Energy, Office of Science, Stanford Precourt Institute for Energy, Basic Energy Sciences, Lawrence Berkeley National Laboratory, SLAC National Accelerator Laboratory, Division of Materials Sciences and Engineering

OpenAlex ID: W7223130988