Abstract
The Mo shell in the ground state of is widely believed to be unoccupied. However, this assumption lacks clear experimental and theoretical corroboration. Using x-ray absorption and emission spectroscopy along with resonant inelastic x-ray scattering, we provide experimental evidence of two-dimensional exhibiting enhanced many-body effects due to its reduced dimensionality. The observed phenomena include many-body effects such as and spin-flip excitations, valence-hole and excited electron, and core-hole and excited electron bound excitonic states. Moreover, density functional theory and ligand field-based calculations were performed to investigate and interpret the experimental spectra. Considering that these many-body effects can only be observed by the interaction of x-ray photons with if the state is partially occupied, our experimental and theoretical approach clearly demonstrates a partial occupation of the Mo state, refuting the assumption that the ground state is a state. The Mo d occupancy is and determined with two different theoretical approaches (density functional theory and multiplet, respectively) and the computed spectra agree very well with our measurements further supporting this finding. Both the two- and three-dimensional samples exhibit strong core-hole effects that reduce the absorption onset at both the Mo and O edge. The band gap of the three-dimensional sample is experimentally found to be 3.1 ± 0.2 eV; however, for the two-dimensional material, strong many-body effects, even at the O edge, prevent an accurate determination of this value. The presence of these quasiparticles influences the band dispersions near the Fermi level, and thus has a key role in the performance of possible -based devices.
- Received 20 February 2022
- Revised 13 May 2022
- Accepted 31 May 2022
DOI:https://doi.org/10.1103/PhysRevMaterials.6.074003
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