Evidence of singlets in a 122-type Fe-based superconductor

Cuprate superconductors often show signature of Zhang-Rice singlet states in their photoemission spectral functions which are believed to play a role in their ground state properties. Here, we studied the core level spectra of a Fe-based superconductor, CaFe_1.9Co_0.1As₂ (T_c = 15 K) in the 122-family employing high-resolution hard x-ray photoemission spectroscopy (HAXPES). While the As core level spectra show negligible change with doping and/or temperature, the Ca 2p HAXPES data show a decrease in surface-bulk difference with Co-doping compared to its parent compound, CaFe₂As₂. The peak position shifts gradually towards lower binding energies with cooling. Interestingly, the Fe 2p spectra show emergence of a new feature at a lower binding energy relative to the screened Fe 2p peak in the doped sample. The intensity of this feature enhances with the decrease in temperature. The property of this feature is very similar to the Zhang-Rice singlets observed in cuprates and the low energy screened feature in manganites. Emergence of this feature in the superconducting composition and its enhancement with the decrease in temperature suggests relevance of the underlying interactions in the ground state properties of these Fe-based superconductors.