  Coupling among conduction electrons (e.g. Zhang-Rice singlet) are often
manifested in the core level spectra of exotic materials such as cuprate
superconductors, manganites, etc. These states are believed to play key roles
in the ground state properties and appear as low binding energy features. To
explore such possibilities in the Fe-based systems, we study the core level
spectra of a superconductor, CaFe$_{1.9}$Co$_{0.1}$As$_2$ (CaCo122) in the
CaFe$_2$As$_2$ (Ca122) family employing high-resolution hard $x$-ray
photoemission spectroscopy. While As core levels show almost no change with
doping and cooling, Ca 2$p$ peak of CaCo122 show reduced surface contribution
relative to Ca122 and a gradual shift of the peak position towards lower
binding energies with cooling. In addition, we discover emergence of a feature
at lower binding energy side of the well screened Fe 2$p$ signal in CaCo122.
The intensity of this feature grows with cooling and indicate additional
channels to screen the core holes. The evolution of this feature in the
superconducting composition and it's absence in the parent compound suggests
relevance of the underlying interactions in the ground state properties of this
class of materials. These results reveal a new dimension in the studies of
Fe-based superconductors and the importance of such states in the
unconventional superconductivity in general.
