Moessbauer studies of the superconducting cobalt/nickel-doped BaFe2As2. Whither go the injected electron(s)?

Moessbauer studies of cobalt- and nickel-doped BaFe2As2 show that the s-electron density at the 57Fe nuclei, as measured by the isomer shift, is the same as that for the parent BaFe2As2. Apparently, the electron population of the d shell, which shields the s-electron density at the nuclei, remains unchanged. We invoke the involvement of p-orbital hybridization with the d orbital in Fe-As bonding. Furthermore, the shrinkage of the lattice on substitution enhances the As-As sp hybridization, providing a path for the migration of additional electrons. The proposed mechanism is consistent with Hall coefficient and thermoelectric effect measurements. (fast track communication)