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

Abstract

Mössbauer studies of cobalt- and nickel-doped BaFe(2)As(2) show that the s-electron density at the (57)Fe nuclei, as measured by the isomer shift, is the same as that for the parent BaFe(2)As(2). 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.

A [sup 57]Fe Mössbauer study of local structure and spin arrangements in antiferromagnetic NaFeAs

“…The obtained distribution function has a bimodal profile which emphasizes an inhomogeneous magnetic state of iron cations (2) Fe (1) within the NaFeAs structure. The same Mössbauer spectra with unresolved Zeeman structure were obtained early for other families of Ba 1-x K x Fe 2 As 2 [13] and AFe 2-x M x As 2 (A = Ca, Sr; M = Co, Ni, Rh) [14,15] arsenides containing the impurity ions in cationic sublattices. The low value of the average hyperfine magnetic field, H Fe 13K = 23.8 ± 0.3 kOe, reflects a very low value of a magnetic moment of iron cations in the NaFeAs structure.…”

Universal pair-breaking in transition-metal-substituted iron-pnictide superconductors

“…Such a contrast has been argued to arise from the aliovalent versus isovalent nature of, respectively, Co and Ru substituents, but recent work has put this into question. Mossbauer studies of BaFe 2−x Co x As 2 and BaFe 2−x Ni x As 2 find no change in d-electron population with substitution [38], while an x-ray absorption study reveals no change in the Fe valence with Co substitution in BaFe 2−x Co x As 2 [39]. Furthermore, recent calculations suggest that substituted d-electrons can remain localized at the substituent sites [5], either still resulting in a rigid band shift [6] or generating a phase diagram strikingly similar to that expected from a rigid band shift [40].…”

Of Substitution and Doping: Spatial and Electronic Structure in Fe Pnictides

“…In particular, it has been discussed controversially whether or not a substitution of Fe by Mn, Co, Ni, or Cu does indeed lead to the widely assumed charge-carrier doping of (Ba,K)(Fe,TM) 2 As 2 : While some investigations [2][3][4] seem to be in favor of a picture where charge carriers arrive at the mostly Fe-derived bands near E F , others [5][6][7] are not. The TM impurity may lead to a change of the Fermi surface, destabilizing magnetism in favor of superconductivity and also questioning a doping effect; [8] furthermore, if the total electron density peaks close to the impurity the latter appears isovalent to Fe.…”