Abstract
Considering model Hamiltonians that respect the symmetry properties of the pnictides, it is argued that
pairing interactions that couple electrons in different orbitals with an orbital-dependent pairing strength inevitably
lead to interband pairing matrix elements, at least in some regions of the Brillouin zone. Such interband
pairing has not been considered of relevance in multiorbital systems in previous investigations. It is also
observed that if, instead, a purely intraband pairing interaction is postulated, this requires that the pairing
operator has the form †k= fkdk,,↑
† d−k,,↓
† , where labels the orbitals considered in the model and fk
arises from the spatial location of the coupled electrons or holes. This means that the gaps at two different
Fermi surfaces involving momenta kF and kF can only differ by the ratio fkF / fkF and that electrons in
different orbitals must be subject to the same pairing attraction, thus, requiring fine tuning. These results
suggest that previously neglected interband pairing tendencies could actually be of relevance in a microscopic
description of the pairing mechanism in the pnictides.
