Conventional superconductivity in Fe-based pnictides: The relevance of intra-band electron-boson scattering

Various experimental data point against the gapless unconventional pairings, since the large intra-band impurity scattering is strongly pair-breaking for them. If the strength of the inter-band impurity scattering is in the Born limit it may be very detrimental for s +/- pairing (hole(h)- and electron(el)-gap are of opposite signs). However, in some single crystals the inter-band impurity scattering may be strong and far beyond the Born scattering limit. In that case the proposed s +/- pairing is much less suppressed and might be realized if the inter-band repulsive interaction (due to either the direct Coulomb interaction or antiferromagnetic spin fluctuations) dominates the attractive one. The present experimental data imply that the intra-band pairing in the h- and in the el-band is a nodeless and conventional one. The latter is inevitably due to a nonmagnetic el-boson attractive interaction (EBI), and must be taken into account. The EBI is either due to phonons (EPI) or to excitons (EEI), or both are simultaneously operative. We discuss their possible interplay briefly. The recent large Fe-isotope effect, if confirmed, favors the EPI intra- and inter-band pairing, additionally amplified by EEI, in which case the s+ pairing (the h- and el-gaps are in-phase) is favored. Otherweise s +/- pairing is realized. In order to test the strength of the inter-band pairing (which increases T-c independently of the sign of the inter-band coupling constant) we propose measurements of the Legget mode by varying doping. Copyright (C) EPLA, 2009