Predictions of Multiband s± Strong-Coupling Eliashberg Theory Compared to Experimental Data in Iron Pnictides

The experimental critical temperatures and the gap values of the superconducting materials LaFeAsO1−xFx, SmFeAsO1−xFx and Ba1−xKxFe2As2, which are exponent of the most important families (1111 and 122) of iron pnictides, can be reproduced by an effective s-wave three-band Eliashberg model in which the dominant coupling is in the interband channel and the order parameter is nodeless but undergoes a sign reversal between hole and electron bands (s± symmetry). In particular, high values of the electron-boson coupling constants and small typical boson energies (in agreement with neutron diffraction experiments) are necessary to reproduce the exact Tc, the experimental gap values and their temperature dependence. The same parameters allow fitting the experimental temperature dependence of the upper critical field and predicting the presence of characteristic structures related to the electron–boson coupling in the Andreev-reflection spectra.