Resistivity in Ba(FeCo)As: Comparison of thin films and single crystals

The temperature dependence of the resistivity of epitaxial Ba(Fe[Formulaomitted]Co[Formulaomitted])[Formulaomitted]As[Formula o mitted] thin films (with nominal doping [Formulaomitted] and [Formulaomitted]) has been analysed and compared with analogous measurements on single crystals taken from literature. The [Formulaomitted] of thin films looks different from that of single crystals, even when the cobalt content is the same. All [Formulaomitted] curves can be fitted by considering an effective two-band model (with holes and electrons bands) in which the electrons are more strongly coupled with the bosons (spin fluctuations) than holes, while the effect of impurities is mainly concentrated in the hole band. Within this model the mediating boson has the same characteristic energy in single crystals and thin films, but the shape of the transport spectral function at low energy has to be very different, leading to a "hardening" of the electron-boson spectral function in thin films, associated with the strain induced by the substrate.