Doping and critical-temperature dependence of the energy gaps in Ba(Fe1−xCox)2As2 thin films

The dependence of the superconducting gaps in epitaxial Ba(Fe1−xCox)2As2 thin films on the nominal doping x (0.04 x 0.15) was studied by means of point-contact Andreev-reflection spectroscopy. The normalized conductance curves were well fitted by using the two-dimensional Blonder-Tinkham-Klapwijk model with two nodeless, isotropic gaps—although the possible presence of gap anisotropies cannot be completely excluded. The amplitudes of the two gaps S and L show similar monotonic trends as a function of the local critical temperature T A c (measured in the same point contacts) from 25 K down to 8 K. The dependence of the gaps on x is well correlated to the trend of the critical temperature, i.e., to the shape of the superconducting region in the phase diagram. When analyzed within a simple three-band Eliashberg model, this trend turns out to be compatible with a mechanism of superconducting coupling mediated by spin fluctuations, whose characteristic energy scales with Tc according to the empirical law 0 = 4.65kBTc, and with a total electron-boson coupling strength λtot = 2.22 for x 0.10 (i.e., up to optimal doping) that slightly decreases to λtot = 1.82 in the overdoped samples (x = 0.15).