The empirical aspects of Eliashberg formalism for the superconducting mechanism in Iron-based superconductors

We have investigated experimentally how properties of NdFeAsO0.8F0.2 superconductor affected due to the substitution of the Ca2+/Nd3+ doping. Based on the XRD data refinement, various structural parameters such as lattice parameters, bond angles, bond length, and etc. were studied. We have determined the upper limit of the calcium solubility in the NdFeAsO0.8F0.2 phase and it is restricted to x=0.05. Also, we have found that the lattice parameters and the cell volume decreased by increasing the calcium content. According to the XRD data analysis, we have argued that these reductions are due to the variations in the bond lengths and the bond angles of (O/F)-Nd-(O/F) and As-Fe-As i.e. alpha, beta upon increasing the calcium dopant. So, we have expected that the superconducting transition temperature (TC) will be sensitive to the calcium doping values. Experimentally, the TC of our samples was reduced from 53 K (for x = 0) to 48 K (for x = 0.01) and 27 K (for x=0.025) and disappeared for our other sample. Then we have studied the dependence of TC and bond angles, bond length, the pnictogen height, and the lattice parameter to examine the available theories from an empirical point of view. The consistency of our experimental results and the theoretical reports based on the spin- and the orbital-fluctuation theories shows that these models play an important role in the pairing mechanism of the iron-based superconductors.