Anomalous behavior of coupling constant near the magnetic phase transition of Ni-doped Ba-122 pnictides

We investigated Ni-doped Ba-122 pnictide single crystals at two different doping levels (BaFe_2−xNi_xAs₂; x = 0.05 (underdoped) and 0.10 (optimally doped)) using optical spectroscopy. We analyze measured optical data with an extended Drude model and a generalized Allen's formalism. We extracted the electron-boson spectral density function using an approximate single-band approach from measured optical scattering rate. We observed that an anomalous behavior in the coupling constant obtained from the electron-boson spectral density function takes place near the magnetic transition temperature of the underdoped sample. This anomaly has not been observed in cuprate system. Nevertheless, overall temperature and doping dependent properties of the electron-boson spectral density of pnictides are similar to those of cuprates. This may indicate that these two different high-temperature superconducting systems share a same mechanism for their superconductivity.