First-principles study of BaFe2-xNixAs2 (x=0, 0.125, 0.25, 1 and 2) superconductor using density functional theory

A metal can be known as a superconductive material throughout their transition temperature (Tc) determination. This is done by the macroscopic thermo/electrodynamic experiment. Upon quantum, electronic band theory cannot tell if the material is a superconductor. This is because superconducting band gap (AEsc) of the conventional superconductor is too small. However, this might not true for the higher Tc, recently discovered namely Fe-based superconductor (FeBS). This new kind of superconductor is labelled to have an unconventional superconductivity (SC). This is because its SC mechanism could no longer explain the conventional phonon mediated SC. Conventionally, spins (Fe) destroyed phonon SC interaction. Fairly, role of spins played for SC is not yet fully understood. Thereby, it is a chance that the FeBS has other kinds of electronic feature related to its SC mechanism. The aim of this research is to verify electronic features contributed to Fe-SC. This is done by calculating various electronic phase properties of BaFe2- xNixAs2 using Density Functional Theory (DFT). According to their Tc determination experiment, BaFe2- xNixAs2 with x=0, 1 and 2 is the parent FeBS, a non-superconductor (NSC) and the nonFe; Ni-based superconductor (NiBS), respectively. It is featured on their calculated band structures that the parent FeBS has paired AEsc while the NiBS has the inter-band pockets of Charge Density Wave (CDW) which is responsible for the non-Fe SC whereby the NSC has none of those. Referring to the electronic phase diagram data, BaFe2-xNixAs2 with x=0, 0.125 and 0.25 is representing phase before, during and after the Fe-SC. It is concluded from the band structures calculation that the FeBS (x=0.125) is not only depicted an obvious paired AEsc but must accompanied by intra-band Spin Density Wave (SDW) nesting pockets to have the spin mediated SC. Also, the SC ended (x=0.25) when it is only intra-band SDW. This proved that spins played role for SC when the atomic Fe distances, its magnetic moment and ordering are precisely favored by the SC interaction. Moreover, direct comparison of the FeBS versus the NiBS is done to understand their mechanism in enhancing SC. Pairing SC mechanism of both superconductors is depicted in their momentum and real space. It is determined from their calculated electronic properties and charge distribution, respectively. This is to demonstrate electrons that responding to SC interaction; super-electrons. It is found that their super-electrons behaved differently. This might be a clue why FeBS (Fe-SC) has higher Tc than the NiBS (non-Fe SC). Overall, the quantum calculation in this firstprinciples study provided insight for super-electrons behavior. Henceforth, it may improve the understanding of unconventional superconductors.