On the s±-Wave Superconductivity in the Iron-Based Superconductors: A Perspective Based on a Detailed Study of Ba0.6K0.4Fe2As2 via the Generalized-Bardeen-Cooper-Schrieffer Equations Incorporating Fermi Energy

Abstract

Guided by the belief that Fermi energy EF (equivalently, chemical potential μ) plays a pivotal role in determining the properties of superconductors (SCs), we have recently derived μ-incorporated Generalized-Bardeen-Cooper-Schrieffer equations (GBCSEs) for the gaps (Δs) and critical temperatures (Tcs) of both elemental and composite SCs. The μ-dependent interaction parameters consistent with the values of Δs and Tcs of any of these SCs were shown to lead to expressions for the effective mass of electrons (m*) and their number density (ns), critical velocity (v0), and the critical current density j0 at T = 0 in terms of the following five parameters: Debye temperature, EF, a dimensionless construct y, the specific heat constant, and the gram-atomic volume. We could then fix the value of μ in any SC by appealing to the experimental value of its j0 and calculate the other parameters. This approach was followed for a variety of SCs—elemental, MgB2 and cuprates and, with a more accurate equation to determine y, for Nitrogen Nitride (NbN). Employing the framework given for NbN, we present here a detailed study of Ba0.6K0.4Fe2As2 (BaAs). Some of the main attributes of this SC are: it is characterized by -wave superconductivity and multiple gaps between 0 - 12 meV; its Tc ~ 37 K, but the maximum Tc of SCs in its class can exceed 50 K; EF/kTc = 4.4 (k = Boltzmann constant), and its Tc plotted against a tuning variable has a dome-like structure. After drawing attention to the fact that the -wave is an inbuilt feature of GBCSEs, we give a quantitative account of its several other features, which include the values of m*, ns, vo, and coherence length. Finally, we also deal with the issue of the stage BaAs occupies in the BCS-Bose-Einstein Condensation crossover.