On the s<sup>±</sup>-Wave Superconductivity in the Iron-Based Superconductors: A Perspective Based on a Detailed Study of Ba<sub>0.6</sub>K<sub>0.4</sub>Fe<sub>2</sub>As<sub>2</sub> via the Generalized-Bardeen-Cooper-Schrieffer Equations Incorporating Fermi Energy

Abstract

Guided by the belief that Fermi energy E F (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 (T c s) of both elemental and composite SCs. The μ-dependent interaction parameters consistent with the values of Δs and T c s of any of these SCs were shown to lead to expressions for the effective mass of electrons (m * ) and their number density (n s ), critical velocity (v 0 ), and the critical current density j 0 at T = 0 in terms of the following five parameters: Debye temperature, E F , 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 j 0 and calculate the other parameters. This approach was followed for a variety of SCs-elemental, MgB 2 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 Ba 0.6 K 0.4 Fe 2 As 2 (BaAs). Some of the main attributes of this SC are: it is characterized by s ± -wave superconductivity and multiple gaps between 0 -12 meV; its T c ~ 37 K, but the maximum T c of SCs in its class can exceed 50 K; E F /kT c = 4.4 (k = Boltzmann constant), and its T c plotted against a tuning variable has a dome-like structure. After drawing attention to the fact that the s ± -wave is an inbuilt feature of GBCSEs, wegive a quantitative account of its several other features, which include the values of m * , n s , v o , and coherence length. Finally, we also deal with the issue of the stage BaAs occupies in the BCS-Bose-Einstein Condensation crossover.

A Fermi Energy-Incorporated Framework for Dealing with the Temperature- and Magnetic Field-Dependent Critical Current Densities of Superconductors and Its Application to Bi-2212

“…This becomes cumbersome if one is simultaneously dealing with more than one such equation. The operator Re provides a much simpler, one-step, alternative, as was also noted in the context of Fe-based World Journal of Condensed Matter Physics SCs [23]. This remark also applies to (39).…”

Generalized BCS Equations: A Review and a Detailed Study of the Superconducting Features of Ba2Sr2CaCu2O8

“…The salient superconducting properties of Ba 0.6 K 0.4 Fe 2 As 2 , which are generic of the Fe-based SCs (also called iron-pnictide SCs), are as follows (for sources of these properties, see [31]): (i) Superconductivity in this SC is due to the s AE -wave state, which signifies that a gap below the Fermi surface and the one corresponding to it above the Fermi surface, Δ h and Δ e , respectively, has opposite signs. By adopting the framework of GBCSEs as outlined above, it has been shown in [31] that one can quantitatively explain all the above features of Ba 0.6 K 0.4 Fe 2 As 2without invoking a new superconducting state for the SC. This is an important remark because, on the basis of the multiband approach, it has been suggested in a recent review article [32] that superconductivity in Fe-based SCs is manifestation of a new state.…”

An Overview of the Multi-Band and the Generalized BCS Equations-Based Approaches to Deal with Hetero-Structured Superconductors

“…It has been shown that [6], A more accurate (but rather elaborate) equation that additionally contains E F explicitly has been derived in [8]; the values of y obtained via these equations differ significantly only when μ/kθ ≈ 0.3 or less.…”

“…Gleaned from [1], summarized then are the reasons for its wide adoption. In Section 3, based on [4] [5] [6] [7] and [8], is…”