Generalized BCS Equations and the Iron-Pnictide Superconductors

Abstract

<p align="justify"> <span style="font-family:Verdana;">A detailed quantitative study of the pnictide composite superconductor (CS) </span><span><span style="font-family:Verdana;">Ba</span>_{<span style="font-family:Verdana;">0.6</span>}<span style="font-family:Verdana;">K</span>_{<span style="font-family:Verdana;">0.4</span>}<span style="font-family:Verdana;">Fe</span>_{<span style="font-family:Verdana;">2</span>}<span style="font-family:Verdana;">As</span>_{<span style="font-family:Verdana;">2</span>}<span style="font-family:Verdana;"> is presented in the frame</span></span><span style="font-family:Verdana;">-</span><span style="font-family:Verdana;">work </span><span style="font-family:Verdana;">of t</span><span style="font-family:Verdana;">he recently derived set of generalized BCS equations</span><span><span style="font-family:Verdana;">. Invoking multiple Debye temperatures to take into account anisotropy of the CS, we address the current experimental data on its </span><i><span style="font-family:Verdana;">T</span>_{<span style="font-family:Verdana;">c</span>}</i><span style="font-family:Verdana;"> and the (not so clear-cut) gap-values via </span></span><span style="font-family:Verdana;">different theoretical scenarios</span><span style="font-family:Verdana;"> that attempt to identify the ion species responsible for pairing in it.</span><span style="font-family:Verdana;"> This is done with the aid of the Bogoliubov</span><span style="font-family:Verdana;">’</span><span style="font-family:Verdana;">s restriction on the BCS dimensionless electron-phonon coupling constant. </span><span style="font-family:Verdana;">Significantly, our study sheds light on</span><span style="font-family:Verdana;"> the gaps which have recently been observed in different iron-pnictide CSs as nodes or line-nodes on the Fermi surface and have evinced considerable interest.</span><span> </span> </p

A Study of Superconducting La&lt;sub&gt;2&lt;/sub&gt;CuO&lt;sub&gt;4&lt;/sub&gt; via Generalized BCS Equations Incorporating Chemical Potential

“…As noted [3]- [5] for all the HTSCs dealt with so far, the first step in their study via GBCSEs is to fix the Debye temperatures of the ions that may be causing pairing. This is done by applying the following equations to each of the sub-lattices of the HTSC comprising layers designated as A x B 1−x : …”

“…This leads to two values for the smaller gap and one for the larger gap via the following equations (Equations (3) and (5) of [5]…”

“…For all the other HTSCs mentioned above, this brings into play two interaction parameters because their sub-lattices contain predominantly two distinct species of ions that can cause electron-phonon interaction to take place, e.g., Y and Ba ions [4] in YBa 2 CuO 7 and any two [3] of the Tl, Ba, and Ca ions in Tl 2 Ba 2 CaCuO 8 . This is also the case for the Bi-based [3] and iron-pnictide HTSCs [5].…”

On a New Equation for Critical Current Density Directly in Terms of the BCS Interaction Parameter, Debye Temperature and the Fermi Energy of the Superconductor

“…In particular, in the latter of these references, the gaps observed in ironpnictide SCs as nodes or line nodes on the Fermi surface have evinced considerable interest. For a quantitative account of the T c and the multiple gaps of a prominent member of the iron-pnictide family, namely Ba 0.6 K 0.4 Fe 2 As 2 , in the framework of the generalized-BCS equations (GBCSEs) [13]-which will be further discussed below, we draw attention to [14]. The purpose of this note is to present an approach in which P c (T)-defined as the momentum at which the binding energy of the CPs vanishes (this is equivalent to the vanishing of the gap [13])-is calculated via the dynamics of CPs.…”

A Study of Heavy-Fermion Superconductors via BCS Equations Incorporating Chemical Potential

“…This is a scenario inspired by the multi-condensate superconductivity approach advocated by Bianconi and collaborators ( [9], and references therein). We now recall that a case has been made for the applicability of the electron-lattice interactions to a wide variety of SCs [10]; also that by employing such interactions, the T c s and multiple gaps of several high-T c SCs have been accounted for via the generalized BCS equations [11]- [14]. Note that complexity of structure is a feature common to both high-T c SCs and HFSCs.…”