Coupled multiple-mode theory for s± pairing mechanism in iron based superconductors.

M N Kiselev,S L Drechsler,D V Efremov,K Kikoin,Jeroen Van Den Brink

doi:10.1038/srep37508

Abstract

We investigate the interplay between the magnetic and the superconducting degrees of freedom in unconventional multi-band superconductors such as iron pnictides. For this purpose a dynamical mode-mode coupling theory is developed based on the coupled Bethe-Salpeter equations. In order to investigate the region of the phase diagram not too far from the tetracritical point where the magnetic spin density wave, (SDW) and superconducting (SC) transition temperatures coincide, we also construct a Ginzburg-Landau functional including both SC and SDW fluctuations in a critical region above the transition temperatures. The fluctuation corrections tend to suppress the magnetic transition, but in the superconducting channel the intraband and interband contribution of the fluctuations nearly compensate each other.

Highlights

The rapidly extending realm of high-Tc superconductors has been enriched recently by a new class of materials, so called iron based superconductors (FeSC).[1,2,3,4,5] Like many other high-Tc materials, these compounds crystallize in strongly anisotropic lattices: one can identify quasi-two-dimensional subsystems which contain the electrons that are subject to Cooper pairing
A specific feature of the electronic structure of FeSC is the multipocket Fermi surface, mostly semi-metallic, some superconducting materials possess either only electron[6,7] or only hole pockets.[8]. Another characteristic feature of FeSC is the interplay between different electronic instabilities of the pristine normal metal phases. Most of these materials are unstable against spin-density wave (SDW) type itinerant antiferromagnetism, and the phase diagrams of doped compounds contains domains of superconductor (SC) and SDW ordering, the latter sometimes being accompanied by structural phase transitions with a potential for orbital ordering.[9,10,11,12]
The derivation of the effective hydrodynamic action in terms of magnetic m(x,t) and superconducting ∆i(x,t) (i=e,h) dynamical fluctuations is done by integrating out the BS equations with respect to the ”fast” degrees of freedom and is presented in Section ”Methods”

Summary

Introduction

The rapidly extending realm of high-Tc superconductors has been enriched recently by a new class of materials, so called iron based superconductors (FeSC).[1,2,3,4,5] Like many other high-Tc materials, these compounds crystallize in strongly anisotropic lattices: one can identify quasi-two-dimensional subsystems which contain the electrons that are subject to Cooper pairing. In this case the approach based on the effective hydrodynamic action is much easier than the direct solution of the BS equations This approach which accounts for the multi-point correlation functions goes beyond the conventional BS paradigm limited by two- and four- point Green’s function.[16,17] The derivation of the effective hydrodynamic action in terms of magnetic m(x,t) and superconducting ∆i(x,t) (i=e,h) dynamical fluctuations is done by integrating out the BS equations with respect to the ”fast” (with the energy of the order of the bandwidth) degrees of freedom and is presented in Section ”Methods”. Within our analysis we start discussing the limiting case us u3 and u3Πs ∼ 1 corresponding to the interplay between two fluctuating modes: one is the s± superconducting and another one is the SDW magnetic This regime is believed to be present in most of FeSC [e.g., in broadly studies doped ”A-122” systems (where A = Ba ,Sr, Ca)] The Lagrangian of the two-mode coupling theory takes the form:. With this numbers the one finds large Ginzburg number Gi(2) ∼ Tc/εF ∼ 0.02 − 0.05 and considerable renormalizations of the critical temperatures(Tc − Ts)/Tc ∼ 0.1

Discussion

Methods