Intra-inter band pairing, order parameter symmetry in Fe-based superconductors : A model study

TL;DR

This study uses a two-band model to show that combined intra- and inter-band pairing mechanisms are essential for understanding the superconducting properties of Fe-based materials, including the single transition temperature and specific heat behavior.

Contribution

It demonstrates that both intra- and inter-band pairing are necessary to accurately describe the superconducting features of Fe-based superconductors, highlighting the importance of combined pairing mechanisms.

Findings

Inter-band pairing is crucial for a single global T_c in multi-gap systems.

The specific heat jump ratio scales with the square of T_c in sign-changing s± wave symmetry.

Combined intra-inter band pairing explains experimental observations better than single mechanism models.

Abstract

In the quest of why there should be a single transition temperature in a multi-gapped system like Fe-based materials we use two band model for simplicity. The model comprises of spin density wave (SDW), orbital density wave (ODW) arising due to nested pieces of the electron and hole like Fermi surfaces; together with superconductivity of different pairing symmetries around electron and hole like Fermi surfaces. We show that either only intra or only inter band pairing is insufficient to describe some of the experimental results like large to small gap ratio, thermal behaviour of electronic specific heat jump etc. It is shown that the inter-band pairing is essential in Fe-based materials having multiple gaps to produce a single global $T_c$. Some of our results in this scenario, matches with the earlier published work \cite{two-band-prb}, and also have differences. The origin of difference between the two is also discussed. Combined intra-inter band pairing mechanism produces the specific heat jump to superconducting transition temperature ratio proportional to square of the transition temperature, both in the electron and hole doped regime, for sign changing s$^{\pm}$ wave symmetry which takes the d+s pairing symmetry form. Our work thus demonstrates the importance of combined intra-inter band pairing irrespective of the pairing mechanism.