Anisotropic but nodeless superconducting gap in the presence of spin density wave in iron-pnictide superconductor NaFe1-xCoxAs

TL;DR

This study reveals an anisotropic yet nodeless superconducting gap coexisting with spin density wave order in NaFe1-xCoxAs, providing microscopic insights into their interplay and challenging existing theoretical models.

Contribution

It demonstrates the coexistence of SDW and superconductivity with an anisotropic nodeless gap in NaFe1-xCoxAs, revealing complex band structure interactions.

Findings

SDW and superconductivity coexist on the same bands

The superconducting gap is anisotropic and nodeless

Contrasts with isotropic gaps in SDW-free samples

Abstract

The coexisting regime of spin density wave (SDW) and superconductivity in the iron pnictides represents a novel ground state. We have performed high resolution angle-resolved photoemission measurements on NaFe1-xCoxAs (x = 0.0175) in this regime and revealed its distinctive electronic structure, which provides some microscopic understandings of its behavior. The SDW signature and the superconducting gap are observed on the same bands, illustrating the intrinsic nature of the coexistence. However, because the SDW and superconductivity are manifested in different parts of the band structure, their competition is non-exclusive. Particularly, we found that the gap distribution is anisotropic and nodeless, in contrast to the isotropic superconducting gap observed in an SDW-free NaFe1-xCoxAs (x=0.045), which puts strong constraints on theory.