Anisotropic but Nodeless Superconducting Gap in the Presence of Spin-Density Wave in Iron-Pnictide SuperconductorNaFe1−xCoxAs

Abstract

The coexisting regime of spin-density wave (SDW) and superconductivity in iron pnictides represents a novel ground state. We have performed high-resolution angle-resolved photoemission measurements on ${\mathrm{NaFe}}_{1-x}{\mathrm{Co}}_x\mathrm{As}$ ($x=0.0175$) 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 nonexclusive. Particularly, we find that the gap distribution is anisotropic and nodeless, in contrast to the isotropic superconducting gap observed in a SDW-free ${\mathrm{NaFe}}_{1-x}{\mathrm{Co}}_x\mathrm{As}$ ($x=0.045$), which puts strong constraints on theory.