In-Gap Quasiparticle Excitations Induced by Non-Magnetic Cu Impurities in Na(Fe0.96Co0.03Cu0.01)As Revealed by Scanning Tunneling Spectroscopy

TL;DR

This study uses scanning tunneling spectroscopy to reveal in-gap quasiparticle states caused by non-magnetic Cu impurities in Na(Fe,Co)As, providing strong evidence for the S+- pairing symmetry in iron pnictide superconductors.

Contribution

It demonstrates the direct observation of impurity-induced in-gap states supporting the S+- pairing model in iron-based superconductors.

Findings

Non-magnetic Cu impurities induce in-gap quasiparticle states.

Experimental data aligns with S+- pairing gap and scalar scattering potential.

Supports the sign-reversal pairing mechanism in iron pnictides.

Abstract

The pairing mechanism in the iron pnictides remains unresolved yet. One of the central issues is the structure of the superconducting order parameter which classifies the community into two different and highly disputed camps. On one hand the picture of pairing based on the magnetic origin predicts a sign reversal gap on the electron and hole Fermi pockets, leading to the S+- pairing. On the other hand, a more conventional S++ pairing gap was suggested based on the phonon or orbital fluctuation mediated pairing. In the superconducting state, the impurities may generate a unique pattern of local density of states in space and energy, which are regarded as the fingerprints for checking the structure of the pairing gap. In this study, we successfully identified the non-magnetic and magnetic impurities in Na(Fe0.97-xCo0.03Tx)As (T=Cu, Mn) and investigated the spatial resolved scanning tunneling spectroscopy. We present clear evidence of the in-gap quasiparticle states induced by the nonmagnetic Cu impurities, giving decisive evidence of the S+- pairing. This is corroborated by the consistency between the experimental data and the first-principles calculations based on the S+- pairing gap with a scalar scattering potential.