Anomalous Zeeman response in coexisting phase of superconductivity and spin-density wave as a probe of extended $s$-wave pairing structure in ferro-pnictide

TL;DR

This study investigates how the anomalous Zeeman response in the coexistence of superconductivity and spin-density wave phases in ferro-pnictides reveals the extended s-wave pairing structure and the effects of magnetic fields on the superconducting gap.

Contribution

It demonstrates that the Zeeman response can confirm the extended s-wave gap structure and shows how magnetic fields anisotropically suppress the superconducting gap in coexisting phases.

Findings

Anisotropic reduction of the superconducting gap with increasing magnetic field.

SDW formation removes nodes in the superconducting gap.

The generated gap in nodal areas is anisotropic, linked to pairing structure.

Abstract

In several members of the ferro-pnictides, spin density wave (SDW) order coexists with superconductivity over a range of dopings. In this letter we study the anomalous magnetic Zeeman response of this coexistence state and show that it can be used to confirm the extended s-wave gap structure as well as structure of superconducting (SC) gap in coexisting phase. On increasing the field, a strongly anisotropic reduction of SC gap is found. The anisotropy is directly connected to the gap structure of superconducting phase. The signature of this effect in quasiparticle interference measured by STM, as well as heat transport in magnetic field is discussed. For the compounds with the nodal SC gap we show that the nodes are removed upon formation of SDW. Interestingly the size of the generated gap in the originally nodal areas is anisotropic in the position of the nodes over the Fermi surface in direct connection with the form of SC pairing.