Emergent Phases of Nodeless and Nodal Superconductivity Separated by Antiferromagnetic Order in Iron-based Superconductor (Ca4Al2O6)Fe2(As1-xPx)2: 75As- and 31P-NMR Studies

TL;DR

This study uses NMR techniques to explore how isovalent P substitution in an iron-based superconductor induces a transition from nodeless to nodal superconductivity, separated by antiferromagnetic order influenced by pnictogen height and electron correlations.

Contribution

It reveals the evolution of superconducting and magnetic phases driven by pnictogen height, highlighting the role of electron correlation in phase emergence in Fe-based superconductors.

Findings

Nodeless superconductivity occurs for 0≤x≤0.4.

Nodal superconductivity appears around x=1.

Antiferromagnetic order exists for 0.5≤x≤0.95, separated by superconducting phases.

Abstract

We report $^{31}$P- and $^{75}$As-NMR studies on (Ca$_4$Al$_2$O$_{6}$)Fe$_2$(As$_{1-x}$P$_x$)$_2$ with an isovalent substitution of P for As. We present the novel evolution of emergent phases that the nodeless superconductivity (SC) in 0$\le x \le$0.4 and the nodal one around $x$=1 are intimately separated by the onset of a commensurate stripe-type antiferromagnetic (AFM) order in 0.5$\le x \le$ 0.95, as an isovalent substitution of P for As decreases a pnictogen height $h_{Pn}$ measured from the Fe plane. It is demonstrated that the AFM order takes place under a condition of 1.32\AA$\le h_{Pn} \le$1.42\AA, which is also the case for other Fe-pnictides with the Fe$^{2+}$ state in (Fe$Pn$)$^{-}$ layers. This novel phase evolution with the variation in $h_{Pn}$ points to the importance of electron correlation for the emergence of SC as well as AFM order.