Simultaneous optimization of spin fluctuations and superconductivity under pressure in an iron-based superconductor

TL;DR

This study uses high-pressure NMR to investigate how spin fluctuations and superconductivity in an iron-based superconductor are simultaneously optimized, revealing a strong link between magnetic correlations and superconducting properties.

Contribution

It provides direct experimental evidence that magnetic correlations and superconductivity are concurrently maximized under pressure, supporting a magnetic origin of superconductivity in this material.

Findings

Spin fluctuations increase with pressure up to 2.2 GPa, then decrease.

Superconducting transition temperature T_c follows the pressure dependence of spin fluctuations.

No magnetic order or structural transition occurs up to 2.5 GPa.

Abstract

We present a high-pressure NMR study of the overdoped iron pnictide superconductor NaFe$_{0.94}$Co$_{0.06}$As. The low-energy antiferromagnetic spin fluctuations in the normal state, manifest as the Curie-Weiss upturn in the spin-lattice relaxation rate $1/^{75}T_1T$, first increase strongly with pressure but fall again at $P > P_{\rm opt} =$ 2.2 GPa. Neither long-ranged magnetic order nor a structural phase transition is encountered up to 2.5 GPa. The superconducting transition temperature $T_c$ shows a pressure-dependence identical to the spin fluctuations. Our observations demonstrate that magnetic correlations and superconductivity are optimized simultaneously as a function of the electronic structure, thereby supporting very strongly a magnetic origin of superconductivity.