Tuning the interplay between nematicity and spin fluctuations in Na$_{1-x}$Li$_x$FeAs superconductors

TL;DR

This study investigates how small doping levels in Na$_{1-x}$Li$_x$FeAs superconductors alter the relationship between nematicity and spin fluctuations, revealing a transition from nematicity promoting to suppressing spin fluctuations.

Contribution

It uncovers a doping-dependent switch in the interplay between nematicity and spin fluctuations, providing new insights into the origin of nematicity in iron-based superconductors.

Findings

For x ≤ 0.02, nematicity promotes spin fluctuations.

For x ≥ 0.03, nematicity suppresses spin fluctuations.

Small doping causes a drastic change in the nematic-spin fluctuation relationship.

Abstract

Strong interplay of spin and charge/orbital degrees of freedom is the fundamental characteristic of the iron-based superconductors (FeSCs), which leads to the emergence of a nematic state as a rule in the vicinity of the antiferromagnetic state. Despite intense debate for many years, however, whether nematicity is driven by spin or orbital fluctuations remains unsettled. Here, by use of transport, magnetization, and $^{75}$As nuclear magnetic resonance (NMR) measurements, we show a striking transformation of the relationship between nematicity and spin fluctuations (SFs) in Na$_{1-x}$Li$_x$FeAs; For $x\leq 0.02$, the nematic transition promotes SFs. In contrast, for $x\geq 0.03$, the system undergoes a non-magnetic phase transition at a temperature $T_0$ into a distinct nematic state that suppresses SFs. Such a drastic change of the spin fluctuation spectrum associated with nematicity by small doping is highly unusual, and provides insights into the origin and nature of nematicity in FeSCs.