Tuning the interplay between nematicity and spin fluctuations in Na1−xLi x FeAs superconductors

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 75As nuclear magnetic resonance (NMR) measurements, we show a striking transformation of the relationship between nematicity and spin fluctuations (SFs) in Na1−xLixFeAs; For x ≤ 0.02, the nematic transition promotes SFs. In contrast, for x ≥ 0.03, the system undergoes a non-magnetic phase transition at a temperature T0 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.

Quantitative characterization of short-range orthorhombic fluctuations in FeSe through pair distribution function analysis

“…In FeSe and other systems, valuable insight can be gained through studies of not only the static nematic phase, but also fluctuations and other signatures of the nematic phase at temperatures above T s [8,9,[26][27][28][29][30][31][32][33][34][35][36][37][38][39][40][41]. These fluctuations may contain clues about the origin of nematicity and its connection to superconductivity.…”

Widespread orthorhombic fluctuations in the (Sr,Na)Fe2As2 family of superconductors

“…The hope is that such fluctuations may reveal important details about the origin of nematicity and ultimately superconductivity. Signatures of high-temperature nematicity above the static nematic phase have in fact been observed in numerous iron pnictide and chalcogenide systems with a variety of experimental probes [17][18][19][20][21][22][23][24][25][26][27][28], indicating that an effort to characterize these nematic fluctuations systematically within and between phase diagrams of representative FeSCs holds great promise.…”

Observation of a Ubiquitous (π, π)-Type Nematic Superconducting Order in the Whole Superconducting Dome of Ultra-Thin BaFe2–x Ni x As2 Single Crystals

“…[49] Although the nematicity is believed to be of electronic origin, [49−59] the driving force for this nematic phase is still under debate, because the coupling of orbital and spin degrees of freedom makes it difficult to identify whether the spin or orbital fluctuations take the leading role. [29,[32][33][34]39,40,44,49,54,60] Since the spin fluctuations favor a sign-changing 𝑠wave superconducting state [61−64] (𝑠 ± ) while the orbital fluctuations favor a sign-preserving 𝑠-wave state [65] (𝑠 ++ ), understanding the nematic phase can shed light on the superconducting mechanism in ironbased superconductors. [49,57,60] More importantly, given the universal presence of the electronic nematic phase and its proximity to the superconducting state, it is considered to have a close relationship with the superconductivity.…”