Tuning the interplay between nematicity and spin fluctuations in Na_1-xLi _x FeAs superconductors

Area of Science:

• Condensed Matter Physics
• Materials Science

Background:

• Iron-based superconductors (FeSCs) exhibit a strong interplay between spin, charge, and orbital degrees of freedom.
• A nematic state typically emerges near the antiferromagnetic state in FeSCs, but its driving mechanism (spin vs. orbital fluctuations) is debated.
• Understanding nematicity's origin is crucial for advancing superconductivity research.

Purpose of the Study:

• To investigate the relationship between nematicity and spin fluctuations (SFs) in Na_1-xLi_xFeAs.
• To determine how doping affects the interplay between nematicity and SFs.
• To shed light on the fundamental nature and origin of nematicity in FeSCs.

Main Methods:

• Transport measurements
• Magnetization measurements
• ⁷⁵As nuclear magnetic resonance (NMR) spectroscopy

Main Results:

• For low Li doping (x ≤ 0.02), the nematic transition enhances spin fluctuations.
• For higher Li doping (x ≥ 0.03), a distinct nematic state emerges that suppresses spin fluctuations.
• A drastic, doping-induced change in the spin fluctuation spectrum associated with nematicity was observed.

Conclusions:

• The study reveals a transformation in how nematicity influences spin fluctuations in Na_1-xLi_xFeAs with doping.
• This finding provides critical insights into the complex relationship between electronic orders in FeSCs.
• The results challenge existing models and offer new directions for understanding nematicity in superconductors.