Dynamic spin-lattice coupling and nematic fluctuations in NaFeAs

TL;DR

This study uses inelastic neutron scattering to explore how spin and lattice dynamics interact in NaFeAs, revealing nematic fluctuations linked to spin-lattice coupling that influence phonon behavior and magnetic correlations.

Contribution

It provides new insights into the dynamic spin-lattice coupling and nematic fluctuations in NaFeAs, highlighting their role in the material's structural and magnetic transitions.

Findings

In-plane transverse acoustic phonons soften near Ts and harden approaching TN.

Spin-spin correlation lengths increase below Ts with weak anomaly at TN.

Evidence of electronic nematic phase associated with dynamic spin-lattice interactions.

Abstract

We use inelastic neutron scattering to study acoustic phonons and spin excitations in single crystals of NaFeAs, a parent compound of iron pnictide superconductors. NaFeAs exhibits a tetragonal-to-orthorhombic structural transition at $T_s\approx 58$ K and a collinear antiferromagnetic (AF) order at $T_N\approx 45$ K. While longitudinal and out-of-plane transverse acoustic phonons behave as expected, the in-plane transverse acoustic phonons reveal considerable softening on cooling to $T_s$, and then harden on approaching $T_N$ before saturating below $T_N$. In addition, we find that spin-spin correlation lengths of low-energy magnetic excitations within the FeAs layer and along the $c$-axis increase dramatically below $T_s$, and show weak anomaly across $T_N$. These results suggest that the electronic nematic phase present in the paramagnetic tetragonal phase is closely associated with dynamic spin-lattice coupling, possibly arising from the one-phonon-two-magnon mechanism.