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 Ts≈58 K and a collinear antiferromagnetic (AF) order at TN≈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 Ts, and then harden on approaching TN before saturating below TN. 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 Ts, and show weak anomaly across TN. 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.
@article{arxiv.1805.03493,
title = {Dynamic spin-lattice coupling and nematic fluctuations in NaFeAs},
author = {Yu Li and 1 Zahra Yamani and 2 Yu Song and 1 Weiyi Wang and 1 Chenglin Zhang and 1 David W. Tam and Tong Chen and Ding Hu and Zhuang Xu and Songxue Chi and Ke Xia and Li Zhang and Shifeng Cui and Wenan Guo and Ziming Fang and Yi Liu and Pengcheng Dai},
journal= {arXiv preprint arXiv:1805.03493},
year = {2018}
}