Nematic fluctuations in iron arsenides NaFeAs and LiFeAs probed by As75 NMR

Abstract

$^{75}\mathrm{As}$ NMR measurements have been made on single crystals to study the nematic state in the iron arsenides NaFeAs, which undergoes a structural transition from a high-temperature (high-$T$) tetragonal phase to a low-$T$ orthorhombic phase at ${T}_{\mathrm{s}}=57$ K and an antiferromagnetic transition at ${T}_{\mathrm{N}}=42$ K, and LiFeAs having a superconducting transition at ${T}_{\mathrm{c}}=18$ K. We observe the in-plane anisotropy of the electric field gradient $\ensuremath{\eta}$ even in the tetragonal phase of NaFeAs and LiFeAs, showing the local breaking of tetragonal ${C}_{4}$ symmetry. Then, $\ensuremath{\eta}$ is found to obey the Curie-Weiss (CW) law as well as in $\mathrm{Ba}{({\mathrm{Fe}}_{1\ensuremath{-}x}{\mathrm{Co}}_{x})}_{2}{\mathrm{As}}_{2}$. The good agreement between $\ensuremath{\eta}$ and the nematic susceptibility obtained by electronic Raman spectroscopy indicates that $\ensuremath{\eta}$ is governed by the nematic susceptibility. From comparing $\ensuremath{\eta}$ in NaFeAs and LiFeAs with $\ensuremath{\eta}$ in $\mathrm{Ba}{({\mathrm{Fe}}_{1\ensuremath{-}x}{\mathrm{Co}}_{x})}_{2}{\mathrm{As}}_{2}$, we discuss the carrier-doping dependence of the nematic susceptibility. The spin contribution to nematic susceptibility is also discussed from comparing the CW terms in $\ensuremath{\eta}$ with the nuclear spin-lattice relaxation rate divided by temperature $1/{T}_{1}T$. Finally, we discuss the nematic transition in the paramagnetic orthorhombic phase of NaFeAs from the in-plane anisotropy of $1/{T}_{1}T$.