Nematic fluctuations in iron arsenides NaFeAs and LiFeAs probed by 75As NMR

As-75 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-s = 57 K and an antiferromagnetic transition at T-N = 42 K, and LiFeAs having a superconducting transition at T-c = 18 K. We observe the in-plane anisotropy of the electric field gradient eta even in the tetragonal phase of NaFeAs and LiFeAs, showing the local breaking of tetragonal C-4 symmetry eta Then, eta is found to obey the Curie-Weiss (CW) law as well as in Ba(Fe1-xCox)(2)As-2. The good agreement between eta and the nematic susceptibility obtained by electronic Raman spectroscopy indicates that eta is governed by the nematic susceptibility. From comparing eta in NaFeAs and LiFeAs with eta in Ba(Fe1-xCox)(2)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 eta with the nuclear spin-lattice relaxation rate divided by temperature 1/T1T. Finally, we discuss the nematic transition in the paramagnetic orthorhombic phase of NaFeAs from the in-plane anisotropy of 1/T1T.