Nematic fluctuations in NaFe_1-xNi_xAs

We report elastroresistivity measurements on single crystals of NaFe_1−xNi_xAs. The nematic susceptibility is defined as proportional to the change of resistivity under uniaxial pressure, whose temperature dependence can be well fitted by a Curie-Weiss-like function. Surprisingly, the sign of nematic susceptibility changes from positive to negative with just slight Ni doping, which is contrary to the expectation that electron doping should always lead to positive nematic susceptibility. Compared to our previous results on BaFe_2-xNi_xAs₂ system, although the mean-field nematic transition temperature from the Curie-Weiss fitting of the nematic susceptibility becomes zero around optimal doping, our results do not support the presence of a nematic quantum critical point in this system. We will also provide discussions on the possible influence of nematic fluctuations on the zero-pressure resistivity.