Fig. 1

Characterization of the Na_1−xLi _x FeAs single crystals. a Crystal structure of Na_1−xLi _x FeAs. b Variation of the c-axis lattice parameter of the Na_1−xLi _x FeAs single crystals at 300 K with a linear dashed guide line. Inset shows the ab-plane image of a single crystal in a millimeter scale. c Zero-field cooled magnetizations at low temperatures at H _ab  = 10 Oe, revealing SC transition and shielding fractions. d–f Temperature dependence of normalized resistivity, ρ/ρ_300K, enlarged resistivity near the SC transition, and dρ/dT for selected samples. Black and magenta arrows in d, f denote SDW (T_SDW) and structural (T _S ) transitions, respectively, while blue arrows in e reflect the \(T_{\mathrm{c}}^\rho\) evolution with x. Note that although previous studies of NaFeAs have observed a local inflection point in the ρ curve at T_SDW (or a sharp dip in dρ/dT), we observed weak minimum for our NaFeAs due to the presence of twinning