Physical properties and phase diagram of NaFe1−x V x As*

We grew a series of NaFe1−x V x As (0 ≤ x ≤ 0.03) single crystals and performed the measurements of resistance, magnetic susceptibility, and specific heat to study the superconducting phase diagram by doping V into NaFeAs. Both the structural and the spin-density-wave (SDW) transitions are slightly suppressed by V-doping. While superconducting transition temperature is enhanced to the maximum value of ∼ 15 K when the optimal doping level x = 0.007 and then is suppressed rapidly with further V-doping, displaying a small superconducting dome. Our results suggest that V-impurities should act as strong magnetic scattering centers which cause the sharp suppression of superconductivity in NaFe1−x V x As.