Phase diagram and physical properties of NaFe₁₋ₓCuₓAs single crystals

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A series of high quality NaFe₁₋ₓCuₓAs single crystals has been grown by a self-flux technique, which were systematically characterized via structural, transport, thermodynamic, and high pressure measurements. Both the structural and magnetic transitions are suppressed by Cu doping, and bulk superconductivity is induced by Cu doping. Superconducting transition temperature (Tc) is initially enhanced from 9.6 to 11.5 K by Cu doping, and then suppressed with further doping. A phase diagram similar to NaFe₁₋ₓCoₓAs is obtained except that insulating instead of metallic behavior is observed in extremely overdoped samples. Tc's of underdoped, optimally doped, and overdoped samples are all notably enhanced by applying pressure. Although a universal maximum transition temperature (Tcmax) of about 31 K under external pressure is observed in underdoped and optimally doped NaFe₁₋ₓCoₓAs, Tcmax of NaFe₁₋ₓCuₓAs is monotonously suppressed by Cu doping, suggesting that impurity potential of Cu is stronger than Co in NaFeAs. The comparison between Cu and Co doping effect in NaFeAs indicates that Cu serves as an effective electron dopant with strong impurity potential, but part of the doped electrons are localized and do not fill the energy bands as predicted by the rigid-band model.