A series of high-quality NaFe${}_{1-x}$Cu${}_x$As single crystals have been grown with the 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. The superconducting transition temperature ($T_c$) is initially enhanced from 9.6 to 11.5 K by Cu doping, and then suppressed with further doping. A phase diagram similar to NaFe${}_{1-x}$Co${}_x$As is obtained except that semiconducting instead of metallic behavior is observed in the extremely overdoped samples. $T_c$s of underdoped, optimally doped, and overdoped samples are all notably enhanced by applying pressure. Although a universal maximum transition temperature ($T_c^{\max }$) of about 31 K under external pressure is observed in underdoped and optimally doped NaFe${}_{1-x}$Co${}_x$As, $T_c^{\max }$ of NaFe${}_{1-x}$Cu${}_x$As is monotonically suppressed by Cu doping, suggesting that impurity potential of Cu is stronger than that of 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.

• Received 23 July 2013

DOI:https://doi.org/10.1103/PhysRevB.88.094516