Phase Diagram and Physical Properties of Co-doped and Cu-doped NaFeAs Single Crystals
A.F. Wang, J. J. Lin, X. G. Luo, Y. J. Yan, J. J. Ying, Z. J. Xiang, G. J. Ye, P. Cheng, F. Chen, J. Q. Ma, X. F. Lu, B. Lei, and X. H. Chen
Hefei National Laboratory for Physical Science at Microscale and Department of Physics,
University of Science and Technology of China,
Hefei, Anhui 230026, People’s Republic of China
Abstract: Series of high-quality NaFe1−xCuxAs and NaFe1−xCoxAs single crystals have been grown with the self-flux technique, and were
systematically characterized via structural, transport, thermodynamic, and high-pressure measurements.Both Cu and Co doping suppress
the structural and magnetic transitions and induce bulk superconductivity in NaFeAs. The superconducting transition temperature (Tc) is
initially enhanced from to 11.5 K by Cu doping and to 20 K by Co doping,and then suppressed with further substitution. The phase
diagramsof NaFe1−xCoxAs and NaFe1−xCuxAs are similar, except that’s emiconducting instead of metallic behavior is observed in the
extremely overdoped NaFe1−xCuxAs samples.In both doped system, Tc 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 andoptimally doped NaFe1−xCoxAs, Tcmax of NaFe1−xCuxAs is monotonically suppressed by Cu doping,
suggestingthat 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.