  We have successfully grown single crystalline NaFeAs with cobalt or phosphor
doping. Bulk superconductivity occurs in NaFe$_{0.95}$Co$_{0.05}$As at 19 K
while much higher transition temperature is observed in
NaFeAs$_{0.8}$P$_{0.2}$, in which the Tc of 33 K is even higher than the
highest value realized by applying pressure in NaFeAs and all other
isoelectronic element substituted samples. We discuss these behaviours by
comparison with 122 and 1111 systems. We hope that our findings will help
improve our understanding of iron-based superconductivity.
