Electron doping evolution of structural and antiferromagnetic phase transitions in NaFe1-xCoxAs iron pnictides

We use transport and neutron diffraction to study the electronic phase diagram of NaFe1-xCoxAs. In the undoped state, NaFeAs exhibits a tetragonal-to-orthorhombic structural transition below Ts followed by a collinear antiferromagnetic (AF) order below TN. Upon codoping to form NaFe1-xCoxAs,Ts and TN are gradually suppressed, leading to optimal superconductivity near Co-doping x=0.025. While transport experiments on these materials reveal an anomalous behavior suggesting the presence of a quantum critical point (QCP) near optimal superconductivity, our neutron diffraction results indicate that commensurate AF order becomes transversely incommensurate with TN>Tc before vanishing abruptly at optimal superconductivity. These results are remarkably similar to electron-doping and isovalent-doping evolution of the AF order in BaFe2-xNixAs2 and BaFe2(As1-xPx)2, thus suggesting a universal behavior in the suppression of the magnetic order in iron pnictides as superconductivity is induced.