Phase diagram and neutron spin resonance of superconducting NaFe1−xCuxAs

We use transport and neutron scattering to study the electronic phase diagram and spin excitations of ${\mathrm{NaFe}}_{1\ensuremath{-}x}{\mathrm{Cu}}_x\mathrm{As}$ single crystals. Similar to Co- and Ni-doped NaFeAs, a bulk superconducting phase appears near $x\ensuremath{\approx}2%$ with the suppression of stripe-type magnetic order in NaFeAs. Upon further increasing Cu concentration the system becomes insulating, culminating in an antiferromagnetically ordered insulating phase near $x\ensuremath{\approx}50%$. Using transport measurements, we demonstrate that the resistivity in ${\mathrm{NaFe}}_{1\ensuremath{-}x}{\mathrm{Cu}}_x\mathrm{As}$ exhibits non-Fermi-liquid behavior near $x\ensuremath{\approx}1.8%$. Our inelastic neutron scattering experiments reveal a single neutron spin resonance mode exhibiting weak dispersion along $c$ axis in ${\mathrm{NaFe}}_0.98{\mathrm{Cu}}_0.02\mathrm{As}$. The resonance is high in energy relative to the superconducting transition temperature ${T}_{\mathrm{c}}$ but weak in intensity, likely resulting from impurity effects. These results are similar to other iron pnictides superconductors despite that the superconducting phase in ${\mathrm{NaFe}}_{1\ensuremath{-}x}{\mathrm{Cu}}_x\mathrm{As}$ is continuously connected to an antiferromagnetically ordered insulating phase near $x\ensuremath{\approx}50%$ with significant electronic correlations. Therefore, electron correlations is an important ingredient of superconductivity in ${\mathrm{NaFe}}_{1\ensuremath{-}x}{\mathrm{Cu}}_x\mathrm{As}$ and other iron pnictides.