# Phase diagram and neutron spin resonance of superconducting NaFe$_{1-x}$Cu$_x$As

## TL;DR

This study investigates the phase diagram and spin excitations in NaFe$_{1-x}$Cu$_x$As, revealing superconductivity near 2% Cu doping, a transition to insulating phases at higher doping, and the role of electronic correlations in superconductivity.

## Contribution

It provides the first detailed phase diagram and neutron scattering analysis of NaFe$_{1-x}$Cu$_x$As, highlighting the importance of electronic correlations and impurity effects in its superconducting and magnetic properties.

## Key findings

- Superconductivity appears near 2% Cu doping with suppressed magnetic order.
- A high-energy, weakly dispersive neutron spin resonance is observed.
- The system transitions to an insulating antiferromagnetic phase near 50% Cu doping.

## Abstract

We use transport and neutron scattering to study the electronic phase diagram and spin excitations of NaFe$_{1-x}$Cu$_x$As single crystals. Similar to Co- and Ni-doped NaFeAs, a bulk superconducting phase appears near $x\approx2\%$ 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 $x\approx 50\%$. Using transport measurements, we demonstrate that the resistivity in NaFe$_{1-x}$Cu$_x$As exhibits non-Fermi-liquid behavior near $x\approx1.8\%$. Our inelastic neutron scattering experiments reveal a single neutron spin resonance mode exhibiting weak dispersion along $c$-axis in NaFe$_{0.98}$Cu$_{0.02}$As. The resonance is high in energy relative to the superconducting transition temperature $T_{\rm c}$ but weak in intensity, likely resulting from impurity effects. These results are similar to other iron pnictides superconductors despite the superconducting phase in NaFe$_{1-x}$Cu$_x$As is continuously connected to an antiferromagnetically ordered insulating phase near $x\approx 50\%$ with significant electronic correlations. Therefore, electron correlations is an important ingredient of superconductivity in NaFe$_{1-x}$Cu$_x$As and other iron pnictides.