A Mott insulator continuously connected to iron pnictide superconductors

TL;DR

This paper demonstrates that NaFe$_{1-x}$Cu$_x$As near $x\approx 0.5$ is a Mott insulator with antiferromagnetic order, and shows a continuous transition from this insulating state to superconductivity as doping decreases, emphasizing electron correlations in high-$T_c$ superconductivity.

Contribution

It provides the first evidence of a Mott insulating state in an iron-based superconductor, connecting it smoothly to superconductivity through doping.

Findings

NaFe$_{1-x}$Cu$_x$As near $x\approx 0.5$ is a Mott insulator.

Superconductivity emerges as doping decreases from the Mott insulating state.

The insulating behavior persists above the N\'eel temperature, indicating a Mott insulator.

Abstract

Iron-based superconductivity develops near an antiferromagnetic order and out of a bad metal normal state, which has been interpreted as originating from a proximate Mott transition. Whether an actual Mott insulator can be realized in the phase diagram of the iron pnictides remains an open question. Here we use transport, transmission electron microscopy, X-ray absorption spectroscopy, and neutron scattering to demonstrate that NaFe$_{1-x}$Cu$_x$As near $x\approx 0.5$ exhibits real space Fe and Cu ordering, and are antiferromagnetic insulators with the insulating behavior persisting above the N\'eel temperature, indicative of a Mott insulator. Upon decreasing $x$ from $0.5$, the antiferromagnetic ordered moment continuously decreases, yielding to superconductivity around $x=0.05$. Our discovery of a Mott insulating state in NaFe$_{1-x}$Cu$_x$As thus makes it the only known Fe-based material in which superconductivity can be smoothly connected to the Mott insulating state, highlighting the important role of electron correlations in the high-$T_{\rm c}$ superconductivity.