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, resonant inelastic X-ray scattering and neutron scattering to demonstrate that NaFe1−xCuxAs near x≈0.5 exhibits real space Fe and Cu ordering, and are antiferromagnetic insulators with the insulating behaviour persisting above the Néel temperature, indicative of a Mott insulator. On decreasing x from 0.5, the antiferromagnetic-ordered moment continuously decreases, yielding to superconductivity ∼x=0.05. Our discovery of a Mott-insulating state in NaFe1−xCuxAs 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-Tc superconductivity.
Highlights
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
Neutron scattering, transmission electron microscopy (TEM), X-ray absorption spectroscopy (XAS) and resonant inelastic X-ray scattering (RIXS), we demonstrate that heavily Cu-doped NaFe1 À xCuxAs exhibits Fe and Cu ordering, and becomes an AF insulator when x approaches 0.5
The behaviour in NaFe1 À xCuxAs is entirely different from the bipartite magnetic parent phases seen in the iron oxypnictide superconductor LaFeAsO1 À xHx, where magnetic parent phases on both sides of the superconducting dome are metallic antiferromagnets[33]
Summary
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. The parent compounds of iron pnictide superconductors such as AFe2As2 (A 1⁄4 Ba, Sr, Ca) and NaFeAs have crystal structures shown in Fig. 1a,b, respectively[2]. They exhibit a tetragonal-to-orthorhombic structural phase transition at temperature Ts, followed by a paramagnetic to AF phase transition at TN (TsZTN) with a collinear magnetic structure, where the spins are aligned antiferromagnetically along the a axis of the orthorhombic lattice (Fig. 1c)[22,23].
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