NaFe0.56Cu0.44As: A Pnictide Insulating Phase Induced by On-Site Coulomb Interaction

Abstract

In the studies of iron-pnictides, a key question is whether their bad-metal state from which the superconductivity emerges lies in close proximity with a magnetically ordered insulating phase. Recently it was found that at low temperatures, the heavily Cu-doped NaFe1-xCuxAs (x > 0.3) iron-pnictide is an insulator with long-range antiferromagnetic order, similar to the parent compound of cuprates but distinct from all other iron-pnictides. Using angle-resolved photoemission spectroscopy, we determined the momentum-resolved electronic structure of NaFe1-xCuxAs (x = 0.44) and identified that its ground state is a narrow-gap insulator. Combining the experimental results with density functional theory (DFT) and DFT+U calculations, our analysis reveals that the on-site Coulombic (Hubbard) and Hund's coupling energies play crucial roles in formation of the band gap about the chemical potential. We propose that at finite temperatures charge carriers are thermally excited from the Cu-As-like valence band into the conduction band, which is of Fe 3d-like character. With increasing temperature, the number of electrons in the conduction band becomes larger and the hopping energy between Fe sites increases, and finally the long-range antiferromagnetic order is destroyed at T > TN. Our study provides a basis for investigating the evolution of the electronic structure of a Mott insulator transforming into a bad metallic phase, and eventually forming a superconducting state in iron-pnictides.

Correlation-driven metal-insulator transition in proximity to an iron-based superconductor

“…The temperature dependence of σ 1 (ω → 0) displays no anomalies around the Néel transition temperature, indicating that the electronic band structure is unchanged across T N . This observation is consistent with the recent results of angle-resolved photoemission spectroscopy (ARPES) on the same compound [37]. The high-temperature behavior of σ dc , determined in the transport measurements in Ref.…”

“…4(g)], on the other hand, the aforementioned fluctuations become quenched and the electronic bands experience exchange splitting. The occupied states of this electronic band structure are in good agreement with the results of ARPES measurements on the same compound [37]. This band structure further reveals that the soft absorption edge identified earlier is formed by flat valence and conduction bands separated by 0.8 eV.…”

“…[22]). The high degree of crystalline order is further confirmed by the relatively sharp quasiparticle peaks observed in the ARPES measurements on the same compound in the antiferromagnetic state [37].…”

“…At low doping levels superconductivity emerges from a high-temperature metallic phase and reaches optimum once the SDW order of 195121-2 the parent compound has been sufficiently suppressed [21]. At doping levels near x = 0.5 NaFe 1−x Cu x As exhibits insulating behavior in the dc resistivity up to room temperature [21,22,37] with an activation energy on the order of 10 meV. In the same region of the phase diagram neutron diffraction investigations have revealed the presence of real-space ordering of nonmagnetic copper and magnetic iron ions into alternating stripes [22] [see Fig.…”

Phase diagram and neutron spin resonance of superconducting NaFe1−xCuxAs

“…1(a)] 18 , heavily doped NaFe 1−x Cu x As is a candidate system to tune the strength of electronic correlations 19 . Recent transport 18 , scanning tunneling microscopy (STM) 20 , angle-resolved photoemission spectroscopy 21 , and optical conductivity 22 measurements demonstrated that with significant (> 10%) Cu doping, NaFe 1−x Cu x As acquires an insulating ground state. Diffraction measurements revealed short-range Fe-Cu cation order and magnetic order develops as the system becomes insulating.…”