Charnukha, A.Yin, Z.P.Song, Y.Cao, C.D.Dai, PengchengHaule, K.Kotliar, G.Basov, D.N.2017-12-212017-12-212017Charnukha, A., Yin, Z.P., Song, Y., et al.. "Correlation-driven metal-insulator transition in proximity to an iron-based superconductor." <i>Physical Review B,</i> 96, no. 19 (2017) American Physical Society: https://doi.org/10.1103/PhysRevB.96.195121.https://hdl.handle.net/1911/98916We report the direct spectroscopic observation of a metal to correlated-insulator transition in the family of iron-based superconducting materials. By means of optical spectroscopy we demonstrate that the excitation spectrum of NaFe1−xCuxAs develops a large gap with increasing copper substitution. Dynamical mean-field theory calculations show a good agreement with the experimental data and suggest that the formation of the charge gap requires an intimate interplay of strong on-site electronic correlations and spin-exchange coupling, revealing the correlated Slater-insulator nature of the antiferromagnetic ground state. Our calculations further predict the high-temperature paramagnetic state of the same compound to be a highly incoherent correlated metal. We verify this prediction experimentally by showing that the doping-induced weakening of antiferromagnetic correlations enables a thermal crossover from an insulating to an incoherent metallic state. Redistribution of the optical spectral weight in this crossover uncovers the characteristic energy of Hund's-coupling and Mott-Hubbard electronic correlations essential for the electronic localization. Our results demonstrate that NaFe1−xCuxAs continuously transitions from the typical itinerant phases of iron pnictides to a highly incoherent metal and ultimately a correlated insulator. Such an electronic state is expected to favor high-temperature superconductivity.engArticle is made available in accordance with the publisher's policy and may be subject to US copyright law. Please refer to the publisher's site for terms of use.Journal articlePhysRevB-96-195121https://doi.org/10.1103/PhysRevB.96.195121