The localization of charge carriers by electronic repulsion was suggested by Mott in the 1930s to explain the insulating state observed in supposedly metallic NiO. The Mott metal–insulator transition has been subject of intense investigations ever since1,2,3—not least for its relation to high-temperature superconductivity4. A detailed comparison to real materials, however, is lacking because the pristine Mott state is commonly obscured by antiferromagnetism and a complicated band structure. Here we study organic quantum spin liquids, prototype realizations of the single-band Hubbard model in the absence of magnetic order. Mapping the Hubbard bands by optical spectroscopy provides an absolute measure of the interaction strength and bandwidth—the crucial parameters that enter calculations. In this way, we advance beyond conventional temperature–pressure plots and quantitatively compose a generic phase diagram for all genuine Mott insulators based on the absolute strength of the electronic correlations. We also identify metallic quantum fluctuations as a precursor of the Mott insulator–metal transition, previously predicted but never observed. Our results suggest that all relevant phenomena in the phase diagram scale with the Coulomb repulsion U, which provides a direct link to unconventional superconductivity in cuprates and other strongly correlated materials.
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We thank K. Kanoda and R. Valentí for fruitful discussions. Part of the work is supported by the Deutsche Forschungsgemeinschaft via DR228/41-1 and DR228/48-1. We also thank the Deutscher Akademischer Austauschdienst for support. This work was partially supported by JSPS KAKENHI grant no. JP16H06346. We acknowledge the Russian Ministry of Education and Science (Program ‘5 top 100’). We also acknowledge support from the Croatian Science Foundation project IP-2013-11-1011. J.A.S. acknowledges support from the Independent Research and Development program from the NSF while working at the Foundation and from the National High Magnetic Field Laboratory User Collaboration Grants Program. Work in Florida was supported by the NSF grant no. DMR-1410132, and the National High Magnetic Field Laboratory through the NSF cooperative agreement no. DMR-1157490 and the State of Florida. Parts of the text and results reported in this work are reproduced from the thesis of A.P.33 at the University of Stuttgart, and accessible at https://doi.org/10.18419/opus-9487.
The authors declare no competing interests.
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Pustogow, A., Bories, M., Löhle, A. et al. Quantum spin liquids unveil the genuine Mott state. Nature Mater 17, 773–777 (2018). https://doi.org/10.1038/s41563-018-0140-3
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