Quantum spin liquids unveil the genuine Mott state

• Published in: Nature materials Vol. 17; no. 9; pp. 773 - 777
• Main Authors: Pustogow, A., Bories, M., Löhle, A., Rösslhuber, R., Zhukova, E., Gorshunov, B., Tomić, S., Schlueter, J. A., Hübner, R., Hiramatsu, T., Yoshida, Y., Saito, G., Kato, R., Lee, T.-H., Dobrosavljević, V., Fratini, S., Dressel, M.
• Format: Journal Article
• Language: English
• Published: London Nature Publishing Group UK 01.09.2018; Nature Publishing Group; Springer Nature - Nature Publishing Group
• Subjects: 639/301/1034/1038; 639/301/119/2795; 639/766/119/995; 639/766/119/999; 639/766/483/640; Antiferromagnetism; Asymmetry; Bandwidths; Biomaterials; Chemistry and Materials Science; Condensed Matter; Condensed Matter Physics; Cooling; Cuprates; Current carriers; Energy; High temperature; Insulators; Laboratories; Letter; Materials Science; Metal-insulator transition; Nanotechnology; Optical and Electronic Materials; Phase diagrams; Phase transitions; Physics; Quantum phenomena; Solids; Spin liquid; Strongly Correlated Electrons; Unconventional superconductivity
• ISSN: 1476-1122; 1476-4660; 1476-4660
• DOI: 10.1038/s41563-018-0140-3

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 since 1 – 3 —not least for its relation to high-temperature superconductivity 4 . 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. A thorough analysis of the optical and transport properties of several two-dimensional organic conductors and insulators with varying on-site correlation strengths and bandwidths led to a quantitative phase diagram for pristine Mott insulators.