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# π-electron S = ½ quantum spin-liquid state in an ionic polyaromatic hydrocarbon

## Abstract

Molecular solids with cooperative electronic properties based purely on π electrons from carbon atoms offer a fertile ground in the search for exotic states of matter, including unconventional superconductivity and quantum magnetism. The field was ignited by reports of high-temperature superconductivity in materials obtained by the reaction of alkali metals with polyaromatic hydrocarbons, such as phenanthrene and picene, but the composition and structure of any compound in this family remained unknown. Here we isolate the binary caesium salts of phenanthrene, Cs(C14H10) and Cs2(C14H10), to show that they are multiorbital strongly correlated Mott insulators. Whereas Cs2(C14H10) is diamagnetic because of orbital polarization, Cs(C14H10) is a Heisenberg antiferromagnet with a gapped spin-liquid state that emerges from the coupled highly frustrated Δ-chain magnetic topology of the alternating-exchange spiral tubes of S = ½ (C14H10)•− radical anions. The absence of long-range magnetic order down to 1.8 K (T/J ≈ 0.02; J is the dominant exchange constant) renders the compound an excellent candidate for a spin-½ quantum-spin liquid (QSL) that arises purely from carbon π electrons.

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## Acknowledgements

This work was sponsored by the World Premier International (WPI) Research Center Initiative for Atoms, Molecules and Materials, Ministry of Education, Culture, Sports, Science and Technology of Japan. We acknowledge financial support from the Japan Science and Technology Agency under the ERATO Isobe Degenerate π-Integration Project, the Mitsubishi Foundation, the Japan Society for the Promotion of Science (JSPS) under the Scientific Research on Innovative Areas ‘J-Physics’ Project (No. 15H05882), the European Union/JST SICORP-LEMSUPER FP7-NMP-2011-EU-Japan project (contract no. NMP3-SL-2011-283214), the UK Engineering and Physical Sciences Research Council (grant nos EP/K027255 and EP/K027212) and the Slovenian Research Agency (grant no. N1-0052). We thank the ESRF for access to synchrotron X-ray facilities, the Royal Society for a Newton International Fellowship (G.K.) and a Research Professorship (M.J.R.), A. N. Fitch for help with the synchrotron XRD experiments, H. Okazaki with the magnetic measurements and K. Kamarás with the infrared measurements.

## Author information

Authors

### Contributions

K.P. and M.J.R. conceived and designed the project. K.P. directed and coordinated the research. Y.T. and M.M. interpreted and discussed all the results and carried out the final structural and magnetic work. H.T., N.T., T.K., Y.N. and R.A. carried out the calculations. A.Š. synthesized the materials. D.A. carried out the EPR spectroscopy and G.K. the vibrational spectroscopic work. A.J.C.B. and A.Š. carried out early structural and magnetic work. K.P. wrote the paper with input from all the authors.

### Corresponding authors

Correspondence to Matthew J. Rosseinsky or Kosmas Prassides.

## Ethics declarations

### Competing interests

The authors declare no competing financial interests.

## Supplementary information

### Supplementary information

Supplementary information (PDF 2391 kb)

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Takabayashi, Y., Menelaou, M., Tamura, H. et al. π-electron S = ½ quantum spin-liquid state in an ionic polyaromatic hydrocarbon. Nature Chem 9, 635–643 (2017). https://doi.org/10.1038/nchem.2764

• Accepted:

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• DOI: https://doi.org/10.1038/nchem.2764

• ### Redox-controlled potassium intercalation into two polyaromatic hydrocarbon solids

• F. Denis Romero
• M. J. Pitcher
• M. J. Rosseinsky

Nature Chemistry (2017)

• ### Synthesis successes

• Roser Valentí
• Stephen M. Winter

Nature Chemistry (2017)