Letter | Published:

A spin–orbital-entangled quantum liquid on a honeycomb lattice

Nature volume 554, pages 341345 (15 February 2018) | Download Citation

Abstract

The honeycomb lattice is one of the simplest lattice structures. Electrons and spins on this simple lattice, however, often form exotic phases with non-trivial excitations. Massless Dirac fermions can emerge out of itinerant electrons, as demonstrated experimentally in graphene1, and a topological quantum spin liquid with exotic quasiparticles can be realized in spin-1/2 magnets, as proposed theoretically in the Kitaev model2. The quantum spin liquid is a long-sought exotic state of matter, in which interacting spins remain quantum-disordered without spontaneous symmetry breaking3. The Kitaev model describes one example of a quantum spin liquid, and can be solved exactly by introducing two types of Majorana fermion2. Realizing a Kitaev model in the laboratory, however, remains a challenge in materials science. Mott insulators with a honeycomb lattice of spin–orbital-entangled pseudospin-1/2 moments have been proposed4, including the 5d-electron systems α-Na2IrO3 (ref. 5) and α-Li2IrO3 (ref. 6) and the 4d-electron system α-RuCl3 (ref. 7). However, these candidates were found to magnetically order rather than form a liquid at sufficiently low temperatures8,9,10, owing to non-Kitaev interactions6,11,12,13. Here we report a quantum-liquid state of pseudospin-1/2 moments in the 5d-electron honeycomb compound H3LiIr2O6. This iridate does not display magnetic ordering down to 0.05 kelvin, despite an interaction energy of about 100 kelvin. We observe signatures of low-energy fermionic excitations that originate from a small number of spin defects in the nuclear-magnetic-resonance relaxation and the specific heat. We therefore conclude that H3LiIr2O6 is a quantum spin liquid. This result opens the door to finding exotic quasiparticles in a strongly spin–orbit-coupled 5d-electron transition-metal oxide.

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Acknowledgements

We thank Y. Motome, M. Udagawa, R. Valentí, A. Gibbs, Y. B. Kim, A. Smerald and N. Shannon for discussions, and U. Wedig, Y. Ishikuro, T. Nishioka and S. Nakatsuji for experimental support and discussions. This work was partly supported by the Japan Society for the Promotion of Science (JSPS) KAKAENHI (numbers 24224010, 26707018, 15K13523, JP15H05852, JP15K21717 and 17H01140) and the Alexander von Humboldt foundation.

Author information

Author notes

    • K. Kitagawa
    •  & T. Takayama

    These authors contributed equally to this work.

Affiliations

  1. Department of Physics, University of Tokyo, Bunkyo-ku, Hongo 7-3-1, Tokyo 113-0033, Japan

    • K. Kitagawa
    • , A. Kato
    • , R. Takano
    •  & H. Takagi
  2. Max Planck Institute for Solid State Research, Heisenbergstrasse 1, 70569 Stuttgart, Germany

    • T. Takayama
    • , Y. Matsumoto
    • , S. Bette
    • , R. Dinnebier
    • , G. Jackeli
    •  & H. Takagi
  3. Graduate School of Integrated Arts and Sciences, Kochi University, Akebonocho 2-5-1, Kochi 780-8520, Japan

    • Y. Kishimoto
  4. Institute for Functional Matter and Quantum Technologies, University of Stuttgart, Pfaffenwaldring 57, 70569 Stuttgart, Germany

    • G. Jackeli
    •  & H. Takagi

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Contributions

T.T. and A.K. prepared the sample and performed the bulk experiments. K.K., R.T. and Y.K. carried out the NMR measurements. Y.M. carried out the low-temperature specific heat measurements. S.B. and R.D. performed structural analysis. G.J. gave theoretical inputs. T.T., K.K., Y.M. and H.T. wrote manuscript and all authors commented on it. H.T. designed and supervised the experiments.

Competing interests

The authors declare no competing financial interests.

Corresponding author

Correspondence to H. Takagi.

Reviewer Information Nature thanks M. Mourigal and S. Todadri for their contribution to the peer review of this work.

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DOI

https://doi.org/10.1038/nature25482

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