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Unconventional thermal metallic state of charge-neutral fermions in an insulator

Nature Physics volume 15, pages 954–959 (2019)Cite this article

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Abstract

Quantum oscillations in transport and thermodynamic parameters at high magnetic fields are an unambiguous signature of the Fermi surface, the defining characteristic of a metal. Recent observations of quantum oscillations in insulating SmB6 and YbB12, therefore, have been a big surprise—despite the large charge gap inferred from the insulating behaviour of the resistivity, these compounds seemingly host a Fermi surface at high magnetic fields. However, the nature of the ground state in zero field has been little explored. Here, we report the use of low-temperature heat-transport measurements to discover gapless, itinerant, charge-neutral excitations in the ground state of YbB12. At zero field, sizeable linear temperature-dependent terms in the heat capacity and thermal conductivity are clearly resolved in the zero-temperature limit, indicating the presence of gapless fermionic excitations with an itinerant character. Remarkably, linear temperature-dependent thermal conductivity leads to a spectacular violation of the Wiedemann–Franz law: the Lorenz ratio is 104–105 times larger than that expected in conventional metals, indicating that YbB12 is a charge insulator and a thermal metal. Moreover, we find that these fermions couple to magnetic fields, despite their charge neutrality. Our findings expose novel quasiparticles in this unconventional quantum state.

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Fig. 1: The resistivity of YbB12.
Fig. 2: The heat capacity of YbB12.
Fig. 3: The thermal conductivity of YbB12.
Fig. 4: The thermal Hall angle of YbB12.

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Data availability

The data that support the plots within this paper and other findings of this study are available from the corresponding authors upon reasonable request.

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Acknowledgements

We thank K. Behnia, D. Chowdhury, P. Coleman, J. Knolle, H. v. Löhneysen, E.-G. Moon, R. Peters, S. Sebastian, T. Senthil and L. Taillefer for fruitful discussions. This work was supported by Grants-in-Aid for Scientific Research (KAKENHI) (nos. 25220710, 15H02106, 15H03688, 16K13837, 18H01177, 18H01180 and 18H05227) and on Innovative Areas ‘Topological Material Science’ (no. 15H05852) from the Japan Society for the Promotion of Science (JSPS). This work at Michigan is mainly supported by the Office of Naval Research through the Young Investigator Prize under Award No. N00014-15-1-2382 (electrical transport characterization), by the National Science Foundation under Award No. DMR-1707620 (magnetization measurement) and by the National Science Foundation Major Research Instrumentation award under No. DMR-1428226 (the equipment of the electrical transport characterizations). The development of the torque magnetometry technique in intense magnetic fields was supported by the Department of Energy under Award No. DE-SC0008110. A portion of this work was performed at the NHMFL, which is supported by National Science Foundation Cooperative Agreement No. DMR-1644779, the Department of Energy and the State of Florida. J.S. thanks the Department of Energy for support from the BES programme ‘Science in 100 T’. The experiment at the NHMFL is funded in part by a QuantEmX grant from ICAM and the Gordon and Betty Moore Foundation through grant GBMF5305 to Z.X., T.A., L.C., C.T. and L.L. We are grateful for the assistance of T. Murphy, H. Baek, G. Jones, F. Balakirev, R. McDonald, J. Betts and J.-H. Park of the NHMFL.

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Authors and Affiliations

  1. Department of Physics, Kyoto University, Kyoto, Japan

    Y. Sato, Y. Kasahara, T. Taniguchi, S. Kasahara, H. Murayama & Y. Matsuda

  2. Department of Physics, University of Michigan, Ann Arbor, MI, USA

    Z. Xiang, L. Chen, T. Asaba, C. Tinsman & Lu Li

  3. MPA-CMMS, Los Alamos National Laboratory, Los Alamos, NM, USA

    T. Asaba

  4. Department of Advanced Materials Science, University of Tokyo, Chiba, Japan

    O. Tanaka, Y. Mizukami & T. Shibauchi

  5. Institute of Quantum Beam Science, Graduate School of Science and Engineering, Ibaraki University, Mito, Japan

    F. Iga

  6. Los Alamos National Laboratory, Los Alamos, NM, USA

    J. Singleton

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Contributions

F.I. grew the high-quality single-crystalline samples. Y.S., Y.K., S.K. and H.M. performed the thermal transport measurements. T.T., S.K., O.T. and Y.Mizukami performed the heat capacity measurements. Z.X., L.C, T.A., C.T., J.S. and L.L. performed the high-field resistivity measurements. Y.S., Z.X., Y.K., T.T., S.K., H.M., Y.Mizukami, T.S., L.L. and Y.Matsuda analysed the data. Y.S., Y.K., T.S., J.S., L.L. and Y.Matsuda prepared the manuscript.

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Correspondence to Lu Li or Y. Matsuda.

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Sato, Y., Xiang, Z., Kasahara, Y. et al. Unconventional thermal metallic state of charge-neutral fermions in an insulator. Nat. Phys. 15, 954–959 (2019). https://doi.org/10.1038/s41567-019-0552-2

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