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

Y. Sato, Z. Xiang, Y. Kasahara, T. Taniguchi, S. Kasahara, L. Chen, T. Asaba, C. Tinsman, H. Murayama, O. Tanaka, Y. Mizukami, T. Shibauchi, F. Iga, J. Singleton, Lu Li, Y. Matsuda

Research output: Contribution to journalArticlepeer-review

24 Citations (Scopus)


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.

Original languageEnglish
Pages (from-to)954-959
Number of pages6
JournalNature Physics
Issue number9
Publication statusPublished - Sept 1 2019
Externally publishedYes

ASJC Scopus subject areas

  • Physics and Astronomy(all)


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