Unconventional thermal metallic state of charge-neutral fermions in an insulator
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.
- Publication:
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Nature Physics
- Pub Date:
- July 2019
- DOI:
- 10.1038/s41567-019-0552-2
- arXiv:
- arXiv:1905.05357
- Bibcode:
- 2019NatPh..15..954S
- Keywords:
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- Condensed Matter - Strongly Correlated Electrons
- E-Print:
- 7 pages, 4 figures. This is the original submitted version. Final version is accepted for publication in Nature Physics