paper

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

arXiv:1905.05357 · doi:10.1038/s41567-019-0552-2

Abstract

Quantum oscillations (QOs) in transport and thermodynamic parameters at high magnetic fields are an unambiguous signature of the Fermi surface, the defining characteristic of a metal. Therefore, recent observations of QOs in insulating SmB $_{6}$ and YbB $_{12}$ , in particular the QOs of the resistivity $I ¨ _{xx}$ in YbB $_{12}$ , have been a big surprise, pointing to the formation of a novel state of quantum matter. Despite the large charge gap inferred from the insulating behaviour of $I ¨ _{xx}$ , 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 YbB $_{12}$ . At zero field, despite $I ¨ _{xx}$ being far larger than that of conventional metals, a sizable linear temperature dependent term in the thermal conductivity is clearly resolved in the zero-temperature limit ( $I^º_{xx} / T (T \to 0) = I^º_{xx}^{0} / T \neq = 0$ ). Such a residual $I^º_{xx}^{0} / T$ term at zero field, which is absent in SmB $_{6}$ , leads to a spectacular violation of the Wiedemann-Franz law: the Lorenz ratio $L = I^º_{xx} I ¨ _{xx} / T$ is $1 0^{4}$ - $1 0^{5}$ times larger than that expected in conventional metals. These data indicate that YbB $_{12}$ is a charge insulator but a thermal metal, suggesting the presence of itinerant neutral fermions. Remarkably, more insulating crystals with larger activation energies exhibit a larger amplitude of the resistive QOs as well as a larger $I^º_{xx}^{0} / T$ , in stark contrast to conventional metals. Moreover, we find that these fermions couple to magnetic field, despite their charge neutrality. Our findings expose novel gapless and highly itinerant, charge-neutral quasiparticles in this unconventional quantum state.

7 pages, 4 figures. This is the original submitted version. Final version is accepted for publication in Nature Physics