Recent observations of quantum oscillations (QOs) in Kondo insulators SmB_6 [1], [2] and YbB_{12} [3] have been a big surprise since they seem to host Fermi surfaces, which is a defining character of a metal. Though there have been several theoretical proposals to explain this mystery so far, it is far from settled because of lack of experimental inspections by other means of measurements. In this talk, I will present low-temperature heat-transport measurements to discuss low energy excitations in the ground state of YbB_{12}. At zero field, despite the resistivity rho_{xx} being far larger than that of conventional metals, sizeable T-linear dependent terms in the specific heat C and thermal conductivity K_{xx} in the zero-temperature limit are clearly resolved, leading to a spectacular violation of the Wiedemann-Franz law: the Lorenz ratio L = K_{xx} rho_{xx}/T is 10^4-10^5 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 larger amplitudes of the resistive QOs as well as a larger K_{xx}/T (T rightarrow 0), in stark contrast to conventional metals. Moreover, we find that these fermions couple to magnetic field despite their charge neutrality, whereas no discernible thermal Hall angle is resolved within our resolution up to 12T. Our findings expose novel gapless and highly itinerant, charge-neutral quasi-particles in this unconventional quantum state.

[1] G. Li et al., Science 346, 1208 (2014).
[2] B. S. Tan et al., Science 349, 287 (2015).
[3] Z. Xiang et al., Science 362, 65 (2018).