Unconventional thermal metallic state of charge-neutral fermions in Kondo insulators

Kondo lattice materials, where localized magnetic moments couple to itinerant electrons, provide a very rich backdrop for strong electron correlations. Among them, recent observations of quantum oscillations at high magnetic fields in both transport and thermodynamic parameters in a Kondo insulator ytterbium dodecaboride YbB₁₂ have come as a great surprise-despite the large charge gap, this compound seemingly hosts a Fermi surface [1]. Here, to explore the nature of the ground state in zero fields, we present low-temperature thermodynamic and thermal transport properties of two Kondo insulators, non-magnetic YbB₁₂ and magnetic YbIr₃Si₇. In YbB₁₂, sizeable linear T-dependent terms in the heat capacity and thermal conductivity are resolved in the zero-temperature limit, indicating the presence of gapless fermionic excitations with an itinerant character. Remarkably, the coefficient of the linear T-dependent thermal conductivity spectacularly violates the Wiedemann–Franz law, indicating that YbB₁₂ is a charge insulator and a thermal metal, i.e. the possible presence of charge-neutral fermions [2]. In YbIr₃Si₇, two distinct antiferromagnetic phases can be tuned by applying a magnetic field. In the low-field phase, we observed a finite linear T-dependent thermal conductivity, while it exhibits a sharp drop below 300 mK in the high-field phase, indicating a transition from a thermal metal into an insulator/semimetal driven by the magnetic transition. The results suggest that spin degrees of freedom directly couple to the neutral fermions, whose emergent Fermi surface undergoes a field-driven instability at low temperatures [3].