*

Solid-state materials can be classified into two groups: insulators and metals. Insulators do not conduct the electric charge and exhibit diverging resistivity at low temperatures, whereas metals sustain conduction electrons that conduct electricity well down to zero temperature. Surprisingly, recently we discovered that the Kondo insulator YbB₁₂, a long-known material, belongs to neither category.

Metals are defined as materials possessing a Fermi surface, which can be detected by observing characteristic quantum oscillations of magnetization and electrical resistivity in a strong magnetic field. Such oscillations, which are consequences of the quantization of the orbital motion of conduction electrons, are absent in insulators that have no Fermi surfaces. 

We have recently uncovered, however, a profoundly peculiar behavior in YbB₁₂. This material is unmistakably an insulator with an electric resistivity magnitude far beyond that of metals. However, both its magnetization and its resistivity exhibit quantum oscillations [1], in sharp contrast to Kondo insulator SmB₆ [2]. Analysis of the oscillations reveals that they arise from a bulk channel that hosts strongly correlated fermionic excitations. The same Fermi surface is likely to contribute to an itinerant quasiparticle term in both the heat capacity and the thermal conductivity [3]. Moreover, this channel probably survives a field-induced insulator-to-metal transition and gives rise to the unconventional quantum oscillations observed in a high-field exotic metallic state in YbB₁₂ [4].

Our results reveal the existence of an exotic quasiparticle band in Kondo insulator YbB₁₂: it behaves like the normal electrons in metals in many aspects, yet conducts electricity much less efficiently. The investigation of such highly unusual fermions in solid states will arouse great attention in the physics community.