Kondo Insulator YbB₁₂: Neutral Quasiparticles and Unconventional Charge Transport in Magnetic Fields of up to 75 T

YbB₁₂ has attracted a great deal of attention over the past few years. At low temperatures it exhibits significant bulk fermionic contributions to both its heat capacity and thermal conductivity. In high magnetic fields, it shows both de Haas-van Alphen and Shubnikov-de Haas oscillations suggestive of a three-dimensional Fermi surface. Despite these ostensibly metallic properties, YbB₁₂ is a Kondo insulator. Moreover, it behaves in a way that is at first sight counter-intuitive: the higher its zero-temperature resistivity, the stronger the magnetic quantum oscillations. Having accumulated a large quantity of resistivity, Hall effect and magnetization data on YbB₁₂ in magnetic fields of up to 75 T and temperatures down to 20 mK, a consistent picture is emerging of a Fermi-liquid-like ensemble of electrically neutral quasiparticles that coexist with charge carriers that remain in a nonmetallic, or even “bad metallic” state. The neutral quasiparticles suffer successive Lifshitz transitions and undergo Landau quantization in applied magnetic fields; eventually they begin to die slowly after YbB₁₂ passes through a Kondo-insulator-to-Kondo-metal transition at around 45 T. The manner of their death and their other properties provide tight constraints for theories of neutral quasiparticles in mixed-valence systems.