Strong electronic interactions in condensed-matter systems often lead to unusual quantum phases. One such phase occurs in the Kondo insulator YbB₁₂, the insulating state of which exhibits phenomena that are characteristic of metals, such as magnetic quantum oscillations¹, a gapless fermionic contribution to heat capacity^2,3 and itinerant-fermion thermal transport³. To understand these phenomena, it is informative to study their evolution as the energy gap of the Kondo insulator state is closed by a large magnetic field. Here we show that clear quantum oscillations are observed in the resulting high-field metallic state in YbB₁₂; this is despite it possessing relatively high resistivity, large effective masses and huge Kadowaki–Woods ratio, a combination that normally precludes quantum oscillations. Both quantum oscillation frequency and cyclotron mass display a strong field dependence. By tracking the Fermi surface area, we conclude that the same quasiparticle band gives rise to quantum oscillations in both insulating and metallic states. These data are understood most simply by using a two-fluid picture in which neutral quasiparticles—contributing little or nothing to charge transport—coexist with charged fermions. Our observations of the complex field-dependent behaviour of the fermion ensemble inhabiting YbB₁₂ provide strong constraints for existing theoretical models.

凝聚态系统中的强电子相互作用通常会导致不寻常的量子相。一个这样的阶段发生在 Kondo 绝缘体 YbB ₁₂中，其绝缘状态表现出金属特有的现象，例如磁量子振荡¹、对热容量的无间隙费米子贡献^2,3和巡回费米子热传输³。为了理解这些现象，研究它们的演变是有益的，因为近藤绝缘体状态的能隙被大磁场封闭。在这里，我们表明在 YbB _{12中产生的高场金属态中观察到了清晰的量子振荡}; 尽管它具有相对较高的电阻率、较大的有效质量和巨大的 Kadowaki-Woods 比，但这种组合通常会排除量子振荡。量子振荡频率和回旋加速器质量都表现出强烈的场依赖性。通过跟踪费米表面积，我们得出结论，相同的准粒子带会在绝缘和金属状态下产生量子振荡。这些数据最简单地通过使用两流体图来理解，其中中性准粒子（对电荷传输贡献很少或没有贡献）与带电费米子共存。_{我们对居住在 YbB 12}中的费米子系综复杂的场相关行为的观察为现有的理论模型提供了强有力的约束。