In metals, orbital motions of conduction electrons on the Fermi surface are quantized in magnetic fields, which is manifested by quantum oscillations in electrical resistivity. This Landau quantization is generally absent in insulators. Here we report a notable exception in an insulator — ytterbium dodecaboride (YbB12). The resistivity of YbB12exhibits distinct quantum oscillations despite having a much larger magnitude than in metals [1]. This unconventional oscillation is shown to arise from the insulating bulk, even though the temperature dependence of the oscillation amplitude follows the conventional Fermi liquid theory of metals. The large effective masses indicate the presence of a Fermi surface consisting of strongly correlated electrons. Quantum oscillations are also observed in the magnetization of YbB12 [1]. Our result reveals a mysterious dual nature of the ground state in YbB12: it is both a charge insulator and a strongly correlated metal.

[1] Z. xiang et al., Science 362, 65 (2018).