  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). Despite much larger than that of metals, the resistivity
of YbB12 exhibits profound quantum oscillations. This unconventional
oscillation is shown to arise from the insulating bulk, yet the temperature
dependence of their amplitude follows the conventional Fermi liquid theory of
metals. The large effective masses indicate the presence of Fermi surface
consisting of strongly correlated electrons. Our result reveals a mysterious
bipartite ground state of YbB12: it is both a charge insulator and a strongly
correlated metal.
