  The detection of Landau-level-like energy structures near the chemical
potential of an insulator is essential to the search for a class of correlated
electronic matter hosting charge-neutral fermions and Fermi surfaces, a
long-proposed concept that remains elusive experimentally. Here we introduce
and demonstrate that the magneto-thermoelectric response of a quantum insulator
can reveal critical information not available via other approaches. We report
large quantum oscillations (QOs) in the Seebeck response of the hole-doped
insulating state of monolayer tungsten ditelluride (WTe2) in magnetic fields.
The QOs remarkably undergo sign-changes as the field is swept, mimicking those
in metals with Landau quantization. The sign-change in the thermoelectric
response directly implies the presence of a field-induced Landau-level-like
structure at the chemical potential of the insulator. Our results reinforce
WTe2 as a platform for investigating insulating Landau levels and mobile
neutral fermions in two-dimensional insulators.
