Polarized photoluminescence is studied in ZnO/MgZnO heterostructures with different two-dimensional electron gas densities at 0.5 K and in high magnetic fields. At high carrier density, the luminescence spectrum has the shape of the conventional Landau fan diagram, whereas at lower density it shows anomalous behavior: a single dominant line with unexpected splittings at integer filling factors, inconsistent with either cyclotron or Zeeman energies.
Berezhnoy et al. (Sun,) studied this question.