Antilevitation of Landau levels in vanishing magnetic fields

We report in this Rapid Communication an antilevitation behavior of Landau levels in vanishing magnetic fields in a high quality heterojunction insulated-gated field-effect transistor. We found, in the Landau fan diagram of electron density versus magnetic field, the positions of the magnetoresistance minima at Landau-level fillings $\ensuremath{\nu}=4\ensuremath{-}6$ move below the ``traditional'' Landau-level line to lower electron densities. Moreover, the even and odd filling factors show quantitatively different behaviors in antilevitation, suggesting that the exchange interactions may be important.