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 ν=4-6 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.