Level crossing in the two-dimensional electron system in the presence of gigantic Zeeman effect and quantum Hall ferromagnet

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The main purpose of this research project is clarifying the physical properties peculiar to the "magnetic" two-dimensional electrons gas (2DEG) originating from the exchange interaction between the 2D electrons and the localized moments of magnetic ions. We have grown modulation-doped heterostructures consisting of the well layer of diluted magnetic semiconductor Cd_<1-x>Mn_xTe and the barrier layer of Cd_<1-y>Mg_yTe by molecular beam epitaxy (MBE) and have investigated the magneto-transport and magneto-optical properties. In order to obtain high-mobility "magnetic" 2DEG, we have first tried several inventions in the MBE growth of non-magnetic heterostructures of CdTe/Cd_<1-y>Mg_yTe. As a result, it was confirmed that the cleaning the surface of a GaAs (001) substrate using hydrogen plasma is effective for removing the oxidized surface layer. By adopting this hydrogen cleaning method also in the growth of Cd_<1-x>Mn_xTe/Cd_<1-y>Mg_yTe, we have succeeded in fabricating high-mobility mag … More netic 2DEG. In the magneto-transport and magneto-optical measurements under high magnetic fields, we have found anomalous behaviors peculiar to magnetic 2DEG, such as an abrupt jump in the magnetic field dependence of energy of the excitonic luminescence at a magnetic field of an integer filling factor. In addition, we have achieved a continuous control of the 2D electron density in the field-effect transistor structure with a gate electrode on the top of the magnetic 2DEG sample fabricated by the photolithography and we etching. In the magneto-transport and magneto-optical measurements under the continuous change of the 2D electron density by applying a gate voltage, we have found anomalous behaviors in the magnetic field dependence of magnetoesistivity, Hall resistivity, and the luminescence energy, which cannot be explained by ordinary integer quantum Hall effect. We have analyzed that these anomalous behaviors are due to the effect of quantum Hall ferromagnet, which originates from the crossing between Landau levels with different spin orientations. Less