Experimental observation of localization phenomena in low dimensional systems in high magnetic fields

Prof.dr.ir. J.C. Maan, High Field Magnet Laboratory, University of Nijmegen, Toernooiveld 7, 6525 ED Nijmegen.


Two dimensional systems in the extreme quantum limit when only a few magnetic levels remain occupied, become insulating at low temperatures. Typical observations are a diverging resistance beyond some critical magnetic field. Here we want to present some different experimental approaches. In SiGe heterostructure we study diffusion thermoelectric power and observe that while the resistivity becomes infinite beyond the critical magnetic field, thermoelectric power vanishes under certain conditions. This observation gives some important clues to the origin of the localization. In a second experiment we show that a system with two weakly coupled two dimensional electron gases in GaAs heterostructures (i.e. a '2.5' dimensional system), does not show a diverging resistance in the field range where single two dimensional layers would have become insulating. By adding an in-plane component of the magnetic field by tilting the sample the interlayer coupling is suppressed and an angle induced metal insulator transition is observed. This last experiment was the first done in the new 33T high magnetic field installation.