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The quantum Hall liquid-Hall insulator transition has been studied using a gate-controlled periodical lattice of antidots. This system allows us to probe deep into the phase-space of the transition at both low and high magnetic fields. It is shown, that the transition occurs when the longitudinal and Hall conductivities are equal to each other. Thus, contrary to the scaling delocalisation model, renormalisation occurs only when ω c τ =1. The Hall resistivity at the transition point is close to its classical value, as predicted by the Chern-Simons-Landau-Ginzburg theory of the quantum Hall effect.

Porous silicon layers have been investigated using reflectance and photoluminescence techniques at room and liquid nitrogen temperatures. We observe a noticeable anisotropy of porous silicon samples. The measured parallel and normal components of the refractive index for samples with moderate porosity were 1.25 and 1.30, respectively. Possible mechanisms for the origin of the photoluminescence are discussed.

Magnetoresistance due to weak localisation effects and universal conductance fluctuations (UCFs) has been studied in a system with two quantum wells separated by a tunnelling barrier. We find a weak temperature dependence of the phase-breaking time, which disagrees with the linear dependence of the electron-electron scattering rate on T predicted for the diffusive case. Mesoscopic samples show conductance fluctuations in a magnetic field parallel to the plane of the interfaces. It is suggested that the influence of the magnetic field on the tunnelling rate is responsible for these oscillations.

Magnetoresistance of two-dimensional (2D) electrons in potential superlattice which has a form of stripes with length of 5–10 μm and width 0.15–0.2 μm etched through heterostructures GaAs/AlGaAs has been studied. Magnetoresistance oscillations were found with positions of maxima depending on the length of stripes. The negative magnetoresistance due to weak localization effects was studied. The temperature dependence of the coherence length of electrons was not in agreement with the theory for the two-dimensional or one-dimensional case.