Magnetotransport anomalies in dilute two-dimensional electron systems: An experiment and a model

Magnetotransport characteristics of dilute two-dimensional hole gases (2DHGs) in Si/SiGe heterostructures are shown to have two types of anomalous behaviour. (1) With increasing magnetic field, the normal sequence of quantum Hall effect states is interrupted by the appearance of the insulating state in the region of filling factor two, which in turn is replaced by the i=1 quantum Hall effect state. Tilting the sample plane relative to the magnetic field direction increases dramatically the degree of insulating behaviour. (2) In a similar heterostructure, but with a higher areal density of holes, the insulating state is not observed. However the measured, almost quantised, value of the Hall conductivity turns out So be different from that expected from the shubnikov-de Haas oscillation periodicity. A new model which describes quantum transport in terms of percolation phenomena, predicting a nonmonotonic dependence of Hall conductivity on magnetic field, is shown to be capable of explaining the data.