Quantum Hall and insulating states of a 2-D electron–hole system

We review recent results from magneto-transport studies of an InAs/GaSb based electron–hole system. In high magnetic field, the system shows two types of overall behaviour depending on whether the effective occupancy νe−νh=νeff is zero or finite. When νe−νh is finite the resistivity has a metallic temperature dependence and the Hall resistance is quantized at a finite value. When νe−νh=0 however, the resistivity shows an insulating temperature dependence. Since the condition νe−νh=νeff for a given value of νeff can be achieved with different values of the individual occupancies, the system can oscillate between insulating and metallic behaviour with magnetic field. In the insulating states where νe−νh=0, the system exhibits novel behaviour, where the Hall resistance becomes symmetric under field reversal, and fluctuates reproducibly with magnetic field. Geometry dependence measurements show that in this state the sample interior becomes extremely insulating, and conduction is dominated by the mesa edge. The transport is one dimensional, and the chirality usually associated with the Hall effect is lost. Current driven breakdown is studied for quantum Hall states where νe−νh is finite. There is a strong dependence of the breakdown behaviour on channel width and the relative electron and hole densities. This suggests a strong involvement of the sample interior in stark contrast to the insulating states.