The magnetoresistance of semimetallic InAs͞GaSb superlattices has been measured in parallel magnetic fields. The temperature dependence reveals for the first time the existence of a minigap at the anticrossing points between the electron and hole dispersion relations. At high magnetic fields, samples which are close to intrinsic show surprisingly large decreases in resistance of up to 70%. The magnetoresistance also depends on crystal orientation. We present a theoretical model which explains the experimental observations and estimates the minigap to be of order 7 meV.
The electrical transport properties of a bipolar InAs/GaSb system have been studied in magnetic field. The resistivity oscillates between insulating and metallic behaviour while the quantum Hall effect shows a digital character oscillating from 0 to 1 conducatance quantum e 2 /h. The insulating behaviour is attributed to the formation of a total energy gap in the system. A novel looped edge state picture is proposed associated with the appearance of a voltage between Hall probes which is symmetric on magnetic field reversal.The quantum Hall effect has become well known since its first observation by von Klitzing et al [1] but still raises a number of fundamental questions, including the role of edge states which ensure the possibility of dissipationless conduction [2] [3]. In this letter we examine the quantum Hall effect and magnetotransport properties of a bipolar system of coupled electrons and holes and demonstrate that such a system shows qualitatively different behaviour to that observed for single carriers. Such systems have generated considerable interest due to the possibilities of gap formation by both excitonic [4] [5] and single particle [6] interactions. The electron-hole system introduces the new possibility that the edge states of the electron and hole systems may interact, breaking the normal quantum Hall conditions. The first observation of quantum Hall plateaux in an electron-hole system by Mendez et al [7] found that conventional plateaux were formed at quantum numbers corresponding to the difference in the occupancies of the electron and hole Landau levels. Subsequently Daly et al [8] found that in superlattices with closely matched electron and hole densities the special case of zero Hall resistance could be observed. In this letter we examine the behaviour of insulating states formed in a structure containing one layer each of electrons and holes which interact via interband mixing. If mixing occurs between edge states a total energy gap may appear for the system leading to the insulating behaviour. Several gaps can result from the mixing between different electron and hole Landau levels and as a result the system displays oscillatory metallic and insulating behaviour as a function of magnetic field and the Hall conductivity follows a binary sequence oscillating from 0 -1 -0 conductance quanta.The samples studied consisted of a single layer of InAs sandwiched between thick layers of GaSb. This system exhibits a broken gap lineup with the conduction band edge of the InAs 150meV below the valence band edge of the GaSb. The samples are grown by Metal Organic Vapour Phase Epitaxy (MOVPE) and have a relatively low level of impurities so that the majority of charge carriers are created by intrinsic charge transfer from the GaSb layers to the InAs layer. Typical structures are grown onto semi-insulating GaAs followed by 2µm of GaSb to achieve lattice relaxation. The active layer of InAs is typically 30nm thick, followed by a 90nm GaSb cap. Fitting the low field magnetoresistance and Hall effect to...
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