The incoherent mechanism of negative magnetoresistance for variable-range-hopping transport in two-dimensional electron systems

Abstract

The experimental data on hopping magnetotransport in dilutely doped δ layers for both perpendicular and parallel orientations of magnetic field B are discussed. The temperature dependence of the resistance, InRαT ^−1/3, indicates that the variable-range-hopping mechanism applies. At strong enough fields (B > 2·5 T) the resistance for both orientations rises rapidly with increasing B because of the orbital shrinkage of the impurity wavefunctions. In the range of intermediate fields (1 T < B < 2T) the resistance in the perpendicular orientation appears to be several times less than in the parallel orientation. Such an ‘inverted’ relation cannot be accounted for by the mechanism proposed by Nguyen, Spivak and Shklovskii which is based on the interference of alternative tunnelling paths. To explain the experimental results the effect of magnetic field on the energy positions of the impurity levels is taken into account. By reducing the overlap between the neighbouring sites, a magnetic field pushes the energy levels to the centre of the impurity band. Then the density of states at the Fermi level increases and the resistance decreases. Since the reduction in the overlap in the perpendicular orientation is stronger than in the parallel orientation, the mechanism proposed allows one to explain qualitatively the features of the experimental data.