Magnetoconductance in the variable-range-hopping regime due to a quantum-interference mechanism

Results of systematic magnetoconductance measurements on highly disordered In2O3-x films are described. Measurements were performed as a function of magnetic field, electric field, temperature, system dimensionality, and amount of static disorder. It is shown that in the hopping regime, the low-field magnetoconductance is always positive, and anisotropic in sufficiently thin films. The latter feature is suggestive of a nonlocal (orbital) mechanism. We demonstrate that the spatial range of phase coherence, involved in the phenomenon, scales with the hopping length. This length may be controlled by either the temperature or the electric field. It is further shown that several aspects of the experimental results support the basic ideas of a newly proposed quantum-interference mechanism. An intuitive physical description of the reason for the positive magnetoconductance is discussed based on the percolation model for the hopping transport.