Numerical calculations for large positive magnetoresistance ratios R(B)/R(0) in insulating materials

Numerical calculations are presented for predicting the large positive magn etoresistance ratios R(B)/R(O) which are often observed in insulating mater ials. The magnetic field causes shrinkage of the localized electronic wave function, resulting in less overlapping between the hopping sites and hence a relatively large increase of the resistance in the insulating material. Two specific cases are considered: (a) an insulating 3D sample that exhibit s a "Mott" variable-range hopping law in its zero-field resistance and (b) an insulating 3D sample that exhibits an "Efros-Shklovskii'' variable-range hopping law in its zero field resistance. The numerical calculations are t abulated. The general 3D "soft gap" case is also discussed. (C) 2001 Elsevi er Science B.V. All rights reserved.