We report on low-temperature magnetotransport measurements on the amorphous semiconductor $a-{\mathrm{Gd}}_x{\mathrm{Si}}_{1-x}$ with $x\sim 0.13,$ on the insulating side of the $T=0\mathrm{}$ metal-insulator transition for this material. The samples exhibit a negative magnetoresistance of more than five orders of magnitude at 1 K, which grows exponentially larger at lower temperatures. The temperature dependence of the conductivity displays an activated form indicative of variable range hopping in the presence of a Coulomb gap in all magnetic fields from 0 to 9 T, while the characteristic temperature $T_0$ of the hopping conductivity decreases from over 300 K in 0 T to 6 K in 8.5 T. This enormous magnetoresistance must arise from an exchange interaction between the conduction electrons and the local Gd moments which are randomly oriented in zero field and become (partially) aligned in 9 T, and its consequent influence on the electron-electron interaction. However, it remains to be determined whether the effect of this magnetic interaction is a relative shift of the Fermi energy relative to the mobility edge and a modification to the Coulomb gap or a change in the bandwidth and its consequent effect on the electronic density of states.

• Received 1 September 1998

DOI:https://doi.org/10.1103/PhysRevB.59.R3929