Low-temperature magnetoresistance in insulating a-GdxSi1-x alloys

We report on low-temperature mag magnetotransport measurements on the amorp hous semiconductor a-GdxSil-x with x similar to 0.13, on the insulating sid e of the T=0 metal-insulator transition for this material. The samples exhi bit 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 va riable range hopping in the presence of a Coulomb gap in all magnetic field s from 0 to 9 T, while the characteristic temperature T-0 of the hopping co nductivity 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 condu ction electrons and the local Gd moments which are randomly oriented in zer o 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 Coulo mb gap or a change in the bandwidth and its consequent effect on the electr onic density of states.