Negative magnetoresistance of neutron-transmutation-doped gallium arsenide at variable-range hopping

Investigations of anomalous magnetoresistance (mr) in neutron-transmutation-doped GaAs are performed at T = 0.05 to 4.2 K in the vicinity of the metal-insulator transition (MIT). Without as well as with magnetic field a variable-range hopping-law ρ(T) = ρ0 exp (T1/T)1/2 is found with T1 → 0 at n → nc. At low magnetic field a negative mr and for the first time T1(H) < T1(O) is found. At higher magnetic fields the negative mr saturates and eventually changes into a positive mr with T1(H) > T1(O). The dependence of T1 on n as well as on H originates from the scaling behaviour of the localization radius a and the dielectric constant ε0 at the MIT. From the extrapolation of the T1-curves versus n(T = 300 K) to zero without and with magnetic field nc(H) is determined. In the range of negative mr no shift of nc with H is found. For positive mr, however, nc(H) > nc(O) = 2.18 x 1016 cm-3 is obtained. The scaling behaviour of the conductivity of barely metallic samples extrapolated to T = 0 gives for the critical index ν = 0.84 ± 0.05. The comparison of ν with the critical index b for the scaling relation of T1 gives the critical index ζ for the dielectric constant ζ = 1.55 and ζ/ν = 1.94. (author)