  Large positive (P) magnetoresistance (MR) has been observed in parallel
magnetic fields in a single 2D layer in a delta-doped GaAs/AlGaAs
heterostructure with a variable-range-hopping (VRH) mechanism of conductivity.
Effect of large PMR is accompanied in strong magnetic fields by a substantial
change in the character of the temperature dependence of the conductivity. This
implies that spins play an important role in 2D VRH conductivity because the
processes of orbital origin are not relevant to the observed effect. A possible
explanation involves hopping via double occupied states in the upper Hubbard
band, where the intra-state correlation of spins is important.
