Influence of parallel magnetic fields on a single-layer two-dimensional electron system with a hopping mechanism of conductivity

Large positive (P) magnetoresistance (MR) has been observed in parallel magnetic fields in a single two-dimensional (2D) layer in a δ-doped ${\mathrm{G}\mathrm{a}\mathrm{A}\mathrm{s}/\mathrm{A}\mathrm{l}}_x{\mathrm{Ga}}_{1-x}\mathrm{As}$ 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 intrastate correlation of spins is important.