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

Large positve (P) magnetoresistance (MR) has been observed in parallel magn etic fields in a single two-dimensional (2D) layer in a delta-doped GaAs/Al xGa1-xAs heterostructure with a variable-range-hopping (VRH) mechanism of c onductivity. Effect of large PMR is accompanied in strong magnetic fields b y a substantial change in the character of the temperature dependence of th e conductivity. This implies that spins play an important role in 2D VRH co nductivity because the processes of orbital origin are not relevant to the observed effect. A possible explanation involves hopping via double occupie d states in the upper Hubbard band, where the intrastate correlation of spi ns is important.