  We present a spin-boson theory for magnetotransport in organic semiconducting
materials, on the basis of a coupling between charge carriers' spin and a local
bosonic environment, which is shown to be an irreducible ingredient in
understanding of the anomalous organic magnetoresistance (OMR). Among those
composing this environment triplet-excitons play a basic role. The incoherent
hopping rate between molecules is calculated to give out the fundamental
behavior of OMR. The underlying mechanism is revealed from the calculation of
entanglement, represented by the von Neumann entropy, between the carrier's
spin and bosons. We also obtain the dependence of OMR on the bias voltage, the
spin-boson coupling, and the boson frequency. The results obtained from the
theory are in good agreement with experiments.
