Microscopic theory of organic magnetoresistance based on kinetic equations for quantum spin correlations

The correlation kinetic equation approach is developed that allows describing spin correlations in a material with hopping transport. The quantum nature of spin is taken into account. The approach is applied to the problem of the bipolaron mechanism of organic magnetoresistance in the limit of large Hubbard energy and small applied electric field. The spin relaxation that is important to magnetoresistance is considered to be due to hyperfine interaction with atomic nuclei. It is shown that the line shape of magnetoresistance depends on short-range transport properties. Different model systems with identical hyperfine interaction but different statistics of electron hops lead to different line shapes of magnetoresistance including the two empirical laws H-2/(H-2 + H-0(2)) and H-2/(vertical bar H vertical bar + H-0)(2) that are commonly used to fit experimental results.