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Organic magnetoresistance under resonant ac drive

We study the spin dynamics of an electron-hole pair in a random hyperfine magnetic field and an external field, B0, under a resonant drive with frequency omega0 = gamma B0. The fact that the pair decays by recombination exclusively from a singlet configuration, S, in which the spins of the pair-partners are entangled, makes this dynamics highly nontivial. Namely, as the amplitude, B1, of the driving field grows, mixing all of the triplet components, the long-living modes do not disappear, but evolve from T+, T- into (T+ +/- sqrt(2) T0 + T-)/2. Upon further increase of B1, the lifetime of the S-mode is cut in half, while the T0-mode transforms into an antisymmetric combination (T- - T-)/sqrt(2) and acquires a long lifetime, in full analogy to the superradiant and subradiant modes in the Dicke effect. Peculiar spin dynamics translates into a peculiar dependence on B1. In particular, at small B1, the radiation-induced correction to the current is linear in B1.