Quantum treatment of the Anderson-Hasegawa model in the presence of superexchange

We revisit the Anderson-Hasegawa double-exchange model and critically examine its exact solution when the core spins are treated quantum mechanically. We show that the quantum effects, in the presence of an additional superexchange interaction between the core spins, yield a term, the significance of which has been hitherto ignored. The importance of this term is further assessed by numerically exact computation for a four-spin system.