We characterize the two-site marginals of exchangeable states of a system of quantum spins in terms of a simple positivity condition. This result is used in two applications. We first show that the distance between two-site marginals of permutation invariant states on N spins and exchangeable states is of order 1/N . The second application relates the mean ground state energy of a mean-field model of composite spins interacting through a product pair interaction with the mean ground state energies of the components.PACS numbers: 05.30.−d, 03.67.−a, 02.50.Cw * This paper is dedicated to A Verbeure on the occasion of his 65th anniversary.

Modified electrodes were fabricated by depositing a monolayer containing porphyrin-viologen linked compound on ITO electrodes by Langmuir-Blodgett method. Visible light irradiation on the modified electrodes afforded stable anodic photocurrents. The photocurrents clearly increased in the presence of magnetic fields. The magnetic field effects on photocurrents, as observed for the first time, are ascribed to the contribution of triplet radical pairs.

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