Double exchange in iron-sulfur clusters and a proposed spin-dependent transfer mechanism
AbstracL Mossbauer studies of the electron transport protein ferredoxin I1 have revealed that the reduced Fe3S4 cluster contains one localized Fe3+ site and one delocalized Fe2+-Fe3+ pair. Analysis of the spin coupling of this mixed-valence system has shown that the coupling is dominated by double exchange. We have recently proposed a spin Hamiltonian that takes Heisenberg-Dirac-van Vleck exchange as well as double exchange into account. Hamiltonian can be constructed from an effective Hamiltonian derived from perturbation theory. In applying this approach to mixed-valence systems with three paramagnetic centers we have obtained a term which to our knowledge has not been considered in the literature. The mechanism, which we propose to call exchangetransfer, yields for a spin-only d l -d l -d2 system the expression za ,b , c Cacbtab(1 4sb*sc) where tab is an operator that transfers the excess electron located on the d2-site b to site a; sb and sc are spin operators of the excess electron and the electron on site c, respectively. the product of the transfer integrals connecting the sites a and c and the sites b and c and K is the intra-atomic exchange integral. The mechanism achieves transfer of the excess electron from b to a by a "detour" through an excited state on site c. This spin