Electron-hole pair exchange interactions

The free valencies of a crystal can form pairs, each such pair wandering through the crystal as an entity until it breaks up. Such formations are well known in the theory of the solid state. A pair of opposite valencies in an ionic crystal (electron - - hole bound by Coulomb interaction) forms what is called a Mott exciton. A pair of like valencies (election + electron or hole + hole bound by exchange interactions) forms a so-called doublon. Such formations have recently been investigated, 12, IS). 

This result proves to be valid for all MV — d" pairs with n < 4, provided that the spin core is defined as an ion without extra electron. It is also valid for n > 4 but in such case the spin core must be defined as an ion without extra hole. The ferromagnetic effect of the double exchange is illustrated by Fig. 7. Along with the double exchange, the isotropic exchange interaction plays an important role in MV clusters. This interaction is described by the HDVV spin Hamiltonian ... 

EPR. This trend reflects the exchange interaction between the electron and hole. For given concentrations of donors and acceptors, there is a distribution in the separations between pairs. Slow microwave-modulation frequencies can emphasize the importance of the sharper, more distant pairs (see FIGURE 2). So far, the best donor linewidth of about 1 mT has precluded the observation of N-hyperfine splitting in the 3C polytype, which EPR has shown to be 0.12mT. 

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