J−Jcoupling and local-field effects on N-N pair spectra in GaP

We present an investigation of local fields which split the exciton states bound to N-N pairs in GaP. We show how both $J-J$ coupling and local-field effects compete to give the experimental splitting pattern. For all N-${\mathrm{N}}_i$ pairs with $i\le 7$, we deduce (i) the magnitude of the $J-J$ coupling and (ii) the splitting of the valence band experienced by the hole. Assuming in first order that the field surrounding the hole is a pure strain field associated with the relaxation of lattice around the N-N pair, we deduce the shear components of the deformation tensor experienced by the bound hole.