Jahn-Teller coupling at the silicon vacancy

The one-electron linear Jahn-Teller coupling model for the silicon vacancy fails to explain three key features of the negative charge state: the symmetry and hyperfine interaction for the ground state, the existence of six equivalent distortions, and the difference in the activation energies for the trigonal and tetragonal distortion reorientations. The inclusion of higher-order Jahn-Teller coupling can remedy these failures by splitting the degeneracy between the (Q(theta),Q(zeta)) type distortions involving an elongation along the tetragonal axis and those involving a compression along the tetragonal axis. One such higher-order coupling scheme is detailed.