In order to investigate the quantum state of the Si vacancy, a cluster model was introduced which included the electron–lattice coupling between electrons and Jahn–Teller distortions together with the Coulomb interaction between electrons. By using the numerical diagonalization method, account was taken of both the non-adiabatic and the correlation effects. It was found that the 3-fold orbital degeneracy in the ground state of a Si vacancy was stable against the Jahn–Teller distortions because of the strong quantum fluctuation due to the non-adiabatic coupling between electrons and Jahn–Teller distortions. The obtained result was a striking contrast to the previous theoretical results within the adiabatic approximation and was consistent with the newly observed experiment.

Nonadiabatic Interaction between Electrons and Jahn–Teller Distortions in a Silicon Vacancy. Y.Yamakawa, K.Mitsumoto, Y.Ōno: Journal of Magnetism and Magnetic Materials, 2007, 310[2], 993-5