Possibility of a metastable state for a transition-metal impurity in semiconductor

A transition-metal impurity in a semiconductor can be shifted from its high symmetry substitutional position due to the Jahn-Teller interaction. An interplay of the spin-orbit and Jahn-Teller interactions may lead to a double-well potential for such an impurity in the configuration coordinates. One of these new states can be metastable and characterized by extremely long relaxation time especially at low temperatures. This allows us to explain the observation of the line complementing the intracenter transition line to the forbidden energy gap both in the luminescence and in the photoluminescence excitaton spectra. Various other puzzling experimental observations are discussed and explained on the basis of this mechanism.