[26] B. Stebe and A. Ainane, Superlattices Microstruct. 5 , 545 (1989). [27] G. Finkelstein et al., Phys. Rev. Lett. 74 , 976 (1995). [28] H. Buhmann et al., Phys. Rev. B 51 , 7969 (1995). [29] A. Shields et al., Phys. Rev. B 51 , 18049 (1995). [30] P. Folkes, J. Luminescence 115 , 104 (2005). 9