1. R. M. de la Cruz and R. Pareja, in Positron Annihilation, Ed. by L. Dorikens-Vanpraet, M. Dorikens, and D. Segers (World Sci., Singapore, 1989), p. 702.
2. R. Suzuki, T. Mikado, H. Ohgaki, et al., Phys. Rev. B 49, 17 484 (1994).
3. V. P. Shantarovich, I. B. Kevdina, Yu. A. Novikov, et al., Fiz. Tverd. Tela (St. Petersburg) 38, 2686 (1996) [Phys. Solid State 38, 1473 (1996)].
4. Y. Itoh, H. Murakami, and A. Kinoshita, Appl. Phys. Lett. 63, 2798 (1993).
5. Y. Itoh, H. Murakami, and A. Kinoshita, Hyperfine Interact. 84, 121 (1994).