1. J. Feldmann, R. Sattmann, E. O. Göbel, et al., Phys. Rev. Lett. 62, 1892 (1989); J. Feldmann, J. Nunnenkamp, G. Peter, et al., Phys. Rev. B 42, 5809 (1990).
2. E. Finkman and R. Planel, in Proceedings of the 23rd International Conference on the Physics of Semiconductors (ICPS23), Ed. by M. Scheffler and R. Zimmermann (World Sci., Singapore, 1996), Vol. 3, p. 1723.
3. S. V. Zaĭtsev, V. D. Kulakovskiĭ, A. A. Maksimov, et al., Pis’ma Zh. Éksp. Teor. Fiz. 66, 351 (1997) [JETP Lett. 66, 376 (1997)].
4. A. Waag, F. Fisher, H.-J. Lugauer, et al., J. Appl. Phys. 80, 792 (1996).
5. M. Nagelstraßer, H. Dröge, H.-P. Steinrück, et al., Phys. Rev. B 58, 10 394 (1998).