1. L. Esaki and R. Tsu; IBM J. Research Develop. 14, 61 (1970).
2. For example, L. Esaki, in recent topics in semiconductor physics, edited by H. Kamimura and Y. Toyozawa (World Sc., Singapore, 1983) p. 1.
3. Y. Aoyagi, A. Doi, T. Meguro, S. Iwai and S. Namba, in Proc. 1st Int. Conf. on Electronic Materials, eds. T. Sugano et al., p. 242 (Materials Research Society, Pittsburgh, 1989).
4. H. Watanabe, in Proc. 20th Int. Conf. on Physics of Semiconductors, to be published (World Sc. Singapore, 1990).
5. T. Nakayama and H. Kamimura, J. Phys. Soc. Jpn. 54, 4726 (1986).