1. L. Esaki and R. Tsu, IBM J. Res. Dev. 14, 61 (1970).
2. For a comprehensive review, see Synthetic Modulated Structures, eds. L.L. Chang and B.C. Giessen (Academic, New York, 1985); see also papers in Surface Science 142, (1984).
3. L. Esaki, Proceedings of the 17th International Conference on the Physics of Semiconductors, eds. J.D. Chadi and W.A. Harrison (Springer-Verlag, New York, 1985) p. 473.
4. Inst. Phys. Conf. Ser. No. 56;C Weisbuch,1981
5. For a recent review, see L. Esaki, IEEE J. Quantum Electron 22, 1611 (1986).