1. Allen, S. J., Jr., Störnier, H. L., and Hwang, J. C. M., 1983, Phys. Rev. B, 28: 4875.
2. Alsmeier, J., Sikorski, Ch., and Merkt, U., 1988, Phys. Rev. 5, 37: 4314.
3. Alsmeier, J., Batke, E., and Kotthaus, J. P., 1989, Phys. Rev. 5, 40:12574.
4. Alsmeier, J., Batke, E., and Kotthaus, J. P., 1990a, Phys. Rev. B, 41:1699.
5. Alsmeier, J., Kotthaus, J. P., Klapwijk, T. M., and Bakker, S., 1990b, in “Proc. 20th Int. Conf. Phys. of Semiconductors (Thessaloniki),” to be published.