1. V. J. Goldman, D. C. Tsui and J. E. Cunningham, Phys. Rev. Lett. 58, 1256 (1987)
2. T. C. L. G. Sollner, Phys. Rev. Lett. 59, 1622 (1987).
3. See also C. A. Payling, E. Alves, L. Eaves, T. J. Foster, M. Henini, O. H. Hughes, P. E. Simmonds, J. C. Portal, G. Hill and M. A. Pate, Proc. 3rd Int. Conf. on Modulated Semiconductor Structures, Montpellier, France. J. Physique C5, 289 (1987).
4. G. A. Toombs, E. S. Alves, L. Eaves, T. J. Foster,. Henini, O. H. Hughes, M. L. Leadbeater, C. A. Payling, F. W. Sheard, P. A. Claxton, G. Hill, M. A. Pate and J. C. Portal, 14th Int. Symposium on Gallium Arsenide and Related Compounds, Crete, 1987. To be published by Institute of Physics.
5. V. J. Goldman, D. C. Tsui and J. E. Cunningham, Phys. Rev. B35, 9387 (1987).