1. R. Ratnalingam and J. B. Sousa, J. Low Temp. Phys. 4, 401 (1971).
2. M. A. Campos Tomé, D. Phil. Thesis, Oxford, 1973.
3. H. M. Rosenberg and M. J. Metcalfe, Phys. Lett. 33A, 311 (1970).
4. N. G. Aliyev and N. V. Volkenshteyn, Soviet Phys.?Solid State 7, 2068 (1966).
5. N. G. Aliyev and N. V. Volkenshteyn, Soviet Phys.?JETP 22, 17 (1966).