1. A. D. B. Woods and R. A. Cowley,Rep. Prog. Phys. 36, 9 (1973).
2. R. Scherm, W. G. Stirling, A. D. B. Woods, R. A. Cowley, and G. J. Coombs,J. Phys. C: Solid State Phys. 7, L341 (1974).
3. W. G. Stirling, R. Scherm, F. Volino, and R. A. Cowley,Proc. 14th Int. Conf. Low Temp. Phys. (North-Holland, Amsterdam, 1975).
4. W. G. Stirling, R. Scherm, P. A. Hilton, and R. A. Cowley,J. Phys. C: Solid State Phys. 9, 1643 (1976).
5. B. N. Esel'son, Yu. Z. Kovdrya, and V. B. Shikin,Sov. Phys.?JETP 32, 37 (1971).