1. M. Nielson, J. P. McTague, W. Ellenson, J. Phys.(Paris), 32, C4–10 (1977)
2. for a review see J. G. Dash and M. Schick in The Physics of Liquid and Solid Helium, Part II, ed. K.H. Bennemann & J.B. Ketterson, (Wiley 1978 )
3. B. Cowan, L.A. El-Nasr, M. Fardis, A. Hussain, Phys.Rev.Lett. 58, 2308 (1987)
4. J. Saunders, C.P. Lusher, B.P. Cowan, Phys.Rev.Lett. 64, 2523 (1990)
5. H. Godfrin, unpublished.