1. R.P. Feynman, inProg. Low Temp. Phys., Vol. 1, ed. C.J. Gorter (North-Holland, Amsterdam, 1955) p. 17.
2. see, for a review, R.J. Donnelly,Quantized Vortices in Helium II (C.U.P., Cambridge, 1991).
3. J.T. Tough, inProg. Low Temp. Phys., ed. D.F. Brewer (North-Holland, Amsterdam, 1982), Vol. 3, p. 133.
4. K.W. Schwarz, Phys. Rev. B38, 2398 (1988).
5. R. Aarts, Ph.D. Thesis (Eindhoven University, 1993), unpublished.