1. J.-P. Jan, W. B. Pearson, and M. Springford, inLow Temperature Physics?LT9 (Part B), J. Daunt, D. O. Edwards, F. J. Milford, and M. Yagub, eds. (Plenum Press, New York, 1965), p. 776.
2. J.-P. Jan, W. B. Pearson, and Y. Saito,Proc. Roy. Soc. (London)297, 275 (1967).
3. Å. Karlsson,J. Low Temp. Phys. 1, 59 (1969).
4. M. Springford, J.-P. Jan, W. B. Pearson, and I. M. Templeton,Bull. Am. Phys. Soc. 8, 469 (1963).
5. J.-P. Jan and W. B. Pearson,Can. J. Phys. 42, 220 (1964).