1. P. W. Anderson and P. Morel, Phys. Rev. 123, 1911 (1961).
2. D. D. Osheroff, R. C. Richardson, and D. M. Lee, Phys. Rev. Lett. 28, 885 (1972).
3. F. Steglich et al., Phys. Rev. Lett. 43, 1892 (1979).
4. H. Ott et al.,in 18th International Conference on Low Temperature Physics,Kyoto (Japanese Journal of Applied Physics, Tokyo, 1987).
5. B. Sarma, M. Levy, S. Adenwalla, and J. Ketterson, in Physical Acoustics (Academic Press, New York, 1992), Vol. XX.