1. W.A. Phillips, Amorphous Solids: Low-Temperature Properties ( Springer-Verlag, Berlin, 1981 ).
2. C.C. Yu and A.J. Leggett, Comm. Cond. Mat. Phys. 14, 231 (1988).
3. E.R. Grannan, M. Randeria and J.P. Sethna, Phys. Rev. B41 7799 (1990).
4. U. Buchenau, H.M. Zhou, N. Nicker, K.S. Gilroy and W.A. Phillips, Phys. Rev. Lett. 60, 1318 (1988).
5. H. Ikari, Phys. Lett. Al23 179 (1987).