1. J.H. Bishop, D.W. Cutter, A.C. Mota and J.C. Wheatley: Journ. Low Temp. Phys. 10, 379 (1973)
2. W.C. BTck, A.C. Mota, J.C. Wheatley, J.H. Bishop and P.M. Brewster: Journ. Low Temp. Phys. 4, 391 (1971)
3. A.J. Leggett and M. Vuorio: Journ. Low Temp. Phys. 3, 359 (1970)
4. J.P. Harrison: Journ. Low Temp. Phys. 37, 467 (1979)
5. J.P. Harrison and J.P. Pendrys: Phys. Rev. B8, 5940 (1973)