1. R. W. Stark and C. B. Friedberg,Phys. Rev. Letters 26, 556 (1971).
2. C. B. Friedberg and R. W. Stark, inLow Temperature Physics LT-13 (Proc. XIIIth Intern. Conf. Low Temp. Phys., Boulder, Colorado, 1972) (Plenum, New York, 1973).
3. R. P. Feynman, R. B. Leighton, and M. Sands,The Feynman Lectures in Physics (Addison Wesley Publishing Co., Reading, Massachusetts, 1964) Ch. 37, Vol. I and Vol. II, Ch. 15.
4. R. W. Stark and L. M. Falicov, inProgress in Low Temperature Physics, C. J. Gorter, ed. (North-Holland, Amsterdam, 1967), Vol. V, and references therein.
5. J. C. Kimball, R. W. Stark, and F. M. Mueller,Phys. Rev. 162, 600 (1967).